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On the “Sisyphean Evolution of Darwin’s Finches”

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

Author’s note: Are Darwin’s finches “a particularly compelling example of speciation” as well as “evolution in action”? In a series of posts, I offer some notes on the question of whether macroevolution is happening on the Galápagos Islands. Please find the full series here.

The evolutionary biologists Bailey McKay (Chapman Fellow at the American Museum of Natural History) and Robert Zink (Bell Museum at the University of Minnesota) have opposed the general extrapolation to macroevolution. See their article “Sisyphean evolution of Darwin’s finches” (Biological Reviews 90: 689-698; 2014), for which they were awarded the Katma Award. About the award ceremony we read: 

They [McKay and Zink] present a detailed morphological analysis to complement previous genetic analyses of the six putative species of ground finch in the genus Geospiza that form the Darwin’s Finch complex, and report that there is insufficient genetic and morphological divergence among populations to support species-level taxonomic ranks for these finch populations. Instead, in opposition to deep-rooted conventional thinking by evolutionary biologists, McKay and Zink propose that populations of Darwin’s finches are “transient morph” that have diverged in bill size and body size under strong selection for ability to use local seed resources, but that shifting adaptive landscapes and gene flow among islands constantly erode morphological and genetic differences among populations and thwart speciationMcKay and Zink call this process of formation and dissolution of locally adapted populations “Sisyphean evolution” after the Greek king Sisyphus who was condemned to roll a boulder toward the top of a hill only to have it inevitably slide back to where it began. McKay and Zink make a strong case that there is only one species of ground finch, and that rather than unveiling the process of speciation, the Darwin’s Finch case study shows that local adaptation and morphological divergence under the influence of natural selection are not sufficient to initiate speciation.1 [Emphasis added.]

The authors themselves note in their contribution (2014, p. 689): 

We argue that instead of providing an icon of insular speciation and adaptive radiation, which is featured in nearly every textbook on evolutionary biology, Darwin’s ground finch represents a potentially more interesting phenomenon, one of transient morphs trapped in an unpredictable cycle of Sisyphean evolution. Instead of revealing details of the origin of species, the mechanisms underlying the transient occurrence of ecomorphs provide one of the best illustrations of the antagonistic effects of natural selection2 and introgression.

Behe Addresses the Genetics

As for genetics, see Michael J. Behe in 2019: 

Standing variation consists of the mutant genes that are already present in a population and can be called upon by natural selection to help a species adapt to changed environmental circumstances, obviating the need for a new mutation. For example, the most highly selected mutant gene associated with thick-versus thin-beaked Galápagos finches did not first arise when Peter and Rosemary Grant were studying the finches in the 1970s. It actually arose about a million years ago and has been present in the group ever since.

…For example, that mutant protein that is most strongly associated with thin- versus thick-beak genes in Darwin’s finches, ALX1has only two changed amino acid residues out of 326 compared to the wild type protein. Both of those are predicted by computer analysis to be damaging to the protein’s function. Yet, apparently no better solution to the task of changing finch beak shape has come along in a million years, even though an enormous number of mutations would be expected to occur in the bird population during that time. 

Why not? Well, consider that an army platoon that takes an unoccupied hill has a much easier task than an opposing force that later wants to displace them. Similarly, a likely big factor in finch evolution is that the quick and dirty mutations have already been established. So, in order to supplant them a new mutation would have to be better right away than the fixed ones. That is, its selection coefficient compared to mutation-free ALX1 would have to be greater than the damaging ones. There is no known correlation, however, between the strength of the selection coefficient and whether a mutation is constructive or degradative. Thus, we have no reason to think standing variation would be supplanted.3

For more information, see here:

Variations in ALX1 may play a key role in beak morphology. Two haplotypes4 for this gene are observed within the different species of finches. The B haplotype is almost exclusively found in individuals with a blunt beak while the P haplotype is neatly associated with individuals having a peaked beak. There are 335 fixed differences aggregated in the vicinity of the gene between B and P haplotypes including 2 missense mutations at positions 112 and 211. In the medium ground finch, Geospiza fortis, which exhibits high intraspecies variation in beak shape, both haplotypes have been reported but a significant association with beak shape has also been observed.

The Finch Beak Myth

These scientific data are followed by the myth: “Finch beak morphology observed on the Galápagos Islands was used by Charles Darwin to formulate his theory of evolution.” See Jonathan Wells against this nonsense! From Zombie Science (2017), p. 67: 

Although the Galápagos finches had little impact on Darwin’s thinking [he doesn’t even mention them in The Origin of Species], biologists who studied them a century later called them “Darwin’s finches” and invented the myth that Darwin had correlated differences in the finches’ beaks with different food sources (he hadn’t). According to the myth, Darwin was inspired by the finches to formulate his theory of evolution, though according to historian of science Frank Sulloway “nothing could be further from the truth.” [Emphasis added.]

Next, “A Paradigm of the Limits of Natural Selection?”

Notes

  1. https://academic.oup.com/condor/article/118/1/209/5153217
  2. Strangely enough, the authors themselves try to explain even this example of “Sisyphean evolution” by natural selection. See for relativization of the selection, Lönnig, http://www.weloennig.de/OmnipotentImpotentNaturalSelection.pdf. Anyway, in this example, too, natural selection is by no means as strict as it is repeatedly claimed: “The connection between beak shapes and feeding ecology in birds was much weaker and more complex than we expected and that while there is definitely a relationship there, many species with similarly shaped beaks forage in entirely different ways and on entirely different kinds of food” (emphasis added). http://www.bristol.ac.uk/news/2019/january/adaptation-of-bird-beaks-.html. “The observation that Galapagos finch species possessed different beak shapes to obtain different foods was central to the theory of evolution by natural selection, and it has been assumed that this form-function relationship holds true across all species of bird….However, a new study published in the journal Evolution suggests the beaks of birds are not as adapted to the food types they feed on as it is generally believed.” Behe: “Truth is, birds use their beaks for many functions besides just picking food — essentially, everything. Linking beak shape solely to feeding behavior is simplistic. How could such a myth survive for so long? Answer: by assumption, without empirical rigor.” https://evolutionnews.org/2019/02/a-theory-in-crisis-darwinian-anomalies-accumulate/
  3. https://evolutionnews.org/2019/03/response-to-my-lehigh-colleagues-part-2/.  https://evolutionnews.org/2018/12/2-of-our-top- stories-of-2018-behes-darwin-devolves-topples-foundational-claim-of-evolution/. As for the many functions of the ALX1 gene, see https://en.wikipedia.org/wiki/ALX1 and https://www.genecards.org/cgi-bin/carddisp.pl?gene=ALX1. https://www.uniprot.org/uniprot/P0DMV5. Some points from the original article (abstract): “We find extensive evidence for interspecific gene flow throughout the radiation. Hybridization has given rise to species of mixed ancestry. A 240 kilobase haplotype encompassing the ALX1 gene that encodes a transcription factor affecting craniofacial development is strongly associated with beak shape diversity across Darwin’s finch species as well as within the medium ground finch (Geospiza fortis), a species that has undergone rapid evolution of beak shape in response to environmental changes. The ALX1 haplotype has contributed to diversification of beak shapes among the Darwin’s finches and, thereby, to an expanded utilization of food resources” (emphasis added). https://www.nature.com/articles/nature14181. “…ALX1, has only two changed amino acid residues out of 326 compared to the wild type protein. Both of those are predicted by computer analysis to be damaging to the protein’s function” (emphasis added).
  4. “A haplotype is a group of genes within an organism that was inherited together from a single parent. The word ‘haplotype’ is derived from the word ‘haploid,’ which describes cells with only one set of chromosomes, and from the word ‘genotype,’ which refers to the genetic makeup of an organism. A haplotype can describe a pair of genes inherited together from one parent on one chromosome, or it can describe all of the genes on a chromosome that were inherited together from a single parent. This group of genes was inherited together because of genetic linkage, or the phenomenon by which genes that are close to each other on the same chromosome are often inherited together. In addition, the term ‘haplotype’ can also refer to the inheritance of a cluster of single nucleotide polymorphisms (SNPs), which are variations at single positions in the DNA sequence among individuals.” https://www.nature.com/scitable/definition/haplotype-haplotypes-142/https://en.wikipedia.org/wiki/Haplotype. The simplest but still imperfect definition: “Haplotype: a set of genetic determinants located on a single chromosome.”