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

We know that speciation has occurred because many new species have appeared in the history of life. Evolutionary biologist Ernst Mayr wrote, “Darwin called his great work On the Origin of Species, for he was fully conscious of the fact that the change from one species into another was the most fundamental problem of evolution.”¹ According to evolutionary biologist Douglas Futuyma, speciation “is the sine qua non of diversity” required for evolution. Speciation “stands at the border between microevolution — the genetic changes within and among populations — and macroevolution.”²

But How Does Speciation Occur? 

Part of the problem is that the term species is notoriously difficult to define. A definition applicable to plants and animals won’t necessarily work for bacteria, and definitions applicable to living things won’t necessarily work for fossils. As of 2004, several dozen definitions were in use among biologists and paleontologists.³ The definition most often used by evolutionary biologists is the “biological species concept,” according to which species are groups of interbreeding natural populations that are reproductively isolated from other such groups.⁴

If species are defined this way, then in one sense speciation has been observed in the laboratory. Normally when two different species hybridize, either naturally or artificially, the hybrids are sterile because the maternal and paternal chromosomes are too dissimilar and cannot pair up in cell division. Occasionally, however, the hybrid undergoes chromosome doubling, or polyploidy. With matching sets of chromosomes that can undergo cell division, the hybrid may then be fertile and constitute a new species under the biological species concept. In the first decades of the 20th century, Swedish scientist Arne Müntzing used two plant species to make a hybrid that underwent chromosome doubling to produce hempnettle, a member of the mint family that had already been found in nature.⁵

Speciation by polyploidy is called secondary speciation to distinguish it from primary speciation — the splitting of one species into two. According to Douglas Futuyma, polyploidy “does not confer major new morphological characteristics…[and] does not cause the evolution of new genera” or higher levels in the biological hierarchy.⁶ So although secondary speciation by polyploidy has been observed in flowering plants, it is not the solution to Darwin’s problem. The solution would be primary speciation by variation and selection, which has not been observed.

Darwin and Incipient Species

In 1940, geneticist Richard Goldschmidt argued that “the facts of microevolution do not suffice for an understanding of macroevolution.” He concluded, “Microevolution does not lead beyond the confines of the species, and the typical products of microevolution, the geographic races, are not incipient species.”⁷

Darwin used the term incipient species to refer to a variety of one species he thought was in the process of becoming a new species: “I believe a well-marked variety may be justly called an incipient species.”⁸ But how can we possibly know whether two varieties (or races) are in the process of becoming separate species? Saint Bernards and Chihuahuas are two varieties of the dog species (Canis lupis familiaris) that, for anatomical reasons, do not interbreed naturally. Are they on their way to becoming separate species? The Ainu people of northern Japan and the !Kung of southern Africa are members of the human species (Homo sapiens sapiens). Although people from both groups could undoubtedly interbreed, without modern technology, which affords mass movement of people around the globe, they would be (for all practical purposes) reproductively isolated geographically, linguistically, and culturally. Are they therefore incipient species? Clearly, Darwin’s term incipient species is a theoretical prediction, not evidence.

Origin of a New Species?

We sometime read in the news media that scientists have finally observed the origin of a new species. Such cases, however, are invariably either examples of incipient speciation, or cases in which scientists have inferred from already-existing species how they might have split in the past.⁹ Observational evidence for primary speciation is still missing.

In 1992, evolutionary biologist Keith Stewart Thomson wrote, “A matter of unfinished business for biologists is the identification of evolution’s smoking gun,” and “the smoking gun of evolution is speciation, not local adaptation and differentiation of populations.” Before Darwin, Thomson explained, the consensus was that species can vary only within certain limits; indeed, centuries of artificial selection had seemingly demonstrated such limits experimentally. “Darwin had to show that the limits could be broken,” wrote Thomson, and “so do we.”¹⁰

In 1996, biologists Scott Gilbert, John Opitz, and Rudolf Raff wrote: 

Genetics might be adequate for explaining microevolution, but microevolutionary changes in gene frequency were not seen as able to turn a reptile into a mammal or to convert a fish into an amphibian. Microevolution looks at adaptations that concern the survival of the fittest, not the arrival of the fittest. 

They concluded, “The origin of species — Darwin’s problem — remains unsolved.”¹¹

Evidence of Primary Speciation

English bacteriologist Alan Linton went looking for evidence of primary speciation and concluded in 2001:

None exists in the literature claiming that one species has been shown to evolve into another. Bacteria, the simplest form of independent life, are ideal for this kind of study, with generation times of twenty to thirty minutes, and populations achieved after eighteen hours. But throughout 150 years of the science of bacteriology, there is no evidence that one species of bacteria has changed into another…Since there is no evidence for species changes between the simplest forms of unicellular life, it is not surprising that there is no evidence for evolution from prokaryotic [e.g., bacterial] to eukaryotic [e.g., plant and animal] cells, let alone throughout the whole array of higher multicellular organisms.¹²

In 2002, evolutionary biologists Lynn Margulis and Dorion Sagan wrote, “Speciation, whether in the remote Galápagos, in the laboratory cages of the drosophilosophers [those who study fruit flies], or in the crowded sediments of the paleontologists, still has never been directly traced.”¹³ So evolution’s smoking gun is still missing.

Next, the concluding entry in the series, “Darwin’s One Wrong Argument.”

Notes

1. Ernst Mayr, The Growth of Biological Thought (Cambridge, MA: Harvard University Press, 1982), 403.
2. Douglas J. Futuyma, Evolution (Sunderland, MA: Sinauer Associates, 2005), 401.
3. Jerry A. Coyne and H. Allen Orr, Speciation (Sunderland, MA: Sinauer Associates, 2004), 25.
4. Mayr, The Growth of Biological Thought, 273; Coyne and Orr, Speciation, 26-35.
5. Arne Müntzing, “Cytogenetic Investigations on Synthetic Galeopsis tetrahit,” Hereditas 16 (1932), 105-154.
6. Futuyma, Evolution, 398.
7. Richard Goldschmidt, The Material Basis of Evolution (New Haven, CT: Yale University Press, 1940), 8, 396.
8. Charles Darwin, Origin of Species, 1st ed., 52, http://darwin-online.org.uk/content/frameset?pageseq=67&itemID=F373&viewtype=side (accessed August 23, 2020).
9. Jonathan Wells, The Politically Incorrect Guide to Darwinism and Intelligent Design (Washington, DC: Regnery, 2006), 52-55.
10. Keith Stewart Thomson, “Natural Selection and Evolution’s Smoking Gun,” American Scientist 85 (1997), 516-518.
11. Scott F. Gilbert, John M. Opitz, and Rudolf A. Raff, “Resynthesizing Evolutionary and Developmental Biology,” Developmental Biology 173 (1996), 357-372.
12. Alan H. Linton, “Scant Search for the Maker,” The Times Higher Education Supplement (April 20, 2001), Book Section, 29.
13. Lynn Margulis and Dorion Sagan, Acquiring Genomes: A Theory of the Origins of Species (New York: Basic Books, 2002), 32.