Evolution of Populations Mrs. Williams Freshman Biology

Variation Inherited differences between individuals of a population Can be Physical characteristic Biochemical characteristic Behavioral characteristic If there is no variation for a trait, it is said to be fixed

Gene Pool All of genes found within a population Relative frequency of alleles- proportion of gene pool that the allele makes up

Sources of Variation Mutation Gene Shuffling Creates new variation by changing parts of the genetic code Gene Shuffling Creates new variation by the reshuffling of genes during sexual reproduction Chromosome segregation Crossing-over

Single Gene Traits Traits are coded for by a single gene If trait has simple Mendelian (dominant/recessive) inheritance, there are 2 phenotypes possible. If trait has incomplete dominance or codominance, there are 3 phenotypes possible. If trait has multiple alleles, # of phenotypes depends on # of alleles For example: ABO blood type have 3 alleles with 4 phenotypes possible

Natural Selection on Single Trait Genes Occurs if the phenotypes are not equal in their fitness Relative frequencies within the gene pool change as some phenotypes are selected for (or some are selected against) This is evolution (a change in allele frequencies within a population over time) Natural Selection Refresher

Polygenic Traits Trait is coded for by more than one gene Various phenotypes possible Phenotypes form a bell curve

Natural Selection of Polygenic Traits Directional Selection One phenotype extreme is selected for (or one against) Bell curve is shifted to the left or right

Natural Selection of Polygenic Traits (continued) Stabilizing Selection Both phenotype extremes are selected against (average phenotype is selected for) Bell curve narrows

Natural Selection of Polygenic Traits (continued) Disruptive Selection Average phenotype is selected against (extremes are selected for) Bell curve splits into two peaks

Genetic Drift Change in allelic frequencies due to random effects Effects are seen more in smaller populations

Genetic Drift: Bottleneck Effect Event randomly removes large numbers of individuals from a population Many variations can be lost

Genetic Drift: Founder Effect Small part of the population removes itself (or is removed) from the larger population

Genetic Equilibrium Allelic Frequencies remain the same Also referred to as “Hardy-Weinberg Equilibrium” No evolution Random mating Large population No immigration or emigration No mutation No natural selection

Speciation Species - group of individuals that can breed together and produce a fertile offspring Speciation is the process of forming new species from existing species To occur: Populations of one species must be isolated from each other long enough to accumulate enough changes to become two species

Types of Isolation Geographical Isolation Populations are separated by a geographical barrier and cannot mate and share genes

Types of Isolation Behavioral Isolation Temporal Isolation Populations are separated by behavioral differences and don’t mate with each other to share genes Temporal Isolation Populations reproduce at different times so they cannot mate together and share genes

This leads to… Reproductive Isolation Cannot mate and produce a fertile offspring Occurs because individuals cannot Mate together or Create a zygote Create a viable offspring Create a fertile offspring