Originally Posted by Thao
But mamabadger, that is so very simplistic. Speciation doesn't happen in a "leap" as you say (which of course would be absurd) but over a long period of time in small, incremental changes. And they don't have to have completely different DNA in order to not be able to breed with each other; small changes in morphology or behavior (preferences) are enough to make it so two populations do not breed. It has been observed in both plants and animals.
Ways it can happen:
temporal isolation (individuals develop preferences for mating at different times, so don't mate with each other)
assortative mating (individuals develop preferences for certain characteristics and so don't mate with individuals without those characteristics)
Here is an article with lots of examples of observed speciation. Will you read it?http://www.talkorigins.org/faqs/faq-speciation.html
In this case, and in the ring species references from Orangebird, species is being defined in the same way: as two populations who, for whatever reason, will no longer breed with each other. (The only exception is that of insects whose condition was manipulated in a laboratory, and whose situation would likely not occur naturally.) I was not talking about categories who will not
breed, but those who have actually become genetically distinct.
For example, take two "species" of ducks, mallards and black ducks. We have lots of mallards here, but no black ducks; out west, they have black ducks but no mallards. They have never had contact until a few years ago, when habitat loss resulted in the black ducks moving east. When they contacted the mallard population, the two groups interbred with no difficulty. They look quite different, but were always genetically the same species.
The same is true for dogs and wolves, who remain capable of producing young together; they are categorized as different species, but are genetically still one species.
There are also types of songbirds which appear identical, but never interbreed. Two populations were separated, and now the mating song of one group has no appeal to the females of the other group. They never mate; but are they genetically distinct? Since they look the same and have the same behavior patterns, it's likely that they are. The only way to know is to mix the egg of one group with the sperm of another in a petri dish, and see if they are compatible. That
is what I mean by the same species.
However, we do have species that are not
compatible in this way. The sperm of a moose will never fertilize the egg of a rabbit, and not just because the animals refuse to mate with each other. The development of a new species in that
sense still poses the same problems. Changes in appearance or behavior can happen gradually, as you say. Changes in genetic code, from compatible with one species to compatible with a different one, cannot just develop in increments, over time. An animal with a genetic code different from that of its parents may represent a new species, but it is one which will die out in a single generation. An animal with the same, compatible genetic code is still the same species, regardless of outward changes or mating habits. The only way a new species (not a variation, a species
) can form is if at least two individuals of opposite sexes develop the same genetic variation during the same period of time and successfully mate and produce young.