The ultimate source of diversity are mutations
Mutations are also the source of genetic variability in
mitosis and prokaryotic binary fission. Mutations can be a simple as errors in replication substituting the wrong nucleotide, inserting an additional building block or deleting a building block. Mutations, however, can also affect large segments of chromosomes through insertion, deletion or duplication. The rate of mutations depends on the mechanism of replication, but is heavily influenced by environmental factors such as radiation and unstable molecules (radicals) that randomly attach nucleic acids on other molecules in cells.
The rate at which mutations accumulate effects the fate of a cell or population. Many microorganisms, particularly viruses, often show a high rate of mutation, which helps them change their surface structure and evading the defense mechanisms of their hosts. Larger, sexually reproducing organisms have low mutation rates, but use sexual reproduction to maintain a high variability of already existing mutations within a population. While for most instances the parental
chromosomes (homologous chromosomes) are largely identical, they
always differ somewhat, because some of the genes are mutated in
one parent, but not the other. These genes are referred to as alleles
and sexual reproduction ensures random distribution of acquired
mutations throughout a population. The distribution of such mutations
or alleles in a population of a species can be tracked by Mendelian
Different mutations affect organisms in different ways. Mutations sometimes have no immediate effect on the functioning of an organisms. These are called silent mutations. Other mutations, those that affect protein coding genes, or the regulatory elements on chromosomes, do affect the functioning of organisms, often in a detrimental effect, causing diseases or even death (lethal mutations).
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Lukas K. Buehler