​ ardy Weinberg Principle
H
In the mechanism of Evolution, the basic ideas are centered around the
concepts proposed by Charles Darwin(Natural Selection), Hugo
deVries(Saltation), Thomas Malthus, Gregor Mendel, and other prominent
scientists. It is believed that the work of Thomas Malthus on the
relationship between population and natural resources has influenced
Charles Darwin to develop key features of natural selection; Population
size, in reality, is limited, which is possible when an individual of the
population compete for resources, which ultimately gives rise to the
concept of "Survival of the fittest".
While proposing the mechanism of Evolution and reasons for variation and
speciation, Mendel's ideas of the existence of inheritable "factor"
influencing phenotype were either passed over or kept in silence by darwin.
Around the early 20th century Hugo deVries' works on primrose led the
discovery of concepts on mutations. He defined mutation as sudden
random and directionless variation in an individual in a population. The
concept of Saltation(Single-step significant mutation) proposed by Hugo
deVries contrasted with Darwin's gradual changes in the process of
speciation. These two ideas were later united under the Modern Evolution
theory, which is also called Neo Darwinism.
The existence of factors proposed by Mendel was later confirmed by the
discovery of RNA by Friedrich Miescher and DNA by James Watson.
These genetic materials pile together with histone protein to form genes
locus in a chromosome. The phenotypes' mechanism resulting from these
alleles was proposed by Gregor Mendel, who introduced the concept of
dominance and recessive traits for a gene. He supported his theory by
performing an experiment on Pisum sativum (pea plant) in his garden. He
classified some of the characters, such as pea coats, pea shape, and
others. Into dominant and recessive traits. During his experiment, he found
it difficult to distinguish and count the number of Homozygous Dominant
and Heterozygous in the sample because their phenotype is dominant.
Later Hardy and Weinberg came with a principle to resolve this problem.
They proposed "allele frequency remains constant through generations"
and called it Hardy-Weinberg principle. Mathematically, the algebraic
equation denoting this principle is as follows:
p^2 + 2pq + q^2 = 1 ;
Here the Sum total of all allelic frequencies is 1.
p is the frequency of Dominant allele A.
q is the frequency of the Recessive allele a.
p^2 is the probability of the occurrence of AA.
2pq is the probability of the occurrence of Aa.
q^2 is the probability of occurrence of aa.
It is similar to the binomial expansion (p + q)^2.
When the allele frequency measured differs from the actual value indicates
the extent of evolutionary changes.
Hardy-Weinberg equilibrium is affected by five factors:
1. Gene Migration/ Gene flow:
It is referred to as the change in gene frequency due to the migration of the
population to another place. Also, the loss of new genes/alleles from the
old population and gained by a new population multiple times is gene flow.
2. Genetic Drift:
The gene flow that occurs by chance is called Genetic Drift.
3. Mutation:
Sudden random, directionless change in the process of cell division is
called a mutation.
4. Genetic Recombination:
During meiotic division (gametogenesis), genetic recombination adds a
new allele to the existing chromosome by coupling.
5. Natural Selection:
Natural selection processes in which heritable variations are enabling
better survival are enabled to reproduce and leave large progenies.
*If any of the above factors are changed, HWP is broken, and allele
frequency is altered; therefore, Evolution will occur, which always occurs
because it is most unlikely the factors are unaffected.
If the change in allele frequency is significant, it may lead to the formation
of new species. Gradually the drifted population becomes the founder;
hence the effect is called the Founder effect.
Further natural selection transforms the altered gene frequency into a new
population. Therefore as soon as the Hardy-Weinberg principle is broken,
Evolution starts to occur, and when pressures are from the environment,
natural selection occurs. The pressure could be overpopulation, resource
competition, changing environment, and predators.
The natural selection is of 3 types:
1.Stabilizing:
Individuals with a common form of the trait have the highest fitness.
Example: Robin's egg clutch of 4 eggs is best because too few might be
less probable to survive, and too many might result in resource scarcity.
2.Directional selection:
The individual that displays a more extreme form of the trait have higher
fitness than the individual with an average form of the trait
Example: Peppered Moth population of England during the Industrial
Revolution had higher frequencies of darker color because it was well
camouflaged in soot-covered trees. While the light-colored are easily seen
and eaten by birds.
3.Disruptive
Individuals with either extreme variation of a trait have higher fitness than
individuals with the average form of the trait.
Example: Chinook Salmon males compete with each other to fertilize the
female egg, large fish have the advantage because they are strong fighters
whereas smaller fish also have an advantage because they can sneak in
while the large fish are fighting, and fertilizing the egg without being seen.