BIO Chp. 13
Modern Synthesis or Neo-Darwinism
Integration between the units of evolution (genes) and the mechanism of evolution (natural selection)... a synthesis. *An evolutionary theory proposing that mutations produce variations and that natural selection determines which variations will survive in order to produce biological evolution* - Explains Darwin's observation of variation in terms of mutation.
But what prevents natural selection from eliminating all variation as it selects against unfavorable genotypes? Why aren't less adaptive alleles eliminated as the "best" alleles are passed to the next generation?
It turns out that the tendency for natural selection to reduce variation in a population is countered by mechanisms that maintain variation.
Why is it important to have large populations in order to have a Hardy-Weinberg equilibrium population?
Large populations allow fluctuations due to chance to *have less of an impact.*
Refer to the evolutionary tree in the picture. Are crocodiles more closely related to lizards or birds?
Look for the most recent common ancestor of these groups. Crocodiles are more closely related to birds because they share a more recent common ancestor with birds than with lizards.
Why is "survival of the fittest" misleading?
"survival of the fittest" is misleading if we take it to mean direct competition between individuals. The phrase instead refers to organisms that are just better "fitted" to their environment - the ones that have a higher chance of survival and reproduction.
How to calculate allele frequencies
(# of homozygotes x 2) + (# of heterozygotes)/ Total # of alleles *If diploid*, multiply total by 2
What was Darwin's phrase for evolution? What does it mean?
*Descent with modification.* An ancestral species could diversify into many descendant species by the accumulation of adaptations to various environments.
Explain how a duplicated mutation plays a major role in evolution.
*Duplication of a gene through errors in meiosis can provide an important source of genetic variation.* If a repeated segment of DNA can persist over the generations, mutations may accumulate in the duplicate copies without affecting the function of the original gene, eventually *leading to new genes with novel functions.*
What type of fossils reveal ancient organisms' behaviors?
*Fossil imprints* that remain after the organism decays. - - Footprints, burrows, and fossilized feces (known as coprolites)
anatomical homology (x2)
*Homologous features* - similar features with different functions evolved from the same structure in a common ancestor. *Vestigial structures* - remnants of more developed structures that were present and functional in presumed ancestors.
Homoplasy *What are the related terms?*
*Homoplastic features* demonstrate that organisms with separate (distant) ancestries may adapt in similar ways to similar environments; *convergent evolution.* Ex. Aardvark, giant anteater, and pangolin. - different continents; similar structures
Why can organisms maintain variation?
*Most eukaryotes are diploid.* - two sets of chromosomes prevents genetically uniform populations - "hiding" of recessive alleles maintains huge pool of alleles that may be favored if environment changes.
Evolution definition
*The accumulation of inherited (i.e. genetic) changes within populations over generations.* Does not refer to changes that occur in an individual within its lifetime. Is not limited to speciation that may only occur over very long periods of time.
Darwin's greatest contribution to biology was his explanation of . . .
*how* life evolves.
Darwin's theory of evolution
- the idea that living species are descendants of ancestral species that were different from present-day ones - the genetic changes in a population from generation to generation
How have humans caused genetic drift in species?
*overhunting and habitat destruction* may create severe bottlenecks for other species.
Macroevolution
- (big changes) - significant evolutionary changes in populations that result in new species; *speciation.*
Microevolution
- (small changes) - *changes in allele or genotype frequencies* that occur in a population over a few generations - - - Changes in allele frequencies from one generation to the next or adaptations may result
Inactive pseudogenes
- *genes that have become inactivated by mutations* "Organisms may also retain genes that have lost their function, even though homologous genes in related species are fully functional."
In multicellular organisms, where must a mutation occur in order for it to affect the population's genetic variability?
- *only mutations in cells that produce gametes* because they are what passes the genetic information on to offspring
How is this supported today?
- All forms of life use the *same genetic language of DNA and RNA*, and the *genetic code.* - Organisms that are extremely dissimilar *share homologous genes* inherited from a very distant common ancestor.
Bottleneck effect
- Genetic drift resulting from a drastic reduction in population size. - Typically, the surviving population is no longer genetically representative of the original population.
Founder effect
- Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool is not reflective of that of the original population. The smaller the group, the less likely that the genetic makeup of the colonists will represent the gene pool of the larger population they left. *reduces genetic variability due to small population size.*
Using DNA for evolution studies
- If *two species have closely matched homologous genes*, biologists conclude that these *sequences must have been inherited from a relatively recent common ancestor.* - Conversely, the *greater the number of sequence differences* between species, the *more distant* is their last common ancestor.
Fossil record
- The chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata. It is incomplete.
Fossils and what do they tell us?
- are imprints or remains of organisms that lived in the past - they document differences between past and present organisms and the fact that many species have become extinct.
Microevolution vs. Macroevolution
- change in the gene pool of a population over several generations - large scale changes in a population that leads to the evolution of a new species !! Microevolution *leads* to macroevolution !!
Natural Selection in Hardy-Weinberg equilibrium population.
- means that all individuals in a population be equal in ability to reproduce This is probably never met in nature because *populations consist of varied individuals, and some variants leave more offspring than others.*
What are the three main causes of evolutionary change?
- natural selection, genetic drift, and gene flow
Strata
- rock layer formed when a new layer of sediment covers an older one and compresses it - singular, stratum
Types of homologies (x3)
-Anatomical -Developmental -Molecular
Directions of gene flow
-Immigration = individuals move into a population. -Emigration = individuals move out of a population. *note* • Gene flow tends to work against natural selection and genetic drift.
Molecular comparisons between diverse organisms have allowed biologists to . . .
. . . develop hypotheses about the evolutionary divergence of major branches on the tree of life
Who and what all influenced Charles Darwin?
1) Jean-Baptiste de Lamarck 2) Charles Lyell 3) Thomas Malthus 4) Observation and practice of animal husbandry 5) The Voyage of the HMS Beagle 1831-1836 - Galapagos organisms, while different, resembled those of South America more than the rest of the world ~ *spatial similarities and differences.*
What are five things that could cause evolution?
1)Small population; Genetic drift 2)Nonrandom mating (sexual selection) 3)Mutation 4)Migration/Gene Flow 5)Natural selection
Four Conditions for Natural Selection to Ocurr
1. *Variation* - the individuals of a population exhibit variation. Some of this variation is heritable. Genetic variation is the raw material for evolution. 2. *Overproduction* - in every generation each species has the capacity to produce more offspring than can survive. 3. *Limits on population growth* - due to limited resources (and other constraints) not all offspring survive to reproduce. 4. *Differential reproductive success* - better adapted individuals will more likely to survive, reproduce, and pass their traits on to the next generation.
What are the *five main conditions* to have a Hardy-Weinberg equilibrium population - Factors that change a gene pool
1. *Very large population.* - The smaller the population, the more likely that allele frequencies will fluctuate by chance from one generation to the next. 2. *No gene flow between populations.* - When individuals move into or out of populations, they add or remove alleles, altering the gene pool. 3. *No mutations.* - By changing alleles or deleting or duplicating genes, mutations modify the gene pool. 4. *Random mating.* - If individuals mate preferentially, such as with close relatives (inbreeding), random mixing of gametes does not occur, and genotype frequencies change. 5. *No natural selection.* - The unequal survival and reproductive success of individuals (natural selection) can alter allele frequencies.
What can affect evolutionary change, but doesn't happen very often? (x2)
1. Although *new genes/alleles* originate by mutation, these random/rare events probably change allele frequencies little within a population 2. *Nonrandom mating* can affect the frequencies of homozygous and heterozygous genotypes, but by itself usually does not affect allele frequencies.
What mechanisms maintain variation?
1. Balancing selection 2. Heterozygote advantage
Evidence of evolution (x6)
1. Fossil Record 2. Biogeography (Geographic distribution of organisms) 3. Homoplasy (Phenotypic convergence) 4. Selective breeding/artificial selection 5. Homologies 6. Experimental evidence of adaptation
2 important points about natural selection
1. Natural selection is more an editing process than a creative mechanism. 2. Natural selection is contingent on time and place - It favors those heritable traits in a varying population that fit the current, local environment. If the environment changes, different traits may be favored.
Though natural selection leads to adaptation, there are several reasons why nature abounds with organisms that seem to be less than ideally "engineered" for their lifestyles. List the 4 reasons
1. Selection can act only on existing variations. 2. Evolution is limited by historical constraints. 3. Adaptations are often compromises. 4. Chance, natural selection, and the environment interact.
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Evolutionary tree
A branching diagram that reflects a hypothesis about evolutionary relationships among groups of organisms.
Genetic drift
A change in the gene pool of a population due to chance. Effects of genetic drift are most pronounced in *small populations.* *Decreases variability*
Mutation
A change in the genetic information of a cell *The ultimate source of genetic diversity*
Intrasexual selection
A direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex. - frequently found in species where the winning individual acquires a harem of mates.
Vestigial structures
A feature of an organism that is a historical remnant of a structure that served a function in the organism's ancestors - "leftover" structures that are perhaps of no importance to the organism
Sexual selection
A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.
Population
A group of individuals belonging to one species and living in the same geographic area and can potentially interbreed.
Natural Selection
A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits.
Paleontologist
A scientist who studies fossils
Humans owe much of their physical versatility and athleticism to their flexible limbs and joints. But we are prone to sprains, torn ligaments, and dislocations. Why?
Adaptations are compromises: Structural reinforcement has been compromised as agility was selected for.
Why would natural selection tend to reduce genetic variation more in populations of haploid organisms than in populations of diploid organisms?
All alleles in a haploid organism are phenotypically expressed and are hence screened by natural selection.
Males with the most elaborate ornamentation may garner the most mates. How might choosing such a mate be advantageous to a female?
An elaborate display may signal good health and therefore good genes, which in turn could be passed along to the female's offspring.
What types of animals do you think would be most represented in the fossil record? Explain your answer.
Animals with hard parts, such as shells or bones that readily fossilize, and those that lived in areas where sedimentary rock may form
What does strata also tell us?
Because younger strata are on top of older ones, the relative ages of fossils can be determined by the layer in which they are found. *Thus, the sequence in which fossils appear within layers of sedimentary rocks is a historical record of life on Earth.*
How do fossils form?
Casts form when a dead organism captured in sediment decomposes and leaves an empty mold that is later filled by minerals dissolved in water. The minerals harden, making a replica of the organism.
Who came up with the theory of evolution?
Charles Darwin
Note
Chromosomal mutations that delete, disrupt, or rearrange many gene loci at once are almost certain to be harmful.
Imagine that you are studying a very large population of moths that is isolated from gene flow. A single gene controls wing color. Half of the moths have white-spotted wings (genotype WW or Ww) and half of the moths have plain brown wings (ww). There are no new mutations, individuals mate randomly, and there is no natural selection on wing color. How will p, the frequency of the dominant allele, change over time? A. p will fluctuate rapidly and randomly because of genetic drift. B. p will increase; the dominant allele will eventually take over and become most common in the population. C. p will decrease because of genetic drift. D. p will neither increase nor decrease; it will remain more or less constant under the conditions described.
D
Suppose there is a trait that has considerable variation (e.g., tail length) in a population of birds. Males with the longest tails have more offspring that all other males. This is an example of _____________. a. Stabilizing selection b. Directional selection c. Disruptive selection d. Not enough information to answer
Directional selection
What are the three ways natural selection can alter variation in a population?
Directional selection, stabilizing selection, disruptive selection
Why can't an individual evolve?
Evolution involves changes in the genetic makeup of a population over time. An individual's genetic makeup rarely changes during its lifetime.
How does gene flow affect populations?
Gene flow tends to reduce differences between populations. For example, humans today move more freely about the world than in the past, and gene flow has become an important agent of microevolutionary change in previously isolated human populations.
What part of organisms is directly exposed to the environment?
Evolutionary fitness is related to genes, but it is an *organism's phenotype* that is directly exposed to the environment.
True or false. Individuals evolve. Explain.
False, populations evolve - group of organisms evolve over time as adaptive traits become more common in the group and other traits change or disappear.
True or false. Evolution is goal directed
False. Evolution does not lead to perfectly adapted organisms. Natural selection is the result of environmental factors that vary from place to place and over time. A trait that is favorable in one situation may be useless in different circumstances.
Homoplastic features
Features with similar functions that evolved independently in distantly related organisms
How does natural selection differ from genetic drift, gene flow, and mutations?
Genetic drift, gene flow, and even mutation can cause microevolution, but only by chance could these events result in improving a population's fit to its environment. In natural selection, only the events that produce genetic variation (mutation and sexual reproduction) are random. Thus, the process of natural selection *IS NOT RANDOM.* *ONLY* natural selection consistently leads to adaptive evolution—evolution that results in a better fit between organisms and their environment.
Heterozygote advantage
Greater reproductive success of heterozygous individuals compared with homozygous; tends to preserve variation in gene pools.
What did people believe before Charle's Darwin?
Greek philosopher Aristotle's view - species are fixed, permanent forms that do not evolve.
How did Jean-Baptiste de Lamarck influence Charles Darwin?
He was a French scientist that published Philosophie Zoologique (1809). He believed . . . -Organisms pass traits acquired during their lifetimes to their offspring. -Organisms endowed with a vital force that drove them to change toward greater complexity and perfection.
How did Thomas Malthus influence Darwin?
He was a clergyman and economist. He wrote An Essay on the Principle of Population as it affects the Future Improvement of Society (1798). He believed . . . - Conflict between population size and food supply generates famine, disease, and war which alters population growth. A struggle for existence.
How did Charles Lyell influence Darwin?
He was an English geologist. He wrote Principles of Geology (1830). He believed . . . - the Earth did not originate in present form - slow pace of geologic processes still occurring today, indicated the Earth was very old.
What is homology? How does the concept of homology relate to molecular biology?
Homology is similarity in different species due to evolution from a common ancestor. Similarities in DNA sequences or proteins reflect the evolutionary relationship that is the basis of homology.
sexual dimorphism
Marked differences between the secondary sex characteristics of males and females
Compare artificial selection and natural selection.
In artificial selection, humans choose the desirable traits and breed only organisms with those traits. In natural selection, the environment does the choosing: Individuals with traits best suited to the environment survive and reproduce most successfully, passing those adaptive traits to offspring.
Fossil Record (example)
Multicellular ('complex') organisms in younger rocks Unicellular ('simple') organisms in older rocks
Which is *least* likely to alter allele and genotype frequencies in a few generations of a large, sexually reproducing population: gene flow, mutation, or natural selection? Explain.
Mutation. Because mutations are rare, their effect on allele and genotype frequencies from one generation to the next is likely to be small.
What is the ultimate (original) source of genetic variation? What is the source of most genetic variation in a population that reproduces sexually?
Mutation; unique combinations of alleles resulting from sexual reproduction
What else did Charles Darwin propose?
Natural selection
In what sense is natural selection more an editing process than a creative process?
Natural selection cannot create beneficial traits on demand but instead "edits" variation in a population by selecting for individuals with those traits that are best suited to the current environment.
Directional selection
Natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than other individuals. - shifts the overall makeup of the population by acting against individuals at one of the phenotypic extremes.
Disruptive selection
Natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.
Why do we consider natural selection to be the most important mechanism of evolutionary change?
Natural selection is the only process that consistently leads to adaptive change in populations.
Stabilizing selection
Natural selection that favors intermediate variants by acting against extreme phenotypes
Balancing selection
Natural selection that maintains stable frequencies of two or more phenotypic forms in a population
New info. Lecture
New info. from Lecture
What is the advantage to females of being choosy?
One hypothesis is that females prefer male traits that are correlated with "good genes."
Example of molecular homology
Prokaryotes and Eukaryotes both have ribosomes used for protein production. This process is highly conserved across all groups of life and has changed very little, indicating its crucial role to life of the cell.
What anatomical feature did scientists predict in fossils of species transitional between terrestrial and aquatic mammals?
Reduced hind limb and pelvic bones
intersexual selection
Selection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; also called mate choice.
Homologies
Similarities between organisms based on descent from a *recent common ancestor.*
Homology
Similarity in characteristics resulting from a shared ancestry.
Relative fitness
The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population.
developmental homology
Species that differ as adults often bear striking similarities during *embryonic stages* Examples •Presence of gill ridges in human embryos indicates that humans evolved from an aquatic animal with gill slits •Human embryos have long bony tails
Explain how the phrase "survival of the fittest" differs from the biological definition of relative fitness.
Survival alone does not guarantee reproductive success. An organism's relative fitness is determined by its number of fertile offspring and thus its relative contribution to the gene pool of the next generation.
What was Darwin's boldest hypothesis?
That ALL life forms are related.
What role does the environment play in natural selection in a population?
The environment determines which organisms leave the most offspring.
Which term in the Hardy-Weinberg equation—p2, 2pq, or q2—corresponds to the frequency of individuals who have no alleles for the disease PKU?
The frequency of individuals with no PKU alleles is p2.
How might gene flow between populations living in different habitats actually interfere with each population's adaptation to its local environment?
The introduction of alleles that may not be beneficial in a particular habitat prevents the population living there from becoming fully adapted to its local conditions.
What does sexual selection have to do with organisms' characteristics?
The males and females of an animal species obviously have different reproductive organs. But they may also have secondary sexual characteristics, *noticeable differences not directly associated with reproduction or survival.*
How does the outcome of evolution by natural selection differ from the outcomes of evolution by genetic drift and gene flow?
The outcomes of genetic drift and gene flow are randomly determined. The outcome of natural selection is non-random.
Artificial selection
The selective breeding of domesticated plants and animals to promote the occurrence of desirable traits in the offspring.
Hardy-Weinberg equilibrium
The state of a population in which frequencies of alleles and genotypes remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
Molecular biology
The study of biological structures, functions, and heredity at the molecular level.
Gene flow
The transfer of alleles from one population to another as a result of the movement of individuals or their gametes. *Gene flow increases genetic variability* and two populations become more similar. Gene flow tends to *work against natural selection and genetic drift.*
Explain why the following statement is incorrect: "Antibiotics have created resistant bacteria."
The use of antibiotics did not cause bacteria to make new alleles. Rather, antibiotic use has increased the frequency of alleles for resistance that were already naturally present in bacterial populations.
Since it is rare, when does a mutated allele usually became helpful?
This kind of effect is more likely when the *environment is changing* in such a way that mutations that were once disadvantageous are favorable under the new conditions.
Which organisms are the "fittest" ?
Those that produce the largest number of viable, fertile offspring and thus pass on the most genes to the next generation.
True or false. An entire organism can be fossilized.
True. In rare instances, an entire organism, including its soft parts, is encased in a medium that prevents bacteria and fungi from decomposing the body. Examples include insects *trapped in amber* (fossilized tree resin) and mammoths, bison, and even prehistoric humans *frozen in ice* or *preserved in bogs.*
True or false. Natural selection can amplify or diminish *only* heritable traits.
True. An organism may become modified through its own interactions with the environment during its lifetime, and those acquired characteristics may help the organism survive. But *unless coded for in the genes of an organism's gametes,* such acquired characteristics cannot be passed on to offspring. A championship female bodybuilder will not give birth to a muscle-bound baby.
How could evolution never or rarely occur in a population?
When different populations of the same species are geographically isolated from each other - Ex. Each population of Galápagos tortoises is restricted to its own island.
Concluding Note
With all these constraints, we cannot expect evolution to craft perfect organisms. Natural selection operates on a "better than" basis. Evidence for evolution is seen in the imperfections of the organisms it produces as well as in adaptations.
Adaptations
a (an evolutionary) modification (a trait) that improves the chances of survival and reproductive success in a given environment. - - increases an organism's "fitness".
superbug
a bacteria that is resistant to all antibiotics
Theory
a widely accepted explanatory idea that is broader in scope than a hypothesis, generates new hypotheses, and is supported by a large body of evidence.
Gene pool
all copies of every type of allele at every locus in all members of the population.
What is the product of evolution?
biodiversity
Types instances that cause genetic drift (x2)
bottleneck effect and founder effect
Fresh assortments of existing alleles arise every generation from *three* random components of sexual reproduction:
crossing over, independent orientation of homologous chromosomes, and random fertilization
Homologous Structures
features that often have different functions but are structurally similar because of common ancestry. ex. forelimbs of humans, cats, whales, and bats
What makes up an organism's phenotype?
its physical traits, metabolism, and behavior
Where do alleles come from?
mutations
How to calculate genotype frequencies (Hardy-Weinberg equation)
p^2 + 2pq + q^2 = 1
What about sexual selection intrigued Darwin??
some of these mate-attracting features do not seem to be otherwise adaptive and may in fact pose some risks. - For example, showy plumage may make male birds more visible to predators.
Population genetics
the study of genetic variability within a population and the forces that act on it.
Heritability
the transmission of traits from parents to offspring via genetic information; also known as inheritance
What are the two essential components of artificial selection?
variation and heritability
Biogeography
• Study of the geographical distribution of extinct and modern species •Isolated continents and island groups have evolved their own distinct plant and animal communities - - *Endemic* ~ naturally only in particular location
What are the three types of variability that can occur in a population? What are their equations?
•Genotype frequency - - p2 + 2pq + q2 = 1 •Phenotype frequency - - # of dominant individuals/total # of individuals •Allele frequency - - p + q = 1
Molecular homology
•Similarities in cells at the molecular level indicate that living species evolved from a common ancestor or interrelated group of common ancestors •All living species use DNA to store information •Certain biochemical pathways are found in all or nearly all species
