Evolution of Populations Ch 16

Where does Variation come from? Mutations errors in mitosis & meiosis environmental damage Meiosis mixing of genes through crossing over and independent assortment.

Natural Selection and Populations -Single gene trait: controlled by single gene with two alleles Examples: widow’s peak, hitchhiker’s thumb, tongue rolling -Polygenic trait: controlled by 2 or more genes with 2 or more alleles Examples: height, hair color, skin color, eye color.

Natural Selection Affecting Polygenic Traits A. Directional Selection: Individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait Example: Galapagos finches – beak size Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness

Natural Selection Affecting Polygenic Traits B. Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve Example: human birth weight

Natural Selection Affecting Polygenic Traits C. Disruptive Selection: individuals at both ends of the curve have highest fitness. Example: birds where seeds are either large or small

Evolution and Speciation How can natural selection lead to the formation of new species???

Speciation Natural selection and chance events can change the relative frequencies of alleles (# times an allele occurs in the gene pool) in a population and lead to speciation. Speciation is the formation of new species. A species is a group of organisms that breed with one another and produce fertile offspring.

What Factors are involved in the formation of New species?? The gene pool of two populations must become separated for them to become new species.

Reproductive Isolation As new species evolve, populations become reproductively isolated from each other. When members of the two populations can’t breed with each other and produce fertile offspring, reproductive isolation has occured

Isolating Mechanisms How can reproductive isolation occur? Behavioral Isolation: when two populations are capable of interbreeding, but have differences in courtship rituals or other reproductive behaviors. Ex: Eastern and western meadow larks have overlapping ranges but don’t breed because they have a different matting call.

Isolating Mechanisms How can reproductive isolation occur? 2. Geographic Isolation: two populations are separated by geographic barriers, like rivers, mountains, or bodies of water. Ex: grand canyon squirrels: Colorado river separated 2 populations 10,000 years ago.

Isolating Mechanisms How can reproductive isolation occur? Temporal isolation: when two or more species reproduce at different times (different months or seasons) Ex: Northern Leopard Frog, North American Bullfrog Mates in April Mates in July

Genetic Drift Genetic Drift: random change in allele frequency that occurs in small populations Genetic drift can be caused by: 1. Founder Effect 2. Bottleneck Effect

1. Founder Effect When a new population is started by only a small group of individuals. example: colonization of New World just by chance some rare alleles may be at high frequency; others may be missing skew the gene pool of new population

2. Bottleneck Effect When a large population is drastically reduced by a disaster. Famine, natural disaster, loss of habitat. Loss of variation by chance event. Alleles lost NOT due to fitness!

Testing Natural Selection & Speciation in Nature Can natural selection actually be observed in nature?? YES! Important studies of natural selection in action have been done in the Galapagos Islands, with the descendants of the finches that Darwin first studied Darwin’s Hypothesis: The finches he saw were different from each other, but he hypothesized that they all had a common ancestor

Testing Darwin’s Hypothesis Two assumptions of Darwin’s finch hypothesis: For beak size and shape to evolve, there must be enough heritable variation in those traits for natural selection to act on them Difference in beak size and shape must produce differences in fitness, allowing natural selection to occur

Testing Darwin’s Hypothesis The Galapagos Islands Researchers from Princeton University--Peter and Rosemary Grant--tested Darwin’s finch hypothesis The Grants focused on the Medium Ground Finch on Daphne Major, one of the Galapagos islands

The Grants’ Observations During the rainy season plenty of food During droughts food becomes scarce, & birds with bigger beaks have better chance of survival This shows that beak size can be changed by natural selection See video 13min

A few finches, founders (species A), travel from S A few finches, founders (species A), travel from S. America to the Galapagos Survive and reproduce A group of birds from Species A travels to another island in the Galapagos- Geographic Isolation. The two groups no longer share a gene pool

Speciation in Galapagos Finches 3. Seed sizes on the second island favor birds with bigger beaks. Species B evolves over time. 4. If birds from population B migrate back to the original island, they will not mate with birds from population A Populations A and B are separate species

Speciation in Galapagos Finches 5. Ecological Competition: If species A and B compete for seeds on the first island, they continue to evolve in ways that increase the difference between them. A new species--C-- might evolve

Continued Evolution The process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galapagos Island chain.

The End. Answer your review questions. Name and describe 3 types of selection that affect polygenic traits. 2) What is speciation? 3) Name and describe 3 types of reproductive isolation.