Evolution of Populations

Chapter 16

• The study of genetics helps scientists understand the relationship between inheritance and evolution• Scientists know that… –genes control traits and that many

genes have at least two forms, or alleles

-members of all species are heterozygous for many genes

• In genetic terms, evolution is any change in the relative frequency of alleles in a population

• A population is a group of individuals of the same species that can interbreed• Members of a population share a

gene pool- all the genes, and their

different alleles, in the population

• The number of times the alleles occur in a gene pool compared to the number of times that other alleles for the same gene occur is the relative frequency of the allele

• The two main sources of genetic variation are mutations and gene shuffling• A mutation is any change in a

sequence of DNA• Gene shuffling occurs during

gamete formation (crossing-over)

• It can produce millions of different gene combinations, but doesn’t change the allele frequency in the population• Both mutations and gene

shuffling increase genetic variation by increasing the number of different genotypes

• The number of phenotypes for a trait depends on how many genes control the trait• A single-gene trait is a trait

controlled by only one gene• If there are two alleles for the

gene, two genotypes are possible

• In humans, a single-gene trait is the presence of a widow’s peak

• Allele for a widow’s peak is dominant over the allele for hairline with no peak.

• As a result, there are only two phenotypes – having a widow’s peak or not

• A polygenic trait is controlled by two or more genes

• Each gene of a polygenic trait may have more than one allele

• Polygenic traits form many phenotypes• Variation in a polygenic trait in a

population often forms a bell-shaped curve with most members near the middle

• example: height in humans

16.2 Genetic Change

•Natural selection on single-gene traits can lead to changes in allele frequencies…evolution

Natural Selection on Polygenic Traits

• Directional Selection

Stabilizing Selection

Disruptive Selection

Genetic Drift/Founder Effect

Hardy-Weinberg Principle

•Allele frequencies in a population will remain constant unless one or more factors cause them to change•Genetic Equilibrium

5 Conditions to Maintain G.E.

1. Must be random mating2. Population must be very large3. No movement into or out of the

population4. No mutations5. No natural selection