Download Allele Frequencyнаmeasure of how common a certain allele is in a

Genetic Variation
­A population with a lot of genetic variation, most likely had a wide range of phenotypes.
­The greater the variation in phenotypes, the more likely that some individuals can survive in a changing environment.
Example: If an unusually cold winter occurs, penguins that are shorter and rounder will be able to stay warmer more easily.
But if there is a shortage of food, taller, slimmer penguins might be better divers for food.
Genetic variation is stored in a populations gene pool.
­Gene pool­ the combined alleles of all the individuals in a population.
­Different combinations are formed when individuals mate and have offspring.
Allele Frequency­ measure of how common a certain allele is in a population.
To calculate: Count the number of times an allele occurs in a gene pool
Divide by the total number of alleles for that gene in the gene pool.
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Gg
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G=Green
g=Brown
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Sources of Genetic Variation:
1. Mutation­ random change in DNA of a gene, can cause a new allele to form. If in a reproductive cell it can be passed on. Increases genetic variation in a gene pool.
2. Recombination­ during meiosis the parent's alleles are arranged in new ways. The shuffling of alleles results in many genetic combinations.
Normal distribution­ frequency is highest near the mean value and decreases toward each extreme end of the range.
Example: Human heights, most people are of average height with a few being very short or very tall.
This would be the distribution if all the phenotypes for a trait had equal chances of survival.
However, if environmental changes occurred a certain phenotype might have an advantage.
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Microevolution­ observable changes in the allele frequencies of a population over time. Occurs in a single population, mostly by natural selection.
Directional Selection­ favors phenotypes of one of the extremes of a trait.
­the once rare phenotype becomes more common.
Example:
Drug­resistant bacteria­ in the 1940's a trait for drug­
resistance existed among bacteria, but there was no advantage for being drug resistant. But once antibiotics came into use, this trait became a huge advantage. Overuse of antibiotics led to more resistant phenotypes. Today over 200 types of bacteria have some degree of antibiotic resistance.
Stabilizing Selection­ the intermediate phenotype is favored and becomes more common in the population.
­there is something in natural selection, that both extremes are not advantageous.
Example: Gall flies lay their eggs in goldenrod and the plant tissue swells around it and forms a gall.
Large galls attract downy woodpeckers and they eat the larvae inside.
Small galls attract wasps that lay their eggs inside the gall and once hatched, they eat the gall fly larvae.
So, gall flies that produce middle­sized galls became more common. This decreases the genetic diversity in the gall fly population.
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Disruptive Selection­ occurs when both extreme phenotypes are favored, while individuals with the intermediate are selected against by nature.
Example:
lazuli bunting­ a type of bird in North America­ has 3 color phenotypes:
*Bright blue­ attracts mates because of bright colors, but also is attacked by the dominant male.
*Blue­Brown­ Does not attract mates because of the mottled color, but is also attacked by the dominant male.
*Brown­ Win a mate because the dominant males don't attack them.
*Since the 2 extremes can either mate or not be attacked, their phenotypes are favorable.
­This can lead to the formation of a new species.
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