Mechanisms of Mechanisms of EvolutionEvolution

EvolutionEvolution

EvolutionEvolution occurs as a occurs as a population’s population’s genes & their frequencies change over genes & their frequencies change over time (due to mutations)time (due to mutations)

Gene pool-Gene pool- all of the alleles of the all of the alleles of the population’s genespopulation’s genes

Allelic frequency-Allelic frequency- % of any specific allele % of any specific allele in the gene poolin the gene pool

ex. % T or % tex. % T or % t

Genetic EquilibriumGenetic Equilibrium

If a population’s If a population’s frequency of allelesfrequency of alleles remain the sameremain the same over generations than it over generations than it is said to be in is said to be in genetic equilibriumgenetic equilibrium

According to scientists this population is According to scientists this population is not evolvingnot evolving because the phenotypes because the phenotypes would remain the same.would remain the same.

Types of mechanisms of evolutionTypes of mechanisms of evolution

1. 1. MutationsMutations- can be caused by - can be caused by

environmental factors like chemicals,environmental factors like chemicals,

radiation, uv raysradiation, uv rays

-usually lead to death; but some -usually lead to death; but some

mutations may produce mutations may produce some useful some useful

gene variationgene variation that can become a part of that can become a part of

the gene pool & get passed onthe gene pool & get passed on

Types of mechanisms of evolutionTypes of mechanisms of evolution

2. 2. Natural Selection-Natural Selection-Individuals with certain Individuals with certain useful variations such as speed are useful variations such as speed are moremore likelylikely to survive ( to survive (survival of the fittestsurvival of the fittest) in ) in their env. passing those variations on to the their env. passing those variations on to the next generationnext generation Overtime, offspring w/ certain variations Overtime, offspring w/ certain variations (adaptations) make up most of the population, (adaptations) make up most of the population, & may look entirely different from their & may look entirely different from their ancestorsancestors

Types of Natural SelectionTypes of Natural Selection Stabilizing Selection-Stabilizing Selection- favors average favors average

individuals. This type of selection reduces individuals. This type of selection reduces variation in a populationvariation 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”.

Types of Natural SelectionTypes of Natural Selection

Directional Selection-Directional Selection- favors one of the favors one of the extreme variations of a trait & can lead to extreme variations of a trait & can lead to the rapid evolution of a populationthe 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.

Types of Natural SelectionTypes of Natural Selection Disruptive Selection-Disruptive Selection- favors favors both both

extreme variationsextreme variations of a trait; resulting of a trait; resulting eventually in no intermediate form of the eventually in no intermediate form of the trait & leads to evolution of 2 new speciestrait & 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

Types of mechanisms of evolutionTypes of mechanisms of evolution 3. 3. Genetic Drift-Genetic Drift- allele frequencies change by chance allele frequencies change by chance

eventsevents Types of genetic driftTypes of genetic drift 3A). 3A). Founder effect-Founder effect- certain individuals leave a certain individuals leave a

population and begin a new population in a different population and begin a new population in a different location; usually the new pop. has less genetic variation location; usually the new pop. has less genetic variation than the larger pop.than the larger pop.

ex. Amish population- some have short arms w/ extra ex. Amish population- some have short arms w/ extra fingers/toes (polydactylism) due to a recessive allele fingers/toes (polydactylism) due to a recessive allele (chance event) from 1 amish settler(chance event) from 1 amish settler

Types of mechanisms of evolutionTypes of mechanisms of evolution 3. 3. Genetic DriftGenetic Drift (continued)- allele (continued)- allele

frequencies change by chance eventsfrequencies change by chance events

Types of Genetic Drift-Types of Genetic Drift-

3B). 3B). Bottleneck effect-Bottleneck effect- some chance some chance

disaster (ex. Flood) destroys organismsdisaster (ex. Flood) destroys organismsBottleneck 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.

Types of mechanisms of evolutionTypes of mechanisms of evolution

4. 4. Gene Flow-Gene Flow- transport of alleles by transport of alleles by migrating individuals from one population migrating individuals from one population to another populationto another population

ex. ex. Migration Migration intointo or or outout of a of a populationpopulation may be responsible for a may be responsible for a marked change in allele frequencies.marked change in allele frequencies.

ex. Individual leaves a population, its ex. Individual leaves a population, its alleles are lost from the gene poolalleles are lost from the gene pool

Types of mechanisms of evolutionTypes of mechanisms of evolution

5. 5. Non-random mating-Non-random mating- in all populations in all populations organisms usually select mates non-randomly organisms usually select mates non-randomly for traits that are easily seen (looks, personality, for traits that are easily seen (looks, personality, etc.) etc.)

This means that some organisms are more This means that some organisms are more desirable to mate with than others.desirable to mate with than others.

Animal breeders do essentially the same thing Animal breeders do essentially the same thing when they intentionally try to improve varieties when they intentionally try to improve varieties or create new ones by carefully making sure or create new ones by carefully making sure mating is not random.mating is not random.

Evolution is usually a consequence if any of Evolution is usually a consequence if any of the 5 mentioned mechanisms occur.the 5 mentioned mechanisms occur.