From a single ultimate ancestor, many hundreds of millions of separate species have, at one time or another, evolved. Even creatures as different as, say, snails and monkeys, are derived from ancestors who originally diverged from a single species in a speciation event. Since the 1940s it has been widely accepted that the first step in the origin of species is normally geographical separation. A species is accidentally divided into two geographically separated populations. 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.
It is controversial whether geographical separation is always necessarily implicated in speciation. Darwin was impressed by the fact that qualities of sexual attractiveness were often the reverse of qualities leading to individual survival. A male who manages to persuade a female to mate with him rather than with a rival is likely to contribute his genes to future gene pools. Genes for sexually attractive tails willy-nilly have an advantage that compensates for their admitted disadvantages. If I were to give an award for the single best idea anyone has ever had, I'd give it to Darwin, ahead of Newton and Einstein and everyone else.