Lesson Overview 17.1 Genes and Variation

Genotype and Phenotype in Evolution Population – mating group of organisms of the same species Gene pool – all genes (and their alleles) present in a population Allele frequency - # of times allele occurs in a population Changes as population evolves over time Natural selection operates on individuals, but causes a change in the allele frequency

For example, this diagram shows the gene pool for fur color in a population of wild boar.  

Populations evolve not individuals!! Populations and Gene Pools Evolution-change in the relative frequency of alleles in the gene pool of a population over time.   Natural selection operates on individuals, but causes a change in allele frequencies in populations. Populations evolve not individuals!!

Sources of genetic variation Main source of genetic variations in populations is mutations!! They occur randomly Only heritable mutations matter for evolution Other sources of variation include: 1. Genetic recombination- crossing over and independent assortment in meiosis 2. Lateral gene transfer- (bacteria only) Bacteria swap plasmids between members of the same generation, then pass them to their offspring

Natural Selection & Phenotype An organism’s genotype and environmental conditions makes up its phenotype. Natural selection operates on variation in organisms’ phenotypes. Phenotypes that are better suited for the environment will have higher fitness

Single-Gene and Polygenic Traits Number of phenotypes for a trait depends on how many genes control the trait. Single-gene trait – trait controlled by one gene. Ex. Banded or un-banded shell. NS on these traits leads to changes in allele and phenotypic frequencies.

Polygenic trait – trait controlled by two or more genes. Ex. Height in humans. NS on these traits affects fitness of phenotypes.

NS on polygenic traits leads to selection in populations When NS on polygenic traits affects the fitness of phenotypes, it leads to selection: 1. Directional selection 2. Stabilizing selection 3. Disruptive selection

Types of selection in populations Directional selection – organisms at one end of The curve have a higher fitness than those in the middle or at the other end Stabilizing selection – organisms in the Center have highest fitness Disruptive selection – organisms at the ends of curve have highest fitness

NS is not the only source of change in allele frequencies Genetic drift – change in allele frequency that occurs in small populations due to random chance *Genetic bottleneck – change in allele frequency following a dramatic reduction in population size *Founder effect – change in allele frequency following migration of a small subgroup out of the population to start a new population