1. Mechanisms of Evolution

2. Evolutionary Change
Populations evolve, not individuals.
Any change in gene (and allele) frequencies within a population or species can lead to evolution.
Allele Frequency – number of copies of an allele compared to the total number of alleles in a population.

3. Factors that Cause Evolutionary Change
Mutation
Gene flow (migration)
Non-random mating
Genetic Drift
Natural Selection

4. 1. Mutation
Provide the variation that can be acted upon by natural selection.
Mutations provide the raw material on which natural selection can act.
Only source of additional genetic material and new alleles.
Examples of mutations: Antibiotic resistance in bacteria, poison resistance in the Norway rat (p. 351).

5. 2. Gene Flow (migration)
Gene flow is the movement of alleles between populations.
Gene flow occurs when individuals join new populations and reproduce.
Gene flow keeps neighboring populations similar.
Low gene flow increases the chance that two populations will evolve into different species.

6. Non-random mating
During non-random mating, individuals in a population select mates, often based on their phenotypes. Females often choose males with preferred phenotypes.
In-breeding is also a form of non-random mating.
In-breeding increases the proportion of homozygotes in a population. Harmful recessive alleles are more likely to be expressed.
A mating between first cousins increases the risk of albinism 10-fold.
Albinism – a recessive genetic condition.

7. Genetic Drift
Genetic drift is the random fluctuation in allele frequency between generations due to chance.
In each generation, some individuals may, just by chance, leave behind a few more descendants (and genes)than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.
Its affects are more pronounced in smaller populations.

8. Genetic Drift

9. Genetic Drift: The Founder Effect
A new population is formed by only a few individuals or founders.
The allele frequencies in the expanded population are often very different from those of the founder population.
The Amish population of Philadelphia and Pennsylvania was founded by a few families in the 1700’s. The new population has a high frequency of Ellis-van Crevald syndrome: symptoms include short-limb dwarfism,
and polydactyly (additional fingers or toes)

11. Genetic Drift: The Bottleneck Effect
A genetic bottle neck is one form of genetic drift in which a population “crashes” then rebounds.
The survivors only have a fraction of the alleles from the original population, therefore genetic diversity is lost.
Animals known to be affected by genetic bottlenecks include the northern elephant seal, cheetah, and some human populations.

12. Genetic Drift: The Bottleneck Effect

13. Natural Selection
Natural selection occurs when an organism is subject to environment pressure. Populations have a range of phenotypes , the fittest survive and pass on their genes, producing a population that is better adapted to its environment.
Hmwk Questions: p. 352 # 1-4 and p. 356 # 7-12