Evolution of Populations

Population

Group of individuals of the same species living in the same area

Gene Pool

All genes and alleles present in a population

Genotype Frequency

Percentage of each genotype (ex: AA, Aa, aa) in a population

Phenotype Frequency

Percentage of individuals showing a specific observable trait

Allele Frequency

Percentage of a specific allele in a population compared to all alleles for that gene

Genetic Equilibrium (Hardy-Weinberg Equilibrium)

Condition where a population is not evolving and allele frequencies remain constant; requires: large population, random mating, no mutations, no migration, and no natural selection

Hardy-Weinberg Conditions

Requirements for genetic equilibrium: no evolution occurs if population is large, mating is random, no mutations occur, no gene flow occurs, and no natural selection occurs

Genetic Drift

Random change in allele frequencies in a population, especially in small populations

Founder Effect

A type of genetic drift where a small group starts a new population with allele frequencies different from the original population

Bottleneck Effect

A type of genetic drift where a population is drastically reduced by a disaster, leaving survivors that may not represent the original gene pool

Gene Flow

Movement of alleles into or out of a population through immigration and emigration

Immigration

Movement of individuals into a population, increasing gene flow

Emigration

Movement of individuals out of a population, decreasing gene flow

Single-Gene Traits

Traits controlled by one gene with two or a few possible phenotypes (ex: tongue rolling, earlobe attachment)

Polygenic Traits

Traits controlled by multiple genes, resulting in a range of variation (ex: height, skin color, eye color, hair color)

Stabilizing Selection

Natural selection that favors average individuals and selects against extreme traits

Directional Selection

Natural selection that favors one extreme phenotype over others

Disruptive Selection

Natural selection that favors both extreme phenotypes and selects against the average

Sources of Genetic Variation

Mechanisms that create genetic diversity: mutation, genetic recombination during meiosis, and gene flow

Mutation

A permanent change in DNA that can create new alleles and increase genetic variation

Genetic Recombination

Shuffling of genes during meiosis (crossing over and independent assortment) that increases variation

Species

A group of organisms that can mate and produce fertile offspring

Prezygotic Barriers

Reproductive isolation mechanisms that prevent fertilization (before a zygote forms), such as different mating seasons, behaviors, or geographic separation

Postzygotic Barriers

Reproductive isolation mechanisms that occur after fertilization, resulting in hybrid offspring that are weak, sterile, or unable to survive

Reproductive Isolation

Mechanisms that prevent species from interbreeding and producing fertile offspring