Chapter 17 Evolutionary Processes
What is likely to happen after a founder event occurs?
Each time a founder event occurs, a founder effect is likely to accompany it, changing allele frequencies through genetic drift.
What factor dictates how much of an effect gene flow has?
The further populations are apart the less likely gene flow is to occur and the less equalization occurs.
If females are choosy to protect their investment and males invest little, so they should be willing to mate with almost any female what two things does this lead too?
1.) CHOICE by females 2.)COMPETITION between males
In reference to the bateman-trivers theory. (theory of sexual selection), what are the two broad types of sexual selection?
1.) Female choice 2.)Male-male competition. If male fitness is limited by access to mates, then any allele that increases a males attractiveness to females or success in male - male competition should increase rapidly in the population. Thus, sexual selection should act more strongly on males than on females.
What two predictions are made by the bateman-trivers theory (theory of sexual selection)
1.) If females invest a great deal in each offspring. then they should protect that investment by being choosy about their mates. Conversely if males invest little in each offspring then they should be willing to mate with almost any female 2.) If there are an equal number of males and females in the population, and if males are trying to mate with any female possible, then males will compete with each other for mates.
Does sexual selection result in evolution?
Alleles are increased or decreased in frequency and result in evolution as a result of sexual selection. Because sexual selection tends to be much more intense in males than females, males tend to have many more traits that function only in courtship or male-male competition. Stated another way, sexually selected traits often differ sharply between the sexes... This leads to Sexual dimorphism as previously mentioned in seals.
Describe what gene flow does as an evolutionary mechanism?
As an evolutionary mechanism, gene flow usually has one outcome: it increases the similarity of allele frequencies in the source population and the recipient population. When alleles move from one population to another, the populations tend to become more alike. Essentially, in the words of the textbook "gene flow homogenizes allele frequencies among populations". In otherwords, gene flow tends to equalise allele frequencies between two populations.
Explain how bottlenecks are similiar to the founder effect
Bottlenecks are similar to the founder effect. Instead of being except instead of being split off from the population, the large population undergoes a rapid reduction in size as a result of a catasrophe of sorts. The result is small populations that have fewer genes within the smaller population.. An important difference however between the two is that the loss of genetic variation is accelerated in population/genetic bottlenecks. Not only is there a less of genetic diversity due to smaller populations, but there is also a loss of diversity to genetic drift. This means that genetic diversity can be lost quicker.
We know from previous notes that natural selection affects individuals and evolution acts on populations. Beneficial alleles increase in population and detrimental alleles will decrease. With the above in mind describe the 3 different patterns of selection. List them here and describe them below in their own seperate Q cards.
Directional selection Stabilising selection Disruptive selection
In regards to disruptive selection one of the patterns exhibited by natural selection..... What can disruptive selection lead to?
Disruptive selection can lead to speciation, (the formation of new species).
Describe what happened with disruptive selection in black-bellied seedcrackers.
Distribution of bills of black-bellied seedcrackers. Individuals with eithervery short or very long beaks survive best and that birds with intermediate phenotypes are at a disadvantage. Only two sizes of seed are available to the seedcrackers: large and small. Birds with small beaks crack and eat small seeds efficiently. Birds with large beaks handle large seeds efficiently. But birds with intermediate beaks have trouble with both, so alleles associated with medium sizsed beaks are subject to purifying selection
Describe what is meant by the statement "heterozygote advantage":
Each allele recognises slightly different disease-causing organisms Heterozygote advantage - A1A2 and B1B2 should allow you to fight off more diseases
What is evolution DEFINED as
Evolution is defined as a change in allele frequencies.
As previously described at the top of our Q cards, what does evolutionary change indicate? What 4 things can this be due too?
Evolutionary change = change in allele frequency in a population. Due to: Natural Selection - ADAPTATION Genetic drift Gene flow Mutation Remember that the 3 below natural selection are considered random events and thus they can be non-adaptive. Where as natural selection will always be adaptive.
What do criteria do females select from?
Female "chooses" male based on characteristics that might indicate overall MALE FITNESS Size Colour Behavior e.g. song, ritual dancing Nuptial gifts e.g. Chocolates Good parenting skills
What assumptions does the hardy-weinber model make?
For a population to conform to the hardy weinberg principles, none of the five mechanisms of evolution can be acting on the population. In addition, the model assumes that mating is random with respect to the gene in question. 1.) No natural selection at the gene in question - Each generation survives and contributed equal numbers of gametes to the gene pool, no matter what their genotype 2.) No genetic drift, or random allele frequency changes, affecting the gene in question - Assum that we draw alleles in their exact frequencies P and Q and not at some different values caused by chance. 3.) No gene flow - no new alleles added by immigration or lost trough emigration 4.) No mutation - in the alleles of the population 5.) Random mating must be present
What is gene flow?
Gene flow is the movement of alleles from one population to another. It occurs when individuals leave one population, join another, and breed.
Give an example of an organism who experienced the effects of gene flow.
Great tits from the netherlands and great tits on island vlieland
Why does natural selection occur?
Mutations result in adaptations that contribute to biological fitness, the individuals that posses these traits produce more offspring etc...
Evolution occurs due to what 4 reasons?
Natural Selection - ADAPTATION Genetic drift Gene flow Mutation
An important side note
Natural selection is the only mechanism that acting alone can result in adaptation. Mutation, gene flow, and genetic drift do not favour certain alleles over others. They are simply random events.
What is the most fundemental attribute of natural selection?
No matter how natural selection occurs, though, its most fundamental attribute is the same: it increases fitness and leads to adaptations.
Refer to table 5.3 for quick review of evolutionary mechanisms
refer.
Calculate the % loss of A1 in each population that occured as a result of genetic drift. 20 offspring - 16 A1 and 24 A2 2000 offspring - 1996 A1 and 2004 A2
% loss of A1 in each population? 16/24= 2000/2004
Give 3 reasons that explain why mutations are imporant
1.) Gene flow increases genetic diversity in a recipient population if new alleles arrive with immigrating individuals. But gene flow may decrease genetic variation in the soruce population if alleles leave with emigrating individuals. 2.) Genetic drift tends to decrease genetic diversity over time, as alleles are randomly lost or fixed. 3.) Most forms of selection favour certain alleles and lead to a decrease in overall genetic variation. In summary, if most of the evolutionary mechanisms lead to a loss of genetic diversity over time, what restores it? In particular, where do entirely new alleles come from? The answer to both of these questions is mutation.
Describe the 4 ways gene frequencies can change
1.) NATURAL SELECTION - increases the frequency of certain alleles. the ones that contribute to reproductive success in a particular environment 2.) GENETIC DRIFT - causes allele frequencies to change randomly. In some cases, drift may cause alleles that decrease fitness to increase in frequency. 3.) GENE FLOW - Occurs when individuals leave one population, join another, and breed. Allele frequencies may change when gene flow occurs, because arriving individuals introduce alleles to their new population and departing individuals remove alleles from their old population. 4.) MUTATION - modifies allele frequencies by continually introducing new alleles. The alleles created by mutation may be beneficial or detrimental or have no effect on fitness.
What two fundamental claims does the hardy weinberg principle make?
1.) if the frequencies of alleles A1 and A2 in a population are givien by P and Q, then the frequencies of genotypes A1A1,A1A2,A2A2, will be given by p2,2pq, and q2 for generation after generation 2.) When alleles are transmitted via meiosis and random combination of gametes, their frequencies do not change over time. For evolution to occur, some other factor or factors must come into play. (Refer to 5 hardy weinberg assumptions).
What are genetic bottlenecks
A genetic bottleneck is a sudden reduction in the number of alleles in a population. Drift occurs during genetic bottlenecks and causes a change in allele frequencies.
What can come as a result of decreased gene flow after populations have serparated?
As gene flow decreases you are more likely to see different populations evolving serpately from each other which can lead to separate evolutionary paths and thus different species eventually.
Describe the gene pool concept
Assumes random mating. All of the gametes produced in each generation go into a single group called the gene pool and then combine at random to form offspring. To determine which genotypes would be present in the next generation and in what frequency, Hardy and Weinberg simply had to calculate what happened when two gametes were plucked at random out of the gene pool, many times, and each of these gamete pairs was then combined to form offspring.
Give an example of a bottle neck scenario. (Achromatopsia)
Atoll in South Pacific, 1775 Typhoon and famine wiped out population Several thousand to about 20 individuals One survivor had ACHROMATOPSIA, a recessive disorder Normal population - recessive gene <1% After typhoon/famine - recessive gene 2.5% On the atoll after the population crash of 1775: BOTTLENECK effect - allele ~3x higher than normal population Current population: 1 in 20 people have the condition (5%) Allele at a frequency of over 20% GENETIC DRIFT - assuming no advantage or disadvantage to the condition
Give an example of stabilising selection. Think of the "Babies" example. Describe where the previously aformentioned "purifying" took place here.
Babies of average size "slightly over 3kg" survived best. Mortality was high for very small babies and very large babies. This is persuasive evidence that birth weight was under strong stabilizing selection in this population.Alleles associated with high birth weight or low birth weight were subject to purifying selection, and alleles associated with intermediate birth weight increased in frequency.
A founder effect is said to have occured because a small subset of the population splits from the rest of the population. How can this occur? Give an example and explain how founder effects and founder events correlate with each other.
Founder effects can take place when small populations move to an island, an example of this would be iguanas on floating vegetation reaching an island. This results generally results in a rapid, significant change in allele frequencies as a consequence of chance.
More on genetic drift.... the lab experiment mentioned in the notes, and more specifically the emphasis placed on sample size.
Fruit flies - 4 males and 4 females Looked at a gene that determined bristle type Normal Forked 16 generations 96 trials Initially, both alleles at a frequency of 0.5 70 populations LOST one of the alleles, purely by chance Significance of small populations when populations get below around 1000 we start worrying about genetic feasibility because genetic diversity can be lost by random chance to genetic drift..
How can genes move between populations?
Genes can move between populations by pollen, sperm in water, flying etc. Some examples of this would be the great tits located in the netherlands who fly between islands.
What is genetic drift?
Genetic drift is defined as any change in allele frequencies in a population that is due to chance. The process is aptly named, because it causes allele frequencies to drift up and down randomly over time. When drift occurs, allele ferquencies change due to blind luck - "sampling error". Drift occurs in every population, in every generation.
What is genetic variation? Explain why a loss of genetic variation is a bad thing
Genetic variation is the number and relative frequency of alleles that are present in a particular population. The reason is simple: lack of genetic variation in a population is usually a bad thing. This is because selection can only occur if heritable variation exists in a population. If genetic variation is low and the environment changes for example due to the emergence of a new disease causing virus, a rapid change in climate, or a reduction in the availability of a particular food source, it is unlikely that any alleles will be present that have high fitness under the new conditions. As a result, the average fitness of the population will decline. If the environmental change is severe enough, the population may even be faced with extinction.
What is balancing selection?
Heterozygote advantage is one mechanism responsible for a more general phenomenon known as balancing selection. When balancing selection occurs no single allele has a distinct advantage and increases in frequency. Instead there is a balnace among several alleles in terms of their fitness and frequency. Balancing selection also occurs when: 1.) The environment varies over time or in different geographic area occupied by a population. Meaning that certain alleles are favoured by natural selection at different times or in different places. As a result. overall genetic variation in the population is maintained or increased. 2.) Certain alleles are favoured when they are rare, but not when they are common, a pattern known as frequency dependent selection. For example, rare alleles responsible for colouration in guppies are favoured because predators learn to recognize common color patterns. Alleles for common colours get eliminated; alleles for rare colors increase in frequency. As a result, overall genetic variation in the population is mainted or increased.
What are population bottlenecks? What can cause them?
If a large population experiences a sudden reduction in size a population bottleneck is said to occur. Bottlenecks can be caused by disease outbreaks, natural catasrophes such as floods or fires or storms, and other events can cause population bottlenecks.
What 2 things can be indicated by use of the hardy-weinberg principle
If differences ARE seen between observed and expected frequencies: Indicates either: Evolutionary change OR Non-random mating
What is a founder effect?
If the group of individuals that immigrated to a new geographic area establishes a new population and that group is small enough, the allele frequencies in the new population are almost guaranteed to be different from those in the soruce population. A change in allele frequencies that occurs when a new population is established is called a founder effect.
Describe what is meant by the "Fundamental Asymmetry of Sex"
In most species, females invest much more in their offspring than do males. Eggs are large and energetically expensive, females produce relatively few young over the course of a lifetime. A female's fitness is limited not by the ability to find a mate but primarily by her ability to gain the resources needed to produce more eggs and healthier young. Sperm are simple to produce, males can father an almost limitless number of offspring. Thus a male's fitness is limited not by the ability to acquire the resources needed to produce sperm but by the number of females he can mate with.
Is gene flow advantageous?
It's relative to the situation. In the case of great tits on vlieland, immigrants bring in alleles that have relatively low fitness in the island environment. Natural selection and gene flow are working in opposition: selection favours small clutch sizes and other traits that affect survival in the island environment; gene flow constantly introduces alleles that have high fitness in mainland habitats but low fitness on the island. The homogenizing effect of gene flow is bad for island birds. Gene flow can be advantageous too however, If a population has lost alleles due to genetic drift then the arrival of new alleles via gene flow should increase genetic diversity. If increased genetic diversity results in better resistance to infections by bacteria or viruses or other parasites, gene flow would increase the average fitness of individuals. Gene flow is random with respect to fitness - the arrival or departure of alleles can increase or decrease average fitness, depending on the situation.
In what ways do males compete for a female?
Males "fight" for the right to mate Also, indication of MALE FITNESS E.g. Fighting for the right to mate Avoid fighting by comparing features to indicate fitness Guarding females to prevent other males mating
Explain how mutations occur? What do mutations do?
Mutations occur when DNA polymerase makes an error. Mutation constantly introduces new alleles into populations in every generation. Mutation is an evolutionary mechanism that increases genetic diversity in populations. Despite the fact that it increases genetic diversity, mutation is random with respect to the affected allele's impact on the fitness of the individual. Changes in the makeup of chomosomes or in specific DNA sequences do not occur in ways that tend to increase fitness or decrease fitness. Mutation just happens. Mutations generally result in deleterious alleles, because they are acting on organsims that are well adapted to their current habitat, random changes in genes usually result in products that do not work as well as the alleles that currently exist. Deleterious alleles tend to be eliminated by purifying selection. On rare occasions, however, mutation can result in a beneficial allele that allows individuals to produce more offspring. Beneificial alleles should increase in frequency in the population due to natural selection. How fast it increases in the population is dependent on how advantageous it is.
What is sexual dimorphism?
Refer to any trait that differs between males and females. Example would include sea lions. Males are huge compared to female. Male fights for right to reproduce.
Different patterns of selection - Directional selection
Relate this to the examples regarding M. tuberculosis and or the medium finches and the alternation el nino and la nina currents. In this type on natural selection, the average phenotype of the population changes in one direction. Directional selection tends to reduce the genetic diversity of populations. If directional selection continues over time, the favoured alleles will eventually reach a frequency of 1.0 (always) and unfavorable (0) 1.0 = fixed alleles. 0.0 = lost alleles
Different patterns of selection - Stabilising selection
Selection against EXTREMES NO change in the average Reduces genetic diversity Reduces both extremes in a population. There is no change in the average value of a trait over time, and genetic variation in the population is reduced. Think of the goldilocks zone.
Different patterns of selection - Disruptive selection
Selects against the average and for the extremes Results in a split population Increases genetic variation The opposite effect that stabilizing slection has. Instead of favoring phenotypes near the average value and eliminating extreme phenotypes, it eliminates phenotypes near the average value and favours extreme phenotypes. When disruptive selection occurs, the overall amount of genetic variation in the population is maintained.
When does sexual selection occur, what is it?
Sexual selection occurs when individuals within a population differ in their ability to attract mates. It favours individuals with heritable traits that enhance their ability to obtain mates.
In addition to directional selection listed above, describe what purifying selection is
Takes place when the pattern for natural selection is directional selection. Genetic diversity will be reduced. If directional selection continues over time, the favoured alleles will eventually approach a frequency of 1.0, unfavourable alleles will reach 0.0. Alleles at 1.0 are said to be fixed. Alleles at 0.0 are said to be lost. When disadvantageous alleles decline in frequency an example of purifying selection is taking place.
What can be used to measure gene frequencies?
The Hardy-Weinberg Model
Describe what the hard-weinberg principle is used as?
The hardy weeinberg principe acts as a null hypothesis when researchers want to test whether evolution or nonrandom mating is occuring at a particular gene
What is the negative side effect of founder events and founder effects.
The new population is likely to have different allele frequencies than the source population by chance. However, the subset of alleles is a small amount and subsequently there is lower genetic diversity.
What causes genetic drift in populations? Why is size an important factor in the severity of this?
The sampling process that occurs during fertilization occurs in every population in every generation in every species. Basically when producing sperm and eggs or however you produce gametes it is entirely random as to which gets fertilized. With smaller numbers however, the chance that this causes change increases. With smaller numbers you are very likely to get larger variations in chance and thus more genetic drift. Larger populations average these differences out.
What is a founder event?
When a group of individuals immigrates to a new geographic area and establishes a new population, a founder event is said to occur.