There are usually two ways in which a species is created. A large number of species have evolved simply by a single stroke in the evolutionary process – through polyploidy. Polyploidy involves multiplication in the number of gene bearing units – the chromosomes.
This happens either within a pre-existing species or sometimes in a hybrid between two species. The polyploids are unable to form fertile hybrids with the parent species because polyploidy introduces enough differences in their genetic makeup. They have to interbreed among themselves and establish as a new species. The second important process is geographical speciation which takes a much longer time.
When populations or group of organisms are isolated by some barrier such as a mountain range or extension of an arm of sea the differences in the environment which are inevitably there, cause the isolated populations to change. In due course of time, enough differences accumulate which make interbreeding between the two populations impossible. Thus even when the barrier between the two populations is removed, they co-exist as separate species.
Nature has maintained species diversity at an approximately even level throughout the course of organic evolution, although this consistent level has been punctuated by accelerated extinction rates after periods 10-50 million years apart. What this approximately even level should actually be? How many species can occur in a definite area under ideal conditions? Though these questions cannot be answered with precision but an approximation or a guess founded on sound logic is possible.
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Within a cluster of islands the number of species of birds (or reptiles or ants or any other equivalent group) found on an island approximately rises by one fourth root of the area of the island in kms. Thus, the number of species present in an isolated habitat tends to rise by x. (area)0.25 as it area increases. Here exponent 0.25 may vary depending upon local conditions between 0.15 to0.35.
According to this theory of island biogeography in a typical case (where the exponent is 0.25) there is a doubling in the number of species for every ten fold rise in its area. (Mc Arthur and Wilson 1967). In a study of ant population of Hispaniola, 37 genera were discovered in amber from Miocene period – about 20 million years old. Exactly 37 genera occur on the island today also.
However, 15 of the original genera have become extinct and is replaced by 15 others which have invaded the Island. Thus an even level of diversity of ant populations has been maintained since last 20 million years on the island (Wilson 1985).
As fossil records reveal, in a particular taxonomic category, species have a remarkably constant life span. The probability that a particular species will become extinct within a certain span of time after it split off as a new species is roughly constant. Thus the percentage of species being lost in each period of time is approximately similar.
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This regular pattern has, however, been disturbed during the past 260 million years by major episodes of extinction which have been estimated to occur rather regularly at an interval of 26 million years (Raup and Sepkoski 1984).
The estimate of extinction rates in the Plaeozoic and Mesozoic marine fauna ranges between one to ten extinctions for every million years, depending upon major taxonomic categories (Raup 1986, Raup and Sepkoski 1984). As compared to these figures, the current rate of extinction of species would be about 1,000 to 10,000 time higher, which has no parallel in about a half billion years of the history of the biosphere.