Friday, September 04, 2009
Mutant Creationist Arguments
Dr. Steven Novella of Neurologica Blog has a nice post that connects a new study (nicely explained by Larry Moran), using the latest genetic sequencing techniques to give a more accurate estimate of the mutation rate in humans, and creationist claims that the majority of mutations are harmful and, therefore, evolution could not occur by natural selection for lack of sufficient viable variants to select from:
Creationists, as they often do, habitually misunderstand the nature of "mutations." They see them as deviations from a perfect "normal" state. But in reality, all gene variations are just that – variations. No one variation is any more perfect or normal than another. There is likely, for most proteins, a large variety of base pair and amino acid sequences that function just fine, or may have only subtle differences. There is also context – one sequence may have a survival advantage in one environment but be a detriment in another.
Suppose a 4-foot-wide ditch in your backyard, running to the horizon in both directions, separates your property from that of your neighbor's. If one day you met him in your yard and asked how he got there, you would have no reason to doubt the answer, "I jumped over the ditch." If the ditch were 8 feet wide and he gave the same answer, you would be impressed with his athletic ability. If the ditch were 15 feet wide, you might become suspicious and ask him to jump again while you watched; if he declined, pleading a sprained knee, you would harbor your doubts but wouldn't be certain that he was just telling a tale. If the "ditch" were actually a canyon 100 feet wide, however, you would not entertain for a moment the bald assertion that he jumped across.
But suppose your neighbor -- a clever man -- qualifies his claim. He did not come across in one jump. Rather, he says, in the canyon there were a number of buttes, no more than 10 feet apart from one another; he jumped from one narrowly spaced butte to another to reach your side. Glancing toward the canyon, you tell your neighbor that you see no buttes, just a wide chasm separating your yard from his. He agrees, but explains that it took him years and years to come over. During that time buttes occasionally arose in the chasm, and he progressed as they popped up. After he left a butte it, usually, eroded pretty quickly and crumbled back into the canyon. Very dubious, but with no easy way to prove him wrong, you change the subject to baseball.
This little story teaches several lessons. first, the word jump can be offered as an explanation of how someone crossed a barrier, but the explanation can range from completely convincing to totally inadequate depending on details (such as how wide the barrier is). Second, long journeys can be made much more plausible if they are explained as a series of smaller jumps rather than one great leap. And third, in the absence of evidence of such smaller jumps, it is very difficult to prove right or wrong someone who asserts that stepping stones existed in the past but have disappeared.
I suspect, instead, that Behe, consciously or unconsciously, included such a metaphor because it is a type that strikes a deep chord in his intended audience. The very same one that Stephen Jay Gould recounted in "Hooking Leviathan by Its Past":
Every creationist book on my shelf actually cites the absence of and inherent inconceivability of transitional forms between terrestrial mammals and whales. Alan Haywood, for example, writes in his Creation and Evolution [Haywood, Alan 1985. Creation and Evolution. London: Triangle Books]:
Darwinists rarely mention the whale because it presents them with one of their most insoluble problems. They believe that somehow a whale must have evolved from an ordinary land-dwelling animal, which took to the sea and lost its legs . . . A land mammal that was in the process of becoming a whale would fall between two stools -- it would not be fitted for life on land or at sea, and would have no hope for survival.
There are no "stools" but, instead, giant benches -- sometimes called "pews" -- that life has and continues to slide its collective butt along in a never ending attempt to find the most comfortable available seat.
Reading this a question rose unbidden in my mind. No idea why.
How sure are we that humans are one species?
If we found groups of people who weren't inter-fertile would they be different species or would it be similar to Old English.
In Olden days (pre-1500s say) English people in neighbouring villages could talk to each other but travel 4 or 5 villages and they found it hard (if not impossible) to understand each other but they all spoke English and were all described as English speakers/people.
It kinda depends on what definition of "species" you use. The fertility "test" is Ernst Mayr's "biological species concept" and is widely accepted but there are some problems with it. For a multicellular animal such as humans, though, it works well enough. The toughest case would be if we found a population of purported humans who could breed with everyone else but only produced sterile offspring (like horses and donkeys).
In fact, humans are very close genetically and are certainly one species.
As to the issue of isolation, there is the quip that English is the result of Norman soldiers attempting to get laid by Anglo-Saxon bar wenches. When it comes to sex, people will overcome language barriers. Since we are a relatively large, slow-breeding species it doesn't take all that much gene flow between populations to prevent genetic isolation.
I was more thinking of a ring species. Though given what we know of our relatedness I'll happily assume we are one species.