Migration, Genetic Drift - Evolutionary Biology - Lecture Notes, Study notes of Evolutionary biology

Download Migration, Genetic Drift - Evolutionary Biology - Lecture Notes and more Study notes Evolutionary biology in PDF only on Docsity! 1 Migration, Genetic Drift and Nonrandom Mating I. Introduction –Greater Prairie Chicken Example (Tympanuchus cupido pinnatus) A. Classic example of a species that uses a lek in mating. 1. A lek represents a polygynous mating system where males aggregate in a particular area and exhibit their courtship displays to females that visit the group. One male may mate with more than one female. 2. The decline in Greater Prairie Chicken populations can be attributed to the following 3 violations of H-W. They are 1) migration, 2) genetic drift and 3) nonrandom mating. II. Migration – the movement of alleles between populations. A. The one-island model of migration as an example. B. Example of the impact of alleles entering a small population. C. An example with water snakes (Nerodia sipedon) 1. Individuals vary in appearance ranging from strongly banded to undbanded. a. Banding pattern is controlled (more or less) by a single locus with two alleles. b. Banding is dominant over no bands. 2. On the mainland, virtually all of the snakes are banded, but on the island, the unbanded variant predominates. a. Unbanded pattern are more cryptic and presumably less vulnerable to predators. 3. If being unbanded should confer greater fitness, then why isn’t there a greater frequency of unbanded snakes on islands? a. Migration of individuals from the mainland. b. The natural selection against banded individuals on the islands would be expected to fix the allele frequency of being unbanded to 1. However, the effects of immigration are opposing natural selection. c. The result of constant introductions of alleles from the mainland is that this tends to homogenize the allele frequencies on the island. d. If natural selection did not oppose the effects of immigration, then the allele frequency on the island would come to resemble that on the mainland. III. Genetic Drift – Chance in a small population can lead to evolution. 1. Evolution is not a chance event. 2. Drift on the other hand, is a random process. Whether an individual has an appropriate phenotype or not, it may be lost from the population. a. Hence, genetic drift is a chance event that leads to changes in the allele frequency of a population without leading to adaptation of the population. A. A classroom example of genetic drift –red and black beans in a bag. Did your sampling violate one of these conclusions of H-W? 1. The allele frequencies in a population will not change, generation after generation. 2. If the allele frequencies in a population are given by p and q, the genotype frequencies will be given by (p2 + 2pq + q2). Docsity.com 2 B. The source of small populations – the founder effect. 1. A population may be small because a portion of it was moved (or moved itself) to a new location. a. Original population was sampled, but not thoroughly. There has been sampling error. b. Not all of the alleles have been sampled in the frequency that they occurred in the original population. c. As a result, the new allele frequency differs from the old allele frequency. This is known as the founder effect. 2. Genetic drift can also occur when an existing population is severely reduced in size. a. If the loss is due to an indiscriminate event, such as an environmental catastrophe, then this too will result in a sampling error. b. A reduction in the population size and the resulting change in allele frequency is called a bottleneck effect. c. It has been argued that only rare alleles are lost due to bottlenecking. C. Genetic Drift over time – Random Fixation of Alleles and Loss of Heterozygosity. 1. Because the fluctuation in allele frequency is due to a sampling error, then each generation will experience a different allele frequency due to chance. 2. Each generation, allele frequencies will randomly wander between 0 and 1. Over time, this results in two major effects, one causing the other: a. Eventually alleles drift to fixation (1) or loss (0). b. The frequency of heterozygotes declines D. Random Fixation of Alleles. 1. As the frequency of an allele fluctuates between 0 and 1, it will inevitably meet one of two fates. a. The frequency of the allele will reach 0, and the allele will be lost forever (unless it is introduced through mutation or migration). b. The frequency of the allele will reach 1, in which case, the allele is said to be fixed. 2. Is there anything about the an allele that will help us predict the probability that it will drift towards fixation? a. Each allele’s chance of becoming fixed is Ν2 1 3. The initial frequency of an allele is equal to its chance of being fixed. a. If there are x copies of A1 and y copies of A2 and z copies of A3 b. And each allele has a Ν2 1 chance of being the one that drifts to fixation. c. Then the probability that A1 drifts to selection is x x Ν2 1 = Ν2 x Docsity.com