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Evolution under the influence of natural selection leads to adaptive improvement. Evolution, whether under the influence of natural selection or not, leads to divergence and diversity. From a single ultimate ancestor, many hundreds of millions of separate species have, at one time or another, evolved. The process whereby one species splits into two is called speciation. Subsequent divergence leads to ever wider separation of taxonomic units—genera, families, orders, classes, etc. Even creatures as different as, for example, snails and monkeys, are derived from ancestors who originally diverged from a single species in a speciation event.
It is widely accepted that the first step in speciation is normally geographical separation. A species is accidentally divided into two geographically separated populations. Often there may be subpopulations isolated on islands, generalized to include islands of water in land (lakes), islands of vegetation in deserts (oases), etc. Even trees in a meadow may be effective islands to some of their small inhabitants. Geographical isolation means no gene flow, no sexual contamination of each gene pool by the other. Under these conditions the average gene frequencies in the two gene pools can change, either because of different selection pressures or because of random statistical changes in the two areas. After sufficient genetic divergence while in geographical isolation, the two subpopulations are no longer capable of interbreeding even if later circumstances chance to reunite them. When they can no longer interbreed, speciation is said to have occurred and a new species (or two) is said to have come into being. This "biological" definition of the species cannot be used for organisms that do not reproduce sexually. It has been controversially suggested that natural selection itself may reinforce the divergence between incipient species by penalizing any tendency to hybridize. It is also controversial whether geographical separation is always necessarily implicated in speciation (it is agreed that it usually is).