Darwin was puzzled why nature comes in neat species instead of a blur
If every living thing descends from others by tiny steps, why isn't the natural world a smooth smear of in-between forms? Charles Darwin raised exactly this worry in the chapter of his 1859 book devoted to problems with his theory. Explaining how distinct species arise, and stay distinct, is still a lively field.
Speciation is the process by which populations evolve into separate species. The biologist Orator F. Cook coined the word in 1906 for the splitting of one lineage into two. Most biologists now agree that the key ingredient is reproductive isolation: once two groups can no longer exchange genes, they are free to drift apart for good.
Geography offers four broad routes. In allopatric speciation a population is split by a barrier such as a rising mountain range, and the halves diverge under different pressures and mutations. Peripatric speciation is the special case of a small breakaway group on the fringe, where chance effects in tiny populations may matter a lot. In parapatric speciation neighbours still meet at the edges, but hybrids fare poorly, so selection favours avoiding each other. Sympatric speciation happens with no separation at all, as when insects in one place specialise on different host plants. East Africa's Rift Valley lakes, home to over 800 described cichlid species and possibly more than 1,600, are the best-known example.
Change can be startlingly fast. Three-spined sticklebacks colonised isolated lakes and streams after the last glacial period, and over roughly 10,000 generations their fins, bony plates, jaws and colours have diverged more than those of some separate fish genera. In plants, doubling the chromosome number can create a new species in one generation, instantly unable to breed with its parents.
One proposed answer to Darwin's riddle involves sex. A sexual species split into many narrowly adapted forms would leave each with few members and trouble finding mates, so rarity carries a cost. Animals also tend to shun partners with unusual traits, which often signal harmful mutations, keeping each species uniform enough for field guides to work. Asexual organisms pay no such price, and they indeed show the continuous variation Darwin expected, which makes sorting them into species notoriously hard.
Source: Speciation
