Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
Matthew Cobb has kindly called my attention to this piece from the BBC about this soon to be published paper (pdf) in the Journal of Zoology by Graham Mitchell, S.J. van Sittert, and John Skinner on the neck of the giraffe. How giraffes got their long necks is a venerable question in biology, having been discussed by, among others, Lamarck , Wallace, and Darwin. The paper by Mitchell and colleagues deals with an hypothesis proposed in 1996 by Robert E. Simmons and Lue Scheepers: that the long neck of giraffes results from sexual selection by male-male competition (see chap. 6 of WEIT for a general discussion of sexual selection). At first it may seem obvious that long necks are for reaching leaves and shoots way up in the trees, and, indeed, this is the most popular idea. But before dismissing sexual selection, consider the behavior called ‘necking’, displayed by two male giraffes in the following clip.
It is evident that serious, and violent, combat occurs between males, and Simmons and Scheepers record several accounts of serious injury and death resulting from necking. Mitchell and colleagues, nonetheless, conclude that sexual selection is not the cause of long necks:
Support for this theory [the sexual selection hypothesis] would be that males invest more in neck and head growth than do females. We have investigated this hypothesis in 17 male and 21 female giraffes with body masses ranging from juvenile to mature animals, by measuring head mass, neck mass, neck and leg length and the neck length to leg length ratio. We found no significant differences in any of these dimensions between males and females of the same mass, although mature males, whose body mass is significantly (50%) greater than that of mature females, do have significantly heavier (but not longer) necks and heavier heads than mature females. We conclude that morphological differences between males and females are minimal, that differences that do exist can be accounted for by the larger final mass of males and that sexual selection is not the origin of a long neck in giraffes.
Mitchell et al. also dispute the importance of predation on giraffes, which Simmons and Scheepers thought was higher on males as a consequence of the physiological and anatomical costs of such an outsized structure. Many people might think, What could catch and kill a giraffe? The answer: lions. In the following clip, an adult giraffe is killed by a group of lions.
Some human populations in South America, Africa, and Southeast Asia are of extraordinarily small stature: they are called pygmies. All of them inhabit rainforests that are warm and humid. Although they bear a common name, genetic work shows that each group has evolved independently, so it is better to speak of “the pygmy phenotype” (“phenotype” refers to any aspect of an organism that can be observed or measured). A population is said to show the pygmy phenotype when the height of adult males averages about 160 cm (5′ 3″) or smaller. (The smallest pygmy population comprises the Efe hunter-gatherers of the Congo, where adult males and females are 143 and 136 cm tall respectively [4′ 8″ and 4′ 6″]).
A new paper in Trends in Ecology and Evolution summarizes what is known about the distribution, genetics, and evolutionary basis of the pygmy stature. The first thing we learn is that we’re not absolutely sure if the height difference is due to genetic differences between pygmies and populations of “normal” stature. Could it instead be due to differences in nourishment alone? The evidence is against this because pygmy populations show no clinical signs of malnutrition. Still, other nongenetic environmental factors could be responsible. The best way to show that the height difference is based on genes is, of course, to rear pygmy and non-pygmy infants in a common environment and show that the height difference remains; surprisingly, this has not been done (though surely there are some pygmy children brought up in different environments— the authors don’t discuss this).
Why are pygmies so short? If the this phenotype is indeed genetically based, the obvious hypothesis is that natural selection in warm tropical environments causes humans to evolve smaller size. But why? The authors give four hypotheses:
1. There is a scarcity of food in the rainforest, and this selects for smaller individuals who are able to maintain their bodies with fewer calories. The evidence for this hypothesis is mixed: pygmy populations don’t especially suffer a dearth of calories, although this may be due to their recent trading for food with other populations outside the rainforest.
2. Living in a hot, humid environment selects for smaller bodies because smaller individuals have a higher ratio of body surface are to body volume. This allows them to lose, through sweating and heat transfer, relatively more heat than larger individuals. The weakness of this idea is that among pygmy populations there is no correlation between body size and ambient humidity.
3. It is easier to move through dense, tangled forest if you are small. Bending down repeatedly while walking apparently uses quite a bit of energy. There is only anecdotal evidence for this idea, but it may be true.
4. If there is high mortality, then it may pay you to mature and reproduce early because otherwise you could die and leave no genes. The authors note that infant mortality up to age 5 of African rainforest poulations are 27-40%, about twice that of nearby populations that live in other habitats. This idea predicts that among populations, there will be a positive correlation between life expectancy and average adult height. This is indeed observed, providing some support for the idea.
Of course, all or some of these factors could work together. At present, we have some fruitful ideas about why the pygmy phenotype evolved, but nothing definitive yet. It is surprising that of the many differences in appearance between human ethnic groups– differences that involve skin color, hair configuration, facial configuration, height, body configuration, and physiology –the only one whose evolution we understand fairly well is pigmentation (see WEIT for the explanation, based on sun exposure). The rest is mystery. In my book I broach the idea that sexual selection may account for some of these, though it’s hard to explain the short pygmy phenotype this way (why would the target of sexual selection be correlated with humid, forested habitats?).
Note: Another population that was abnormally short were the “hobbits”: Homo floresiensis, a population (described as a species) of humans that lived on the Indonesian island of Flores. Again described in WEIT, this species is based on a single skull from one individual and an arm bone from another. Their apparent height was about 106 cm (3′ 6″), they weighed about 50 pounds, and were the size of a modern 5-year-old child. They lived about 18,000 years ago, when H. sapiens of modern stature already lived throughout the world. Although some think that the single tiny individual was really diseased and not a “normal” individual, it does appear from the arm bone that they really were this small. But they had one feature not present in modern pygmy populations: very small brains, about half the size of modern human brains (the brains of pygmies scale roughly the same as short humans elsewhere). H. floresiensis did not represent the pygmy phenotype, and were more likely an ancient population of a different species of Homo that, isolated on its island, was bypassed by the evolution of other populations into the modern human phenotype.
Here is a figure from Perry and Dominy’s paper showing the distribution of pygmy populations throughout the world (red dots) and some pictures of pygmy individuals:
Figure 1. Association of the human pygmy phenotype with tropical rainforest habitats. (a) Approximate locations of small-bodied hunter-gatherer populations discussed in this article, with mean adult male stature estimates [6], [10], [65], [78] and [79]. The smallest modern human statures (mean adult male height < 155 cm) are always associated with tropical rainforests (red circles). Some hunter-gatherer populations occupying savanna-woodlands (black circles) are also relatively small, such as the Hiwi of the Venezuelan llanos, the Hadza of Tanzania and the !Kung San of Botswana and Namibia. Precipitation data are from the Tropical Rainfall Measuring Mission (Goddard Space Flight Center, National Aeronautics and Space Administration; http://trmm.gsfc.nasa.gov). (b) Yanomamö male, Venezuela (photograph by Raymond Hames, with permission). (c) Efe male, Democratic Republic of Congo (photograph by William Wheeler, with permission from the National Anthropological Archives, Smithsonian Institution). (d) Batek male, Malaysia, with white-handed gibbon (Hylobates lar) hunted by blowdart (photograph by Kirk Endicott, with permission).
Photo and caption from Perry, G. H., and N. J. Dominy. 2009. Evolution of the human pygmy phenotype. Trends in Ecology & Evolution 24:218-225.
Research hot off the lab bench: My friend Graham Coop at The University of California at Davis sends me this note that they may be zeroing in on the genes responsible for the pygmy phenotype:
We’ve just published a large scan for selection across various human populations: two of the of the top hundred SNPs [single nucleotide polymorphisms] whose allele frequency most differentiate Bantu populations from pygmy populations fall next to genes in the insulin growth factor signaling system (discussed on the bottom of page 6). Obviously these will require much more followup, but these seem like reasonable candidates for genomic regions habouring variation affecting height in pygmies.
There’s a nice article by Nicholas Wade in today’s NY Times about the evolution of horns, antlers, and other organs of weaponry and display. Much of the article explicates a recent review article, The Evolution of Animal Weapons, by Douglas Emlen of the University of Montana. Wade writes
Dr. Emlen noticed a tendency for weapons to start out small, like mere bumps of bone, and then to evolve to more ornate form. The small weapons are actually quite destructive since their only role is to attack other males. But the more baroque weapons, even though they look more fearsome, seem to cause lesser loss of life.
There’s a truly fabulous slide show of beetle weaponry by Emlen and his collaborators accompanying the online article, and a nice graphic on the diversity of weaponry in the print and online versions. A taste of the slide show:You really must see the whole thing.
Last Thursday’s Times Higher Education Supplement (the UK one) has a series of short pieces on Darwin and his legacy. Perhaps the most interesting is by Tim Birkhead, the noted evolutionist who works on sperm competition in birds and has written a number of academic and popular books on the topic. Birkhead claims that Darwin was reluctant to publish work on sexual selection—and missed entirely the topics of sperm competition between males as well as the relative promiscuity of females—because of the prudishness of his Victorian milieu as well as of his daughter Henrietta. I’m not a historian of science and so can’t evaluate these claims, but Birkhead’s piece is provocative and well worth reading, as are the other pieces in this supplement.
You really must see