Download Population Genetics

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Population Genetics
Population genetics is the branch of evolutionary biology responsible for investigating processes that
cause changes in allele and genotype frequencies in populations based upon Mendelian inheritance.
Four different forces can influence the frequencies: natural selection, mutation, gene flow(migration),
and genetic drift. A population can be defined as a group of interbreeding individuals and their
offspring. Population genetics was a vital ingredient in the modern evolutionary synthesis, its
primary founders were Sewall Wright, J. B. S. Haldane and R. A. Fisher, who also laid the foundations
for the related discipline of quantitative genetics.
Founder Effect
The founder effect refers to the loss of genetic variation when a new colony is established by a very
small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in
1952, using existing theoretical work by those such as Sewall Wright. As a result of the loss of genetic
variation, the new population may be distinctively different, both genetically and phenotypically,
from the parent population from which it is derived. In extreme cases, the founder effect is thought to
lead to the speciation and subsequent evolution of new species.
Genetic Drift
The genetic drift or allelic drift is the evolutionary process of change in the allele frequencies (or
gene frequencies) of a population from one generation to the next due to the phenomena of
probability in which purely chance events determine which alleles (variants of a gene) within a
reproductive population will be carried forward while others disappear. Especially in the case of
small populations, the statistical effect of sampling error during random sampling of certain alleles
from the overall population may result in an allele, and the biological traits that it confers, to become
more common or rare over successive generations, and result in evolutionary change over time.
The concept of Genetic drift was first introduced by Sewall Wright in the 1920s, and is now held to be
one of the primary mechanisms of biological evolution. It is distinct from natural selection, a nonrandom evolutionary selection process in which the tendency of alleles to become more or less
widespread in a population over time is due to the alleles' effects on adaptive and reproductive
success.
Gene Pool
The gene pool is the complete set of unique alleles in a species or population. A large gene pool
indicates extensive genetic diversity, which is associated with robust populations that can survive.
Meanwhile, low genetic diversity can cause reduced biological fitness and an increased chance of
extinction. When all individuals in a population are identical with regard to a particular phenotypic
trait they are known as Monomorphic.
Hardy-Weinberg Principle
Hardy-Weinberg principle states that when no evolution occurs in a population the allele and
genotype frequencies do not change from one generation to the next. No evolution refers to no
mutation, no gene flow, no natural selection, and no genetic drift. To be in equilibrium two more
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assumptions need to be made that random mating occurs and there are discrete, non-overlapping
generations.
Hardy–Weinberg principle for two alleles: the horizontal axis shows the two allele frequencies p
and q, the vertical axis shows the genotype frequencies and the three possible genotypes are
represented by the different graphs
The Hardy–Weinberg principle states that the genotype frequencies in a population remain constant
or are in equilibrium from generation to generation unless specific disturbing influences are
introduced. Those disturbing influences include non-random mating, new mutations, selection,
random genetic drift and gene flow. Genetic equilibrium is a basic principle of population genetics..
The Hardy-Weinberg principle can be used to calculate the frequency of particular alleles based on
frequency of, say, an autosomal recessive disease. This concept is also known by a variety of names:
HWP, Hardy–Weinberg equilibrium, HWE, or Hardy–Weinberg law. It was named after G. H.
Hardy and Wilhelm Weinberg.
Derivation
Consider two alleles, A and a, with frequencies p and q, respectively, in the population. The different
ways to form new genotypes can be derived using a Punnett square (also known as a Prout Square),
where the fraction in each is equal to the product of the row and column probabilities.
Punnett square for Hardy–Weinberg equilibrium
A (p)
Females
A (p)
AA (p²)
a (q)
Aa (pq)
a (q)
Aa (pq)
aa (q²)
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The final three possible genotypic frequencies in the offspring become:



These frequencies are called Hardy-Weinberg frequencies (or Hardy-Weinberg proportions). This is
achieved in one generation, and only requires the assumption of random mating with an infinite
population size.
Sometimes, a population is created by bringing together males and females with different allele
frequencies. In this case, the assumption of a single population is violated until after the first
generation, so the first generation will not have Hardy-Weinberg equilibrium. Successive generations
will have Hardy-Weinberg equilibrium.
Deviations from Hardy-Weinberg equilibrium
Violations of the Hardy–Weinberg assumptions can cause deviations from expectation. How this
affects the population depends on the assumptions that are violated. Generally, deviation from the
Hardy-Weinberg equilibrium denotes the evolution of a species.
1. Random mating
The HWP states the population will have the given genotypic frequencies (called Hardy-Weinberg
proportions) after a single generation of random mating within the population. When violations of
this provision occur, the population will not have Hardy-Weinberg proportions. Three such violations
are:
2.Inbreeding - which causes an increase in homozygosity for all genes.
3.Assortative mating - which causes an increase in homozygosity only for those genes involved in
the trait that is assortatively mated (and genes in linkage disequilibrium with them).
4.Small population size, which causes a random change in genotypic frequencies, particularly if the
population is very small. This is due to a sampling effect, and is called genetic drift.
The remaining assumptions effect the allele frequencies, but do not, in themselves, effect random
mating. If a population violates one of these, the population will continue to have Hardy-Weinberg
proportions each generation, but the allele frequencies will change with that force.
a.
Selection, in general, causes allele frequencies to change, often quite rapidly. While directional
selection eventually leads to the loss of all alleles except the favored one, some forms of
selection, such as balancing selection, lead to equilibrium without loss of alleles.
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b.
c.
Mutation will have a very subtle effect on allele frequencies. Mutation rates are of the order
10−4 to 10−8, and the change in allele frequency will be, at most, the same order. Recurrent
mutation will maintain alleles in the population, even if there is strong selection against them.
Migration genetically links two or more populations together. In general, allele frequencies
will become more homogeneous among the populations. Some models for migration
inherently include nonrandom mating (Wahlund effect ).
Population bottleneck
A population bottleneck or genetic bottleneck is an evolutionary event in which a significant
percentage of a population or species is killed or otherwise prevented from reproducing, and the
population is reduced by 50% or more, often by several orders of magnitude.
Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the
population size. They also increase inbreeding due to the reduced pool of possible mates (see small
population size).
A slightly different sort of genetic bottleneck can occur if a small group becomes reproductively
separated from the main population. This is called a founder effect.