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Classification and evolution Typeit
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A is often referred to as a group of organisms that can interbreed to produce fertile offspring. It is also the smallest taxon in the hierarchy that starts with domain, kingdom and , and continues down the ranks past , a group of closely related genera.
Organisms were originally grouped based on observable characteristics, but are now classified based on their evolutionary relationships and the time since two species last shared a common . Despite this, or perhaps because of this, closely related species often share a number of features. For example, all organisms can be sorted into one of five kingdoms:
Variation in organisms can arise as a result of , changes to the base sequences of DNA. It is called variation if it occurs within a species, and variation if the variation can take only one of a limited number of forms, with no intermediates. In some cases, variation may generate a different version of a gene, called an , which may be advantageous to an individual and confer greater reproductive success. Natural selection may then act to spread it throughout the population since it is more likely to be inherited in the next generation than a different version of the gene. This theory was developed independently by Darwin and , and is now backed up by evidence from , the remains of dead organisms.
As a result of selection, organisms develop different adaptations to their environment, which may be anatomical, physiological or . However, it should be noted that variation is not solely due to genetics, and can also be influenced by the .