Reasons to Believe

Natural Selection: Driver of Evolution or Stasis?

The theory of natural selection as a driver of evolution goes back to Charles Darwin’s visits on the ship Beagle to the Galápagos Islands. There, his studies of finches persuaded him that natural selection leads to the transmutation of species. In other words, through natural selection, one species can transmute into an entirely different species.

Darwin was not on the Galápagos Islands long enough to test through observations in real time and under real-world conditions (as opposed to controlled laboratory experiments) to verify whether or not natural selection really does transform one species into a brand new one that never existed before. Now, 180 years after the completion of the voyage of the Beagle, several such real-time, real-world observations have been performed.

The most recent of these studies was published by four American zoologists in the October issue of The Auk.1 The four zoologists measured changes and heritability of bill length, bill depth, flipper length, and foot length over a 28-year period in adult Magellanic penguins in the Punta Tombo region of Argentina. For 21 of the 28 years, they detected no natural selection. In the seven years where they did see some natural selection, “the direction and intensity of selection on traits varied.”2 That is, the natural selection was not causing a permanent change in the species.

The four zoologists noted that the penguins they studied live in an ever-changing environment and that the dynamic nature of the penguins’ environment maintains the high degree of morphological variation they observed. They concluded, “The temporal variability in selection likely fosters stability of morphology through time, a pattern that might not be evident in short-term studies.”3

In another long-term field study, one on great reed warblers in Sweden, a team of three ecologists concluded that the populations they observed were “subject to low levels of directional selection and higher levels of stabilizing selection.”4 They predicted that the populations are “changing very little if anything.”5

In a 30-year-long study of two populations of Darwin’s finches (the medium ground finch and the cactus finch) on the Galápagos island of Daphne Major, two evolutionary biologists observed that the two finch species “changed several times in body size and two beak traits.”6 However, for both species the changes “varied from unidirectional to oscillating, episodic to gradual.”7 They concluded that “the phenotypic states of both species at the end of the 30-year study could not have been predicted at the beginning.”8

Ornithologists have noted for some time that the medium ground finch on the Galápagos island of Santa Cruz exhibits bimodal traits. A long-term study showed that natural selection may “simultaneously maintain the current bimodality while also constraining further divergence.”9

What are we to make of all these long-term studies of natural selection? One obvious conclusion is that many more such long-term field studies are needed, not just for species of birds but for all different kinds of animals and plants. Another is that what looks like a linear unidirectional change over a time span of a few months may instead be a small segment of a long-term sinusoidal variation about a mean that occurs over time spans of decades, centuries, or millennia.

An obvious lesson is that it is a mistake to build a model for the history of life on Earth based only on short-term field studies. Rather than the transmutation of species through natural selection that Darwin deduced from his few months on the Galápagos Islands, the long-term field studies suggest that natural selection maintains stasis, the stabilizing of a species’ morphological traits over time.

From a biblical creation perspective, God endows the different kinds of life he creates with a capacity for natural selection so that through natural selection the kinds of life can effectively adapt to random changes in their habitats. For each kind of life, there is an optimal set of morphological traits. God designs the capacity for natural selection to maintain the morphological traits, keeping them close (as close as changes in the environment will permit) to this optimal set. Long-term field studies appear to be sustaining this biblical perspective.

Endnotes

  1. Laura Koehn et al., “Natural Selection on Morphology Varies among Years and by Sex in Magellanic Penguins (Spheniscus magellanicus),” The Auk 133 (October 2016): 783–805, doi:10.1642/AUK-16-50.1.
  2. Ibid., 783.
  3. Ibid.
  4. Mikael Åkesson, Staffan Bensch, and Dennis Hasselquist, “Genetic and Phenotypic Associations in Morphological Traits: A Long Term Study of Great Reed Warblers Acrocephalus arundinaceus,” Journal of Avian Biology 38 (January 2007): 58–72, doi:10.1111/j.2006.0908-8857.03669.x.
  5. Ibid.
  6. Peter Grant and Barbara Rosemary Grant, “Unpredictable Evolution in a 30-Year Study of Darwin’s Finches,” Science 296 (April 2002): 707–11, doi:10.1126/science.1070315.
  7. Ibid.
  8. Ibid.
  9. Andrew Hendry et al., “Disruptive Selection in a Bimodal Population of Darwin’s Finches,” Proceedings of the Royal Society B 276 (February 2009): 753–59, doi:10.1098/rspb.2008.1321.

 

Subjects: adaptation, Adaptaton, Evolution