Nature‘s Microevolutionary Gems Part 2: Bird-Sized Evolutionary Change
|Links to our 9-Part Series Responding to Nature‘s Evolution Evangelism Packet:
• Part 1: Evaluating Nature’s 2009 “15 Evolutionary Gems” Darwin-Evangelism Kit
In Nature‘s evolution-evangelism packet, two of Nature‘s “evolutionary gems” looked at birds. The first such gem showed “Differential dispersal in wild birds” — but before we get caught up in the jargon, let’s just cut to the main question: What sort of evolutionary change was observed? From reading Nature‘s evolution-evangelism packet, one is told that the “findings illustrate the large effect of immigration on the evolution of local adaptations and on genetic population structure” or that “evolutionary differentiation can be rapid and occur over surprisingly small scales.” So exactly what was this rapid, large evolutionary change?
In one study it turns out that female members of the bird species Parus major (common name: “great tit”) bred on the western end of the Dutch island of Vlieland tend to lay 1.15 ± 0.14 eggs per clutch more than females bred in populations on the east end of the islands. You read that right. The birds native to the island are still reproductively compatible with “immigrant” birds. The fact that evolutionary biologists consider a difference of 1.15 � 0.14 eggs per clutch to be a “large effect” on a population shows just how desperate they are to find evidence of biological change in nature. (See Erik Postma & Arie J. van Noordwijk, “Gene flow maintains a large genetic difference in clutch size at a small spatial scale,” Nature 433:65-68 (January 6, 2005).)
Another study cited in this “gem” promised to show “marked evolutionary differentiation” at “small spatial and temporal scales.” Readers learned that over a span of about 35 years, great tits from the eastern part of the Wytham woodland in southern England saw a decrease in adult body size that amounted to a net average change of about 1 gram (less than 10 percent of total body mass). Fledgling birds likewise saw a small change in body mass. (Birds in the northern part of the wood did not experience such a change.) (See Dany Garant, Loeske E.B. Kruuk, Teddy A. Wilkin, Robin H. McCleery & Ben C. Sheldon, “Evolution driven by differential dispersal within a wild bird population,” Nature 433:60-65 (January 6, 2005).)
Recall that Nature‘s introduction to the packet boasts that “all life evolved by natural selection” and claims that this is “a fact, in the same way that the Earth orbits the Sun is a fact.” Nature claimed the packet would show “just what is the evidence for evolution by natural selection.” But if such featured “evolutionary gems” are among the best that evolutionary scientists have to offer, which is what the packet implies, then that leaves a large gap between the observed data and Nature‘s grand claims.
Meanwhile, what treatment of microevolutionary changes would be complete without a discussion of Darwin’s Galapagos finches? The packet explains that “When Charles Darwin visited the Galapagos Islands, he recorded the presence of several species of finch that all looked very similar except for their beaks,” further noting that “Darwin speculated that all the finches had a common ancestor that had migrated to the islands.” We’ve all heard this story before — but is there more too it?
According to the British Natural History Museum, “Mockingbirds from the Galapagos Islands, not finches, gave Charles Darwin his ideas about evolution. … Darwin’s finches are the better-known birds connected with helping Darwin come to his conclusions on evolution. However, it was the little-known mockingbirds that were the key.” Likewise, historian of science Frank Sulloway debunks the finch myth, stating that, “far from being crucial to his evolutionary argument, as the legend would have us believe, the finches were not even mentioned by Darwin in the Origin of Species.” (See: Frank J. Sulloway, “Darwin and His Finches: The Evolution of a Legend,” Journal of the History of Biology, Vol. 15(1):1-53 (Spring, 1982).)
Island Biogeography observes that it is “unclear whether [the mockingbird] genus (Nesomimus) is sufficiently distinct morphologically to warrant separation from the mainland genus (Mimus)” (Robert J. Whittaker, Island Biogeography: Ecology, Evolution, and Conservation, p. 96 (Oxford University Press, 1998)), or as Explore Evolution explains, the mockingbirds “show only small-scale variations in existing traits.”
Setting aside Nature‘s perpetuation of common myths about the Galapagos finches, modern-day field studies of these finch species are commonly cited as examples of evolution. So again we must ask, How much evolutionary change are we talking about? Here, the packet is somewhat forthright, acknowledging that we’re in fact only talking about “small differences in the depth, width or length of the beak.” The packet then refers readers to a study that investigated the genetic basis of these small changes in beak morphology. It’s an interesting paper, but as the packet explains, these changes in beak morphology may be caused by mere “differing expression of the gene for calmodulin, a molecule involved in calcium signaling that is vital in many aspects of development and metabolism.” (See Arhat Abzhanov, Winston P. Kuo, Christine Hartmann, B. Rosemary Grant, Peter R. Grant,, and Clifford J. Tabin, “The calmodulin pathway and evolution of elongated beak morphology in Darwin’s finches,” Nature, Vol. 442:563-567 (August 3, 2006).)
So what’s at issue here is mere differential expression of a gene (rather than the evolution of an entirely new gene) causing “small differences” in beak morphology. For the Darwin-critic, this is interesting since such differences in beak shape are well-known throughout a variety of bird groups (such as Hawaiian honeycreepers), making it unsurprising that commonly observed forms of biodiversity have a small-scale genetic basis. None of this suggests how large-scale evolutionary change could occur. While the origin of the beaks (and the birds) themselves may remain unexplained, perhaps diverse beak morphologies are designed to evolve.