Mechanisms of Evolution The “Process” in Populations

Lamarck’s Theory: acquired characteristics Evolution: change through time All species come from pre-existing species with modifications Over time there are differences in ancestral & descendant species

Mechanisms of Evolution Natural selection Changes in allele frequencies Natural Gene Variation Genetic variation is the “rule” Polymorphism Traits display variation DNA sequence polymorphism Sequencing DNA Variations in coding & non-coding sequences

Hardy-Weinberg Principle 1908 G.H. Hardy & G. Weinberg Original proportions of genotypes in a population will stay constant if… Population is large Random mating occurs No mutations No genes are input from other sources No selection occurs

Hardy-Weinberg Equation p - # of dominant alleles q - # of recessive alleles p2 - # of homozygous dominant individuals 2pq - # of heterozygous individuals q2 - # of homozygous recessive individuals p + q = 1 p2 + 2pq + q2 = 1

Examples

Within a population of butterflies, the color brown (B) is dominant over the color white (b). 60% of all the butterflies are brown. Calculate the H-W frequencies of BOTH alleles and the different genotypes.

In a large population of biology instructors 396 are red-sided and 557 are tan-sided. Assume red-sided is a totally recessive trait. What are the allele frequencies of each allele. The expected genotypic frequencies The number of heterozygous individuals. Conditions happen to be really good this year for breeding and next year there are 1245 instructors. If all H-W conditions are met, how many of these would be red-sided? Tan-sided?

H-W Continued… After a few generations the population of biology instructors was re-assessed and 450 are red-sided and 505 are tan-sided. Assume red-sided is a totally recessive trait. What is the NEW p & q? Has evolution occurred? How do you know?

What does this mean? Are the conditions that Hardy & Weinberg set up realistic? So what is the relevance? This equation allows us to measure evolutionary change. If frequencies do not stay constant then there has been evolution

5 Agents of Evolutionary Change Mutation Gene flow Non-random mating Genetic drift Selection ** These follow Hardy-Weinberg criteria!!

Mutation So low  little effect Ultimate source of genetic variation Not affected by natural selection 1-10 times per 100,000 cell divisions Very slow process – environments change too rapidly

Gene Flow Movement of alleles from one population to another Powerful agent Movement of an organism to a new place Drifting of gametes of plants or immature stages of animals Mating of individuals in adjacent populations Can counter natural selection

Nonrandom Mating Certain genotypes mate with another genotype more frequently than random Inbreeding - increases proportion of homozygotes Self-fertilizing plants  homozygous Outcrossing  leads to heterozygotes Increases expression of recessive alleles

Genetic Drift Frequencies of alleles change by chance alone Small populations Bottleneck Effect Flooding, drought, disease, war Lose genetic variability Founder Effect Individuals disperse to new areas Some alleles from original pop. may be lost

Selection Artificial – breeder selects Natural – nature selects 3 conditions Variation must exist between individuals Variation results in differences in # of offspring surviving Variation must be genetic

What does Nature Select For? Avoid predators Match climate conditions Pesticide resistance

Natural Selection & Evolution Not the same thing!! Natural selection – process Evolution – historical record of change through time (outcome) Process may lead to outcome Not the only thing that can lead to outcome (evolution)

Heterozygote Advantage Natural selection  removes alleles Heterozygote advantage favors both alleles Maintains both alleles in population Sickle Cell Anemia – homozygous recessive Heterozygotes  advantage against the affects of malaria parasite Recessive allele is maintained in the heterozygous

Limits?? Genetic variation is required Selection can move faster than variation May have no genetic basis Selection acts on phenotypes – do not eliminate recessive alleles

Critical Thinking Apply Darwin’s idea of natural selection & the Theory of Evolution to explain the following situation… Many species of lemurs have large eyes which allow them to survive as nocturnal creatures. Lemurs that are not nocturnal have smaller eyes. Use Darwin’s ideas of natural selection to explain how the nocturnal lemurs evolved to have large eyes. Ring-Tailed Lemur Nocturnal Lemur

Types of Selection Disruptive Directional Stabilizing Fitness Eliminates intermediate 2 phenotypically distinct groups Directional Eliminates one extreme Stabilizing Eliminates both extemes Favors intermediate Fitness # of surviving offspring left in the next generation Attracting mates # offspring prodeuced per mating

Summary – Darwin’s Ideas What things/conditions are required of a population or in a population in order for natural selection to occur?