Evolution, Natural Selection and Speciation

Theory of Evolution and Natural Selection

Charles Darwin, along with Alfred Russel Wallace, laid out the theory of evolution by natural selection. This theory postulates that species evolve over time through the gradual accumulation of small genetic changes.
Natural selection is the mechanism that drives this evolutionary change. Organisms best adapted to their environment are more likely to survive, reproduce, and pass their beneficial traits to their offspring.
Adaptations are characteristics that increase an organism’s ability to survive and reproduce in an environment.
Variations in a population may arise from spontaneous mutations or genetic recombination during sexual reproduction.
If a variation provides an advantage, it may increase in frequency in a population over time - a process known as adaptive evolution.

Genetic Drift and Gene Flow

Genetic drift is another mechanism of evolution involving random changes in allele frequencies from one generation to the next. It particularly affects smaller populations and can lead to decrease in genetic diversity.
Gene flow, or migration, occurs when individuals move between populations and interbreed, thereby introducing new genetic information into a population and potentially altering gene frequencies.

Types of Natural Selection

Directional selection occurs when an extreme phenotype is favoured and the average phenotype of the population shifts in one direction.
Stabilising selection favours intermediate phenotypes and reduces phenotypic variation, thereby maintaining status quo.
Disruptive selection, on the other hand, favours extreme phenotypes over intermediates and can lead to speciation.

Speciation and Allopatric Isolation

Speciation is the process by which one species splits into two or more species. This may occur when a population becomes isolated.
In allopatric speciation, a geographic barrier physically isolates members of a population, leading to the evolution of new species.
Over time, the isolated populations may diverge genetically and morphologically due to different selection pressures in their environments.
Reinstating contact between these diverged populations may result in reduced interbreeding, reinforcing the reproductive isolation and distinctness of the species.

Sympatric Speciation and Polyploidy

Sympatric speciation occurs in populations that live in the same geographic location. This can happen due to a radical change in the genome, like polyploidy, or due to disruptive selection based on different ecological niches.
Polyploidy is the condition of having more than two complete sets of chromosomes. This commonly occurs in plants and results in a new species that can no longer interbreed with its parent population.