Evolution of Populations

Populations are the units of evolution

Population

A group of individuals of the same species living in the same place at the same time.

Species

A group of populations whose members are capable of interbreeding and producing fertile offspring.

Population genetics

An important turning point for evolutionary theory the 1920’s.

Developed in the 1920’s

A field that combines Darwin’s and Mendel’s ideas by studying how populations change over time.

The Gene Pool

In studying evolution at the population level, geneticists focus on the GP

Total collection of genes in a population at any one time.

The Gene Pool

Sources of Genetic VariationMutationsGene ShufflingCrossing overSexual reproduction

Single Gene Traits

The number of phenotypes produced for a given trait depends on how many genes control the trait

Polygenic traits are controlled by two or more genes

Natural Selection can affect the distributions of phenotypes in 3 ways:

Directional selection Disruptive selection Stabilizing selection

Directional Selection

When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

Disruptive Selection

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.

Stabilizing Selection

Takes place when individuals near the center of a curve have higher fitness than individuals at either end

Isolating Mechanisms

•Behavioral

•Geographic

•Temporal

The Process of Speciation

Behavioral

Geographical

Sciurus aberti kaibabenesis

Sciurus aberti

Temporal

Bufo americanus

Bufo fowleri

The Hardy–Weinberg principle states:

Both allele and genotype frequencies in a population remain constant or are in equilibrium from generation to generation unless…

Disturbing influences happen such as non-random mating, mutations, selection, limited population size, random genetic drift and gene flow.

Genetic equilibrium is a basic principle of population genetics.

Hardy-Weinberg principle is like a Punnett square for populations, instead of individuals.

p2 + 2pq + q2 = 1; p + q = 1