In a paper published in Evolution in 1980, John Endler provided experimental evidence to show that guppy color patterns represented a “shifting balance” between the effects of sexual and natural selection. Thirty-six years after the paper was published, I spoke to John Endler about his motivation for doing this work and what we have learnt about this topic since the paper was published.
(Questions send by email on 28 July 2016; initial responses sent on 4 August 2016 and additional responses sent on 2 September 2016)
Citation: Endler, J. A. (1980). Natural selection on color patterns in Poecilia reticulata. Evolution 34: 76-91.
Hari Sridhar: How did you get interested in guppies and the topic of animal colouration?
John Endler: Pure curiosity. According to Caryl Haskins, the natural populations showed clines (geographic variation in colour patterns), and I thought it would be a perfect system to test my theoretical predictions, derived from my just-finished PhD thesis. I then went on to looking at the balance between sexual selection and crypsis (difficult for predators to see) because there were predation intensity gradients going downstream, just like Haskins described.
HS: Today, 36 years after it was published, would you say that the main conclusions of this paper still hold true? If you were to redo this study today would you do anything differently?
JE: Yes, the main conclusions still hold true, and that’s why it keeps being cited. And if I were to redo it today I can’t think of anything that I would change. Of course, nowadays, we can add lots of interesting genome methods.
HS: In the paper you say “The next step will be to measure the differential survival of color pattern classes to enquire whether or not the color patterns which are predicted to be most adaptive on the basis of background matching actually survive better than those which are mismatches to the background.” Has this happened?
JE: Yes, I’ve got lots of data that I’m writing up now. David Reznick (former PhD student) has more on a big collaboration with me and others on some wild populations.
HS: You say “There are three possible and not exclusive reasons [for why there are not more species with the same degree of polymorphism as guppies]: habitat, species recognition, and automimicry.” Today, do we know more about the relative importance of these factors in causing polymorphism?
JE: We now know a lot more about each, but not their relative importance! This is probably because most people are going molecular and neglecting the ecological reasons behind polymorphisms.
HS: What would you say to a student about to read this paper today? What should he or she take away from it?
JE: There are many more papers on the subject since then, all worth reading. Particularly good authors are Sami Merilaita, Johanna Mappes, Nicola Marples, Innes Cuthill and Martin Stevens. These people did, and are still doing, some lovely tests of many of my early ideas, and going considerably beyond that.
HS: There is one intriguing sentence in the first paragraph of your paper: “Until we know more about how and why natural selection occurs, attempts to measure it are quixotic, and discussions of its importance are theandric.” Can you tell us a little more about this sentence and why you decided to frame the issue in this way?
JE: At the time, almost all discussions and studies of natural selection just estimated it over parts of life cycles (incomplete estimates) with no understanding of why it occurred (the Peppered Moth was a notable exception).
HS: Did this paper have a smooth ride through peer-review? Was Evolution the first place you submitted this paper to? How different was the final published version from the first submitted draft?
JE: Yes, it went through with only minor comments. Evolution was the first place I submitted it to. But it was a genuinely pioneering paper.
HS: When was the last time you visited the field sites where you did this study? Have these sites changed a lot since the time you worked there for this study?
JE: In 1991, although my former Ph.D. student, David Reznick, still visits. Yes, most of the study sites have changed enormously. In fact, what I did then couldn’t be done now because many of the low predation sites, most of the medium predation sites, and all of the high predation sites have been ruined by logging, silting, or chemical fishing. Current studies (by Reznick’s group) are at low predation sites as far as possible from human disturbance.
HS: Would you count this paper as one of your favourites among all the papers you have written? If yes, why?
JE: Yes, it was pioneering and produced good clear results of general interest. Also, it was the first formal study on the balance between sexual selection and predation jointly affecting sexually selected colour patterns and varying geographically with predation intensity.