Biol 214 - Microevolution and Genetic Changes in Populations

Tested on the cumulative Final Exam. The change in gene (allele) frequencies within a population over generations.

Population

All the individuals of a single species that live together in the same place and time.

Estimating population size: mark-and-recapture method

N = MC/R

N = total population size

M = # individuals marked

C = # individuals captured

R = # individuals recaught

Microevolution

A heritable change in the genetics of a population.

What underlies natural selection?

Variation. No variation = no selection. Not all variation comes from natural selection.

Heritable Variation

Variation successfully passed down through reproduction.

Phenotypic Variation

Differences in appearance or function that exist within populations of all organisms.

Quantitative Variation

A way to describe/assess phenotypic variation through the use of a continuous graph in the shape of a hump. Wider humps = more variability, narrower humps = less.

Qualitative Variation

A way to describe/assess phenotypic variation through the use of distinct categories, often controlled by few genes. Shows discontinuous differences.

Ex: Mendel’s phenotypic ratios.

Polymorphism

The existence of discrete variants of a character among individuals in a population: when a single species has two or more distinct forms (morphs or phenotypes) coexisting.

Ex: blood types in humans.

Frequency

A way to describe/assess polymorphism by calculating the percentage of each form of a trait.

What leads to phenotypic variation within populations?

1. Genotypic differences

   • The production of new alleles

   • The rearrangement of new alleles

2. Environmental differences

   • Gene expression

3. The interactions between the two.

Are genetic and phenotypic variations perfectly correlated?

No; the environment significantly shapes final observable traits (phenotypes), but puts less selective pressure on genotypes.

Is it possible for organisms with the same genotype to sometimes exhibit different phenotypes?

Yes, since different environmental factors can influence the expression of genotypes.

Ex: hydrogenas.

Production of New Alleles

Most new alleles arise from small-scale mutations in DNA.

Rearrangement of Existing Alleles

Rearrangement can result from larger scale changes in chromosome structure or number and from several forms of genetic recombination.

Genetic Variation in Populations

Nearly half the gene loci in many plant and animal populations are polymorphic. Most genes have multiple versions (alleles) in a species, seen as different traits within one individual (heterozygous), between members of a local group, or across different but related populations.

Gene Pool

The sum of all gene copies at all gene loci in all individuals.

Calculation of Genotype and Allele Frequencies

Genotypic frequency:

• # individuals with the genotype/total individuals

Allelic frequency:

• # alleles (homozygotes have 0 or 2 per individual, heterozygotes have 1)*# individuals/population*2

Null Model

How something does not occur. Used as a theoretical control.

Puzzle of Persistent Recessive Traits

Assumed that natural selection was selecting for dominant/common alleles, and wasn’t sure why recessive ones kept persisting.

Hardy–Weinberg Principle Assumptions

If the conditions of the model are met, the allele frequencies of the population will never change, and the genotype frequencies will stop changing after one generation (a starting population not in equilibrium will change into equilibria after one generation). Microevolution will not occur.

• No mutations are occurring.

• The population is closed to migration from other populations.

• The population is infinitely large.

• All genotypes in the population survive and reproduce equally well.

• Individuals in the population mate randomly with respect to genotypes.

Hardy–Weinberg Principle

An evolutionary rule of thumb that specifies the conditions under which a population of diploid organisms achieves genetic equilibrium.

The equation that predicts the genotype frequencies in the offspring generation:

• p² + 2pq = q²

Mutation

A spontaneous and heritable change in DNA. Introduces new genetic variation within a population and happens rarely; does not change allele frequencies quickly. Unpredictable effect on fitness: usually harmful.

Deleterious Mutation

A mutation that alters an individual’s structure, function, or behavior in harmful ways.

Lethal Mutations

A mutation that causes great harm or death to organisms carrying them.

Neutral Mutations

A mutation that is neither harmful nor helpful.

Advantageous Mutation

A mutation that has some benefit on an individual that carries it.

Gene Flow

Change in allele frequencies as individuals join a population and reproduce; violates Hardy-Weinberg. Movement alone is not enough; immigrants must also reproduce in the population they join. Unpredictable effect on fitness: May introduce beneficial or harmful genes.

Evolutionary importance of gene flow depends on:

• The degree of genetic differentiation between populations

• The rate of gene flow between them.

Genetic Drift

Random changes in allele frequencies caused by chance events. Reduces variation, especially in small populations; can eliminate or amplify rare alleles. Violates the Hardy–Weinberg assumption of infinitely large population size.

Two common circumstances:

• Population bottlenecks (driven by disaster on original population)

• Founder effects (driven by isolation via migration)

Natural Selection

Differential survivorship or reproduction of individuals with different genotypes. One allele can replace another, or allelic variation can be preserved. Positive effect on fitness through evolution of adaptations.

Nonrandom Mating

Choice of mates based on phenotypes and genotypes. Does not directly affect allele frequencies, but usually prevents genetic equilibrium.

Relative Fitness

A method to determine reproductive success.

Intrasexual Selection

Competition within a sex for mates. Intrasexual selection often leads to traits like large size, horns, or antlers used in combat.

Intersexual Selection

Mate choice between sexes. Intersexual selection favors traits like bright plumage or elaborate displays that are attractive to the opposite sex.

Inbreeding

A special form of nonrandom mating. Increases the frequency of homozygous (especially recessive) genotypes, decreases the frequency of heterozygotes.

Balanced Polymorphisms

The maintenance of two or more phenotypes in fairly stable proportions over many generations. Natural selection preserves balanced polymorphisms when

• Hybrid vigor: heterozygotes have higher relative fitness.

• Different alleles are favored in different environments.

• The rarity of a phenotype provides a selective advantage (ex: one white flower amongst a field of pink flowers).

Adaptive Lagging

No organism can be perfectly adapted to its environment because environments change over time. Each generation is adapted to the environmental conditions under which its parents lived.