Question

1. Briefly explain the following:

a) Effective population size.

b) Random genetic drift.

c) Mechanisms which permit maintenance of genetic variability in natural populations.

d) Phylogeny reconstruction methods.

Answer 1

1) Effective population size.

The effective population size is the number of individuals that an idealised population would need to have inorder for some specified quantity of interest to be the same in the idealised population as in the real population.

The formula for the effective population size is.

Ne = (4*Nm*Nf)/(Nm + Nf)

Ne= Effective population size

Nm= The number of breeding males

Nf= The number of breeding females

Effective population size  is one of the most important tool in population genetics and conservation biology.

2) Random Genetic drift.

Genetic drift explains random fluctuations in the numbers of gene variants in a population. Genetic drift happens when the occurrence of different forms of a gene, increases and decreases by chance over time. These variations in the presence of alleles are measured as changes in allele frequencies. Usually gentetic drift occurs in small population where infrequently occuring alleles face a greater chance of being lost. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.

3) Many factors act to increase or maintain the genetic variation in a population. One of the most important factors is mutation. Mutation is the ultimate source of all variation.

Selective neutrality is another important factor which helps in maintaining the genetic variation in a population. It describes situations in which alternate alleles for a gene differ little in fitness. Under neutrality allele frequencies vary over time. Over a long period rand fluctuations in relative frequency of different alleles results in some being eliminated from the population.

Several forms of natural selection also play a major role in maintaining the genetic variation rather than to eliminate it.These include balancing selection, frequency-dependent selection, and changing patterns of natural selection over time and space.

4) Some of the phylogeny reconstruction methods include.

Parsimony analysis - find that tre that explains sequence data with minimum numbers of. Substitutions.( Tree includes hypothesis of sequence at each of the nodes.)

Maximum likelihood analysis- given a model for sequence evolution, find the tree thatbhas highest probability under this model. This approach can also be used to successively refine the model

Bayesian statistics - uses ML analysis to calculate posterior probability for trees, clades and evolutions parameters.

Distance based methods- calculate evolutionary distances between sequences using some evolutionary model. Construct a distance matrix and based on the distance scores construct a Phylogenetic tree.