Is evolution a fact? Oxford biologist Richard Dawkins and many other scientists think so. In his latest book, The Greatest Show on Earth, Dawkins presents what he thinks is the best evidence for the evolutionary paradigm.
Dawkins regards biological evolution as "the greatest show on Earth" because, according to his description, the simple mechanism of natural selection operating iteratively on random genetic variation generated the diversity of life that has existed throughout Earth's history. Because of Dawkins' prominence and influence as the foremost spokesperson for the evolutionary paradigm, it is important to provide a chapter-by-chapter response to the case that he presents for biological evolution.
A few weeks ago I offered some comments on the first chapter of The Greatest Show on Earth. This week I continue my critique, focusing on chapters two and three.
These two chapters form a coherent unit within the book. Dawkins uses them to set the stage to make his case for biological evolution. He leads the reader down the "Primrose Path to Macro-Evolution" (as chapter three is titled).
Dawkins kicks off this chapter with a fascinating discussion of Plato's influence on biology. Of specific note is Plato's theory of forms that resulted in the application of essentialism to the concept of a species. Essentialism is the idea that any entity, like a species, has a defining set of characteristics and properties that all examples of that entity possess.
Contrary to how many people think, Dawkins argues that species are not essential entities. Instead they are a population of individuals that varies over time. He asserts that after enough time passes a species extant today would be unrecognizable to the distant past or in the distant future.
Employing this population approach, biologists think of a species as being defined by its gene pool, the collection of genes that specify the full range of the traits that members of the species possess. There are many more variants of a trait within a population than any one member can harbor. Each individual represents a subset of the traits found within the whole species.
According to biologists, a species evolves when new types of traits are added to the gene pool through mutations or when the frequency of the various traits is altered. In this sense, it is the population that evolves, not individual members of the species. But as the population, and consequently the species, changes, each member continues to represent a subset of the traits specified by the gene pool.
In other words, species are not essential because the defining traits of the species are always changing. Dawkins demonstrates this notion by describing the ability of humans to alter species, in some cases quite dramatically, through the use of artificial selection.
This chapter picks up where the previous one left off by arguing that organisms in the wild perform their own versions of artificial selection. He cites several examples to support his assertion. Some female birds will choose males with plumages that are brightly colored to mate with (sexual selection). As a result, drab-colored males are weeded out of the population over time because their genes are not transferred to the next generation due to fewer mating opportunities. A comparison between sexual selection and the artificial selection performed by dog breeders is drawn. Dawkins also discusses the role that insects play in shaping the color and fragrance of flowers. Both characteristics are used by plants to attract insects to the flowers so that they will pollinate other plants. Those plants that attract more insects will reproduce at a higher frequency than those that fail to attract pollinators. The net effect is that the color and scent produced by the flowers of a given species will be altered over time based on insect preference. This pollination work is compared to the work of horticulturalists attempting to develop new rose varieties.
Dawkins then discusses the role that prey animals play in modifying predators so that the latter creatures become better designed and more skilled as hunters. Those predators that more efficiently capture and consume prey are going to pass the characteristics they possess to their offspring. Through this evolutionary process, predators evolve to be more effective killers, and likewise, prey evolves to be more elusive. In a sense predators shape the evolution of prey and vice versa. For Dawkins, this example illustrates the principle of natural selection, an undirected form of artificial selection. He claims that natural selection is a more generalized form of artificial and sexual selection.
In short, Dawkins attempts to lead the reader down the primrose pathway to evolution by showing that species are not immutable, that they can be shaped by artificial selection, and that undirected, blind versions of selection are at work in the natural realm. In short, evolution–natural selection iteratively operating on random genetic variation–is a fact.
I agree with Dawkins's view that Charles Darwin (and Alfred Russell Wallace) made important contributions to biology by effectively arguing that species are not fixed entities. They are malleable, capable of changing over time in response to changes in the environment, resource availability, predatory pressure, mate preference, etc.
As Dawkins elegantly illustrates in The Greatest Show on Earth, there are numerous examples of artificial, sexual, and natural selection that prove the plasticity of species. Thanks to work in population genetics–and Darwin's ideas about reproductive fitness and success–the mechanism that drives these changes is well understood.
But just because species are malleable doesn't mean they aren't essential. Essentialism maintains that members of a group can possess other properties that aren't necessary to categorize them within the group or to preclude them from the group. This more sophisticated understanding of essentialism makes it possible for a species to change, but within boundaries that still make them essentially part of the group.
Work by breeders and studies of natural selection in the field suggest that species are indeed defined by boundaries and that microevolutionary changes can't be extrapolated much beyond the speciation regime. As I pointed out a few weeks ago, the scale of the biological changes that take place in microevolution and speciation are radically different than the presumed changes that take place for macroevolution. Processes happening at one level can't automatically be extrapolated to other levels without proper validation.
To illustrate the plasticity of species, Dawkins cites the common example of the large number of diverse dog breeds and the ability of humans to use artificial selection to sculpt the canine gene pool. Ironically, the efforts of dog breeders demonstrate that there are biological boundaries beyond which dogs can't be pushed even with the best human efforts. English bulldogs illustrate this point. Because of their unusually large heads, these dogs can only be born via caesarian section. The "evolution" of English bulldogs is only possible because humans have figured a way to work around the problems caused by their large heads utilizing a nonnatural means to birth this breed. Hypothetically, if natural selection drove canines to have disproportionately large heads, at some point the evolutionary process would come to a halt, because the large-headed puppies could never pass through the birth canal. Dawkins writes, "what happens under domestication is that animals are artificially shielded from many of the risks that shorten the lives of wild animals…artificial selection has pushed them into a zone that natural selection would not have tolerated."
The trade-offs confronting organisms in their quest to survive is another reason why boundaries for species must exist. These trade-offs stymie the directional action of natural selection. From an evolutionary vantage point, evolution must find a compromise between competing selection pressures. As an example, Dawkins discusses strains of laboratory rats that have been forced to evolve teeth resistant to decay. This type of evolutionary change would never happen in the wild, because decay-resistant teeth come at a cost, namely weakened skeletons, etc. In Dawkins's own words: "Perfection in one department must be bought in the form of sacrifice in another department." These trade-offs create boundaries that species cannot traverse by undirected evolution.
Is evolution a fact? Yes, if one means microevolution and speciation. It is debatable if larger scale changes are a fact, however, given the boundaries that seemingly define species–boundaries revealed by artificial selection and studies of natural selection in the wild.
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