We have already discussed some of the reasons why the genetics of a species can change over a long period of time. Charles Darwin's The Origin of Species went into great depth in this matter, and providing substance into the theory of evolution. The key thing to remember about evolution is that it favours more preferable genes in the gene pool, and over time, these preferable characteristics become more exclusive in the gene pool.
This next section rounds up all the factors that can alter the make up of a gene pool
Natural selection will favour genes that are more suited to their environment and become more exclusive in the gene pool over time in such an environment. Different genes will become more exclusive when the environment changes, or the species migrate.
Mutations are random occurrences which change the genome of an organism. They greatly increase genetic diversity, where advantageous mutations are favoured by natural selection and disadvantageous ones are phased out.
Occurs after genetic drift, where two groups of a species become separated and therefore cannot reproduce. The gene pool of these groups differ over time. If these two groups can once again meet up and reproduce, their genetic differences can be merged within the single group and increase genetic diversity.
Can cause in-breeding depression by continuous inbreeding, non-random mating is also known as selective breeding, where the breakthroughs of Mendelian genetics have allowed us to predetermine what genes are present in offspring. As advantageous genes are desired by the breeder, some of the less 'popular' genes are lost due to this random mating, therefore decreasing genetic diversity.
It is important for a species to have a large gene pool, because in the event of danger, some alleles will allow the species to survive and reproduce to produce a larger and more variant gene pool. For example, an extremely contagious disease may threaten 99% of a species, though the remaining 1% may possess an allele that provides them with resistance to the disease. If this allele was not present in the population, then chances are the entire population would be wiped out
Sometimes, species can be split into groups, usually as a result of a geographical factor preventing the two groups from contacting one another. This means that the two groups are unable to reproduce with one another.
As the two groups now live in different environments, natural selection will favour slightly different genes in each of the groups that will favour them in their particular environment. Over time, the difference in gene pool between the two groups can be quite dramatic.
If these two groups once again become re-united, gene migration occurs (see above). If they remain seperated, their genetic differences become greater which can result in the formation of a new species.
When the genetic differences become so great, it can come to the stage where the two groups can no longer reproduce with one another. This results in the formation of a new species (because organisms who are capable of reproducing but cant reproduce with a member of the same species are deemed another species).
Long ago, the land of Earth was all on one continent. Over time these continents separated, with members of the same species drifting away on each continent. Over time, speciation has occurred. This is evident by looking at the marsupials of Australia, who have been isolated from other mammals in their ancestral line, and therefore have many differences to that of mammals on other continents.
Adaptive radiation is the slow change of genotype and phenotype of a species from its common ancestor, meaning that species with a common ancestor become more diversified over time. The next page investigates adaptive radiation.