List the various causes of variations in the progeny of a population. Describe the three different ways in which natural selection operates in nature, with regard to organic evolution.
Answers
Answer:
Five factors known to cause variations in the progeny of a population are:
Genetic migration/Gene flow :
When migrations of a selection of population to another place occur, gene frequencies change in the original as well as the new population. New genes/alleles are added to the new population and these are lost from the population.
Genetic drift :
This theory was proposed by Sewall Wright in 1930. It deals with the gene frequency of a reproducing small population. In a small population not all the alleles, representatives of that species may be present. Thus the inheritance process is in violation of Hardy-Weinberg law.
In a small population, a chance event may increase the frequency of a character that has little adaptive value. Thus genetic drift may be a significant factor in the origin of new species on islands and other isolated populations. Continuous mating within such populations decreases the proportion of heterozygotes and increase the number of homozygotes. However, the small population as a whole may develop characters different from that found in the main population. Such deviations may lead to speciation or formation of new species.
When a small group of individuals due to genetic drift becomes the founders of a new population, the phenomenon is termed as “founder‟s principle”. The new population often has genotype frequencies different from parent population. Sometimes genotypic frequencies may get changed in a small population isolated temporarily due to natural calamities.
When the population regains its original size, the members of the small population may have diverged genetically from the original parental population. Hence interbreeding between members of small and larger population may not be possible. The small population might have evolved into a new species. This type of genetic drift is known as "bottle neck effect".
Mutation :
Mutations are considered as raw materials for evolution. They help to create and provide variations in a population along with genetic recombinations. The collection of genes in a population is referred to as “gene pool”. Mutations enrich the gene pool with new modified genes. A large scale accumulation of such genes will lead to evolutionary modifications.
Genetic recombination :
During meiosis, due to crossing over of chromosomes, genetic arrangements get altered. Such alterations cause reshuffling of gene combinations. Such recombinations are regular events in gametogenesis. Due to such events, new allelic formations happen and after fertilization, variations result in progeny.
Natural selection :
Nature selects the individuals that are well adapted to changing situations. These individuals increase in number by reproduction.
Different ways in which natural selection results in different populations include the following :
* Stabilisation - More individuals acquire mean character value, i.e. variation is much reduced.
* Directional change - More individuals acquire value other than the mean character value.
* Disruption - More individuals acquire peripheral character value at both ends of the distribution curve.
Explanation:
Genetic variation is an important force in evolution as it allows natural selection to increase or decrease frequency of alleles already in the population. ... Genetic variation is advantageous to a population because it enables some individuals to adapt to the environment while maintaining the survival of the population.