The Evolution of Populations: Microevolution

Microevolution

Evolution at the population level (seen in the frequency of alleles changing from generation to generation).

Allele Frequencies

Measure the amount of genetic variation in a population.

Genotype Frequencies

Show how a population’s genetic variation is distributed among its members.

What is Hardy-Weinberg Equilibrium and what are its assumptions?

Describes that if a population is not changing genetically or evolving, then it has H-W Equilibrium.

Assumptions: no mutation, no migration, large/infinite population, random mating, and no natural selection.

What are the equations for H-W Equilibrium and what do they mean?

p+q=1 or 100% (the dominant allele, p, and the recessive allele, q, must equal 1 or 100%).

p²+2pq+q²=1 or 100% (where p² is two homozygous dominant alleles, 2pq is a pair of dominant and recessive alleles, and q² is two recessive alleles). These predict genotype frequencies.

What are changes in allele and genotype frequencies caused by?

Mutation, gene flow, natural selection, genetic drift, and non-random mating.

Mutation

When an allele changes randomly (most either neutral/harmful, and some beneficial—ultimate source of change/evolution because it introduces new alleles into populations).

Gene Flow

Migration of individuals between/among populations (immigrants might add new alleles/change allele frequencies already present).

Genetic Drift

Chance fluctuations in a gene pool (more common/has a stronger influence in smaller populations and random with respect to fitness).

What are the two types of genetic drift?

Bottleneck: when a large population goes through periods when only a small number of individuals survive.

Founder Effect: when a new population is established by few individuals (carrying gene frequencies different to the parent population— i.e., colonization of isolated islands).

Assortative/Non-Randon Mating

Individuals mating non-randomly/based on sexual selection (genotypes change from expected— i.e., males with brighter feathers are chosen more by females to mate with)—can cause sexual dimorphism.

Natural Selection

Differences in reproductive success (survival of the fittest)— only evolutionary agent that adapts populations to their environment (may preserve/cause allele frequencies to change).

What are the three types of natural selection?

Stabilization: individuals with average/moderate versions of a trait survive and reproduce.

Directional: individuals with traits at one extreme survive and reproduce.

Disruptive: individuals with traits at both extremes are favored.

What evolutionary agents reduce genetic variation?

Random genetic drift, stabilizing selection, and directional selection.

Why did sexual reproduction evolve?

Evolved to generate an endless variety of genotypes that can increase survivability of populations (especially in variable/changing environments)— does not influence frequencies of alleles but generates new combinations of genetic material.

Most genetic variation in a species is maintained in ____ through ____.

Subpopulations; trade-offs (i.e., cyanide production in a plant species).

What influences/determines phenotypes?

Both environmental factors and genotypes (nature vs. nurture).

Vestigial Traits

Remnants of features that have lost most/all of their original function but are still present (i.e., tailbone in humans).

Chance Events

Events that happen by chance that influence evolution (i.e., volcanism, meteorite impacts, etc.).