Comment on An apology to PUC by Sean Pitman.
@Eddie:
And where are such barriers for various cryptic species for which there is no readily apparent phenotype block to mating with the production of viable and fertile offspring? – Sean Pitman
For the benefit of those who perhaps are confused by the subject matter, cryptic species refer to two or more species that closely resemble each other to human eyes yet somehow recognize each other as distinct species. They may occur in sympatry (living together), parapatry (narrowly overlapping in range) or allopatry (geogrpahically separated). They never or seldom interbreed because some sort of a reproductive isolating mechanism other than external morphological appearance has “evolved†(if I can use that term here).
Obviously, but how fundamentally important are these non-morphologic differences vs. those morphologic and non-morphologic differences that are very evident in different breeds of dogs or cats or even different ethnic groups of humans? – who also preferentially “breed” within their own particular groups?
I earlier mentioned a classic example, the Alder Flycatcher and Willow Flycatcher, which occur sympatrically in northeastern North America and are virtually identical yet differ in their vocalizations. The distinctive vocalizations of each species represent a behavioral reproductive isolating mechanism that prevents an individual from mating with the wrong species. Many similar examples exist among birds.
So there are birds who prefer to mate with other birds who have a more “attractive” song? – a song that matches their own particular preference? How is this somehow a basis for species classification whereas more striking differences between human ethnic groups, to include mating preferences, are not?
I think you’re simply demonstrating the arbitrary aspects of taxonomy quite nicely… to include its political implications.
Another classic example that I earlier mentioned is the Eastern Gray Tree Frog and Cope’s Gray Tree Frog, which are essentially identical but differ in their number of chromosomes, 48 in the former and 24 in the latter (providing a rare example of polyploidy among animals). Although sympatric throughout much of their ranges, they cannot hybridize because of the different numbers of chromosomes. They can recognize each other through their distinctive vocalizations, which represent a behavioral reproductive isolating mechanism.
This argument is clearly mistaken. Differences in chromosome number does not, by itself, prevent successful breeding or the production of viable and fertile offspring. Also, as I’ve already noted above, variations in chromosome numbers is not all that “rare” among animals.
For example, hybrids of the wild horse (33 pairs) and the domesticated horse (32 pairs) are both viable and fertile, and yet they have an odd chromosome out, just like mules do. Also, foxes have a varied number from 38-78 chromosomes yet can all mate to produce viable and fertile offspring. Yet another example is the 40+ Robertsonian “races” of Mus musculus domesticus found in Europe and North Africa which have chromosome numbers ranging from 22-40. Yet, the number of chromosome arms are the same and banding studies reveal the genes to be homologous. In other words, the information is the same. Another example of a Robertsonian polymorphism is the house musk shrew that lives in the central region of West Malaysia and has a variation in chromosome numbers from 36 to 40. Also, in Southern India and Sri Lanka musk shrews can be found with having between 30 and 32 chromosomes due to Robertsonian-type changes.
The significance of centric fusions (Robertsonian translocations) in domestic animals, with special reference to sheep, is also interesting. A study was done that considered 856 ewes with either a normal chromosome number 2n = 54 or carrying one or more of the three different translocations (centric fusions) t1, t2 and t3 in various heterozygous and homozygous arrangements. Rams, which were used in the matings, were homozygous for one of the translocation chromosomes (2n = 52), double heterozygotes (2n = 52), triple heterozygotes (2n = 51), or were carriers of four translocation chromosomes (2n = 50) and five translocation chromosomes (2n = 49). A remarkably even distribution of segregation products was recorded in the progeny of all combinations of translocation ewes plus translocation rams in those groups in which sufficient animals were available for statistical analysis. Forty-eight chromosomally different groups of animals were mated. Further, the overall fertility of the translocation sheep (measured by conception rate to first service via the lambing percentage and number of ewes that did not breed a lamb) was not significantly different from the New Zealand national sheep breeding data where this study was done. In some groups the poorer reproductive performance could be explained by the age structure of the flock and inbreeding depression, which probably affected the performance of some animals. Sheep with progressively decreasing chromosome numbers, due to centric fusion, 2n = 50, 2n = 49 and 2n = 48, were reported. The 2n = 48 category represents a triple homozygous ewe and a triple homozygous ram and is the first report of the viable evolution of such domestic animals. Less than 1% of phenotypically abnormal lambs were recorded in a total of 1,995 progeny born over 10 years. It is now considered that there is little or no evidence to suggest that centric fusions in a variety of combinations affect the total productive fitness of domestic sheep.
Bruere AN, Ellis PM., Cytogenetics and reproduction of sheep with multiple centric fusions (Robertsonian translocations). J Reprod Fertil 1979 Nov;57(2):363-75. (http://bison.zbs.bialowieza.pl/isacc/suncus/srihtm.htm)
Sean Pitman
www.DetectingDesign.com
Sean Pitman Also Commented
An apology to PUC
@OTNT_Believer:
Must you keep harping on the one statement on Darwin’s finches that few other than yourself and maybe some other creationists disagree with?
I have yet to see you present one phenotypic or genetic difference between any of Darwin’s Finches and other members of the Dome-nest Clade which could not be rapidly realized in a few thousand years. Certainly a 0.3% difference in cytochrome b isn’t a significant problem. I’m not sure what else makes you think that Darwin’s Finches are no uniquely evolved that they could not be explained as originating from Noah’s Ark a few thousand years ago?
As far as your arguments for the date of the first Egyptian dynasty being preceded by over a thousand years of cultural development, it simply doesn’t take very long for groups of humans to develop complex cultures and governments. Also, there are those who argue that the date for the first dynasty is more likely to be less than 4,500 years ago. Either way, the dating of Egyptian dynasties is hardly a very solid basis for challenging the historical SDA position on origins…
Sean Pitman
www.DetectingDesign.com
The “striking phenotypic differences, and even unique genetic differences†among different populations of humans and dogs are the results of random mutation, genetic drift, natural selection or (in the case of dogs) artificial selection–not “some span of reproductive isolation.â€
It doesn’t matter how the reproductive isolation is achieved, be it “artificial” or “natural”. The resulting phenotypic differences are much more obvious between certain breeds of dogs or even various human ethnic groups than between certain “cryptic” species.
The reason why cryptic species are given taxonomic status while various breeds of dogs and human ethnic groups are not seems arbitrary to me. There really is no clear dividing line for taxonomic status on the one hand, but not on the other…
Humans don’t depend on the color or texture of eyes, hair and skin to avoid mating with chimps or apes, or even different groups of humans.
Are you kidding me? Humans are indeed biased in the choice of a mate toward those of similar phenotypic appearance. While this is not a universal rule (as is also the case with many kinds of cryptic species who also experience the occasional hybrids), it is certainly a bias.
When a female poodle is in heat, it doesn’t matter what “breed†a male dog belongs to, it is equally stimulated and could care less about the length, color or texture of eyes, hair, ears, snout, legs, tail, etc. The reproductive isolating method between dogs (genus Canis) and foxes (genus Vulpes) is likely based on olfaction rather than external morphology.
Have you considered the efforts of a Great Dane to mate with a chihuahua? Come on now, there are clear examples of not only artificial but natural reproductive isolate between various breeds of dogs and even between various human ethnic groups. Aborigines have arguably experienced some time of natural isolation, as have numerous other ethnic groups of modern and ancient humans. Unique phenotypic and even genotypic features were realized that are arguably more significant than the differences between the songs or nest structure of cryptic species of birds or the other very minor variations between cryptic species of frogs or giraffes, etc…
Again, don’t pretend like this is entirely objective science. It isn’t. There is a a fair amount of subjectivity in play here…
Sean Pitman
www.DetectingDesign.com
An apology to PUC
@OTNT_Believer:
And the fact that taxonomists use a b it of subjectivity is a new revelation? Come on Sean, you are nitpicking. Of course there is some subjectivity.
Hey, I’m not the one who came out and said that the differences between Darwin Finches and all other birds were so dramatic and clear cut and objectively understood that they could not be reasonably explained in just a few thousand years… or that the Egyptian dynasties are definitively known to go back over 6,000 years (when they probably go back no more than 4,500 years)…
A “bit” of subjectivity involved here? – no?
Sean Pitman
www.DetectingDesign.com
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