Macroevolution is being replaced by a finer version of collective variations over generations. This is termed as microevolution which refers to the changes in allele frequency within populations.

Microevolution encompasses the processes which cause such changes in allele frequencies from one generation to the next. Several such processes occurring over of generations result in evolution of species.

The fundamental unit of evolution is the population. In general terms, a population consists of interbreeding organisms which occupy a specific habitat at a specific time. Frequency of the particular allele of the gene in the population is considered as the allele frequency for the purpose of microevolution.

The differences in phenotype of populations over generations should be the result of genetic variations. Then only evolution is possible. There are five forces which play major roles in contributing to such genetic variations. They include mutation, natural selection, gene flow, genetic drift, and non random mating. In terms of fitness, only natural selection strives to make the individual more fit to the environment.

Mutations are sudden random changes in genetic code which are hereditary in nature and cause a change in phenotype. This can occur at the level of individual genes or nucleotides like point mutations or at the level of chromosomes like the deletions. Most mutations are either neutral or harmful in effects. Beneficial mutations are very rare in nature.

The mutation rates are too slow to cause significant changes in evolution. An Average mutation rate might take at least 70000 generations to reduce the allele frequency of the population by half. They have little effect on the Hardy- Weinberg ratios of the common alleles of genes in a population. But they are the only source of genetic variation resulting in formation of new alleles in a species.

Gene flow or migration
Gene flow results in more genetically similar populations. It refers to movement of alleles from one population to another mostly through natural methods. The resultant populations are more homogenous and have similar allele frequencies.

Genetic drift
It is the random fluctuation in allele frequency between different generations. These effects are more profound in small populations. Allele frequencies may change in generations due to chance. Genetic drift plays a major role in variation in small population since the allelic pool is smaller. Suppose there is a single allele of a particular gene among a group of 20 organisms. The allele frequency is 1/20 = 20 %. On the subsequent generation the chances are less for the allele to be passed on and hence the next generation of organisms will have 0 % allele frequency resulting in loss of allele.

In a large population, the allelic frequencies remain almost same or have little fluctuations only since the probability of finding the allele in the next generation of organisms is higher.

Genetic drift is in other words a slow accumulation of mutations manly due to errors in replication and/or immune selection. When the genetic change is caused due to recombination or reassortment of genes or alleles, it is referred to as a genetic shift.
Founder effect and genetic bottlenecks are the result of genetic drift.

Non random mating
This occurs when the individual organism is biased in mating with relative individuals in the community or population. Preferential mating with relatives is called inbreeding which is the most common form of non random mating. This increases the frequency of homozygosity and hence the probability of recessive alleles to be expressed in the subsequent generation is higher. This results in higher incidence of recessive genetic disorders in the population. The effect is prominent in closed communities. Founder effect and inbreeding have been found to be the major cause of genetic disorders such as microcephaly.

Natural selection

Natural selection was found to be the major driving force of evolution with an increase in fitness of the individual. Some genotypes are adaptive to their environmental habitats and these 'fit' individuals pass on the alleles to the subsequent generation resulting in increase of allele frequency. This is influenced by the natural selection and survival of the fittest concepts.
There are three forms of natural selection- stabilizing, directional, and disruptive selection. In stabilizing selection, the allele frequency is maintained over generations. Directional selection shifts this frequency and increases the alleles in the subsequent generations in most cases. Disruptive selection favors the extreme traits of the organism to increase fitness. This occurs when there are sudden disturbances in the population such as sudden drop in oxygen levels due to meteorite fall. By promoting the traits, the organisms are better able to survive in the harsher environments.

Human brain genes are found to show signs of selection although the phenotypic characters cannot be traced back to the individual genes. Still chances are high that our brains are continuing to evolve by either or all of the methods cited above.

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