Humans still subject to natural and sensual selection (again)

The above headline is nothing new, but something still important to remind people about. (also we say ‘sensual’ instead of ‘sexual selection’ to keep this a family place. Crap, I just said ‘sexual.’) A little over a year ago a popular physicist got in some trouble for saying that humans were impervious to evolution because natural selection was no longer able to act on us smart creatures. Right after the scientist put a big smelly foot in his mouth I explained why this statement was incorrect (at best), and why you should learn biology from biologists rather than theoretical physicists.

I was reminded of this when I came across a study by Alexandre Courtiol and colleagues, out in PNAS yesterday, that examined whether natural and sexual selection were acting on an 18th-19th century Finnish population, based on local church records of births, marriages, etc. Natural selection refers to the differential survival and reproduction of individuals in a population, a disparity that generally arises because individuals may be better- or worse-adapted to their circumstances than others. Sexual (aka sensual) selection is a special type of natural selection, referring to how well individuals are able to acquire mates. Sure enough, Courtiol et al. found such differences between individuals in their Finnish sample. I have only gotten to glance at the paper, so I still need to check how they measured their variables (like fitness or mating success), but the last line of the abstract is what really stuck out at me:

Our results emphasize that the demographic, cultural, and technological changes of the last 10,000 y[ears] did not preclude the potential for natural and sexual selection in our species.

The fat lady in the opera of Human Evolution has yet to sing (this show’s motto would be, “No fat chicks,” if such a statement weren’t sexist and offensive).

ResearchBlogging.orgRead for yourself!
Courtiol, A., Pettay, J., Jokela, M., Rotkirch, A., & Lummaa, V. (2012). Natural and sexual selection in a monogamous historical human population Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1118174109

Variation 2: The female with the flamboyant and massive male

Continuing my investigation into stages of individual development, I’ve stumbled upon a study of the maturation of semi-wild mandrills (Mandrillus sphinx). Mandrills are one of the most visually striking species of Primates (check out this beastly male to the right), and exemplars of the power of Sexual Selection.

Sexual selection is a special subtype of Natural Selection, where the within-species competition here isn’t so much for survival (as in natural selection) but more specifically for reproduction. Sexual selection is believed to be responsible for many differences between the sexes: male primates often (but not always!) have much larger bodies and canine teeth than females, traits that can be beneficial when competing with other males for access to female mates. And/or females may prefer larger-bodied or -canined males for whatever reason. In accord with the power of female preferences, sexual selection is invoked to explain why males of many species are so wildly colored or ornamented.

So mandrills are perhaps the best example of sexual selection in primates. Males’ faces, butts and genitals are brightly colored, spanning the spectrum from blood-red to nearly bioluminescent blue. Conceivably, at some point in mandrills’ evolutionary history most males were drab-colored, but then who comes riding into town on a silver stallion but a mutant male who was more colossal and colorful than the rest, and females were like, “OMG did you see that variegated guy? I want him so bad,” and as a result, this male reproduced more, and the rest of the story writes itself. Coloration may actually communicate information to females about the health or dominance status of the male (e.g. Setchell 2004). I wish I had the time to investigate the physiological bases of how their hair and skin can produce such colors. To revolutionize the tattoo industry.

Mandrills are also remarkable in how much larger males are than females, in terms of canines (Plavcan and van Schaik 1992), molars (Scott et al. 2009) and body size (Wickings and Dixson 1992). And this brings me to my original thought.

The plot to the right tracks growth in body mass (in kilograms) of male and female mandrills (Wickings and Dixson 1992: 132, fig. 1). The male is the top line and the females the bottom one. The arrows indicate timing of sexual maturity. Holy crap, by the time males are sexually mature, they are about 3 times the body mass of females.

The union of the ~25 lb female with the seemingly-paint-splattered, 75 lb male must be a truly terrifying sight.

Things I cited

Plavcan, J., & van Schaik, C. (1992). Intrasexual competition and canine dimorphism in anthropoid primates American Journal of Physical Anthropology, 87 (4), 461-477 DOI: 10.1002/ajpa.1330870407

Scott JE, Schrein CM, & Kelley J (2009). Beyond Gorilla and Pongo: alternative models for evaluating variation and sexual dimorphism in fossil hominoid samples. American journal of physical anthropology, 140 (2), 253-64 PMID: 19358294

Setchell, J. (2005). Do Female Mandrills Prefer Brightly Colored Males? International Journal of Primatology, 26 (4), 715-735 DOI: 10.1007/s10764-005-5305-7

Wickings, E., & Dixson, A. (1992). Development from birth to sexual maturity in a semi-free-ranging colony of mandrills (Mandrillus sphinx) in Gabon Reproduction, 95 (1), 129-138 DOI: 10.1530/jrf.0.0950129

Size DOES matter

Well, at least in mosquito-fish (Gambusia holbrooki). A recent study shows that female mosquito-fish select their mates based on the size of their genitalia. Females tended to ‘associate’ more with males with slightly larger ‘gonopodia’ (I’m no ichthyologist, but I suppose it’s the fish near-equivalent to a mammalian penis). However, this distinction was only made when females had to choose between large males; for small males, gonopodium size had no effect.

What I think is kind of neat about this study is it is an example of sexual selection that actually involves the sexual organ; most other traits in sexual selection studies are things like bird feathers, deer antlers, and the like. I don’t think, however, this female-choice criterion of plays a major role in mammals. But if it does, perhaps this is the true reason why we wear pants?

Kahn AT, Mautz B, and Jennions MD. Females prefer to associate with males with longer intromittent organs in mosquitofish. Biol Lett in press.