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 phenotypes 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… microevolution

Normal Distribution Frequency is highest near the mean value and decreases steadily toward extreme end of range Population follows a normal distribution when it’s not under natural selection for the trait

Natural Selection on Polygenic Traits Directional Selection takes place when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

Stabilizing Selection Occurs when individuals near the center of the curve have higher fitness than individuals at either end of the curve.

Disruptive Selection Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Favors phenotypes at both extremes

Genetic Drift/Founder Effect Random change in allele frequency Looking at small populations * Think about the way a natural disaster would lead to the 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. Must be random mating Population must be very large No movement into or out of the population No mutations No natural selection