Evolution of Populations and Speciation

1. Evolution of Populations and Speciation Unit: V.

2. Part I: Genes and Variation • How common is genetic variation • All organisms have at least two forms of alleles for each trait some of which are easily observable and other that are “invisible” • Population: A group of individuals of the same species that interbreed. • Gene Pool: All of the genes that are present in a population. • Relative frequency: The number of times an allele occurs in a gene pool compared to the other alleles for the same gene. • Evolution: In genetic terms is any change in the relative frequency of alleles in a population.

3. Part I: Genes and Variations • Sources of Genetic Variations: • Mutations: Any change in genetic material • Gene Shuffling: The mixing or reordering of alleles during sexual reproduction. • Are responsible for most heritable genetic differences • All of the genes on the 23 pairs of human chromosomes can produce 8.4 million different combinations of genes.

4. Part I: Genes and Variations • Single Gene vs. Polygenic Traits: • Single Gene Traits: A trait that is controlled by a single gene that has two alleles. Example (TT) (Tt) Tall and (tt) short. • Polygenic Traits: A trait that is controlled by two or more genes often times with each gene having two or more alleles. • The number of phenotypes produced for a given trait depends on how many genes control the trait.

5. Part II: Evolution as Genetic Change • Natural selection on single-gene traits • Natural selection on single gene traits can lead to changes in the allele frequencies and thus to evolution. • If a mutation takes place and that mutation gives the organisms an advantage or disadvantage to survival that mutation may control if that mutation becomes a common occurrence within that species.

6. Part II: Evolution as Genetic Change • Natural selection on polygenic traits • The effects of natural selection on polygenic traits is much more complex because of variations in fitness. • Natural selection can affect the distributions of phenotypes in of polygenic traits in three ways. • Directional Selection: Occurs when individuals at one end of the distribution curve have a fitness than individuals in the middle or other end of the curve. • Stabilizing Selection: Occurs when individuals near the center of a distribution curve have a higher fitness than either end. • Disruptive Selection: Occurs when individuals at the upper and lower ends of the distribution curve have higher fitness than individuals in the middle.

7. Part II: Evolution as Genetic Change • Genetic Drift: The random change in allele frequencies that occur in small organismal populations. • In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. • Over time a series of chance occurrences of the type can cause an allele to become more common in a population. • Founder Effect: The change in allele frequencies as a result of the migration of a small subgroup of a population.

8. Part II: Evolution as Genetic Change • What happens when no change takes place? • Scientists try to use this ideal situation to study what really happens. • Evolution Versus Genetic Equilibrium • Hardy-Weinberg Principle: States that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. • Does not exist in reality and is only used for study purposes. • If alleles do not change evolution would not occur. • Genetic Equilibrium: Allele frequencies remain constant. This requires five conditions to be maintain from generation to generation in order to take place. • Random Mating, Large Population, No Movement Into or Out of the Population, No Mutations, No Natural Selection.

9. Part III. The Process of Speciation • Speciation: The formation of a new species • Factors such as natural selection and random chance events change the relative allele frequencies and lead to the formation of new species. • If a genetic change increases fitness that change will eventually be found in many more individuals in a given population. • Isolating Mechanisms: Factors that separate populations of the same species and thus separate the gene pools of those populations leading to the formation of new species. • Reproductive Isolation: The separation of species or populations so that they cannot interbred • Behavioral Isolation: Different courtship rituals or other behavior that prevents them from interbreeding. • Geographic Isolation: Species separated by physical geographic barrier like a river, mountain, etc. that prevents interbreeding. • Temporal Isolation: Species that reproduce at different times of the year and prevents interbreeding.

10. Part III. The Process of Speciation • Darwin’s Finches • Are prime examples of evolution using the mechanisms of speciation. • Method of evolution of Darwin’s Finches • Founders Arrive  Separation of Pop.  Changes in Gene Pool  Reproductive Isolation Ecological Competition  Continued Evolution. • Shows evolution and the mechanisms of speciation naturally occurring in nature. • Can be used as a model to predict future patterns of evolution and speciation.