Review - For natural selection to occur individuals in a population must vary from one another. The variations may be visible (Eye color) or on the molecular level (Blood type). Only those traits that can be inherited are relevant to natural selection.
Where does genetic variation come from?
1. Genetic variation is random and caused by mutation. Random changes in a gene can result in a new allele. This new allele may be helpful or detrimental.
2. Sexual reproduction results in a unique combination of alleles that each individual inherits. Unique genetic combinations are the result of independent assortment in meiosis, random fertilization and crossing over
Genetic variation is caused by random processes but natural selection is not random!!!
Mechanisms of Evolution
Evolution is a change in a population’s genetic makeup from generation to generation. What causes a gene pool to change over time?
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Secondary sex characteristics may be used to compete with members of the same sex. Examples: Physical combat or ritualized displays. One sex may be choosy in selecting their mates. Peacock tail sexually selected. Sexual selection often accounts for male/female differences in many animal species. There is some evidence that sexual selection accounts for differences in human male/female body size.
Tendency of organism to choose mate that resemble self is assortative mating. People tend to mate assortatively by height (i.e., tall women marry tall men) and skin color. Positive assortative mating tends to exaggerate differences between groups/populations.
3 Results of Natural Selection. Imagine a population of mice with a range of fur colors. If natural selection favors certain colors over others, there are 3 possible outcomes.
a. Directional selection - Shifts the make-up of a population in favor of one extreme. Example: drug resistant bacteria
b. Disruptive selection - Results in a balance between 2 or more contrasting phenotypes in a population.
c. Stabilizing selection - Favors intermediate phenotypes, getting rid of extremes. Usually prevails – resists changes in a well-adapted population.
2. Genetic drift
If a new generation gets its alleles from a large population, the new generation will represent the gene pool of the previous generation. However, the gene pool of a small population may not accurately represent the next generation because of sampling error (chance). This type of change in the gene pool (due to chance) is called genetic drift.
How do populations get so small that genetic drift occurs?
Bottleneck Effect - Natural disaster kills large numbers of the population, resulting in a smaller population that has different frequencies of alleles. By chance some alleles are more represented than others.
Founder effect - A few individuals leave the main population and colonize a new habitat. This smaller colony has a different genetic makeup than the main population. Smaller population is subject to genetic drift until it is large.
3. Gene flow
When fertile individuals move into or out of a population, causing a change in the allele frequency.