>>Suppose for a moment that the domesticated dog was extinct. Suppose also
>>that the incredible diversity of the species canis familiaris existed in
>>the distant past long before the appearance of man--extinct before the
>>first hominids walked the planet. Now what would modern scientists
>>conclude about the fossilized remains of the species? Would they be
>>grouped into the single species which they are? or would they be
>>grouped into a family of related species? or would every diverse form
>>be considered a unique species?
>
>This is a really good question that I hope someone can give us a good
answer to.
>I have brought it up before but there has never been a response -- probably
>because it was usually an aside in a longer post. But I'm _really_
>interested in knowing how paleontologists decide that two fossils are of
>the same, different, or related species. I suspect there are some
>"invariants" like certain ratios and curvatures that might be identifiers
>of common or related species, and I believe dentition is a good clue to
>common or related species, but everything I know about this is hearsay. I
>hope someone in the group can give us a precis of the methods used and
>point us to some references.
>
There are really two different questions here: 1) how do paleontologists
recognize fossil species, and 2) what does the diversity of the domestic dog
represent?
The biological definition of species has to do with whether or not two
organisms will naturally interbreed, i.e. whether they recognize each other
as the same species or not. Occasionally, different species will interbreed
where their ranges overlap, but this occurs rarely enough so that there is
insufficient gene flow between the populations to break down the species
recognition barriers. (For example, white-tailed deer and mule deer in North
America occasionally hybridize, but this is rare enough that the two species
remain distinct.) Paleontologically, interbreeding cannot be determined,
but it can be inferred. Since interbreeding implies gene flow and
reproductive isolation implies no gene flow, paleontological species can be
determine by morphological gaps between populations. For example, if one
group of trilobites had 10 to 12 thoracic segments and another group had 14
to 16, they would probably be called two species. On the other had, if
there were no other morphological differences observable and they varied
from 10 to 16 segments, they would probably be called the same species.
There are exceptions, but in practice, most paleontological species are
defined on the basis of unique morphological characters not shared with
other populations, implying genetic isolation.
The domestic dog is another problem because it is probably not a natural
species but the artificial hybridization of several wild species, probably
from different genera. (Remember, taxonomic categories above the species
level are man-made; several different genera of dogs hybridize in zoos
today.) The resulting vast morphological gradient among dogs is not natural
and would not usually be seen in the fossil record. Again rarely,
hybridization is recognized in the fossil record and termed "reticulate
evolution" (See P.C. Sylvester-Bradley, 1977, Biostratigraphical tests of
evolutionary theory, pp. 41-64, IN Concepts and Methods of Biostratigraphy,
E.G. Kaufman & J.E. Hazel, Eds., Dowden, Hutchinson & Ross
Lee A. Spencer, Ph.D.
Vertebrate Paleontologist
Earth History Research Center
Southwestern Adventist University
Keene, TX 76059
spencerl@swau.edu
Lee A. Spencer, Ph.D.
Vertebrate Paleontologist
Southwestern Adventist University