BIOLOGY STANDARD 8

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