The Scientific American website has several articles on Creationism Vs. Evolution. One of the articles is 15 Answers to Creationist Nonsense from their June 2002 issue. The answers suffer from the same confusion about evolution that I've been addressing for years. It does not distinguish between evolution and natural selection and it fails to mention random genetic drift as a dominant mechanism of evolution. This is most obvious in the response to a question about speciation.
11. Natural selection might explain microevolution, but it cannot explain the origin of new species and higher orders of life.One can easily imagine cases where speciation is driven entirely by natural selection but most of the textbooks are more pluralistic. The standard models have two populations diverging in phenotype due to either natural selection or random genetic drift or a combination of the two mechanisms.
Evolutionary biologists have written extensively about how natural selection could produce new species. For instance, in the model called allopatry, developed by Ernst Mayr of Harvard University, if a population of organisms were isolated from the rest of its species by geographical boundaries, it might be subjected to different selective pressures. Changes would accumulate in the isolated population. If those changes became so significant that the splinter group could not or routinely would not breed with the original stock, then the splinter group would be reproductively isolated and on its way toward becoming a new species.
Natural selection is the best studied of the evolutionary mechanisms, but biologists are open to other possibilities as well. Biologists are constantly assessing the potential of unusual genetic mechanisms for causing speciation or for producing complex features in organisms. Lynn Margulis of the University of Massachusetts at Amherst and others have persuasively argued that some cellular organelles, such as the energy-generating mitochondria, evolved through the symbiotic merger of ancient organisms. Thus, science welcomes the possibility of evolution resulting from forces beyond natural selection. Yet those forces must be natural; they cannot be attributed to the actions of mysterious creative intelligences whose existence, in scientific terms, is unproved.
The standard models postulate that divergence is initiated when two populations become geographically isolated as described above. If the two locales are different then the population that occupies the new environment might undergo adaptive selection, causing the divergence in morphology. However, if the two locales are similar the populations might just diverge by chance when they become geographically isolated.
Speciation occurs when the two populations have diverged to the point where they can no longer interbreed. At this point they become not only geographically isolated but also reproductively isolated.
There's no obvious way that the evolution of reproductive isolation could be due to natural selection. This would require that one population keep testing itself against the other with lack of cross-fertility providing some benefit to individuals in one of the populations. Instead, it's extremely likely that reproductive isolation is due to chance mutations that become fixed by random genetic drift. John Wilkins, our blogger expert on speciation, described this in a 2006 article that introduces sympatric speciation. Here are the relevant parts concerning the much more common mode of allopatric speciation.
Nobody denies, not even the most ardent antiadaptationist [that's me!], that aspects of organisms are strongly subject to selection, whether during speciation or after it. The critical issue is whether selection is a cause of speciation itself.The bottom line here is that much of what we call speciation—especially the crucial reproductive isolation—is probably not due to natural selection. Instead, random genetic drift is the culprit. What this meams is that the answer to the question above is somewhat misleading. As it turns out, natural selection cannot account entirely, or even mostly, for all speciation events.
The allopatric consensus view allows for local adaptation, of course, when isolated from the parent metapopulation. What it denies is that selection for RI [reproductive isolation] occurs - how could it when speciation is occurring without contact with the reproductively isolated populations? There is selection of RI, of course, since RI on that account is a byproduct of changes in the population that are selectively favoured for ecological reasons. But not selection for RI itself [the selection of and selection for distinction is due to Elliot Sober]. So, argue allopatrists such as Jerry Coyne and Allan Orr, selection is not a cause of speciation in allopatry. And this seems right.
... If we think of speciation as "what makes a species" then we get ecological and other selective processes. If we think of speciation as "what makes it not the same species", then the explanatory focus shifts, and here the answer is, in cases when divergent selection is not going on, populations simply drift away from the reproductive reach of the ancestral population.
Some of you might recall a discussion I had in July with my colleague Spencer Barrett on this issue. He acted very annoyed when I suggested that random genetic drift might play an important role in speciation [see Species Diversity, Darwinism at the ROM]. This disagreement was made obvious to me today when I took a poll of my students in our class on Scientific Misconceptions. I asked them what they had learned from Professor Barrett in their first year class on evolution and more than 70% of them defined evolution as adaptation and were unable to identify random genetic drift as a mechanism of evolution. This means I'm going to have to explain evolution before we can discuss the evolution vs creationism controversy.
Go back and look at the second paragraph of the Scientific American answer (above). Isn't it strange that they don't even mention random genetic drift when listing other mechanisms of evolution? What's going on here? Do the science writers1 at scientific American not know about random genetic drift or do they not think that it's a valid mechanism of evolution according to their definition of evolution. I suspect both.
1. The article was written by John Rennie, a science writer who currently serves as editor in chief of Scientific American.
[Image Credits: The top image comes from webpages on evolution at CUNY Brooklynn (New York). The accompanying text reads, ""In small populations, other forces are at work. When a population is small, the presence or absence of a single individual can have a profound effect on the population gene pool. A sudden reduction in population size can also alter the remaining gene pool. This is the bottleneck effect.
A change in the gene pool brought about by chance is a genetic drift.
An extreme form of genetic drift, combined with the bottleneck effect is called the founder effect, which depends on a small group becoming isolated from the larger group, and can rapidly lead to the creation of a new species."
The bottom image comes from another article by John Wilkins, Explanation, that discusses, among other things, the role of stochastic events, such as random genetic drift, in speciation.]
