Aim: What are population genetics and how do they affect evolution?

I. Population Genetics – Genetics today is concerned with inheritance in large groups of sexually reproducing animals. The study of these organisms as a reproducing group is known as population genetics.

A. Key Terms

1. Species – a group of organisms that have many genetic characteristics in common and can interbreed.

2. Population – members of a species that inhabit a specific geographic area.

3. Gene Pool – the sum total of all the genes collectively present within a given population

B. What causes changes in gene pools?

1. Recombination – during meiosis, chromosome pairs separate and then recombine during fertilization to form new combinations of chromosomes in the zygote.

2. Crossing Over – During meiosis, portions of chromosomes exchange places to produce new combinations of genes in the chromatids.

3. Mutations

C. Environmental Changes can change a gene pool.

1. Gene frequency – a number indicating the ratio of a gene to its other alleles in a gene pool.

a. Allele – the alternative genes for a trait. Example, in the case of the height trait, the genes for tallness and shortness.

2. If an organism possesses a gene that helps it survive in its environment, that gene frequency will increase, and vice versa.

3. Changing environments promote changes in the composition of gene pools.

D. Adaptive Radiation – the divergence of a single population into groups having differing traits.

1. With sufficient divergence, a new species may be formed, a process called speciation.

E. Geographic Isolation – isolation of populations by geographic barriers.

1. Mountains, canyons, rivers, highways, climate and even other organisms can serve as barriers for various kinds of plants and animals.

What are some examples of geographic barriers?

2. When a population becomes divided by a barrier, interbreeding cannot occur. Genes will not flow between them and their gene pools may diverge.

F. Reproductive Isolation

1. Due to geographic isolation, the two populations become increasingly different. This may lead to:

a. Differences in reproductive organs

b. Differing courtship patterns

c. Differences in chromosomes

2. When organisms can no longer interbreed due to the above differences, they are said to be reproductively isolated.

G. Geographic Distribution – Depending on where organisms originated, affects how they evolved. Continental Drift.