Evolution of Populations Chapter 16

I. Genes & Variation

1. Genetic Variation is common

Most organisms have many different alleles (forms of a gene) for one trait Genetic variation seems invisible b/c it can be small differences in biochemical processes like cell respiration or metabolism Individuals can be heterozygous for a trait which results in more variation.

II. Variation & Gene Pools

Populations are used to study genetic variation Gene Pool- A common group of genes shared by a population. Ex: the gene pool of a population of maple trees in a park This population of frogs has genes from the same gene pool

Relative Frequency- the number of times an allele occurs in a gene pool in comparison to the number of times the other alleles occur for the same gene. Is expressed as a percentage Relative frequency has nothing to do with the gene being dominant or recessive.

Ex: In a population of cats the relative frequency of the dominate allele (B) is 65% & the relative frequency of the recessive allele (b) is 35% Fig. 1: Imagine that you go to the mountaintop this year, sample these beetles, and determine that 80% of the genes in the population are for green coloration and 20% of them are for brown coloration. You go back the next year, repeat the procedure, and find a new ratio: 60% green genes to 40% brown genes.

Gene Pool for Fur Color in Mice

Evolution is any change in the relative frequency of alleles in a population

Distribution of the O type blood in native populations of the world Distribution of the B type blood allele in native populations of the world Distribution of the A type blood allele in native populations of the world

III. Sources of Genetic Variation

There are two main sources of genetic variation: Mutation Gene Shuffling

Americans are a genetically varied population

Mutations (a change in the sequence of DNA) Can happen b/c of mistakes during DNA Replication Exposure to radiation Exposure to chemicals in the environment Sometimes mutations affect the phenotype of an organism; sometimes they do not Mutations can affect the “fitness” of an organism

Gene Shuffling Occurs during when gametes are made Also occurs crossing over during meiosis most genetic variation results from gene shuffling.

IV. Single-Gene & Polygenic Traits

What Determines How Many Phenotypes A Population Has? Depends on how many genes control the trait Traits can be single-gene or polygenic Single-gene trait- a trait that is controlled by one gene w/ two alleles Ex: a widow’s peak in humans

In this example population having a widows peak is more common than not having a widows peak.

Many traits are polygenic Polygenic trait- a trait that controlled by two or more genes b/c each gene of a polygenic trait has two or more alleles there are many different phenotypes that can result

Variations of hair color in humans. What is your hair color? Human hair color is a polygenic trait

A bell shaped curve represents the distribution of polygenic traits in humans. This is called “normal distribution” Most people are in the middle Very people are on either extreme very light brown or very dark brown. Very tall or very short

Most humans skin color is in the middle of this bell curve