16.2+Evolution+as+Genetic+Change

**__ 16.2 EVOLUTION AS GENETIC CHANGE __**

NATURAL SELECTION ON A SINGLE-GENE(DB) NATURAL SELECTION ON POLYGENIC TRAITS(DB)
 * Can lead to changes in allele frequencies and eventually evolution.
 * EXAMPLE: If a population of lizards is mostly brown, and some red and black lizards begin to appear, the majority might eventually change. If the red lizards stand out more in the environment, and predators can see them more easily, these red lizards might not survive and reproduce as much, so there won't be an abundance of red lizards. However, if the black lizards absorb sunlight easier in colder weather, these black lizards might eventually become the majority. So the curve of the population changes.
 * There are three types of natural selection on polygenic traits. These three types are **Directional Selection**, **Stabilization Selection**, and **Disruptive Selection**.
 * All of these happen when certain parts of the population survive and reproduce better than other parts of the population.
 * Natural Selection on polygenic traits can result in certain organisms in a population surviving, while others fail or even die off completly.

Genetic Drift and The Founder Effect (st) · **G****enetic Drift**: Genetic drift is the change in the relative frequency with which a gene variant allele occurs in a population that results from the fact that alleles in offspring are a random sample of those in the parents, and because of the role of chance in determining whether a given individual survives and reproduces    · **Founder Effect**: the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population When a population is seperated into differnet environments, their allele frequencies will change to make them more suited to their habitat. [|Founder Effect]  Directional Selection (st) When individuals at one end of the curve have higher fitness than individuals in the middle or the other end If the size of seed increases in a population of finches, those with the larger and stronger beaks will survive. Over time, the average beak will become larger to suit more of the population. [|Directional Selection]
 * The range of phenotypes shifts as some individuals fail to survive and reproduce while others succeed.

Stabilizing Selection (pm) When individuals near the center of the curve have higher fitness than individuals at either end of the curve · Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average. · The fitness of the larger or smaller individuals is lower than that of more averaged sized individuals. · [|Graphic Example of Stabilizing Selection] · [|Stabilizing Selection]

**Disruptive Selection** (pm) When individuals at the upper and lower end of the curve have higher fitness than individuals near the middle ·  When a population of birds lives in an area where medium-sized seeds become less common and large and small seeds become more common. <span style="font-family: Symbol; mso-list: Ignore; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· Birds with unusually small or large beaks would have higher fitness. <span style="font-family: Symbol; mso-list: Ignore; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· The result: the bird population splits into two subgroups specializing in eating different-sized seeds. <span style="font-family: Symbol; mso-list: Ignore; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· [|Graphic Example of Disruptive Selection] · [|Disruptive Selection]