Mechanisms of Microevolution The main causes of microevolution are Genetic drift. Gene flow. Mutations. Natural selection.

Genetic Drift Genetic Drift is a change in the gene pool of a small population due to chance. Genetic Drift has the potential to completely eliminate a specific allele in the organism and make them produce only specific offspring. Genetic Drift is based off of chance.

Figure 13.22

The Bottleneck Effect Genetic Drift is/can be due to the bottleneck effect. The bottleneck effect occurs when there is a large amount of death in a population. The remaining organisms are specific, and the ones who died could have lost some major alleles resulting in less variability in future generations. An example of this bottleneck effect would be the cheetah, with all of the hunting and the ice age, cheetahs have become scarce among our planet.

Figure 13.23

Figure 13.24

The Founder Effect Known as the establishment of a new population whose gene pool differs from the parent population and the subsequent genetic drift in the new colony. The smaller the colony, the less its genetic makeup will match the gene pool of the larger population it migrated from. Depending on success, random drift will continue to affect the frequency of alleles until the population is large enough for genetic drift to be minimal. Contributed to the evolutionary divergence of the finches and other organisms that arrived as strays on the remote Galápagos island that Darwin visited

Genetic Drift and Hereditary Disorders in Human Populations The founder effect explains the high frequency of certain inherited disorders in some human populations established by small numbers of colonists. In 1814, 15 people founded a British colony on Tristan da Cunha, in the middle of the Atlantic Ocean. One of the colonists carried a recessive allele for retinitis pigmentosa (form of blindness).. Of the 240 descendants who still lived on the islands in the 1960s, 4 had retinitis pigmentosa, and 9 others were carriers (heterozygous=one recessive allele). That frequency of the retinitis pigmentosa allele was much higher in Tristan da Cunha, compared to Britain (where the colonists came from).

Figure 13.25

Gene Flow Reduces differences between populations Has become an important agent of evolutionary change for humans today

Mutation Changes in an organism's DNA For one gene locus, mutation doesn't have much of a effect Mutations can have a huge impact Essential to evolution Allows for genetic variation

Natural Selection: A Closer Look Of all causes of microevolution, only natural selection promotes adaptation. Darwin explained the basics, it took modern synthesis to fill in the details Hardy-Weinberg equilibrium

Figure 13.27

Three General Outcomes of Natural Selection Directional selection A single characteristic is favored Often occurs when there's a great environmental change or when they migrate An example would be if an environment suddenly became cold and species had to grow lots of hair

Disruptive selection When a species changes to have more extreme phenotypes but not a specific one If a rabbit could have white, grey, or black hair it would have white or black (not the middleground of grey) Results of disruptive selection are usually not stable

Stabilizing selection maintains variation for a particular trait within a narrow range occurs in a relatively stable environment prevails most of the time in relation to the other two occurs when the population is stressed by change in the environment population will either evolve with the new environment or die off due to the drastic change keeps human babies at around 3-4 kg and therefore any baby lighter or heavier has a higher mortality rate

Figure 13.28

Evolution Connection: Population Genetics of the Sickle-Cell Allele Sickle-cell disease Affects about one out of every 400 African Americans. Is more common among African Americans; but why? It is more common among African Americans because the sickle cell allele resists malaria and since there is more malaria in many parts of Africa, more people have this allele to be immune to malaria Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The sickle-cell allele disorder in which oxygen delivery by the blood is impaired owing to abnormally shaped red blood cells a double dose of this allele can prove to be fatal, but heterozygous individuals have a side effect of being immune to malaria making this allele common in certain areas of Africa

Figure 13.29