Evolution, Natural Selection, and Population Genetics: Key Concepts and Definitions

Evolution

Change in allele frequency over time.

Species

A group of interbreeding animals (one definition).

Descent with modification

Change in inherited traits of populations through successive generations.

Natural selection

When individuals with certain traits are more likely to survive, reproduce, and pass on those traits.

Microevolution

Evolution within a species; changes in allele frequencies and traits from one generation to the next.

Macroevolution

Evolution above the level of an individual species; includes speciation and extinction.

Local adaptation

When populations evolve traits that are advantageous in their specific local environment.

Fitness

How well an organism survives and reproduces in a particular environment.

What does "fittest" mean?

The phenotype best suited for a particular environment, not necessarily the biggest or strongest.

What does evolution NOT do?

It does not progress toward an ultimate goal.

What acts on individuals and leads to evolution in populations?

Natural selection.

Convergent evolution

When similar selection pressures produce similar traits in different areas or populations.

Four requirements for natural selection

Variation, heritability, differential survival/reproduction, and differences in survival/reproduction caused by variation.

Requirement 1 of natural selection

Individuals vary in phenotype.

Requirement 2 of natural selection

Variation must be heritable.

Requirement 3 of natural selection

Some individuals survive and reproduce more successfully than others.

Mutation

A change in the DNA base sequence.

Mutation type

Nonadaptive mechanism of evolution.

Effect of mutation on genetic variation

Increases genetic variation.

Why is mutation important for evolution?

It provides the raw material for evolution.

Are mutations directed toward what an environment needs?

No. Mutations are undirected with respect to selection.

What can cause mutations?

Radiation, chemicals, and natural copying errors.

Point mutation

A single-base deletion, insertion, or substitution.

Genetic drift

Random changes in allele frequencies.

Genetic drift type

Nonadaptive mechanism of evolution.

Effect of genetic drift on genetic variation

Decreases genetic variation.

When is genetic drift especially strong?

In small populations.

Bottleneck effect

A population is severely reduced in size and then rebounds with low genetic variation.

Founder effect

A small sample of an original population establishes a new population.

Gene flow

Movement of genes into and out of a population.

Gene flow type

Nonadaptive mechanism of evolution.

Effect of gene flow within populations

Increases genetic variation within populations.

Effect of gene flow among populations

Decreases differences among populations.

How can organisms move genes between populations?

Movement of organisms or gametes such as pollen.

High gene flow

Creates homogeneity among subpopulations and can disrupt local adaptation.

Restricted gene flow

Allows greater divergence through natural selection or genetic drift.

What happens in small populations?

Genetic drift may overwhelm natural selection.

What happens in highly connected populations?

Gene flow may overwhelm selection and drift.

Darwin's contribution

Proposed natural selection as the mechanism of evolutionary change.

What did Darwin miss?

The mechanism of inheritance.

Blending inheritance

The idea that offspring inherit an average of their parents' traits.

Why is blending inheritance problematic?

It would make it difficult for advantageous variation to persist.

Mendelian/particulate inheritance

Inheritance in which hereditary determinants are discrete particles called genes.

Allele

One specific form of DNA sequence at a locus.

Locus

A region of DNA coding for a particular product.

Gene pool

All genes in all organisms in a population.

Population genetics

The study of the dynamics of genes in entire populations.

Modern synthesis

Combination of Mendelian genetics with Darwinian natural selection using population genetic models.

Homozygous

Having two identical alleles for a gene.

Heterozygous

Having two different alleles for a gene.

Dominant allele

An allele expressed in a heterozygote.

Recessive allele

An allele whose phenotype is not expressed in a heterozygote.

Hardy-Weinberg equilibrium

A state where allele and genotype frequencies do not change over generations.

Hardy-Weinberg equilibrium purpose

A null model for population genetics and evolutionary change.

Hardy-Weinberg assumptions

No selection, no mutation, no migration, a very large population, and random mating.

Hardy-Weinberg equation

p + q = 1

Genotype frequency equation

p² + 2pq + q² = 1

What does p represent?

Frequency of one allele.

What does q represent?

Frequency of the other allele.

What does p² represent?

Frequency of one homozygous genotype.

What does 2pq represent?

Frequency of the heterozygous genotype.

What does q² represent?

Frequency of the other homozygous genotype.

When is a population not in Hardy-Weinberg equilibrium?

When a force of evolution is acting and allele/genotype frequencies change.

Peppered moth example

Industrial pollution favored dark moths because they were more cryptic on dark trees.

Industrial melanism

An increase in dark-colored forms due to selection in polluted environments.

Quantitative genetic trait

A trait controlled by multiple genes and also influenced by the environment.

Examples of quantitative traits

Beak size, body size, leaf area, running speed, and disease resistance.

Characteristics of quantitative traits

They have continuous distributions, are polygenic, and are affected by the environment.

Polygenic

Controlled by many genes.

Additive alleles

Alleles whose effects add together to influence a trait.

Formula for phenotypic classes

2n + 1, where n is the number of diallelic additive loci.

How many phenotypic classes are produced by 5 loci?

11 phenotypic classes.

How many phenotypic classes are produced by 100 loci?

201 phenotypic classes.

Directional selection

Selection that shifts the population mean toward one extreme phenotype.

Example of directional selection

Galápagos finches whose mean beak size shifted after environmental changes.

Stabilizing selection

Selection that favors intermediate phenotypes and selects against extremes.

Disruptive selection

Selection that favors both extreme phenotypes and selects against intermediate phenotypes.

Example of disruptive selection

Large-billed finches feed on hard seeds while small-billed finches feed on soft seeds.

Phenotypic variance

The total variation in observable traits within a population.

Variance formula

Σ(xᵢ − x̄)² / (N − 1)

VP

Total phenotypic variance.

VG

Genetic variance.

VE

Environmental variance.

Phenotypic variance equation

VP = VG + VE

Additive genetic variance (VA)

Genetic variance from allele effects that can be inherited.

Dominance variance (VD)

Variance caused by interactions between alleles in heterozygotes.

Interaction/epistatic variance (VI)

Variance caused by interactions among different loci.

Genetic variance equation

VG = VA + VD + VI

Heritability (h²)

The proportion of phenotypic variance attributable to additive genetic variance.

Heritability equation

h² = VA / VP

What does h² = 0 mean?

There is no additive genetic variation contributing to phenotypic variation, or environmental effects account for all variation.

What does h² = 1 mean?

Additive genetic variation accounts for 100% of phenotypic variation.

Is heritability a property of individuals?

No. It is a property of populations or groups and is population- and environment-specific.

Breeder's equation

R = h²S

Response to selection (R)

The change in the population trait mean from one generation to the next.

Selection differential (S)

The difference between the mean of selected individuals and the mean of the original population.

What happens to R when S increases?

R increases.

What happens to R when h² increases?

R increases.

What causes a larger response to selection?

A larger selection differential and greater heritability.

What does directional selection do to genetic variation?

It generally decreases genetic variation.