Those aren't transitionals because there is always room for another transitional between any two so-called transitionals!
How does one define a transitional...strictly speaking?
Here's the question--not flame bait, but I just plain old haven't ever seen it spelled out.
I am approaching the question by considering the definition of a species as "animals genetically similar enough that they can mate and produce fertile offspring" -- leaving aside plants and unicellular critters for the moment. :-)
Are there any hard-and-fast rules governing the demarcation line between species, when the process of transition between old species "A" and new species "B" takes place over, say, a dozen generations?
Say species A could have mated effectively with successive generations 1-4.
Generation 4 could have mated in turn with species A, generations 1-4, and down to generation 6.
Generation 6 could have mated with generation 4 through generation 8.
Generation 8 could have mated with members of generations 6 through 10.
Generation 10 could have mated with anyone from generation 8-12 and produced offspring.
And in the midst of these changs there are accompanying physical differences, say, size of the beak (hat tip to finches, don't you know), or hardness of the pecker ;-)
Is the going approach to wave one's hands and say "the distinction is for practical purposes immaterial, since we *know* that species A is different from species B 12 generations later"?
Or is an attempt made (when possible) to correlate DNA changes in the different generations?
Yes, I realize 12 generations might not be a good example, nor necessarily representative.
But it is a small enough cohort that one can phrase the question in a more or less succinct form.
Oh, and one other question...
When one says that the members of the two species cannot mate...
does it matter what is the efficient cause of the copulative dissonance ;-)
E.g. Great Dane and a Shih Tzu. Either the male is too small to mount the female, or (going the other way) peg A won't fit in slot B.
Or, is there required to be a mismatch in the chromosomes such that fertilization does not succeed?
...and if the latter, has anyone studied the correlation between cladistics and the fertilization process in detail?
Cheers!