CH 25. Population genetics

A population is a group of interbreeding individuals that share a gene pool

A large population usually made of up local populations or demes- local populations are separated. As population sizes change and locations change, the genetic composition also changes.

Migration

After migration, the formation of a new population is called a conglomerate- to find the allele frequencies of the conglomerate, must know the allele frequencies of the donor and recipient populations, must know the proportion of the conglomerate that is due to migrants. Migration can enhance genetic diversity and introduce new alleles to neighboring populations. The change in allele frequency in the conglomerate population, Pc=m(pd-pr), then add pc to pr to find the allele frequency of the conglomerate. Gene flow is the process by which individuals migrate from one population to another and the migrants are able to breed with the recipient population. Bidirectional migration tends to reduce differences in allele frequencies between neighboring populations. In contrast, genetic drift tends to make local populations more disparate

Bottleneck effect and founder effect

Bottleneck effect- Changes in population size may influence genetic drift via the bottleneck effect. A random event can reduce the size of a population greatly- may randomly eliminate most members regardless of genetic composition. The bottleneck is greatly affected by genetic drift because the surviving members may have allele frequencies that differ those of the original population. The smaller population size after the bottleneck is also a factor in drift. The new population will have less diversity. Founder effect- Geography and population size may also influence genetic drift via the founder effect- the founder effect involves migration- a small group of individuals separates from the larger population and establishes a colony in a new location. The founding population will have less diversity and the allele frequencies will be different compared to the original population.

Genetic drift

Changes in allele frequency in a population due to random fluctuations. frequencies of alleles found in gametes vary by generation. Over time, genetic drift either results in the loss of an allele or its fixation at 100%. The process is random. The rate of genetic drift depends on the initial allele frequencies and the population size. Allele frequencies in larger populations fluctuate less because random sampling error has a smaller effect. Even in large populations, genetic drift leads to homozygosity but this takes longer to occur. The number of new mutations in the population depends on the mutation rate and the population size- greater chance in larger populations. The probability that a new mutation will be fixed is 1/2N. The probability of fixation is the same as the initial allele frequency in the population. Loss of a new allele- 1- chance of fixation, 1-(1/2N). Large populations have a greater occurrence of new mutations but each new mutation has a great chance to be eliminated due to genetic drift. A small population chance to get a new mutation is small but once the new mutation happens, its chance for fixation will be greater. Generations needed for a fixation of a new allele, t=4N. Allele fixation takes much longer in larger populations. Smaller isolated populations may be more genetically disparate.

Population genetics

Concerned with changes in genetic variation within a group of individuals over time.

Natural selection

Conditions in nature result in the selective survival and reproduction of individuals that are better suited to their environment. - Allelic variations in the population may arise by mutations that create a new allele for altered protein function. This new protein may confer an advantage for survival. Individuals who survive are more likely to contribute to the gene pool. This may change the allele frequencies over time that can alter the characteristics of the population. Directional selection favors the extreme phenotype at one end of the spectrum- from either a beneficial new allele introduced or prolonged changes in the environment. Multiplying the fitness values with the genotype frequencies from hardy weinberg to find the mean fitness of the population- use this to find the expected genotypic and allele frequencies after one generation of directional selection. Can lead to gene fixation- only one allele remains. Beneficial alleles at low frequencies may be lost due to genetic drift. Examples of directional selection- antibiotic-resistant bacteria, resistant pests Natural selection raises the mean fitness of the population-greater reproductive potential. Natural selection in finch population involved a change in beak size due to drought conditions.

Darwinian fitness is a measure of reproductive success

Darwinian fitness is the likelihood that one genotype will contribute to the gene pool of the next generation compared to other genotypes. Measures reproductive success, not physically fit. Natural selection acts on phenotypes derived from the genotypes. Fitness values based on the chance of surviving to reproductive age- the genotype with the highest reproductive ability is given a fitness value of one and the other genotypes are assigned relative values compared to one.

Disruptive selection favors multiple phenotypes

Diversifying selection favors the survival of multiple genotypes that produce different phenotypes. The environment can play a role in the fitness values of a genotype. Disruptive selection more common in diverse enviroments. Land snails polymorphism

Balancing selection may occur due to the heterozygote advantage or negative frequency dependent selection

Favor the maintenance of multiple alleles in a more homogenous environment- result in polymorphisms Arises due to overdominance or the heterozygote advantage- reach equilibrium when both alleles (p and q) are maintained. Consider the selection coefficient which measures the degree to which a genotype is selected aganist. The genotype with the highest fitness has a selection coefficient of zero. Genotypes at a selective disadvantage have values closer to one. Can explain the occurrence of deleterious alleles in the population- The heterozygote with sickle cell has higher fitness in areas where malaria is endemic. Negative frequency-dependent selection- second mechanism of balancing selection- fitness of a genotype decreases when its frequency becomes higher. Rarer genotypes have a greater fitness than common ones. Produces a balanced polymorphism- no genotype is too common or rare.

At the population level, some genes are monomorphic but most genes are polymorphic

Polymorphism- the observation that many inherited traits display variations within a population. Differences in color and pattern. The cause of polymorphisms may be due to multiple alleles that influence the phenotype. A gene is polymorphic if it exists as multiple alleles. When a single allele is found in at least 99% of all cases, it is termed monomorphic. A polymorphism can involve deletions, duplications, SNPs. Single nucleotide polymorphisms are the most common genetic change- over 90% of the variation among humans. SNPs in most genes. Alleles can differ by the single base pair or by deletions.

Stabilizing selection favors intermediate phenotypes

The extreme phenotypes are selected against and the intermediate phenotypes have the higher fitness value. Tends to lower genetic diversity for a gene because it eliminates alleles that cause great variation in the phenotypes. Clutch size is intermediate.

Genes in populations and the hardy weinberg equation

The gene pool is composed of all the alleles of every gene in the population. Examine allelic variation.

The hardy weinberg equation can be used to find genotypic frequencies based on allele frequencies

Under a given set of conditions, the allele and genotypic frequencies do not change. It is a null model that suggests that evolution is not occurring. Provides a framework to understand changes in allele and genotypic frequencies when the equilibrium is violated. The equation applies to a gene in a diploid species that is found in only two alleles. No change in the allele and genotypic frequencies if- no new mutations, no genetic drift (large population makes it unlikely that allele frequencies are changing due to random sampling error), no migration, no natural selection (all genotypes have the same level of fitness), random mating-without regard to phenotypes or genotypes. 2pq equals the genotypic frequency of heterozygotes. The frequency of an gamete carrying an allele is equal to the allele frequency in that population. Use hardy Weinberg to detect an evolutionary change and find the percentage of heterozygote carriers- A population not in equilibrium suggests an evolutionary change- want to find the evolutionary force at play. Use the observed data to find the expected genotypic numbers assuming Hardy Weinberg. A gene with two alleles has a degree of freedom of one. To find the percentage of heterozygote carriers- first, find the allele frequency of q starting with the percentage of people affected (q2)

Population genetics is concerned with allele and genotype frequences

Want to understand the prevalence of polymorphic genes in the population- find causative factors. Calculations of allele and genotypic frequencies. Genotype frequency- # of people with this particular genotype/ total # of people in the population. Single monomorphic genes have an allele frequency close to one.

Microevolution

microevolution describes changes in the population's gene pool via generations. Introduction of new genetic variation via molecular changes like mutations. Mutations are not a major factor in promoting change in the population. Mutations are rare. Also include- natural selection, genetic drift, migration and nonrandom mating- may alter existing genetic variation, the collective changes from these mechanisms can promote widespread genetic change.