Ecological isolation is allopatric speciation: populations become geographically separated. This is a common dynamic for speciation.
By contrast, sympatric speciation transpires among populations within the same habitat. A subpopulation that is not physically separated from the main population strikes out on its own for whatever reason.
Sympatric speciation was long thought unlikely and rare, as the conventional Darwinist view about speciation was that populations needed physical separation to prevent interbreeding.
Sympatric speciation happens more often than commonly thought. Mole rats in Israel, palms on Lord Howe Island off Australia, and apple maggots in North America are other examples of rapid sympatric speciation.
Under suitable conditions, allopatric and sympatric speciation can occur with similar ease. ~ American biologist Justin Meyer
Lake Constance Sticklebacks
Threespine sticklebacks were introduced into Switzerland’s Lake Constance around 1860. Since then they have split into 2 distinct species. One lives in the main lake. The other lives in the streams that flow into the lake.
These speciated sticklebacks continue to breed in the same streams at the same time of year. They still interbreed. But the two are genetically and physically distinct. The lake dwellers are bigger, with longer spines and tougher armor.
The sticklebacks’ genetic differences are concentrated on parts of chromosomes that are not prone to recombination. Hence, the gene variants that give the two their distinct traits are less likely to get mixed up.
Orca live in all the world’s oceans, without any geographical barriers to keep their populations from interbreeding. But separate groups do not interbreed, despite living in close proximity to one another. These separate groups differ in choice of prey and how they hunt. Orca choose mates that share their customs. It is an ongoing speciation based upon culture which is resulting in physiological changes.
In the lab, North American mallard and pintail ducks will mate; but not in the wild. The drakes of these species have similar coloration, but mallard females dress quite differently than pintails, who have a long, thin tail.
Coloration was not part of the selection process that led to speciation. Mallard drakes and pintail drakes have different mating displays which make for a lady duck’s pick.
Sympatric speciation is common in plants. To adapt to an environmental stress, such as soil changes, plants often resort to autopolyploidy: creating a new set of similar (homologous) chromosomes. Polyploidal offspring share the same habitat as parent plants but are reproductively isolated. Throughout evolutionary history, plants have productively employed polyploidy for adaptive radiation. The diversity and success of flowering plants owes much to autopolyploidy.
Wheat has been modified and hybridized for millennia by humans. There are now strains that are diploid (2 sets of chromosomes), tetraploid (4 sets), and hexaploid (6). Durum wheat, also known as macaroni wheat, is tetraploid, while bread wheat is hexaploid.