Genes and Variation It’s an organism’s phenotype (traits) that make it better fit its environment And these traits come from the alleles it has All of the alleles in a population is known as its gene pool Evolution is a change in the gene pool of a population over time. This involves changes in allele frequency over time.

Sources of Variation Random mutation…alleles can randomly change…we have approx 300 mutations from our parents, but most are unnoticeable. Recombination…independent assortment and crossing over during meiosis make our genes different from our parents

Sources of Variation Lateral gene transfer…genes transferred from 1 organism to another e.g. bacteria often exchange DNA not so much in higher organisms, like humans and most animals and plants

Phenotypes So it’s the phenotypes that we really see in evolution Traits can be caused by 1 gene or many. Most are polygenic … controlled by many genes So you don’t get just 2 phenotypes, like what we studied in genetic unit Get a range of phenotypes, like we saw with bar graphs for height and hand size in human adaptation lab.

Bell Curves With polygenic traits, you get this range of phenotypes rather than just 2 If this curve changes, evolution has occurred

3 kinds of Natural Selection Directional selection Individuals at one end of curve are most fit for environment e.g. birds with bigger beaks get more food

3 kinds of Natural Selection Stabilizing selection Organisms with average trait survive best e.g. babies with medium weights, too heavy or too light = health problems

3 kinds of Natural Selection Disruptive selection Organism at both extremes are fittest e.g. medium sized seeds less common, large and small beaks are advantageous

Genetic Drift So that’s how evolution occurs by Natural Selection Can also occur by accident or chance and not by natural selection Evolution by random chance is called genetic drift Usually happens in smaller populations, because larger ones can recover from these accidents.

Genetic Drift Bottleneck effect When an accident (e.g. disease, forest fire, hurricane) wipes out a large part of the population the allele frequency may change, but not because of natural selection

Genetic Drift Founder effect When a few individuals leave the group and start a new population They may not have all the alleles in the original population, so the allele frequency has changed, but not because of natural selection

p = frequency of dominant allele q= frequency of recessive allele Genetic Equilibrium When a population isn’t evolving, it is in genetic equilibrium Allele frequencies remain constant according to the Hardy-Weinberg Principle: p + q = 1 p2 + 2pq + q2 = 1 p = frequency of dominant allele q= frequency of recessive allele

Genetic Equilibrium Hardy-Weinberg Principle predicts populations will stay in genetic equilibrium unless: There is nonrandom mating (i.e. artificial or sexual selection, rather than natural selection) Small population size (genetic drift has big effects, then) Gene flow (alleles enter or leave the population through immigration (coming in) or emigration (leaving)

Genetic Equilibrium Hardy-Weinberg Principle predicts populations will stay in genetic equilibrium unless: Mutations happen to change the alleles available Natural selection happens where one genotype becomes more fit for the environment, and survives to reproduce more often H-W principle is kind of a “what if”. We know what it takes for evolution to occur, what would it take for evolution NOT to occur?

Speciation A species is a population who members can interbreed and produce fertile offspring. Evolution has occurred when 1 species is so different from another that they can’t mate any more. This is called reproductive isolation

Kinds of Reproductive Isolation Behavioral isolation When organisms act differently and stop mating, e.g. birds in same area use different songs/rituals to attract mates Temporal isolation When organisms in the same area mate at different times from each other, e.g. orchids in rainforest blooming at different times, frogs in wetlands mating at different times

Besides Reproductive Isolation Geographic Isolation When a population is separated by a river, lake canyon, mountain and stop mating, e.g. Darwin’s finches on different islands, plant seeds that get carried to new areas

Example: Darwin’s Finches Birds arrive from mainland onto islands (founder effect) Birds travel to another island (geographic isolation)

Example: Darwin’s Finches Changes in gene pool because of natural selection e.g. different seeds to eat = changes in beak size/shape; different plants = different colored feathers to hide/stand out

Example: Darwin’s Finches Behavioral isolation: birds fly back, but are different now, don’t mate Birds continue to compete, move around and evolve

Molecular clock Very simply, by looking at how many differences there are in DNA, a biologist can tell how long ago the species evolved from each other.

Rates of Evolution Gradualism: very slowly with changes barely noticed Punctuated equilibrium: short bursts followed by long time frames with no change

Kinds of Evolution Divergent evolution What we’ve been talking about all along One common ancestor leads to many descendants Leads to homologous and vestigial structures

Kinds of Evolution Adaptive radiation Evolve to inhabit all habitats and niches e.g. dinosaurs…live on land, in water, in air, in swamps, herbivores, carnivores, etc.

Kinds of Evolution Convergent evolution Organisms from different ancestors evolve to have similar traits leads to analogous structures e.g. grassland birds Darwin observed: ostrich, emu, rhea

Kinds of Evolution Coevolution When organisms rely on each other so much that as one changes, the other does also e.g. plants and the birds/ insects that pollinate them

Extinction If evolution doesn’t happen, species can go extinct: Very slowly, a few species at a time = background extinction. Relatively quickly, many species at once = mass extinction 5 mass extinctions in Earth’s history Last one was 66 mya…dinosaurs Biggest one was Permian...250mya…96% of all species wiped out!

Convergent Evolution

Divergent Evolution

Divergent Evolution

Adaptive Radiation Evolve to fill many niches, e.g. live in many places or eat many different things

Adaptive Radiation