Mechanism of Evolution

The process by which modern organisms have descended from their ancestors, with changes in their allele frequencies, is called evolution. This change in the allele frequencies is due to the four major forces.

  • Natural Selection

  • Genetic Drift

  • Mutation

  • Gene Flow

Populations evolve, not individuals. The mechanism of evolution occurs due to the changes in the gene pool (collection of genes).

Hardy-Weinberg Equilibrium

In the Hardy-Weinberg equilibrium, the allele frequencies, gene versions, genotypes and set of alleles will remain the same over generations. For eg., let us consider a population of beetles. The beetles are present in two colours dark grey and light grey. AA and Aa beetles are dark grey in colour and the aa beetles are light grey. The Hardy-Weinberg equilibrium can here be explained by a mathematical expression:

p2 + q2 + 2pq = 1

AA + aa  + Aa = 1

The Hardy-Weinberg equilibrium is not evolving. Therefore, the populations are not in Hardy-Weinberg equilibrium.

Mechanism of Evolution

Natural Selection

Some individuals with certain traits have higher survival and reproductive rate than others. They pass on these genetic features to their offspring which brings an evolutionary change in the future generations. This selection of the genetic qualities that prove beneficial for survival in future generations is known as natural selection.

Darwin’s Finches is one classic example of natural selection. Darwin’s finches have evolved into 15 different species depending upon their adaptation and feeding habits.

Genetic Drift

It refers to the change in the allele frequency of a population as a matter of chance. It is a random event whose effect is larger in smaller populations and smaller in larger populations. The two examples of genetic drift are the bottleneck effect and founder effect.

  • Bottleneck Effect: It occurs when there is a sudden decrease in the population due to some environmental factors, such as an earthquake, tsunami, epidemics, etc. In this event, some genes are depleted from the population. This causes a drastic reduction in the genetic diversity of the original gene pool. That means that the genetic makeup of the surviving population becomes different from that of the original one.

  • Founder Effect: When a small number of individuals separated from a larger population make up a new population, there is a loss of genetic diversity. They do not carry the genetic diversity of the previous population. Due to this, some genetic traits become more prevalent than the others, which results in genetic diseases in future generations.

Mutations

Mutations are defined as the changes in the DNA sequence of a living organism. The mutations occurring in the germ cells, i.e., the egg or sperm cells only are passed on to future generations. These mutations are inherited by the parents and are present in every cell throughout a person’s life. These are the mutations that lead to evolution. The acquired mutations occur during a person’s life and are present only in some cells. These are caused due to environmental factors such as ultraviolet radiations and do not pass onto future generations. They are not a part of evolution.

Gene Flow

The transfer of genes from one population to another is called gene flow. For eg., pollen moving from one place to another by the action of wind or people moving to different cities or countries. When a person from one country moves to the other and mates with a person there, a transfer of genes occurs between the individuals. This is how the gene flows between different populations. Gene transfer can be horizontal, i.e., transfer of genetic material from one population to another by asexual means. This phenomenon is prevalent in prokaryotes.

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Important Questions

Q.1. What are the four mechanisms of evolution?

A.1. The four mechanisms of evolution are:

  • Mutation
  • Natural Selection
  • Gene Flow
  • Genetic Drift

Q.2. What is Hardy-Weinberg Equilibrium?

A.2. The Hardy-Weinberg equilibrium states that in the absence of evolutionary forces, the allele and genotype frequencies will remain constant over generations.

Q.3. What do you understand by Natural Selection?

A.3. It is the key mechanism of evolution. It is the differential survival and reproduction of individuals due to differences in the phenotype. The heritable traits of the organisms change over generations and the ones which acclimatize with the environment are naturally selected.

Q.4. What is genetic drift?

A.4. It is the change in the frequency of a gene variant in a population due to the random sampling of organisms.

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