From PNAS:

According to three Cambridge scientists, pygmy humans may owe their small size to a rush to reproduce.

The new study by Andrea Migliano, Lucio Vinicius and Marta Lahr from the Leverhulme Centre for Human Evolutionary Studies, University of Cambridge, argues that the standard explanations for the evolution of human pygmy races are completely wrong. Currently, theories about the evolution of pygmyism assume that small body size is an adaptation to special challenges, though there is still controversy about which particular challenge the change in stature was meant to overcome. Suggestions have included thermoregulation (smaller bodies lose heat quicker), locomotion (they can navigate the dense forest floor more easily), or resistance to starvation (they require less food to maintain). The Cambridge study is different in that it considers pygmy humans’ small size to be a secondary effect.

The team has taken measurements of the stature, growth, and fitness of two Philipine pygmy groups and combined it with nearly a century of other anthropological data, and concluded that their body size is the result of a trade off between increasing fertility and later maturity. Basically, growing bigger makes you more fertile but takes longer. Migliano et al argue that the historic circumstances of pygmy tribes included an unusually high rate for young-adult and adult mortality. In the long run, pygmyism did not evolve because it was positively selected for, but as a consequence of the benefit to be had from being able to reproduce much sooner, and hence maturing much earlier.

I don’t buy it, unless they also explain gigantism with the same theory.

Pygmyism isn’t restricted to humans. It occurs in many animals, and is typically associated to a high degree of endemism (genetic isolation). Island fauna’s are an example of the typical setting in which pygmyism occurs; plus related gigantism. You will have dwarf hippo’s or elephants in such a fauna along with giant rats and owls, etc.

Unless their theory also explicitly holds for non-human pygmyism and explains the related gigantism as well, I have my doubts.

I’m not sure it works that way. If increased YA/A mortality does drive the population towards a type that matures early (and thus small), why would you need it to explain gigantism too? It’d be like expecting an evolutionary reason for a giraffe’s long neck to explain the shape of a hippo.

In fact the alternative explanations of insular dwarfism/gigantism are different for each anyway.

National Geographic has an interview with the lead author.

As does the Daily Telegraph.

Am I the only one that is snickering over the link name?

P NAS?  Really??

It’d be like expecting an evolutionary reason for a giraffe’s long neck to explain the shape of a hippo.

It’s the other way around.   Giraffes developed long necks to avoid the bad breath of hippo’s…..

edit - oops, something went wrong here: I seem to have edited David’s post instead of replying with a quote in some strange way…..

They are tied together by the theory that an allele that confers a significantly greater fitness benefit to a carrier that competing alleles for that locus in the population should increase in frequency at the expense of the others.

I suspect insular dwarfism and insular gigantism are frequently found together because they require isolated populations to arise. There is a potential for an organism to go either way, depending on which course benefits it the most. Gigantism, for example, is commonly found in communities where there is low predation, i.e. no natural enemies (the dodo being the textbook example), but that’s by no means the only reason. Australian tiger snakes occur in both giant and dwarf forms on different islands around Tasmania and SE Australia, but this is due more to adaption to local prey size than anything else (Evolution, v59.1, Jan 2005). [Link]

While a single explanation of everything would be nice, I don’t think it’s practical to expect there to be just one mechanism at work in all instances of gigantism or pygmyism in isolated communities. When I was still doing biology, the talk was still of the ‘island rule’, where small animals get bigger and big animals get smaller (Wikipedia tells me that this idea was first put forward by J. B. Foster, I thought it was E. O. Wilson’s), but this is an observation not a causal mechanism. We might equally expect big animals to get smaller, and small animals to get bigger because whatever selective pressure that originally drove them to their ‘mainland’ size is lessened (an “island life is easy” hypothesis) or because selective pressure against their original size was increased (an “island life is tough” hypothesis). Both these causes, sometimes together, have been advanced to explain instances of insular gigantism/dwarfism. A classic bit of evolutionary ‘just-so’ storytelling.

It’s interesting that, at least in carnivores, the island rule disappears when put under quantitative scrutiny. Meiri, Dayan and Simberloff, writing in The American Naturalist, v163, March 2004, concluded “Only little support for the island rule is found when individual populations rather than species are considered. Our data are at odds with those advanced in support of theories of optimal body size. Carnivore size is subjected to a host of selective pressures that do not vary uniformly from place to place. Mass alone cannot account for the patterns in body size of insular carnivores.” [Link]

[Edit: Found link for Evo paper.)

LaMa - 12 December 2007 12:47 PM

It’s the other way around.   Giraffes developed long necks to avoid the bad breath of hippo’s…..

Ah, but that doesn’t explain the Hippo’s shape, does it?

David B. - 13 December 2007 05:13 AM

LaMa - 12 December 2007 12:47 PM

It’s the other way around.   Giraffes developed long necks to avoid the bad breath of hippo’s…..

Ah, but that doesn’t explain the Hippo’s shape, does it?

O yes it does. Hippo’s developed a more sturdy muscular tough-skinned shape, in order to be able to build up higher pressure in the lungs and release it under higher pressure…. The wide mouths favour a large microflora and fauna that give the breath the smell. Have you ever seen those throats?!? 

They are the burping-machines from Hell….