Re: What defines a species or more specifically, to what extent does the DNA of

There is no set definition of what a "species" is. For vertebrates and many other organisms that reproduce sexually with haploid zygotes (sperm and egg, for example), the definition of speicies is a group of individuals that can produce fertile offspring. By that definition horses and donkeys are seperate species because all horses can mate with horses and produce fertile offsrping, but a horse-donkey cross produces sterile offsrping (known as mules). However, domestic dogs, coyotes, and wolves are also considered seperate species even though crosses are fertile. This is because in the wild, coyotes do not interbreed with wolves, so their populaitons are effectively seperate.

The basic problem, is that humans like to label and classify things and make rules for labelling and classifying. Biology does not conform to any set of human-made rules. Even with something that seems very straightforward and simple like classifying human individuals as either male of female presents a few rare problems for those individuals who are X-Y mosaic or XXY or who have other non-typical arrangements of those two sex determining chromosomes. Nothing in biology is black and white or "digital", there are always shades of grey in between making a full spectrum or "analog" data set. It thus becomes a problem to decide where to draw the line between the categories that we need to make and put names on. The lines we draw between species of canines (dogs, wolves, coyotes) might be very good and important for canines but might not work for felines or equines, let alone viruses or fungi.

In most areas of biology, the definitions and names for species were worked out long before it became possible to do the tests we can do today to determine if the groups are truly species (by any given definition). The people who named species of bacteria did not always use the same rules, nor have the same goals in mind, as the people who named species of flowers or species of vertebrates. So there are many defined species and subspecies that are sort of "grandfathered in" and we still consider them seperate species even if we now know that there was an error in the earlier assignment of their species.

If "species" cannot be accurately and unambiguously defined for most organisms, the definitions of "strain", "race", "serovar", "cultivar" and other sub-species level groupings are even more difficult. Genus, Family, Order and other super-species level definitions are also more difficult. But these labels and classifications are useful to humans even if they are known to be flawed. We use language and we need to put good labels on things so we can talk about them, even if we all know that the label is not perfect.