EVO MT1

How do genetic bottlenecks cause dramatic drifts in allele frequencies?

-Allele frequencies generally fluctuate much more during the bottleneck than before or after (found with modeling effect of bottleneck on multiple populations of lizards) -Bottleneck causes divergence between populations. Before the bottleneck allele frequencies are similar in all populations. After the bottleneck, allele frequencies differ greatly from one population to the next

What type of geology did Cuvier believe in?

-Like most science before Darwin, operational theory for geologists was that God was prime cause of all events including earthquakes, volcanoes, floods, etc. -Cuvier accepted existence of fossils, but said fossils were evidence of catastrophic events caused by God to wipe the Earth clean and start over -To Cuvier earth's history was composed of a series of Epochs which all ended in catastrophe (this view point eventually became known as Catastrophism -an example of a catastropism would be that canyons are byproducts of Noah's flood

Who was Hugo de Vries?

-Rediscovered Mendel's work -Developed the concept of genes -Introduced the term mutation -Published The Mutation Theory (1900): -Posits that evolution largely happened due to mutations of very large effect that were acted on by natural selection and changed the phenotype of the offspring. This is wrong as we know, Vries said evolution was very large jumps and conflicts with the idea that evolution is a slow and gradual process as said by Darwin. De Vries is important because he introduces Gregor Mendel to biologists.

What did Sewell Wright state about gene flow?

-Sewell Wright argued that gene flow can play a creative role in evolution by introducing new alleles to novel environment

What is the Morphological species concept?

-Species are defined based on morphological similarities and differences, often via rigourous analysis of the phenotype -Advantages: Widely applicable, can be applied to extant and extinct species, as well as to sexual or asexually reproducing species, it's easy to understand and it's intuitive -Disadvantages: Requires very careful and often tedious taxonomic work as there is no easy answer to the question how much difference is required before two populations are defined as separate species, difficult to apply to small organisms that do not have a large number of measurable morphological traits such as bacteria, archae or fungi. Finally, behavioral or environmental traits that differ among populations may not be recognized although they are important. -It has become clear that there are a number of organisms that do not have clear morphological differences but are nonetheless very different in terms of their genetic divergence within a single nominal species, particularly the case in many aquatic microorganisms. As a result, many biologists have started to describe species not on the basis of reproduction or phenotype, but on quantities that can be incorporated into a phylogenetic analysis.

What did Thomas Huxley say about Darwin's book?

-Thomas Huxley received the book positively and called the book "a machine gun in the armory of liberalism", also said that the sufficiency of Darwin's theory must be tried by the tests of science alone and no other -Huxley was arguing that On the Origin of Species was an important scientific theory and it should be evaluated as such, he knew that it would be controversial to non-scientists

What are postzygotic barriers?

Hybrid zygotes are formed but have reduced fitness

What is the Balance (selectionist view) on the importance of genetic drift in maintaining genetic diversity in populations?

genetic diversity is maintained by natural selection = natural selection in favor of rare alleles and heterozygous individuals at different times and places keeps diversity in populations

Who was Thomas Hunt Morgan?

supported Mendel's views and replicated Mendel & de Vries' experiments using Drosophila (fruit flies) -Morgan popularized Drosophila as a research system due to their short generation times, being easy to care for, and they had large chromosomes that were easy to see under the microscope

What is the significance of Miller et al. (2006)?

surveyed thousands of people in 34 different countries and asked "Did humans evolve from other animals, and most recently share a common ancestor with chimpanzees?", respondents could answer yes, not sure, or no. -Despite the near universal acceptance of the theory of evolution by means of natural selection by biologists, a large portion of people, including the majority of respondents from the U.S. thought the statement was false or were unsure Towards the end of Darwin's lifetime, people knew him as the person that demonstrated that evolution has occurred, but no one believed natural selection was the mechanism that drove evolution -It took work of geneticists during modern synthesis of 1920s-1940s to provide evidence that convinced people natural selection was the mechanism that caused evolutionary change

What is Darwin's Tangled Bank?

"It is interesting to contemplate a tangled bank clothed with many plants of many kinds. With birds singing in the bushes with various insects fitting about and worms crawling through the damper. And to reflect that these elaborately constructed forms so different form each other are different upon eachother in so complex a manner have all been produced by laws acting around us. These laws taken in the largest sense: growth with reproduction, inheritance which is implied by reproduction, variability from the indirect and direct actions in the conditions of life and from use and disuse, a ratio of increase so high as to lead to a struggle for life and as a consequence to natural selection entailing divergence to character and extinction of less improved forms. Thus from the war of nature from famine and death, the most exhalted object with we are capable of conceiving, namely the production of the higher animals directly follows. There is a grandeur in this view of life with its several powers having individually breathed by the creator into a few forms or into one and that whilst this planet has gone cycling on according to the fixed law of gravity. From so simple a beginning, endless forms most beautiful and most wonderful have been and are being evolved.

What is the significance of Hopi Hoekstra and her work with Peromyscus Polionotus mice?

+Hopi Hoekstra's work on deer mice show evidence for natural selection -Hoekstra investigated a species named Peromyscus Polonius which ranged from brown to light coloring of fur -Because mice are hunted by natural predators, we would think natural selection would favor cryptic coat coloration -This prediction assumes differences in coat color -Many different loci interact epistatically to determine coat color in mice -EX. Protein MC1R, a protein well characterized in mammals -MC1R acts as a genetic switch, determining whether dark eumelanin or light phaeomelanin is produced -Allelic variation at the MC1R locus is strongly associated with phenotypic differences in individual mice and their coat colors -In small isolated barrier islands mice range from lighter brown to white while mice in inland Florida population were darker in their coloration Peromyscus system demonstrates the key components of Evolution by Natural Selection through 4 postulates: 1. Is there genetic variation within the population? 2. Is this genetic variation heritable? 3. Is there variance in reproductive success? 4. Is this reproductive success non-random and a function of the intrapopulation genetic variation? Work in Hoekstra's lab showed that all of these postulates are applicable in Peromyscus mice

How do you calculate degrees of freedom in a Chi-squared test?

-Degrees of freedom = df = k - 1 - m where k is the # of classes and m is the # of independent allele frequencies estimated from the data -In a single locus two allele system, the df = 1

What are examples of premating barriers?

-Ecological isolation: species breed at different times (temporal, like Rhagoletis), or in different places (habitat isolation, like sticklebacks) -Mating isolation: potential mates come into contact but do not mate (behavioral in animals, mediated by specialized pollinators in plant) -Physical barriers

How does self-fertilization affect # of heterozygotes/heterozygosity?

-Ex. snails can "self", "selfing" is an extreme form of self-fertilization used if a mate cannot be found -Extreme forms of self-fertilization will result in a decrease in the number of heterozygotes, this is because all snails that are homozygous at a given locus will produce offspring that are also homozygous -However, heterozygous snails will produce heterozygous babies only half of the time, the other half will be homozygous due to the law of independent assortment. Thus, over time, lots of "selfing" or extreme self-fertilization will result in a decrease in the proportion of heterozygous snails in the population

What is the Bankruptcy effect?

-Gambling games at a casino are slightly in favor of the casino, but odds come very close to 50/50 in some games like blackjack depending on the # of decks in play. Thus the player and casino stand to make about equal amounts of money on average. However, you often hear of people losing all of their money, it's rare to hear about a casino that goes bankrupt. The real house advantage is not the odds of a game, but in the amount of money the casino has relative to the player. Bankroll casino > bankroll player. Thus the player is much more likely to hit 0 first. -This has a connection to genetic drift, alleles once they reach a frequency of 0 in a population, cannot rebound, this is called fixation, they're essentially bankrupt. You're more likely to go bankrupt allelicly when you already had a low proportion of alleles in pop. -Higher chance of fixation when allele freq. is LOW Note on bankruptcy effect: An allele is said to reach fixation when it is the only allele in the population. This is related to the bankruptcy effect because an allele that is new in a given population (i.e., either from a new mutation or carried into the population via migration) will be at very low frequency. Genetic drift may cause its frequency to increase, but since it starts so close to zero drift will likely cause it to be removed from a population rather than causing it to increase to an appreciable frequency. This is analogous to what is likely to happen to a person who visits a casino: because they start close to zero (in terms of budget) particularly when compared to the casino's budget, random fluctuations are likely to lead the person visiting the casino to go broke before the casino does.

What does the h coefficient tell us about the type of selection?

-If 0<h<1 we have directional selection -If h<0 we have stabilizing selection (heterozygote better so selection for average phenotype) -If h>1 we have disruptive selection (heterozygote much worse, so selection for completely dominant or recessive phenotypes)

Which Scientists formed the Modern Synthesis view?

-JBS Haldane who showed mathematically that alleles with even slight fitness advantages would dominate a population over time. (ex. he showed peppered moth which was wild type before industrial revolution, however after industrial revolution over 99% of moths were the melanistic (dark type) form because of the soot and ash pollution staining trees black. -Sewell Wright was a field biologist who was good at math, he introduced the metaphor of the adaptive landscape, and developed the concept of effective population size -Theodosius Dobzhansky conducted field research on Drosophila and was the first to note the importance of genetic drift in small populations -Ernst Mayr was an ornithologist who defined species as breeding populations that were reproductively isolated from other such populations, this is basically the Biological Species Concept (BSC)

What are Forces that increase heterozygosity?

-Mutation: Creates new alleles at some rate m. In small populations these are quickly lost due to drift, but in large populations those can accumulate. -Balancing Selection: Selection favors heterozygous individuals; also known as overdominance where h<0 -Outbreeding: (prairie chickens) reverses the effects of inbreeding depression, where rare deleterious alleles are exposed in homozygotes. Delta = 1- ws/w0 (fitness of inbred crosses/fitness of outbred crosses) -Gene flow: Island model, gene flow changes frequency of allele p on the island by deltapi = m(pc-pi) where m is the proportion of the island population that migrates. Eventually, equilibrium is reached when delta pi = 0 and the allele frequencies on the island and continent are equivalent.

What is the summary of mutations and evolution?

-Mutations are important to evolution, but not because they drive the process of evolution as assumed by de Vries because they don't significantly change genotype frequencies -Point mutations provide genetic variation, which in some cases leads to alleles with a selective advantage. In other cases these mutations cause a loss of function, and are removed from the population by selection -Other mutations, such as gene or chromosomal duplication, provide an influx of new genetic material for evolution to act on

What processes can lead to linkage disequilibrium?

-Natural selection can create linkage disequilibrium. Ex. predation wiping out individuals that carry a certain haplotype, then population will be in linkage disequilibrium as population would be missing a haplotype - Genetic drift can create linkage disequilibrium, works in combination with mutation to create new haplotypes -Population admixture can create linkage disequilibrium, depends on the mixing of populations that contain different haplotype frequencies

What was the significance of Aristotle?

-Philosophers such as Aristotle were interested in the natural world and were among the first to argue that human reason and observation could lead to an understanding of it -Believed species were fixed or unchanging with eternal forms or essences named eides

Who was Alfred Russel Wallace (1823-1913)?

-Prompted Darwin to work up the nerve to publish his ideas about evolution -Wallace was a British adventurer from a modest background who made a living collecting specimen for museums and professors -As a collector, Wallace could not avoid thinking of the great diversity of species he encountered -It was obvious to Wallace that no two individuals were alike; here was a great deal of variation within the species he observed -It was in his best interest to keep track of where he found different species -In 1855 he noted "every species has come into existence coincidentally both in time and space with a pre-existing closely allied species." -This means by 1855 Wallace accepts the idea of evolution -Biogeographical break point / Wallace's line: "boundary" (imaginary) between two land masses or islands who show great differences in species which inhibit each respective island -Like Darwin, Wallace was unsatisfied with explanation species were specially created by God -He also concluded creation did not make sense and some other law regulated the distribution of species -By 1858 Wallace is sure species evolved, but he wanted to know how they evolved? -During an outbreak of malarial fever and while he lays feverous, he thinks of essay written by Thomas Malthus and concludes life of wild animals was a struggle for existence and slight differences in individuals give advantages in the struggle they would pass on to their offspring -Wallace felt so certain that this was a solution to the question of evolution that he wrote a letter to Charles Darwin

What are forces that decrease Heterozygosity?

-Self-Fertilization: (snail) heterozygous individuals produce both homozygous and heterozygous offspring, decreasing # of heterozygous individuals over time -Non-Random mating: Heterozygosity of an inbred (HF) population is equal to the expected heterozygosity of an outbred population minus the coefficient of inbreeding: Hf=h0(1-F) -Population genetic structure: Too many homozygotes; can calculate FST by 2pq(1-f) = A12. This is known as the Wahlund effect. -Purifying selection: New alleles that enter population due to migration or mutation are not adaptive and lost due to natural selection -Directional selection: Selection favors one allele over others -Genetic Drift: Fixation probability is equal to the allele frequency in the population. Genetic drift is most effective in small populations because any new allele will have higher initial frequency. (defn.): Variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or don't reproduce

What is Wright's F-statistics?

-Sewell Wright developed a simple approach to measuring population genetic structure -assume that non-random mating leads to population genetic structure -quantify the proportion of heterozygous individuals (freq. A12) -with no substructure (F=0), the frequency A12 = 2pq(1-0) -If there is no substructure, introduce F coefficient into equation and solve for F. 2pq(1-F) = freq. A12 A completely structured population has Fst = 1.

What is the Biological Species concept, popularized by Ernst Mayr?

-Species are groups of interbreeding, or potentially interbreeding, populations (Mayr 1942) -Reproductive isolation (R.I.) is the criteria used to delimit species by taxonomists who apply the BCS -Populations that do not hybridize in nature, or produce infertile offspring, are considered to be good species -To Mayr, the BCS confirms the lack of gene flow, which ensures that these entities are on separate evolutionary trajectories -Forms the legal basis of the US endangered species act and appears to be very reasonable -Why don't all biologists use this concept? -Cannnot be applied to organisms that reproduce asexually such as bacteria or archae, and to many plants which hybridize freely -Cannot be applied to organisms that cannot be easily studied in nature (for verification of RI=reproductive isolation) or to extinct organisms as there is no way of establishing reproductive compatibility -Gene flow is notoriously difficult to investigate -Can be difficult to apply in practice, particularly in populations that are geographically isolated from one another because it makes use of this potentially interbreeding criterion

What was the significance of Young et al (1996)?

-surveyed populations of flowering plants and compared population size with two measures of genetic diversity -Results suggested population size is positively correlated with genetic diversity, which meets our theoretical predictions

What are the 3 possible outcomes after formation of hybrid zones?

1. Fitness of Hybrids = Fitness of Parental Form, thus we predict the hybrid zone will be relatively wide and would last a relatively long time. Parental populations will eventually blend together as the differentiation between the parental population decreases. 2. Fitness of Hybrids > Fitness of Parental Form, In this case the size of the hybrid zone will depend on whether fitness advantage occurs in ecotone or in a new habitat. An ecotone is a transitional region between two distinct habitat types. Hybrids that have a fitness advantage in an ecotone are likely to stable over evolutionary time and may form a new species. Eventually, either a stable hybrid zone or a new species will form. 3. Fitness of Hybrids < Fitness of Parental Forms. Perhaps the most common outcome. Neither parental form should invest resources into production of hybrids due to lower fitness. In this scenario through natural selection against hybrid genotypes should lead to reinforcement of reproduction isolation through selection for assortative mating. Thus, any trait that leads to less reproduction with the opposite parental form should be favored by natural selection. Hybrid zone will be relatively narrow and short lived. If this is the case, then reinforcement of reproductive isolation will lead to speciation.

So natural systems should commonly have directional and stabilizing selection which reduce genetic variation, however in natural populations we find lots of variation. Why is this?

1. Most populations might not yet be at evolutionary equilibrium, while there are favorable mutations that are rising in frequency, these mutations are not yet fixed and therefore there is genetic variation. 2. Most populations might have a balance between deleterious mutations and natural selection, and that selection on many QTLs could be relatively weak. 3. Seems possible that disruptive selection might be more common than we generally realize.

When did Charles Voyage on the HMS Beagle?

1831-1835

When did Darwin marry his cousin Emma Wedgewood?

1839, he had 9 children, two of which died

When was On the Origin of Species published?

1859

What is the difference between allopatric and sympatric speciation?

Allopatric: Sticklebacks from lakes where there is no differentiation between limnetic and benthic forms Allopatric speciation is the evolution of species caused by the geographic isolation of two or more populations of a species. Sympatric: Clear differentiation into limnetic and benthic forms Sympatric speciationis the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region.

What are the assumptions of HWE?

Assumes no forces are acting to change allele frequencies in a population of diploid, sexually-reproducing organisms that reproduce in discrete, non-overlapping generations - No mutation: no new alleles are introduced via point mutations - No selection: alleles are not adaptive - No migration: no new alleles introduced via movement of individuals - No genetic drift: population size is infinitely large (or near so) - No assortative mating: each individual has an equal chance of producing offspring with any many of the opposite sex

What is the General Lineage Concept?

Biological, Morphological, and Phylogenetic species concepts all work well in some systems but have various shortcomings that prevent them from being universally applicable. However each of these have a similarity in that they share a common way of thinking about species. All consider species to be evolutionary lineages. Species concepts are basically in disagreement about the criteria that we use to identify evolutionary lineages. -De Querioz popularized the idea that we should stop fighting about which of these criteria are best because all of them are likely to play some role in the process of lineage diversions. What he means is: -At some point in the past, there was a single species, ex. humans and chimps share common ancestor 5-7 mya. In the present we are easily recognized as separate species as we are genetically, morphologically, behaviorally distinct but the reason for our present distinctness is at various points during the last 5 or so million years we and our sister lineages evolved all of these traits that made us so easy to recognize and diagnose. Worth noting this is strikingly different than the cryptic bat species example as they are much more closely related and have not evolved these different traits. To de Querioz we should not argue about which of these traits is the single best one to use for diagnosing species, rather we should attempt to recognize the underlying evolutionary lineages and study the evolutionary forces that lead to these traits in the first place. As practical people, many biologists find this idea appealing.

Who was Charles Darwin?

Charles Darwin (1809-1882) -Son of prominent Physician Robert Darwin (1766-1848) -Was a poor student at the University of Edinburgh Medical School -Was so poor of a student that he was asked to leave medical school, was then sent to Cambridge with the goal of becoming an Angelican arson as his father wanted (1st choice physician, 2nd choice parson) -Graduated at 22, Robert wanted Charles to marry his cousin and have grand children

How did the Cape de Verde islands influence Darwin?

Charles explored the coast and recognized the Uniformitarian view of geology really explained these islands perfectly, became more convinced that Catastrophism was unnecessary to explain physical features

Who was Richard Owen (anatomist)?

Defined Homology as the basic forms that underlay the anatomical features of vertebrates -A homologous structure is an example of an organ or bone that appears in different animals, underlining anatomical commonalities demonstrating descent from a common ancestor. In other words, it's when very different animals have bones that appear very similar in form or function and seem to be related. -this lead him to question why species that were supposedly intelligently designed for very different lifestyles (ex. mole, bat, dolphin) had very similar anatomies in areas with minor modifications

How do directional, stabilizing, and disruptive selection (either homozygote) affect phenotype mean?

Directional selection changes mean Stabilizing selection does NOT change mean Disruptive selection does NOT change mean, but increases variance

Who was Erasmus Darwin (Charles' Grandfather)?

Erasmus Darwin (Charles's Grandfather) -Wrote book named Zoonomia where he speculated that the earth is very old and that all warm-blooded animals descended from one ancestor, he was ridiculed for this and Darwin noticed

What did Ernst Mayr speculate?

Ernst Mayr also speculated that the founder effect may lead to speciation due to effects of genetic drift

What was the significance of Feder (1990)?

Feder & Bush (1990): Presented evidence for genetic differentiation at 7 loci between hawthorne and apple rhagoletis collected at different sites. Result showed genetic differentiation between flies at many local populations. In addition, certain alleles are more likely to me found in northern populations and others in southern populations. Why might this be the case? In both the apple and hawthorne flies, mating and oviposition are influenced by chemical cues that are tied to the development of the host plant. In plants growing season of course is heavily influenced by latitude as this reflects the amount of sunlight the plant receives. It is clear that both the host plant and the abiotic environment are influencing genetic diversity in rhagoletis flies. However this is not established as ecological speciation because this still does not provide a clear link between the evolution of reproductive isolation and the traits that are under selection by the environment. It would take another paper out of the bush lab to establish this link.

How do you determine linkage disequilibrium?

For populations in linkage equilibrium we expect that the products of the homozygous and heterozygous haplotypes should be the same, we can use this expectation in our test of linkage equilibriu mas follows. Say D = [Fr (AB) x Fr (ab)] - [Fr (Ab) x Fr (aB)] If D = 0, population in linkage equilibrium If D not = 0, population in linkage disequilibrium D is the coefficient of linkage disequilibrium -Just as in HWE tests or kimura's neutral model, we can compare our calculated D to a statistical distribution to see if the observed value is significantly different from 0

What did Rundle & Schluter et al. (2004) show?

Found evidence for both behavioral and phenotypic differences that influence mate choice directly in sticklebacks The stickleback data collected by Schluter and colleagues support the theory that ecological processes can drive speciation via a process of ecological speciation. This is when divergent natural selection pressures drive the accumulation of differences that lead to reproductive isolation between populations that live in different environments. Recently there has been a resurgence in the interest of this theory. Patrick Nosil (2012) argues that the theory of ecological speciation is useful because it makes a key prediction, namely that reproductive isolation is correlated with adapative and ecological divergence when ecological speciation has occurred. Nosil lays out 3 requirements for there to be ecological speciation: 1. A source of divergence selection, 2. A form of reproductive isolation, 3. A way to link the divergence selection to the reproductive isolation -Benthic fresh water sticklebacks had reduced pelvic and dorsal spines as they faced more predators and reduced gill rakers, which are typically used for filter feeding by limnetic and marine forms -Limnetic sticklebacks had longer spines along the dorsal spine due to predation

How does Genetic drift disproportionately affect different size populations?

Genetic drift will cause fluctuations in gene frequencies in all size populations, but these fluctuations are far more dramatic in small populations and are therefore more likely to result in fixation One of the most important ways genetic drift influences evolution is by removing genetic variation from populations -Genetic drift decreases the heterozygosity of a population One way that genetic drift influences evolution is by removing genetic variations from populations, and thus decreasing heterozygosity. If this is true we expect to see population size and genetic diversity is correlated.

Who was Cuvier?

George Cuvier, Natural Theologian: -Natural Theologian who argued that species were unchanging, borrowed idea from Aristotle -Argued that anatomical features were so perfect and optimally balanced that this was clear evidence of natural selection -To Cuvier, any variations within species were defects or deviations from perfect items -Argued Evolution could not occur because all parts of the organism were so perfectly balanced, and clearly designed to work together, if one part changed it would have a diminishing effect on the organism and would influence the functioning of all other parts -Basically said: evolution could not exist because variation was bad

What was THE problem with Darwin's theory? Why did Darwin fail to convince readers that natural selection was the primary force that produced change?

He didn't have a mechanism for inheritance, and used blending inheritance. thus BLENDING INHERITANCE WAS THE PROBLEM 1. He knew little about heredity, and what he thought he knew was often wrong ex. blending inheritance 2. He argued that natural selection was very slow, but at the time the earth was believed to be not that old 3. He gave too much credence to the idea that mutations of large effect induced/influenced evolution

How do you determine Heritability / Broad sense heritability?

Heritability is defined as the ratio between the genetic variability and the phenotypic variation (assumed to be composed of the gene x environment variation) H = VG / VP = VG / (VG + VE) where G = genetic, P = phenotypic, E = environmental. This term is also known as the broad-sense heritability and is important because it provides an avenue for how the environmental processes influence phenotypically plastic traits

What is Genetic Hitchhiking?

Hitchhiking is the process by which a neutral or slightly deleterious allele increases in frequency because it is physically close to (and thus in linkage disequilibrium) with an allele under positive selection

What was Darwin's dilemma?

How can natural selection favor (allow) the evolution of unfit offspring? The best known solution to Darwin's Dilemma is now known as the Bateson-Dobzhansky-Muller model (BDM) and explains the evolution of extrinsic inviability as follows Extrinsic inviability: Bateson-Dobzhansky-Muller model (BDM) 1. Two allopatric populations (A&B) evolve separately (ex. limnetic and benthic forms) 2. Each experiences substitutions over a period of time (genetic differentiation of A&B) 3. Would a gene from A work as well if translocated to B? It would be unlikely to work better. 4. Some of the genes from A should be expected to perform worse in population B because there will have been some local adaptation. The proportion of the A genome that would perform worse is a function of the degree of difference in the local adaptation of A&B 5. It is these genes that lead to extrinsic inviability The BDM model predicts we should observe evidence of reinforcement, or selectively favored traits that confirm mating discrimination -In sympatry (after speciation), females preferentially mate with their own species to produce more or fitter offspring than those females that mate at random -Males benefit from mating preferentially with females of the same species rather than wasting resources on hybrid or heterospecific females -Natural selection will favor alleles that confer mating discrimination

What were Darwin's two goals in On the Origin of Species?

In On the Origin of Species, Darwin had two goals: 1. Convince his audience that species changed through time (e.g. demonstrate the fact of evolution), which he was largely successful in, many were convinced of this. Successful in large part due to amount of evidence + detail he provided in the book. 2. Provide a mechanism that explained evolutionary change (e.g. natural selection), Darwin was a failure in this regard because he argued for blending inheritance and assumed that populations contained little genetic variation, and Darwin's arguments of natural selection don't make much sense if you assume these things -As a result, later edition invoke different evolutionary processes in addition to natural selection, most notably mutations of large effect, and Lamarckism (hypothesis that organism can pass on characteristics it has acquired through use or disuse during its lifetime = inheritance of acquired characteristics).

What is a QTL Map?

In order for biologists to use Quantitative Genetics to investigate populations, we need to develop something called a QTL Map QTL Map: The use of marker loci to scan chromosomes and identify regions that contain genes that contribute to some trait. -Samples are collected from a population that displays a range of phenotypes for the trait in interest -Genetic markers are used to genotype individual samples -Correlation between particular alleles and the trait are recorded -Regions with high correlation (Y-axis) include markers that are near to the genes that code for the trait in question QTL Mapping can be used to test for phenotypic effects -Mimulus cardinalis and M. lewisii are sister taxa that produce fertile hybrids in the lab. They are pollinated by hummingbirds and bees, respectively, so they are separated in nature. Bee-pollination is ancestral.

What is inbreeding depression?

Inbreeding depression results from the exposure of homozygous recessive alleles to purifying selection -Defined as: Delta = 1 - ws/w0 where w sub s is the fitness of inbred crosses and w sub 0 is the fitness of outbred crosses -Inbreeding depression can essentially be thought of as the decrease in fitness as a result of inbreeding -Inbreeding depression is more common in plants, due to selfing and often the proximity between parents and offspring -Animal example: Greater prairie chicken

What is Darwin's view of species?

Intraspecific variation is present within species, and absolutely essential for Darwin's mechanistic explanation of evolutionary change. (We don't use this view)

What are the two types of postzygotic isolation mechanisms?

Intrinsic factors: caused by developmental defects of the hybrids independent of the environment. Ex. gametic incompatibilities due to chromosomal inversions in sticklebacks Extrinsic factors: environmentally dependent, caused by ecological factors that lead to poor performance by hybrids because they occupy a space intermediate to parental niches

What is Isolation by distance?

Isolation by Distance (IBD): The correlation between physical separation and genetic distance as suggested by Sewell Wright in 1943. -Perhaps physical separation is the leading cause in population genetic structure

Absent any other forces, what will migration do to allele frequencies on an island and mainland?

It will even them out -Thus when we observe island populations with different allele frequencies then the continental populations we often assume that selective pressures on the island are different than those on the continent, otherwise we would expect these allele frequencies to be the same

What are postmating, prezygotic barriers?

Mating or transfer of gametes occur, but no zygotes form. -Mechanical: copulation, but no transfer of male gametes due to physical incompatibilities (bacula size in chipmunks) -Gametic isolation: gametes are not transferred due to intrinsic incompatibilities, such as sperm precedence or pollen tube precedence

What is reinforcement?

Mechanism that leads to the reduced fitness of hybrids, and this mechanism forms when one species undergoes allopatric differentiation. If the barrier disappears, the two population are again sympatric where they come into contact and this is known as a hybrid zone. -The key relationship that allows us to understand the dynamics within and ultimate fate of a hybrid zone are the finesses of the hybrids in comparison to the fitness of the parental species -Reinforcement predicts that prezygotic isolation will evolve more rapidly in sympatry. -We expect this scenario to be common in nature since species ranges are very dynamic and change over time

What is Methodological Naturalism?

Methodological Naturalism (Philosophy we use today): States modern science is based on the premise that our senses, and extensions of our senses through the use of instruments, can give us accurate information about the universe

What is the significance of Shapiro et al. (2013)?

Mike Shapiro: -Collected whole genome data from 40 different breeds of pigeon -Observed lots of genetic diversity in pigeons, as he observed heterozygosity suggested large effective population sizes -Certain genes in pigeons associated with certain phenotypes -Gene Phb2 is identified as gene that all pigeons with crests share, and there's only fixed difference in this region, the CR SNP -CR SNP causes change in AA that causes variations -Identified regulatory factors and effects of CR SNP (pronounced Snip)

What was the significance of Bradshaw et al. (1995)?

Mimulus cardinalis and M. lewisii are sister taxa that produce fertile hybrids in the lab. They are pollinated by hummingbirds and bees, respectively, so they are separated in nature. Bee-pollination is ancestral. -Bradshaw et al. (1995) crossed parentals of M. mimulus and M. lewisii to make F1 hybrids, and then crossed the F1s. F1s were essentially all heterozygous, but F2s showed a range of variability in the phenotype. Bradshaw et al. then used a genetic marker to develop a QTL map. Phenotypic traits that were seemingly related to pollination were identified and grouped together by examining the statistical correlation between these traits and the genetic markers. 3 Examples of these traits shown in QTL maps. One QTL map measures traits that attract the pollinator to the flower, such as the color of the petals, the width of the corolla and the width of the petals. Other QTLs are related to things such as the amount of nectar produced or the length of the stamen and pistils of the flower. All of these traits are easily quantified by Bradshaw et al. and clearly related to pollination. Found data that supported Orr's view vs. RA fisher's. Orr's view was that few loci can have a very large effect on phenotype and that QTLs can determine the phenotype. RA fisher believed alleles driven to fixation by natural selection (and thus responsible for adaptive differences) were subtle in their effect on phenotype. In essence, he suggested nearly every phenotypic trait is subject to the actions of many loci or relative small effects Bradshaw et al. posited that if Orr's view is correct, QTLs would exist that determine flower shape, size, and color. and he found QTL maps for this and proved Orr correct

What is Natural Theology?

Natural Theology: philosophy inspired by the Enlightenment, obsessed with reason and empirical observation. Natural Theologians often used observations to justify belief in divine creation -Natural Theologians had an interest in nature and reliance on exploration and observation -However, we would not agree with interpretations Natural Theologians made

What is Ecological speciation?

Nosil 2012 defined Ecological Speciation as the process by which barriers to gene flow evolve between populations as a result of ecologically based divergent selection between environments. - 3 requirements for this: - 1. A source of divergence selection, - 2. A form of reproductive isolation, - 3. A way to link the divergence selection to the reproductive isolation. Gathering evidence to do this is challenging but we are now starting to accumulate systems where this is possible

What did Hugo de Vries think about mutation?

Obsessed with mutation and believed mutation to be prime cause of evolution -Background info to understand de Vries' work -Gene: a segment of DNA that codes for a distinctive type of RNA or protein -Point Mutation: a single-base substitution in the sequence -de Vries imagined mutations as changes that lead directly to new phenotypes, and Darwin came to accept this idea and future editions of On the Origin of Species spent more space discussing mutations of large effect However, both de Vries and Darwin had an incomplete view of mutation, as it turns out, they are important to the process of evolution because they provide the raw material of genetic variation that natural selection can act upon

Who was Dick (Richard) Lewontin?

One of first people to attempt an enzyme assay, worked in the Hubby Lab -Studied the number of different alleles at different Loci -Showed Drosophila Pseudobscura was really variable at many of its Loci -Now question was, are Drosophila unique in this genetic diversity? -Pioneered application of techniques such as gel electrophoresis to questions of genetic variation and evolution

What is the Classic (Modern Synthesis) view of species?

Organismal phenotypes are well adapted to the environment, thus there should be little genetic variation within populations - largely what people found by looking at phenotypes

What are silent (synonymmous) substitution mutations?

Point mutations that don't change the sequence of the amino acid -Point mutations are relatively common in nature

What did Knowleton et al. (1993) show?

Published genetic data from 7 pairs of shrimp species that are distributed on either side of the ismus of panama. Geological evidence suggests that the issmus of panama was formed at least 3 million years before present. Knowlton's work shows there is a substantial amount of genetic divergence between sister species of shrimp found either in the pacific or the Caribbean. She then kept captive breeding colonies in her lab and tested which pairs of species could produce viable offspring. Knowlton found that none could. But for some species pairs this wasn't for a lack of trying, as they would still go through the mating ritual. Found a clear correlation between how similar two species were genetically and how compatible their mating behavior was. To knowlton et al. this suggested that genetic drift was at least partially responsible for the evolution of reproductive isolation because species that were isolated in behavior tend to be isolated in correlation to their degree of genetic separation. She reasoned that genetic drift could fix incompatible alleles, but it would take some time to accomplish. They reasoned that species with greater genetic divergence had been isolated longer and therefore drift had been able to fix the behavioral traits associated with premating isolation. So it appears that at least in this case genetic drift is driving the evolution of reproductive isolation.

What is Quantitative genetics?

Quantitative Genetics field developed by plant breeders to deal with traits like this -By tracking the heritability of traits and artificially selecting for the trait of interest, one can measure the differences in response to natural selection. -In fact, for artificial selection plant and animal breeders do this all the time. -Consider hypothetical mice that are bred for tail length. In each generation we can artificially select the top 1/3rd of the pop. of mice who have the longest tails. If these are the mice that we allow to breed and all other mice are removed from the population. The difference in phenotype under selection, tail length, between individuals who survive and those who do not is known as the selective differential. Selective differential = difference between mean of individuals who survive and those who do not, AKA strength of selection. If we plot the phenotype by the relative fitness, we can estimate the selection gradient by using the slope of the regression line between these values. Quantitative Genetics is useful because we can measure a few things that allow us to quantify the strength of natural or artificial selection -Simply by comparing phenotypes that are distributed continuously, we can compare the average to the expected phenotypic trait that is distributed trait that is distributed continuously. We can compare these values between parents and offspring. -Plot these values in chart, plot tail length of offspring as function of tail length of parents. Then compute difference between average values in offspring. -Scatterplot of midparent vs. midoffspring values of tail length (Pbar and Obar represent average population phenotypes). P* is the average of 10 highest parental values and O* is the average of 10 highest offspring values. Difference between Obar and O* is the response to selection (R).

What is Response to selection (R)?

R = h2S where R is the response to selection, h2 is the variability, and S is the selection differential. Slope of scatterplot = h2 = [O* - Obar] / [P* - Pbar] = R/S Selective differential = difference between mean of individuals who survive and those who do not, AKA strength of selection.

Who is credited with thinking about the inbreeding coefficient/inbreeding treatment?

Sewell Wright

What is Spatial sorting?

Spatial sorting is the process whereby variation in dispersal ability is sorted along density clines and will, in nature, often be a transient phenomenon. Natural selection had selected traits that are associated with dispersal, and this is an example of spatial sorting in action. -You see evidence for this in the size of individuals, greater variance of body and leg size particularly at the leading edge of the population expansion

What is the Aristotelian view of species?

Species were fixed and unchanging, with eteranal essences and evident variation attributed to developmental stressors or defects (commonly affected view at Darwin's time)

What is the difference between taxonomic and evolutionary species?

Taxonomic species are the names given to entities by taxonomists. In a perfect world taxonomic species would be an accurate representation of evolutionary species, but it might be better to think of a taxonomic species as a hypothesis of an evolutionary species that should be reevaluated as data are collected. -Evolutionary species are the collective group of individual organisms

What is the rate of evolution if genetic drift is the only force acting to change allele frequencies?

The MUTATION RATE If genetic drift is the only force acting to change allele frequencies, the rate of substitution is equal to the mutation rate, regardless of population size. While mutations have a lower probability of drifting to fixation in large populations, there are also many more mutations, so in effect all populations accumulate substitutions at the same rate.

What was the central thesis of On the Origin of Species?

The central thesis of Origin of Species concerns how natural selection leads to the formation of new species. This thesis has 3 components: 1. Variation is continual in natural population, and therefore species present difficult problems for taxonomy. -In his book, he points out that species should be easy to recognize if they were truly the result of special acts of creation by a deity. But Darwin knew that it was not easy at all to recognize species. 2. Isolation leads to adaptive change and therefore species are formed largely via the actions of natural selection. He didn't understand heredity, a mechanism for heredity, and he didn't understand genetics, but in his view natural selection was the driving force for speciation. -In his book he clearly links the environmental conditions that are experienced by isolated populations as the primary selective agent that leads to the prevention of intercrossing (hybridization). 3. Intercrossing/Hybridization provides data on genetic isolation and therefore offers a way to identify species -Darwin points out that the inability of two different organisms to create fertile offspring provides a distinction in the categorization of species. Darwin's central thesis is now clear, speciation is the result of natural selection acting on isolated population, this selection leads to divergence, and the formation of maladaptive hybrids that provide both evidence of this process and proof that natural selection is the primary agent.

What do different values for h in the heterozygous effect mean?

The parameter h is the heterozygous effect, which is am measure of fitness of the heterozygote relative to the selective difference between the two homozygotes - h = 0 means A1 dominant and A2 recessive - h = 1 means A2 dominant, A1 recessive - 0<h<1 means incomplete dominance - h<0 means over dominance of heterozygote - h>1 means under dominance of heterozygote

What is Vicariance?

The physical division of a once-contiguous species range via the formation of a barrier to dispersal ex. river, glacier etc. -May suggested vicariant speciation was one of the primary ways species were formed Species ranges can either be contiguous or disjunct, when ranges are disjunct, there may be apparent barriers to dispersal that divide the two populations of the species -Can lead to allopatric speciation

What is genomic conflict?

This theory holds that epistatic interactions between different loci are disrupted in hybrids and thus implies that genomic conflict is the root cause of hybrid incompatibility. At least 3 main classes of genomic conflict have been implicated in hybrid incompatibility. These classes include Cytonuclear interactions: mitochondrial genes inherited from the mother are in conflict with alleles at nuclear genes that are inherited from the father. Since the mitochondrial encodes genes associated with oxidative respiration, their function is vital component of the organism's phenotype. Predator defense genes: In plants, hybrid necrosis is caused by a mismatch between alleles in genes that create compounds used to deter herbivory Essential cell function: Neuroporin genes (eg. Nup96 found by presgraves) or DNA binding genes such as heterochromin that influence the condensation of the chromosomes during meiosis

Who was Thomas Malthus?

Thomas Malthus led Darwin to the idea that there is a struggle for existence between organisms as more individuals are produced than can survive/an environment can support -Thomas Malthus really suggested the idea of a struggle for existence

What is the Phylogenetic/Genealogical species concept?

Treats the ancestry of the species as the diagnostic character, argues one should infer the phylogenetic history of a group and define species based on some criterion related to this -For example, monophyly can be used as a criterion to delimit species - monophyletic groups are lineages that contain all of the known descendants of a common ancestor -Other scientists have offered refined definitions based on patterns of allelic similarity, clade membership, or genealogical clustering -Advantage: Ease of application, PSC can be applied to any species, PSC is also testable - species are named on the basis of statistically-significant differences in the traits used to estimate the phylogeny -Disadvantage: it can take a very long time to gather the data required to estimate a phylogeny with statistical confidence. And it is common for the same set of samples to be diagnosed differently depending on the type of data collected or the variety of phylogenetic species concepts that can be applied by the researcher. Some would argue it takes too much time and creates too many problems.

What is Uniformitarianism and who can it be largely attributed to?

Uniformitarianism proposed geological features result from the action of continuous processes that can be observed to be acting in the present. Such as erosion, glaciation, sedimentation etc. This implied that Earth was much older than suggested by the book of genesis -Attributed largely to Charles Lyell -This geological switch in thinking had important implications in biological thought -Darwin was influenced by the idea that features of the Earth could be explained by gradual accumulation of small changes, also influenced the view that Earth was much older than suggested by book of Genesis in Old Testament

What is the significance of Bustamante et al. (2005)?

Used McDonald - Kreitman test to identify loci that appeared to be under natural selection in humans, and matched selection in human chromosomes, using red to denote purifying selection and blue to denote positive selection. Identified that humans are absolutely still evolving and under selection and identified 813 genes under purifying selection, and 304 genes under positive selection. Many of genes under positive selection relate to olfaction or neural brain protein. Showed many genes on each chromosome are under selection, and they are often physically adjacent to each other.

What is the significance of Pelletier and Carstens et al. (2018)?

Wanted to know what factors promote intraspecific genetic structure. Along with isolation by distance, isolation by environment (which predicts similarity in environment will lead to similarity in genetic structure) was factored. Found that there was a correlation within species between environment and geography on a global scale (r=0.77), as a result many species are structured by both the environment and geographic distance.

Who was William Buckland?

William Buckland (1836) Natural Theologian: Wrote first account of a fossil dinosaur -Buckland argued fossils he discovered were evidence of previous divine creation -Attributed all creatures on planet to divine creation -Oxford trained minister who wrote first account of fossil dinosaur -Buckland, like all natural theologians used his observations to justify his belief of divine creation

Who was Gregor Mendel?

a monk who lived in the Austro-Hungarian Empire at about the same time Darwin was writing on the Origin of Species -Concluded a series of breeding experiments using the garden pea plant -His paper, Experiments in plant hybridization, was published in the proceedings of the Natural History Society of Brunn in 1865. This paper contained information about his series of experiments on garden pea plants which provided many views on genetics and heredity that became very important. -Unfortunately, his paper was written in German and is only referenced 3 times during his life time

What is the summary of genetic drift's ramifications on the evolution of populations?

a. When fluctuations in allele frequencies are caused by random sampling error, every population follows a unique evolutionary path b. Genetic drift is more rapid and dramatic in small populations than in large populations c. Given sufficient time, genetic drift can cause substantial changes in populations even if their sizes are very large -In addition, genetic drift interacts with mutations in an interesting way as explored in the next lecture Genetic Drift influences evolution by influencing the survival of new mutations -A new mutation that enters the population has a probability of fixation of 1.0 minus its initial frequency. -New mutations that occur in small populations have a higher initial frequency, and thus a better chance of increasing in frequency due to genetic drift -However, the number of mutations is a function of population size, so small populations are less likely to have new mutations than large ones

What is the significance of Shine et al. (2011)?

argued that dispersal will limit the rate of range expansion. Because of this individuals that disperse are likely to be those that had parents that were good at dispersing. Over many generations this will change the phenotypes related to dispersal at the leading edge of population expansion. Spatial sorting occurs because gene frequencies that are related to those trait values will change over time at the leading edge of the expanding population

What is the Wright-Fisher Model?

assumes a population of diploid, sexually-reproducing organisms that reproduce in discrete, non-overlapping generations (like HWE) -However, also assumes: -Natural selection does not operate on the trait (s) that influence the locus in question -Mating in the population is random with respect to the locus in question -No mutation occurs in the locus in question -There is no migration occurring into or out of the population -Does NOT assume population sizes are very large or nearly infinite, thus ASSUMES STOCHASTIC SAMPLING / GENETIC DRIFT Therefore, in small populations the W-F model can be seen as a counter point to the HWE model W-F Model like HWE BUT assumes genetic drift / stochastic sampling exists -Due to these sampling effects, allele frequencies could change from generation to generation, and this is possible due to small or non-infinite population size assumption of the Wright-Fisher model Under the Wright-Fisher model species are expected to lose heterozygosity over time just as they would under the standard model -This has been demonstrated in many systems, EX. in the overfished population of New Zealand snappers in the Tazman bay W-F is preferred for estimating heterozygosity in real populations, because we can estimate the expected loss in heterozygosity without knowing allele frequencies, it's simply a function of the term N and the # of observed heterozygous individuals.

What is the significance of the McDonald - Krietman (1991) Test?

developed a slightly better version of dn/ds ratio test. Their test is based on the prediction by neutral theory that the rate of replacement to silent substitutions should remain constant through time, therefore this rate should be the same within a species and between a species. McDonald and Kreitman in 1991 looked at Drosophila and found that there were 6 times as many differences between species than there are within species, and took this to be strong evidence as natural selection. In conclusion, neutral theory is a useful null model that allows researchers to identify examples of natural selection. Rejecting neutral theory allows us to infer that natural selection has occurred.

What is the Neutral Theory of Molecular Evolution?

evolution which predicts that the majority of nucleotide changes that become fixed in a population are neutral with respect to fitness. -Consequently Kimura argued that genetic drift is the dominant force acting on DNA sequence evolution, and as a consequence most of the differences between species at a natural level don't result from natural selection. This matches the theoretical prediction that the rate of evolution is equal to the mutation rate.

What is the significance of Steiner et al. (2007)?

explained the genomic mechanisms that control this system -By crossing white barrier island mouse with dark inland mouse, Steiner and colleagues were able to produce offspring with varying colorations, this shows there is genetic variation for coat color -Steiner also reported a single snip (SNP = single nucleotide polymorphism) that accounts for 36% of the variation in coat color -SNP (single nucleotide polymorphism): a variation in a single nucleotide of a single base pair in a DNA sequence that occurs at a specific position in the genome, where each variation is present to some degree within a population -Polymorphism: the occurrence of different forms, if something is polymorphic it's variable

How are inbreeding and heterozygosity related?

heterozygosity decreases at a rate proportional to the amount of inbreeding Inbreeding will not change allele freq. of a population, but will influence the evolution of a population because there's fewer and fewer heterozygous individuals

What is the degree to which we can think about loci independently a function of?

i. Epistasis: the interaction among multiple genes that influence a phenotype ii. Recombination: Recombination can break up linkage disequilibrium, and bring together advantageous mutations that evolved on different chromosomes -At appreciable rates of recombination, linkage disequilibrium will break down relatively quickly, thus loci that are found to be in linkage disequilibrium are likely to be a result of one of two factors: either recent genetic drift or admixture, or natural selection that acts on one of the two loci. Naturally evolutionary biologists tend to be more excited about the second explanation, so much so that they've made up a couple of new terms to describe this phenomenon

What is the Neutral view on the importance of genetic drift in maintaining genetic diversity in populations?

most alleles at polymorphic loci are functionally and selectively equivalent, and thus maintained by genetic drift

What are Replacement (non-synonymous) substitution mutations?

point mutations that change the sequence of the amino acid

What is the significance of Jones et al. (2012)?

sequenced the genomes of several different sticklebacks including marine, benthic, and limnetic freshwater forms. The results showed a very clear divergence between the marine and freshwater sticklebacks. Jones et al. then compared the genetic differentiation to the differences in phenotype between the limnetic and benthic stickleback. They found that less than 0.5% of the divergence in the genome is correlated with morphometric differences between the benthic and limnetic forms. The remaining genetic variance was consistent with genetic drift and population demographic effects, which makes sense as each lake would likely have a very small founding population and small population are very susceptible to genetic drift. However, there was still a substantial number of snips associated with divergent morphological forms. This Jones et al. data also suggested chromosomal inversions that lead to differences in phenotype as well as reproductive isolation. Jones et al. attribute most of the morphological differentiation to regulatory rather than coding changes. These regulatory changes are likely the result, at least in part, of chromosomal inversions. In this case the chromosomal inversions that are a component of the standing genetic diversity in sticklebacks are selectively advantageous when some of these fish get into a freshwater environment, this provides a clear mechanism for reproductive isolation in addition to the difference in habitat. In summary, the data presented by Jones et al. constitutes strong evidence that marine and freshwater stickleback forms are genetically distinct due to some combination of founder effect and adaptation and provides a potential mechanism for reproductive isolation. However, while adaptive changes appear to be important in the divergence of the limnetic and benthic forms, it is unclear what these forms represent. Are they different morphotypes of the same species, incipient species, or different species entirely? Did adaptation lead to reproductive isolation as predicted by ecological speciation or did some other factor such as the chromosomal inversions cause the reproductive isolation first followed by divergence due to the fish living in different environments.

How is used neutral theory used as a null model?

used as a null model when homologous DNA sequences are compared because it predicts the rates and patterns of substitutions in the absence of natural selection. -By comparing the ratio of nonsynonymous to synonymous substitutions (dN/dS) you can make inferences about if and what type of selection has occurred - dN/dS <1 when replacements are deleterious (purifying selection) - dN/dS = 1 when replacements are neutral - dN/dS > 1 when replacements are advantageous (directional selection)

What did Mendel find about inheritance?

was conducting studies shortly after the publication of on the origin of species that demonstrated a different type of inheritance -Mendel found through crossing heterozygous pea plants that resulted in ratios of (3:1 for Aa x Aa cross) and (9:3:3:1 for dihybrid cross) were a demonstration of two things: -Some information was passed on between parents and offspring in discrete units -Offspring were a combination of this information from the parents We now know this information is encapsulated in DNA sequences Mendel's Law of Segregation states each individual has two gene copies at a given loci which segregate independently during gamete production such that each gamete contains only a single copy (segregation) Mendel's Law of Independent Assortment states gene copies that are inherited in the next generation at one locus do not influence gene copies at other loci (independent assortment) -Important to note that Mendel stated law of gene transmission, but did not state what the source of variation in genes actually was

What is the significance of Denver et al. (2000)?

worked with C. elegans investigated how common point mutations are in nature by sequencing the same genes in generation after generation of the nematode. -Found mutations occurred at a rate of approximately 2.1*10^-8 mutations/site/generation -Of these, 50% of the mutations were indels (base insertion of deletion), and the total worked out to be about 2.1 mutations/genome Denver et al. (2000): did a similar experiment by sequencing same genes in mitochondrial genome generation after generation and found a higher rate of mutations occurred there -approx. 1.6*10^-7 mutations/site/generation -This makes sense because we now believe Mitochondria lack some of the DNA proofreading enzymes found in the nuclear genome of C. elegans, so the mutation rate is likely higher -This higher mutation rate, mitochondrial DNA is often overrepresented in studies of population within animals -The efficacy of DNA copying suggests that enzymes which proofread DNA after it is copied must be under very strong selection -Likely a trade off between efficacy/efficiency and speed of proofreading