Explosion of papers reporting HYBRIDS discovered between two species that had never before believed to be able to have offspring !!!

A “SPECIES” is defined as:

A living organism, derived long ago from the same ancestor –– that has diverged from other species, over a sufficiently long period of evolutionary time –– such that they are no longer able to breed with other species and produce viable, fertile offspring. This lack of reproduction survival is caused by chromosomal changes (deletions, insertions, inversions, cross-overs, etc.).

In 2o1o, a comparison between genomes of Neanderthals (Homo neanderthalensis) and modern human (Homo sapiens), for example, confirmed that these two subspecies had indeed interbred and produced viable “hybrid” offspring, providing their distant descendants with permanent Neanderthal alleles of many genes now identified. Likewise, in 2o11 it was reported that breeding between Homo denisova and Homo sapiens resulted in viable fertile offspring, thus leaving Denisovian alleles in our genomes today. In fact –– from studies of genomes of gorilla, chimpanzee and orangutan –– we are now able to identify the “ancestral allele” in most, if not all, genes of the modern human genome.

The attached article is a semi-lay description of examples of “hybrid formation” in Darwin’s finchs, tropical butterflies, mosquitoes, sunflowers, and at least three neotropical cat species. Bobcats and lynx have frequently hybridized and continue to do so today along the US-Canada border, which makes it confusing which cats are US citizens vs which are Canadian citizens). Further examples mentioned include: flowering plants and ferns; the African serval cat crossed with domestic cats to produce the Savannah cat; the Asian leopard cat with domestic breeds to produce the Bengal cat; northern American wolves interbreeding with domestic dogs; and the red wolf, an endangered species, crossed with and the eastern wolf.

The famous mid-20th century evolutionary biologist Ernst Mayr declared that formation of a new species requires reproductive isolation. Mayr and biologists of that time thought that offspring of any hybrids would be “less fit or even infertile” –– and therefore would not persist.

All these data belie the common idea that animal species cannot hybridize or, if they do, will produce inferior or infertile offspring (e.g. mules). Such reproductive isolation is part of the classic definition of a species. But it is now clear that innumerable animals and plants violate that rule: Not only can they cross with related species, but hybrid descendants are fertile enough to contribute DNA back into the parental species –– a process called introgression. There’s even a “liger,” the result of a zoo mating of a tiger and a lion. But, similar to male mules, male ligers are sterile, supporting the notion that, in nature, hybridization is ordinarily a dead-end street.

In conclusion, the blurring of species lines can create headaches for the comparative anatomist. And certainly this blurring can complicate conservation policies imposed by any government trying to mandate that one (or another) species is endangered and thus must be protected..!!

Science 18 Nov 2o16; 354: 817–821

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