Mechanisms of Evolution

Is this population evolving? What do you think? Is this population evolving?

This population is NOT evolving Genetic Equilibrium - Population in which allelic frequency does NOT change over generations, i.e. population is NOT evolving

Connecting Darwin & Mendel Darwin’s Handicap ~ He didn’t know how heredity worked Mendel’s work (completed during Darwin’s lifetime) was not understood for decades Two gaps in Darwin’s thinking: Didn’t know how traits were inherited Variation in heritable traits was central to his theory, but he had no idea how that variation appeared

Two Main Sources of Genetic Variation 1) Mutations 2) Gene Shuffling

Mutations can be beneficial, harmful, or harmless Any change in a sequence of DNA. Caused by mistakes during replication or environmental factors such as radiation or chemicals. Source of new phenotypes Mutations can be beneficial, harmful, or harmless

2) Gene Shuffling ~ Mixing of genes during the production of gametes in meiosis Recombination as homologous chromosomes move independently during meiosis Crossing over: the exchange of genetic material during prophase I EX: Shuffling a deck of cards; produces many different combinations, but does not change the frequency of each type of allele in the population

Gene Pool & Allelic Frequency Gene Pool: Entire collection of all the alleles in a population Allelic Frequency: The number of times an allele appears in the gene pool

Gene Pool & Allelic Frequency Gene Pool: 12 green alleles and 8 brown alleles Allelic Frequency of green: 12/20 Allelic Frequency of brown: 8/20

Evolution is the change in allele frequency Occurs in one of three ways: Gene Flow Genetic Drift Natural Selection Remember…evolution occurs Over time In populations…NOT individuals On phenotypes

1) Gene Flow ~ The movement of alleles into or out of a population Population’s genetic variation increases when alleles are added to the population (migration) Decreases when alleles leave (emigration)

2. Genetic Drift Random change in allelic frequencies due to chance.

Types of Genetic Drift Founder Effect ~ A small group of individuals colonize a new habitat Allele frequencies change due to the random change in the population

Genetic drift ~ Founder Effect Sample of Original Population Founding Population A Founding Population B Descendants

Types of Genetic Drift Bottleneck ~ a sharp reduction in the size of a population due to environmental events (flooding, earthquake, etc.) Allele frequencies change due to the random chance of surviving the bottleneck event

3) Natural Selection ~ Most common source of evolutionary change Can lead to an increase or decrease in allele frequency in single gene traits Effects are more complex in polygenic traits 3 types

Single gene traits vs. polygenic traits

1. Directional Selection One of the extreme phenotypes is selected against Ex: Rainfall and Bird Beaks Gizmo

2. Stabilizing Selection BOTH of the extreme phenotypes are selected against Ex: Baby’s birth weight, gall fly size

Stabilizing Selection

3. Disruptive Selection The middle phenotype is selected against Ex. Large and Small Bird Beaks

Disruptive Selection

Evolution leads to new species When the gene pool of two populations become separated, they become two separate species. Natural selection is the key mechanism of evolution that leads to speciation

Evolution vs. Genetic Equilibrium Hardy-Weinberg Principle ~ Five conditions are required to maintain genetic equilibrium Random Mating: All members of the population must have an equal opportunity to produce offspring Large Population: To prevent genetic drift No Migration No Mutations No Natural Selection

Hardy-Weinberg Principle p= dominant allele q= recessive allele

Hardy-Weinberg Principle p2 + 2pq + q2=1 ; p+ q = 1 p2 = frequency of homozygous dominant individuals p = frequency of the dominant allele q2 = frequency of homozygous recessive individuals q = frequency of the recessive allele 2pq = frequency of heterozygous individuals 

PROBLEM #1 You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Calculate the following: The frequency of the "aa" genotype. The frequency of the "a" allele. The frequency of the "A" allele. The frequencies of the genotypes "AA" and "Aa."

PROBLEM #1 (aa) is 36%. The frequency of the "aa" genotype. The frequency of the "a" allele. The frequency of the "A" allele. The frequencies of the genotypes "AA" and "Aa."

PROBLEM #2 A very large population of randomly-mating laboratory mice contains 64% white mice. White coloring is caused by the double recessive genotype, "aa". Calculate allelic and genotypic frequencies for this population.

Problem #2 “aa” = 64%

PROBLEM #3 PROBLEM #4.Within a population of butterflies, the color brown (B) is dominant over the color white (b). 40% of all butterflies are white. Calculate the following: The percentage of butterflies in the population that are heterozygous. The frequency of homozygous dominant individuals.

PROBLEM #3 “bb” = 40% The percentage of butterflies in the population that are heterozygous. The frequency of homozygous dominant individuals.