The Major Forces of Evolution Chapter 6

Reminders… Exam 1 next Week Study Guide is up Zoo day set…......Nov. 5th

Evolution Evolution is a change in gene frequency in a population over time What causes this change? Natural selection Migration (gene flow) Genetic drift Mutation

Evolution The first three result in changes in frequency of genes in a population by “redistributing” the existing alleles Mutation is the only one to introduce new information. Another way to add new info is through nonrandom mating

Natural Selection Darwin and Alfred Wallace independently formulated the theory of evolution They discussed the struggle for existence, extinction of species, adaptation and variation He looked at the work of Malthus, who said that resources are limited but population growth is not Darwin realized that selection acts upon an individual This led him to formulate the principle of natural selection

Natural Selection Here are the steps: More individuals in a population are produced that can survive There is variation and some individuals are better adapted to their environment Individuals compete for limited resources The better adapted ones will survive and reproduce, passing down their genes This results in gene frequencies changing over time If populations become isolated, this may result in enough genetic differences to create new species

Natural Selection For natural selection to occur, traits must be: Variable: variation of genes is crucial for selection Heritable: traits must be inherited through genes passed by parents

Natural Selection Remember, natural selection acts on individuals, not species Each individual has differential reproductive success and this results in a change in gene frequencies as well The fitness (reproductive success) of any variation will change as the environment changes. A result of natural selection is adaptation

Mutation A heritable change in the genetic material (DNA) is a mutation Natural selection, together with mutation, can change gene frequency more rapidly Mutations are random

Migration (Gene Flow) Migration occurs constantly among populations within a species Also need reproduction If we talk about a population being made up of genes rather than individuals, we are referring to a gene pool This is all the genes in the population As individuals of each species move among populations, gene flow causes frequencies to change

Genetic Drift One generation’s frequencies may not accurately represent the frequency of it’s previous generation’s genes Two causes are founder effect and population bottleneck Genetic drift is similar to sampling error because the smaller the sample, the less likely it will represent the population at large

Genetic Drift A small sub-population that separates itself from the rest of the population to “found” a new population will not represent accurately the frequency of the parent population this is founder effect If there is a drastic reduction in the number of individuals in a population This is population bottleneck

Genetic Drift This can result in A higher proportion of recessive genes A greater chance of 2 recessive alleles coming together More recessively expressed traits Loss of genetic diversity

Hardy-Weinberg Used to predict allele frequencies Uses a null hypothesis…tries to say evolution is not happening, which shows that it does Homozygous recessive is aa or rr Homozygous dominant is AA or RR Heterozygous is Aa or Rr Dominants are always expressed, even in heterozygous form

Hardy-Weinberg Equation: p=all dominant alleles q=all recessive alleles p+q=1, or 100% of population p2+2pq+q2 = 1

Assignment We will practice Hardy-Weinberg Lab 6.1 Self Test 6.1 Maybe WS….. Test Review