No, you will not see my pseudonymous face or hear my pseudonymous voice on Pervert Justice today. However, my knowledge of biology, greatly enhanced by reading PZ Myers and articles linked by PZ Myers over the last 8ish years, led me to geek-rage on a paragraph in a story that would not have caused any negative reaction in the Crip Dyke of 10 years ago.
The paragraph in question comes from an article on the pop-sci site New Atlas which describes recent evolutionary changes in insular populations of geckos:
Before the dam was built, the geckos in the area had lived mostly off termites, with larger lizard species eating the bigger bugs and leaving the smaller ones to G. amarali. But it turns out that flooding the valley had wiped out those larger lizards, and with less inter-species competition for food, G. amarali adapted to fill the niche they left behind. The geckos grew larger mouths and heads to help them chow down on the newfound bounty of bigger termites.
Well, no. There was no gecko who grew a bigger mouth just to chomp down bigger prey. Ironically, the next paragraph actually does get it right, but too late to prevent my ire at the use of language that is (deliberately?) misunderstood by so many creationists:
It’s a great “Petri dish” example of natural selection at work. Essentially, those G[ymnodactylus] amarali with bigger heads had access to more food, leading to them being more successful at survival and reproduction. Over time, the big-head genes were passed down to later generations in higher numbers, until it became a common characteristic of the island-dwelling geckos.
There was no single gecko that “grew a bigger head”. All the geckos grew exactly the heads that they were going to grow, given sufficient nutrition for healthy growth. What changed was how commonly child-geckos had the big-head genes as a percentage of the entire gecko population on the islands in question. Put simply, big-head geckos had more available food, and thus had more progeny. This is likely down (at least in large part) to two effects, the first is fewer big-head geckos dying for lack of prey before the end of their fertile years/months. The second is that biological cycles that are necessary precursors to reproduction are sometimes delayed or do not occur at all if an organism is not taking in enough energy (or doesn’t have enough stored fats/nutrition). If you have fewer food resources, you’re simply more likely to have that occur.
I don’t know anything about geckos in particular, of course. IANAB. But mechanisms that generate effects delaying or skipping reproductively necessary cycles are present in organisms as distantly related as cephalopods and mammals, so I have no trouble believing that they are present in fellow tetrapods and likely played a role in this change in prevalence of big heads in Brazilian geckos.
As a last note, it’s interesting to see how population separation occurred in these animals. Unlike other separations that might occur when a river changes course, sea levels rise, local ground subsides allowing water in, or upon other natural changes, these separations were a direct result of human activity. The Serra da Mesa Dam in central Brazil stops up the flow of the Rio Tocantins, thereby creating the largest artificial lake in at least Brazil, and I think (though I may be wrong) in South America. Like many large dams, it is located in hilly terrain so as to use the naturally high contours of the surrounding geography to do most of the work containing the waters of the reservoir. However, in the US many dams are built near the edge of a plateau, with few islands resulting. In the central area of Brazil where Serra da Mesa is located, however, the topology is more complex, and filling the lake volume behind the dam resulted in a number of hills becoming major islands sufficient to support a small, independent ecosystem. Apparently the (larger) lizards that formerly ate the larger insect prey on those hills had a lifestyle that required regular movement between areas and became locally extinct after the hills were isolated by new water levels.
The whole situation is quite interesting for what it reveals about human effects on environments, ecosystems, food webs, populations, and, ultimately, individual animals. Humans in Brazil didn’t set out to help some geckos have more children than others. They didn’t set out to increase the head-size-to-body-size ratio of local gecko populations. Yet the evidence is clear that this effect on the frequency of bigger heads in gecko populations is directly attributable to the creation of the Serra da Mesa reservoir.