Evolution(Natural Selection, Genetic Drift, Hardy-Weinberg)

How do genetic variations occur? What is Evolution? Change in the genetic makeup of a population over time. Fitness – ________________________________________________. Populations can evolve, not individuals. Diverse gene pool good for long-term survival of a species. Genetic variations are important! How do genetic variations occur?

Where does Variation come from? Mutations _______________________ errors in mitosis & meiosis environmental damage Sexual reproduction ___________________ genetic recombination new arrangements of alleles in every offspring new combinations = new phenotypes

Genetic variation in a population

Essence of Darwin’s ideas Natural selection ____________variation exists in populations ____________________ more offspring than the environment can support __________________ for food, mates, nesting sites, escape predators differential survival successful traits = adaptations adaptations become more common in population

Lamarckian vs. Darwinian view in reaching higher vegetation giraffes stretch their necks & transmits the acquired longer neck to offspring Darwin giraffes born with longer necks survive better & leave more offspring who inherit their long necks 

Evolution by Natural Selection Environment is always changing _______________________of the population _________________ = a genetic variation favored by natural selection. When allele frequencies change in a population, the species has evolved Natural selection is _______________, because those with most fit traits are selected for survival and reproduction “Survival of the Fittest”

STABILIZING SELECTION Effects of Selection Changes in the average trait of a population _____________ STABILIZING SELECTION ____________ ___________ giraffe neck horse size human birth weight rock pocket mice

Heterozygote Advantage ________________________________________ Ex: Sickle Cell Anemia aa – dies of sickle cell anemia Aa – some side affects BUT resistant to malaria! AA – no disease present BUT prone to malaria

In addition to natural selection, evolutionary change is also driven by random processes…

II. GENETIC DRIFT The smaller the population, the less genetic variety it has. In a very small population, alleles can be lost from one generation to the next, simply by random chance. When a population evolves only because of this type of random sampling error, _______________is taking place.

Genetic Drift Chance events changing frequency of traits in a population not adaptation to environmental conditions not selection _________________ small group splinters off & starts a new colony it’s random who joins the group Bottleneck a disaster reduces population to small number & then population recovers & expands again but from a limited gene pool ____________________________ Founders: When a new population is started by only a small group of individuals. Just by chance some rare alleles may be at high frequency; others may be missing; skew the gene pool of new population. Ex: human populations that started from small group of colonists example: colonization of New World Bottleneck: When large population is drastically reduced by a disaster-famine, natural disaster, loss of habitat…loss of variation by chance event alleles lost from gene pool not due to fitness, narrows the gene pool

Ex: Cheetahs 2 bottlenecks 10,000 years ago Ice Age last 100 years poaching & loss of habitat All cheetahs share a small number of alleles less than 1% diversity

Human Impact on variation Artificial selection/Inbreeding Loss of alleles in gene pool ______________________ Reduces adaptability Overuse of antibiotics/Insecticides Resistance increased

Hardy Weinberg: Population Genetics Using mathematical approaches to calculate changes in allele frequencies…this is evidence of evolution.

Hardy-Weinberg equilibrium Hypothetical, non-evolving population preserves allele frequencies Natural populations are never in H-W equilibrium useful model to measure if evolution is occurring G.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in 1908. Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?" G.H. Hardy mathematician W. Weinberg physician

Evolution of populations Evolution = change in allele frequencies in a population hypothetical: what conditions would cause allele frequencies to not change? ____________________(no genetic drift) no migration (no gene flow in or out) no mutation (no genetic change) _________________(no sexual selection) no _______________(everyone is equally fit) H-W occurs ONLY in non-evolving populations!

Populations & gene pools Concepts a population is a localized group of interbreeding individuals gene pool is _______________in the population remember difference between alleles & genes! ______________is how common is that allele in the population how many A vs. a in whole population

Hardy-Weinberg theorem Frequencies are usually written as decimals! Counting Alleles assume 2 alleles = B, b frequency of dominant allele (B) = ___ frequency of recessive allele (b) = ___ frequencies must add to 1 (100%), so: p + q = 1 BB Bb bb

Hardy-Weinberg theorem Counting Individuals frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), so: ________________ BB Bb bb

H-W formulas Alleles: _________= 1 Individuals: ____________= 1 B b BB

Practice Problem: In a population of 100 cats, there are 16 white ones. White fur is recessive to black. What are the frequencies of the genotypes?

Tips for Solving HW Problems: Solve for q first. Then solve for p. Don’t assume you can just solve for p2 if only given dominant phenotypic frequency. READ carefully!!!