Evolution of Populations

Chapter 16Evolution of Populations

I. Genes & Variation1. Genetic Variation is common

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

II. Variation & Gene Pools

Populations are used to study genetic variationGene 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 poolRelative 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 percentageRelative 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 B type blood allele in native populations of the worldDistribution of the A type blood allele in native populations of the worldDistribution of the O type blood in native populations of the worldIII. Sources of Genetic Variation

There are two main sources of genetic variation:MutationGene Shuffling Americans are a genetically varied population

Mutations (a change in the sequence of DNA)Can happen b/c of mistakes during DNA ReplicationExposure to radiationExposure to chemicals in the environmentSometimes mutations affect the phenotype of an organism; sometimes they do notMutations can affect the fitness of an organismGene Shuffling Occurs during when gametes are madeAlso occurs crossing over during meiosismost genetic variation results from gene shuffling.

IV. Single-Gene & Polygenic TraitsWhat Determines How Many Phenotypes A Population Has?Depends on how many genes control the traitTraits can be single-gene or polygenicSingle-gene trait- a trait that is controlled by one gene w/ two alleles Ex: a widows peak in humans

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

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

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

A bell shaped curve represents the distribution of polygenic traits in humans. This is called normal distributionMost people are in the middleVery 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