Evolution of Populations

Genes and Variation  Gene Pool  Contains all the alleles of all the genes in a population

Genes and Variation  Allele Frequency  The number of times an allele occurs in a gene pool

Sources of Genetic Variation  1. Mutations  A genetic change that will either:  Increase fitness  Decrease fitness  Neutral impact on the individual

Sources of Genetic Variation  2. Sexual Reproduction  Crossing-over

Sources of Genetic Variation  3. Lateral Gene Transfer  Passing of genes to an organism that is NOT an offspring  Example: Bacteria can pass a plasmid to other bacteria

Single Gene and Polygenic Traits  Single Gene Traits  A trait controlled by just one gene  Example: Stripes on a snail

Single Gene and Polygenic Traits  Polygenic Traits  Controlled by two or more genes  Can have many possible genotypes and phenotypes  Example: Human Height-You can be very tall, very short, or any height in between  TT=Tall  Tt=Medium  Tt=Short

How Natural Selection Works  1. Natural Selection on Single-Gene Traits  Can lead to changes in allele frequencies  Can lead to changes in phenotype frequencies

How Natural Selection Works  2. Natural Selection on Polygenic Traits  There are three ways Natural Selection can act on Polygenic Traits  1. Directional Selection  2. Stabilizing Selection  3. Disruptive Selection

How Natural Selection Works  2. Natural Selection on Polygenic Traits  1. Directional Selection  When individuals at one end of the Bell Curve have higher fitness  Example: More large seeds than small seeds, birds with larger beaks will become more common overtime

How Natural Selection Works  2. Natural Selection on Polygenic Traits  2. Stabilizing Selection  When individuals near the center of the bell curve have higher fitness  Example: Small babies are usually less healthy, large babies have a hard time being born, but average size babies are more likely to survive

How Natural Selection Works  2. Natural Selection on Polygenic Traits  3. Disruptive Selection  Most extreme traits are the most likely to survive  Example: medium size seeds are rare, therefore a bird with a small beak or large beak will become more common over time

Genetic Drift  Genetic Drift  A random change in the frequency of alleles in a population

Genetic Drift  Bottleneck Effect  A change in allele frequency following a dramatic loss of population  Example: Floods and diseases may only leave a few individuals alive

Genetic Drift  Founder Effect  Change in allele frequency that results from migration of a small group of a population

Evolution Versus Genetic Equilibrium  Genetic Equilibrium  The population is NOT evolving, but the allele frequency in the gene pool also is NOT changing  Hardy-Weinberg Principle  Allele frequencies in a population will stay in equilibrium unless something causes them to change

What could cause genetic changes in a population?  1. Nonrandom Mating  Individuals must NOT choose a mate based a specific trait  2. Small Populations  Evolution takes place easier in small populations  3. Movement into or out of a population  4. Mutations  5. Natural Selection

The Process of Speciation  What is a species  Biological Species Concept  A population or group of populations whose members have the ability to breed with one another in nature and produce fertile offspring

The Process of Speciation  Speciation  Formation of a new species

The Process of Speciation  1. Geographic Isolation  2. Behavioral Isolation  3. Temporal Isolation

The Process of Speciation  1. Geographic Isolation  When populations are separated by a barrier  Example: River, mountain, or ocean

The Process of Speciation  2. Behavioral Isolation  When two populations develop differences in courtship rituals  Example: Eastern Medowlarks and Western Medowlarks…….same habitat, but won’t mate with each other because of their different mating songs

The Process of Speciation  3. Temporal Isolation  Populations that live in the same habitat reproduce at different times  Example: Wood frogs and Leopard frogs