Mechanisms of Evolution

1. Mechanisms of Evolution

2. Evolution • Evolution occurs as a population’s genes & their frequencies change over time (due to mutations) • Gene pool- all of the alleles of the population’s genes • Allelic frequency- % of any specific allele in the gene pool ex. % T or % t

3. Genetic Equilibrium • If a population’s frequency of allelesremain the same over generations than it is said to be in genetic equilibrium • According to scientists this population is not evolving because the phenotypes would remain the same.

4. Types of mechanisms of evolution • 1. Mutations- can be caused by environmental factors like chemicals, radiation, uv rays -usually lead to death; but some mutations may produce some useful gene variation that can become a part of the gene pool & get passed on

5. Types of mechanisms of evolution • 2. Natural Selection-Individuals with certain useful variations such as speed are more likely to survive (survival of the fittest) in their env. passing those variations on to the next generation • Overtime, offspring w/ certain variations (adaptations) make up most of the population, & may look entirely different from their ancestors

6. Types of Natural Selection • Stabilizing Selection- favors average individuals. This type of selection reduces variation in a population For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease. Like goldilocks (one of the items is “just right”.

7. Types of Natural Selection • Directional Selection- favors one of the extreme variations of a trait & can lead to the rapid evolution of a population Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.

8. Types of Natural Selection • Disruptive Selection- favors both extreme variations of a trait; resulting eventually in no intermediate form of the trait & leads to evolution of 2 new species For example, imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants

9. Types of mechanisms of evolution • 3. Genetic Drift- allele frequencies change by chance events Types of genetic drift 3A). Founder effect- certain individuals leave a population and begin a new population in a different location; usually the new pop. has less genetic variation than the larger pop. ex. Amish population- some have short arms w/ extra fingers/toes (polydactylism) due to a recessive allele (chance event) from 1 amish settler

10. Types of mechanisms of evolution • 3. Genetic Drift (continued)- allele frequencies change by chance events Types of Genetic Drift- 3B). Bottleneck effect- some chance disaster (ex. Flood) destroys organisms Bottleneck effect. From an original peccary population with its own gene pool, a few individuals sqeeze though a "bottleneck". In this case a few peccaries with the genotype bb survive a catastrope. These individuals become the founders (originators) of a new peccary population. As they reproduce, the new gene pool is very different from that of the original population.

11. Types of mechanisms of evolution • 4. Gene Flow- transport of alleles by migrating individuals from one population to another population ex. Migration into or out of a population may be responsible for a marked change in allele frequencies. ex. Individual leaves a population, its alleles are lost from the gene pool

12. Types of mechanisms of evolution • 5. Non-random mating- in all populations organisms usually select mates non-randomly for traits that are easily seen (looks, personality, etc.) • This means that some organisms are more desirable to mate with than others. • Animal breeders do essentially the same thing when they intentionally try to improve varieties or create new ones by carefully making sure mating is not random. • Evolution is usually a consequence if any of the 5 mentioned mechanisms occur.