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Top Scientific Problems with Evolution: Natural Selection

Photo: Darwin's finch, by Victor Gleim, CC BY-SA 4.0 , via Wikimedia Commons.

Editor’s note: We are delighted to present a series by biologist Jonathan Wells on the top scientific problems with evolution. This is the fifth entry in the series, excerpted from the new book The Comprehensive Guide to Science and Faith: Exploring the Ultimate Questions About Life and the CosmosFind the full series so far here.

In the Introduction to On the Origin of Species Darwin wrote, “I am fully convinced that species are not immutable.” He continued, “Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.”1

But Darwin had no evidence for natural selection. In On the Origin of Species, the best he could offer was “one or two imaginary illustrations.”2 So instead of direct evidence for natural selection, Darwin (who himself bred pigeons) based his argument on domestic breeding, or what is often called artificial selection. He noted that “the breeding of domestic animals was carefully attended to in ancient times,” and that “its importance consists in the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by an uneducated eye.”3

The Origin of Species

Yet in all the years of domestic breeding, no one ever reported the origin of a new species, much less a new organ or body plan. In the 1930s, neo-Darwinian biologist Theodosius Dobzhansky used the word microevolution to refer to changes within existing species (such as those observed by domestic breeders), and the word macroevolution to refer to the origin of new species, organs, and body plans. He wrote,

There is no way toward an understanding of the mechanisms of macroevolutionary changes, which require time on a geological scale, other than through a full comprehension of the microevolutionary processes observable within the span of a human lifetime and often controlled by man’s will. For this reason we are compelled at the present level of knowledge reluctantly to put a sign of equality between the mechanisms of macro- and microevolution, and proceeding on this assumption, to push our investigations as far ahead as this working hypothesis will permit.4

Evidence for Natural Selection?

But a “working hypothesis” is not evidence. It wasn’t until the 1950s that British naturalist Bernard Kettlewell discovered what appeared to be the first evidence for natural selection. Peppered moths in the UK exist predominantly in two varieties: dark (“melanic”) and light. Before the 19th-century industrial revolution, melanic forms were rare or absent, but when smoke from industrial cities darkened nearby tree trunks, the melanic form became much more common. This phenomenon, called industrial melanism, was attributed to melanic moths being better camouflaged than light moths and thus less visible to predatory birds: in other words, to natural selection. 

Kettlewell captured some of each variety and marked them with a tiny spot of paint. Then he released them onto dark- or light-colored tree trunks. When he recaptured some the next day, he found that a significantly greater proportion of better-camouflaged moths survived. Kettlewell termed this this “Darwin’s missing evidence.”5 The story, usually illustrated with photos of light- and dark-colored peppered moths on light- and dark-colored tree trunks, was featured for decades in many biology textbooks as compelling evidence for evolution.6

The Habits of Peppered Moths

By the 1980s, however, it had become clear that peppered moths don’t normally rest on tree trunks in the wild. They fly by night and rest during the day in upper branches where they can’t be seen. By releasing moths onto tree trunks in the daytime, Kettlewell’s experiment failed to simulate natural conditions. It turned out that most textbook photographs had been staged by pinning dead moths on tree trunks or by placing live moths in unnatural positions and photographing them before they moved away.7

Better evidence for natural selection came from finches in the Galápagos Islands in the 1970s. The islands were home to what biologists listed as 13 different species of finches, and biologists Peter and Rosemary Grant and their colleagues studied one of these on a single island. The Grants and their colleagues kept detailed records of each finch species’ anatomy, including the length and depth of their beaks. When a severe drought in 1977 killed many of the islands’ plants, about 85 percent of the birds died. The Grants and their colleagues noted that the survivors had beaks that were, on average, 5 percent larger than the population average before the drought, presumably because the surviving birds were better able to crack the tough seeds left by the drought. In other words, the shift was due to natural selection. The Grants estimated that if a similar drought occurred every ten years, the birds’ beaks would continue to get larger until they would qualify as a new species in 200 years.8

The Arrival of the Fittest

When the drought ended and the rains returned, however, food was plentiful, and the average beak size returned to normal. No net evolution had occurred.9 Nevertheless, “Darwin’s finches” found their way into most biology textbooks as evidence for evolution by natural selection.10

So there is evidence for natural selection, but like domestic breeding, it has never been observed to produce anything more than microevolution. As Dutch botanist Hugo de Vries wrote in 1904, “Natural selection may explain the survival of the fittest, but it cannot explain the arrival of the fittest.”11

For the arrival of the fittest, most modern evolutionary biologists rely on mutations.

Next, mutation.


  1. Charles Darwin, Origin of Species, 1st ed., 6, http://darwin-online.org.uk/content/frameset?pageseq=21&itemID=F373&viewtype=side (accessed August 23, 2020).
  2. Darwin, Origin of Species, 1st ed., 90, http://darwin-online.org.uk/content/frameset?pageseq=105&itemID=F373&viewtype=side (accessed August 23, 2020).
  3. Darwin, Origin of Species, 1st ed., 32-34, http://darwin-online.org.uk/content/frameset?pageseq=47&itemID=F373&viewtype=side (accessed August 23, 2020).
  4. Theodosius Dobzhansky, Genetics and the Origin of Species (New York: Columbia University Press, 1937), 12.
  5. H.B.D. Kettlewell, “Darwin’s missing evidence,” Scientific American 200 (1959), 48–53.
  6. Jonathan Wells, “Second Thoughts About Peppered Moths: This classical story of evolution by natural selection needs revising,” The Scientist 13 (May 24, 1999), https://www.discovery.org/a/590/ (accessed August 23, 2020); Jonathan Wells, Icons of Evolution (Washington, DC: Regnery, 2000), 137-157.
  7. Judith Hooper, Of Moths and Men: Intrigue, Tragedy and the Peppered Moth (London, UK: Fourth Estate, 2002); Wells, Zombie Science, 63-66.
  8. Peter T. Boag and Peter R. Grant, “Intense natural selection in a population of Darwin’s finches (Geospizinae) in the Galápagos,” Science 214 (1981), 82-85.
  9. H. Lisle Gibbs and Peter R. Grant, “Oscillating selection on Darwin’s finches,” Nature 327 (1987), 511-513.
  10. Wells, Icons of Evolution, 159-175.
  11. Hugo de Vries, Species and Varieties, Their Origin by Mutation, 2d ed. (Chicago, IL: Open Court Press, 1906), 825-826, https://www.gutenberg.org/files/7234/7234-h/7234-h.htm (accessed August 23, 2020).