Convergence vs. natural selection

Summary of problems:

There is nothing mysterious about convergence. Species facing similar selective pressures would be expected to be similar in certain ways. There is no reference to any particular scientist who would support the claims EE advances, but it is an argument commonly advanced in the creationist literature.

Full discussion:

Explore Evolution says the following about convergence:

Neo-Darwinian biologists use the term "convergence" or "homoplasy" to describe similar structures that are not due to common ancestry but which are found in different types of organisms. They call these features convergent because they think that the evolutionary process has come together (converged) on the same structure two or more times in creatures that exist on very different branches of the Tree of Life. Convergence is a deeply intriguing mystery, given how complex some of the structures are. Some scientists are skeptical that an undirected process like natural selection and mutation would have stumbled upon the same complex structure many different times.
EE, p. 48

No citation is given to the scientists who are supposedly skeptical about the ability of natural selection to explain convergence. Indeed, it's difficult to imagine how anyone who thinks a structure could evolve in one lineage in response to a set of selective pressures would be surprised that the same selective pressures would produce a functionally similar structure in another lineage. Furthermore, in the examples of convergence offered in EE, the convergence does not involve any complexity. The mole paw is a simple modification of shape of the paws found in other mammals. Similarly, the mole cricket's forelimbs are simple modifications of the basic anatomy of the insect forelimb. No new structures are involved, merely the rearrangement of pre-existing structures.

Even though scientists cannot be readily identified making the argument EE presents, that argument is easy to locate in the writings of creationists. In 1925, young earth creationist George McCready Price made a very similar argument to that in EE:

we become convinced that these many similar or identical structures, which must have been evolved quite independently (if evolved at all), make too great a draft on our credulity. At least, these hundreds of examples of "parallel evolution" greatly weaken our confidence in homology, or similarity of parts and organs, as a proof of blood relationship.

There are several distinct types of eyes, each type being quite efficient as organs of seeing. But if we take the eye of the higher animals, we become amazed to find an almost identical structure in the cuttlefish or devilfish, which is really a mollusk. Its eye has all the parts found in the human eye, a retina, a sclerotic, a choroid, a vitreous humor, an aqueous humor, and an adjustable lens, just as in the eye of one of the higher vertebrates. Now I can believe that these similar organs could have been created independently for these very distinct classes of animals. But I cannot believe that this marvelous organ was evolved independently in these two instances by any process of natural development or evolution. I do not think that [Darwin's] mental equilibrium would have been restored if he had considered that this organ must have been evolved quite separately in at least these two instances. Indeed, this process must have been repeated also once more; for the pecten, another kind of shellfish wholly different from the cuttlefish, has two rows of almost equally perfect eyes around the edge of its body. I cannot force myself to believe that these complete organs of sight were separately and independently evolved by any natural development in these three instances.

The argument has been repeated many times by many creationists. In 1970, Evan Shute wrote:

Many resemblances between animals and plants of different genera, families and orders defy evolutionary explanation. There are both differences and similarities between creatures of different kinds. The evolutionist must decide what features are useful as true species criteria and what features are spurious or misleading. A small but interesting sampling of strange similarities between widely diverse living forms is given here, from a study of spinal tracts, ears, placentae, electric organs, kidney function, fern vessels, milk, brown fat, sweat glands, and other systems: It is asserted that these puzzling resemblances are best explained by special creationism rather than by evolutionary convergence.
Evan Shute (1970) "Puzzling Similarities," Creation Research Science Quarterly, 7(3):147-151.

More recently, the newsletter for the old earth creationist group Reasons to Believe claimed:

No known evolutionary mechanism can account for the nature of biological convergence. Convergence has been far too common throughout life s history, has involved exceedingly complex structures, and has occurred in situations in which the forces of natural selection have been vastly different. Biological convergence is an important component in the argument that life, throughout earth's history, is a result of the supernatural activity of a Creator.

In the scientific literature, convergence is far from surprising. In Futuyma's Evolutionary Biology, the second of seven "principles of evolutionary change" is "homoplasy [convergence] is common in evolution":

When a similar character (or character state) in two organisms has not been derived from a corresponding character (or state) in their most recent common ancestor, it is said to be homoplasious. An example of a homoplasious character is the superficially similar eye of vertebrates and of cephalopods (squids, octopods). Both have a lens and retina, but their many profound differences indicate that they evolved independently: for example, the axons of the retinal cells arise from the cell bases in cephalopods, but from the cell apices in vertebrates.

Three more or less arbitrarily distinguished kinds of homoplasy are recognized. In convergent evolution (convergence), independently evolved features are superficially similar, but arise by different developmental pathways. The eyes of vertebrates and cephalopods are an example. Parallel evolution (parallelism) is thought to involve similar developmental modifications that evolve independently (often in closely related organisms, because they are likely to have similar developmental mechanisms to begin with). Evolutionary reversals constitute a return from a [derived] character state to a more ancestral state.

Homoplasious features are often (but not always) adaptations by different lineages to similar environmental conditions. In fact, a correlation between a particular homoplasious character in different groups and a feature of those organisms' environment or niche is often the best initial evidence of the feature's adaptive significance.
Douglas Futuyma (1998) Evolutionary Biology 3rd ed., Sinauer Associates:Sunderland, MA. p. 110-111

There is nothing the least bit surprising about convergence or homoplasy in general. Shared selective pressures ought to produce some degree of similarity in structures. Characteristics used to identify evolutionary relationships are selected to avoid traits likely to be result of convergence, so homoplasy is not, in general, a problem for reconstructing evolutionary history. Convergence, far from being a surprise, is a predicted result of evolutionary processes.

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