|MadSci Network: Genetics|
Good question, Richard.
The common definition of species is a group of organisms which can interbreed to produce fertile offspring. What seems simple in theory is actually pretty messy in practice. In practice there are two groups of scientists: lumpers tend to minimize the number of species by minimizing population differences, while splitters tend to maximize the number of species by treating even small differences as species level splits. The lumper/splitter debates are pretty arcane unless you are a birder trying to maximize your life list, or a paleontologist trying to name a new skeleton.
Sometimes the barriers to producing fertile offspring are not physical (number of chromosomes or body size) but behavioral, as when some bird species have local variations of their mating call which prevent interbreeding between subpopulations. If two subpopulations are separated for long enough behaviorally, it is possible that physical barriers will arise and seal the deal.
To put some of the stats we will discuss presently into context, here are the stats for humans vs. the other great apes. Humans diverged from our nearest relatives, the chimps, about 5 million years ago. Humans have 46 chromosomes (2n), while the other great apes have 48, so a genetic barrier between humans and great apes has arisen in this time. In terms of sequence difference, humans are only about 1.2% from chimps, 1.4% from gorillas and 2.4% from orangutans.
In the genus Canis (Dogs, wolves, jackals, and coyotes) the number of chromosomes is 78 in all known species, so no physical barrier to interbreeding exists in terms of chromosome segregation. Nonetheless, some rather extreme physical barriers due to size (wolves vs foxes) and behavior (pack vs. solitary) exist, which maintain the separation of species in the wild. As for sequence divergence, the gray wolf (the putative ancestor of domestic dogs) is about 1.8% divergent from dogs, while coyotes are about 4% different, a pretty drastic difference if you keep in mind the numbers from the great apes. It's intriguing that this large divergence arose in just 15,000 years of domestication, and I'm not sure whether scientists have adequately explained it.
So the question is not really whether species do interbreed, but whether they can interbreed under artificial conditions.
According to Gray (Mammalian Hybrids, a checklist with bibliography, 1954) all species in the genus Canis have been known to hybridize in captivity. I?m reporting this secondhand, as I can?t find Gray locally, but the source seems reputable. Thus, a dog-coyote hybrid is feasible. As for the foxes, they are out at ~8% sequence divergence, and only have 34 chromosomes, so a dog-fox hybrid probably isn?t viable.
As for a general resource on this stuff, I'm not aware of a good one. Sorry.
Most of my canine stats come from Vila et al, J Hered 1999:90(1)p71-77
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