Nature's Microevolutionary Gems Part 3: Flea and Guppy-Sized Evolutionary Change
The paper cited for this "gem" opens by saying that "The maintenance of genetic variation in traits under natural selection is a long-standing paradox in evolutionary biology." Before getting into the "paradox" and its implications, we must assess the degree of evolutionary change observed. Turns out we're talking about small changes in the coloration spots on male guppies. The picture below tells the entire story of these small-scale differences:
Adapted by permission from Macmillan Publishers Ltd: Nature, Figure 1, Robert Olendorf, F. Helen Rodd, David Punzalan, Anne E. Houde, Carla Hurt, David N. Reznick, & Kimberly A. Hughes, "Frequency-dependent survival in natural guppy populations," Vol. 441:633-636 (June 1, 2006), Copyright 2006.
Keep in mind that it's nearly identical fish like these that Nature boasted would show us "just what is the evidence for evolution by natural selection."
While this is undoubtedly exciting stuff, I found more color variation in the goldfish I caught at the local school carnival back in the third grade. (Those were hardy goldfish -- they lived for years and grew quite large, for a goldfish, until a neighbor fed them tap water during a family vacation and killed them. Two lessons learned: Leave good instructions for petsitters and don't drink unfiltered tap water.)
One of the interesting findings in this paper was that "rare phenotypes had a highly significant survival advantage compared to common phenotypes." If they confer such a strong survival advantage, why are they rare? The evidence suggests that the fitness a trait is not determining its frequency, but rather the frequency of a trait is determining its fitness. If fitness is not the causative agent, then what is causing natural selection? Just as the standout in the crowd often gets noticed, it seems that sexual selection is driving male coloration patterns, as "females have a mating preference for rare or novel males." That's all fine and good, but how is the evolution of small-scale changes in coloration patterns in male-guppies going to explain large-scale evolutionary change?
Another innovative study awakened dormant parasites (bacteria) and their hosts (water fleas) from layers of lake sediment to show that they have undergone an evolutionary "arms races." The article speculated, "It is likely that different genes contributed to different parasite fitness components and thus follow different evolutionary dynamics," and thus the genetic basis for increased parasite infectivity or host resistance was not determined. As a result, it's difficult to assess whether any significant biological changes took place. All the researchers know was that a game of cat-and-mouse occurred between parasite and host. From the beginning to the end of the race, the bacteria remained bacteria and the water fleas remained water fleas. While this does show that the red queen hypothesis is viable, it also shows that the queen tends to send species running down evolutionary dead ends.
(See Ellen Decaestecker, Sabrina Gaba, Joost A. M. Raeymaekers, Robby Stoks, Liesbeth Van Kerckhoven, Dieter Ebert, & Luc De Meester, "Host-parasite 'Red Queen' dynamics archived in pond sediment," Nature, Vol. 450:870-873 (December 6, 2007).)