Chapter 19: The Evolution of Populations

Vocabulary flashcards covering key terms from the lecture notes on population genetics, variation, and microevolution.

Population genetics

The study of how allele and genotype frequencies in populations change over time.

Gene pool

The total collection of alleles across all genes in a population.

Genotypic frequency

The proportion of individuals with a given genotype (AA, Aa, aa) in a population.

Allelic frequency

The proportion of a specific allele (A or a) in the population’s gene pool.

Population

A group of interbreeding individuals of the same species living in the same area.

Hardy-Weinberg equilibrium

A state in which allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary forces.

p (dominant allele frequency)

Frequency of the dominant allele in a population under HW conditions.

q (recessive allele frequency)

Frequency of the recessive allele in a population under HW conditions.

p^2

Frequency of the homozygous dominant genotype (AA) in Hardy-Weinberg equilibrium.

2pq

Frequency of the heterozygous genotype (Aa) in Hardy-Weinberg equilibrium.

q^2

Frequency of the homozygous recessive genotype (aa) in Hardy-Weinberg equilibrium.

HW conditions

Five conditions for Hardy-Weinberg: no mutations, random mating, no selection, very large population, no gene flow.

Microevolution

Change in allele or genotype frequencies within a population over generations.

Natural selection

Nonrandom differential survival and reproduction that changes allele frequencies.

Genetic drift

Random changes in allele frequencies, especially in small populations.

Founder effect

A few individuals start a new population, altering its allele frequencies.

Bottleneck effect

A sharp reduction in population size that reduces genetic diversity.

Gene flow

Movement of alleles into or out of a population via migration or dispersal.

Adaptive evolution

Evolution driven by natural selection that improves adaptation to the environment.

Relative fitness

An individual’s reproductive success relative to others in the population.

Stabilizing selection

Favors average phenotypes and reduces variation.

Directionally selection

Shifts the phenotype distribution toward one extreme.

Diversifying (Disruptive) selection

Favors extreme phenotypes; average phenotypes are selected against.

Frequency-dependent selection

Fitness of a phenotype depends on its frequency; rare forms may be favored.

Sexual selection

Selection for traits that increase mating success rather than survival.

Intrasexual selection

Competition among individuals of the same sex for mates.

Intersexual selection

Mate choice by one sex, often females, influencing mating.

Sexual dimorphism

Distinct differences between sexes due to sexual selection.

Heterozygote advantage

Heterozygotes have higher fitness than either homozygote, maintaining variation.

Maintaining genetic variation

Mechanisms that preserve variation: diploidy, diversifying selection, heterozygote advantage, and frequency-dependent selection.

Diploidy

Having two alleles per gene; recessive alleles can be hidden in heterozygotes.

Mutations

Changes in DNA that create new alleles; source of genetic variation.

Point mutation

A single-base change in DNA that can create a new allele.

Chromosomal alterations

Large-scale changes to chromosomes that can alter genes and phenotypes.

Discrete genetic variation

Variation with two or more alleles at a single gene locus.

Continuous variation

Phenotypes produced by the combined effects of two or more genes (polygenic).

Phenotypic variation

Variation in observable traits due to genetics and environment.

Phenotypic plasticity

Environment can influence phenotype expression without changing genotype.

Mendelian inheritance

Inheritance of discrete units (genes) with segregation and independent assortment.

Chromosome theory of inheritance

Genes reside on chromosomes; supported by Sutton-Boveri and Morgan.

Mutations are typically harmful

Most new alleles are deleterious; some are neutral or adaptive depending on context.

Beneficial alleles

Alleles that increase fitness in a given environment.

Isolated population

A population separated from others, leading to drift and divergence.

Fixed allele

An allele with a frequency of 1 in the population.

Homozygous

Having two identical alleles for a gene (AA or aa).

Heterozygous

Having two different alleles for a gene (Aa).

Carrier

A heterozygous individual who carries a recessive allele and usually has no phenotype.

Tay-Sachs allele carrier frequency

In a population, carriers occur according to Hardy-Weinberg; for a recessive allele, carrier frequency is 2pq when p+q=1.

Null hypothesis in Hardy-Weinberg

HW equilibrium serves as the null model; deviations indicate microevolution.

Random mating

Mating occurs without regard to genotype; one HW condition.

Non-heritable variation

Phenotypic variation caused by environmental factors, not genetics.

Discrete variation

Variation with two or more distinct phenotypes determined by a single gene.

Polygenic/continuous variation

Traits controlled by many genes; continuum of phenotypes.

Phenotype

Observable traits of an organism.

Genotype

Genetic makeup of an organism for a given trait.

Crossovers

Exchange of genetic material during meiosis, increasing genetic diversity.

Independent assortment

Random orientation of chromosome pairs during meiosis, increasing diversity.

Random fertilization

Random union of sperm and egg, adding to genetic variation.

Sources of genetic recombination

Crossovers, independent assortment, and random fertilization.

Morgan

Scientist whose work with fruit flies linked genes to chromosomes, supporting the chromosome theory.