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An Atheist Skeptical about Evolution
It's a fundamental datum of biological science that like produces like; any offspring must be the same species as its progenitor. Hence, the child is the same species as its parent, and as its grand-parent, its great-grand-parent, etc.
But inter-specific Evolution (vs. intra-specific, the kind which Darwin actually investigates in On the Origin of Species) is predicated on the view that a series of descendants can, after epochal-scale time has elapsed, differ specifically from a distant ancestor, owing to mutation.
But how does a gradual series of mutations - with each mutant being a mutated "version" of its parent - culminate in a new species, since like produces like? (the child is the same species as its parent, the parent is the same species as its parent, the parent's parent is the same species as its parent; hence the child is the same species as its parent's parent's parent; etc).
Doesn't this view lead to the conclusion that everything is the same species, or nothing is any species? But the fact of differing species was the starting point, for which Evolution was adduced as an explanation. Does - can - it really explain inter-specific change?
February 18, 2004 | Permalink
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Hi Paul,
Species are usually considered distinct if when crossed, they cannot breed fertile offspring. Hence, the horse and donkey are different species, since mules are infertile. Similarly, the lion-tiger cross creates a "liger" which is also sterile. More divergent creatures cannot produce living offspring for a wide variety of reasons - inability to fertilize ova, inability of chromosomes to line up on the mitotic spindle (i.e. different #'s of chromosomes) etc.
The modern crocodile is not so different from the species that existed epochs ago. Transported by time machine they could no doubt mate with their ancient ancestors. Transported by the same time machine, you or I would have more than physical difficulty in mating successfully with our reptilian predecessors.
Spontaneous mutations in a population are random alterations in the DNA which will differ among individuals. They will persist if there is no selection pressure against them. However, if there is no selection pressure for them, a given mutation in a single individual will be diluted in the gene pool over time, and therefore will not have significant impact vis a vis "speciation".
Because these random differences occur with similar probability over eons, they are useful in "dating" evolutionary divergence over time within a species or across species. Their impact on speciation, as per Darwin, occurs only in so far as selection pressure operates in their favor.
Posted by: BF at Feb 21, 2004 7:24:32 PM
Hi BF,
Good pt. - thanks for pointing out my carelessness re: the matter of cross-breeding.
However, my point doesn't rise or fall with that error, so far as I can see. I was focusing on "prosaic" reproduction: mates of the same species reproduce, with the offspring being a "specimen" of the same kind as the progenitors.
The notion of Inter-specific Evolution asks us to imagine that "prosaic" pattern over unimaginably vast stretches of time, finally yielding (literally) a new specimen. While - I'd say unquestionably - intra-specific evolution is evinced in the mutation-dynamic to which you refer, I see no reason to conclude that a "specific shift" could ever occur; especially because of the simple syllogism which I relied upon in my initial account.
Restated, in light of your correction, it would go like this: any specimen capable of reproducing of necessity is specifically the same as it parents. But then, its parents - by definition, fertile - would be specifically the same as each their own parents. And so on ...
Hence, in the case of homo sapiens I see no reason - and no room, logically -to conclude to a "reptilian ancestor," but perhaps the case can be made. I encourage you to make it, if you are so inclined!
Nice to have you as a visitor/interlocutor here.
Posted by: Paul Craddick at Feb 21, 2004 8:01:22 PM
Hi Paul,
Glad to visit.
Based on the definition of species, offspring could only be said to be the "same" as their parents in so far as they would be able to produce viable offspring with their parents. But Oedipus could not clone himself by coupling with Mom! In higher life forms, sexual reproduction insures that offspring are not "carbon copies" of their parents.
I'm not ready to buy the notion that the prosaic pattern repeated over unimaginably vast stretches of time would necessarily yield a new specimen in the sense of a new species - not on the basis of random mutation alone. What would be the mechanism for the transformation in the absence of selection pressure?
The age of the fossil record is about as long as one might reasonably consider - say a few hundred million years. Cockroaches have existed this long and do not appear terribly different in form from those of today, despite the continuous occurence of random mutations over this period.
Xerox machines would operate on the basis you propose - constant addition of small distortions over long periods of time would eventually yield an unreadable document. In biological systems, in the absence of selection pressures, those small changes would be diluted over time. Only if they had an impact on survival, would they be filtered in or filtered out - on the basis of selection pressure.
Posted by: BF at Feb 22, 2004 9:39:06 AM
BF,
I'm not sure exactly how to take your second paragraph.
Are you saying that you agree with me as to the problematic nature of Inter-specific evolution, or are you saying that I've stated the issue factitiously because I've omitted to consider selection pressure?
If the latter, I didn't mean to omit it, and I don't think my argument tacitly relies on that omission. (I take it "selection pressure" here means that in regards to prevailing conditions a mutation privileges the mutant vs. its [non-mutated, but otherwise specifically "identical"] competitors, and hence provides reinforcement to the mutation). My overall point concerns how to understand the effect of accumulated mutations on species-membership.
In connection with your first paragraph, I didn't mean to suggest a perfect isomorphism between offspring and parent, but rather that they have "species-identity"; despite individuating differences, they are the same "kind" of thing (My favored locution was "specifically the same" because of the common etymological root of "species" and "specific" [and "specimen," etc.]).
This is all very interesting to me, not least because one upshot of the discussion so far is that the modern biological notion of "species," as you lay it out, is much more "parsimonious" than the notion which I employ, in light of Aristotle's notions of form, substance, etc.
I find your Xerox analogy very intriguing, and I will definitely give some thought to it.
Thanks for your insights.
Posted by: Paul Craddick at Feb 22, 2004 8:05:12 PM
Well, yes I am saying I have a problem with the notion of inter-specific evolution as posed, not because of a semantic or logical fallicy in the abstract notion of the concept - but more concretely, because it cannot occur operationally. Organic evolution in the absence of natural selection is a non-starter.
Posted by: BF at Feb 24, 2004 7:48:53 PM
Alas, every "thread" seems to unravel ... I'll leave it to others to decide whether my argument in any way turns on ignoring - rather than, as I intended, presupposing - natural selection.
I fear we've gotten bogged down here in an Ignoratio Elenchi ...
Posted by: Paul Craddick at Feb 25, 2004 6:32:58 AM
Hello,
speciation occurs when isolated populations of the same species are subject to different selective pressures.
For starters, try http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
They have some great examples toward the middle and bottom of the page.
A relatively small number of mutations - often even just one mutation - may prevent individuals from mating with their parents, thus creating a new species.
For instance, polyploids are common among plants. (see also http://www.geocities.com/RainForest/Vines/2259/colchicine.htm -- Colchicine is a natural toxin used to induce such mutations). Wheat, rye etc. are such polyploids that have been cultivated early by humans; they do, however, occur in nature as mutants of grass species)
Note also the horse, zebra, Przewalski horse, and donkey. They have remarkably similar genomes, but different numbers of chromosomes. Such aberrations can occur because of non-disjunction during meiosis (Down syndrome in humans is an example of such a mutation). Once again, one single mutation *can* create a species barrier within a population and allows these two populations to diverge further by means of natural selection.
Hope that helps ;)
Posted by: Björn at Mar 5, 2004 6:22:51 AM