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| Growth of the West Indian fighting conch (Strombus pugilis) and morphometrics (height, width and lip thickness) taken in this study. (a–d) Juvenile shells with lips less than 1.8 mm. (e–h) Adult shells with lips thicker than 1.8 mm. (e) Dorsal view showing measurement of outer lip thickness. (f) A typically small mature contemporary shell from Isla Carenero with characteristic scorch markings from cooking. Arrow indicates location of outer lip thickness measurement. (g) One of the largest mature contemporary shells from Isla Popa. (h) A large pre-human mature shell. (a–c,h) are pre-human, whereas (d–g) are contemporary. Scale bar, 2 cm. |
Scientists working for the Smithsonian Tropical Research Institute have discovered a lovely example of environmental change driving evolution, this time the environmental change being the presence of humans who predated on the Caribbean fighting conch, Strombus puglis, as a source of food.
Juveniles of this species live in muddy sediments and only emerge to mate when they reach sexual maturity - at which point they become easy for humans to catch - but only after they have developed a thick outer lip as a protection from predators. This means that having attained sexual maturity can be assessed in ancient shells by measuring the thickness of this lip.
Careful observation and measurement of modern shells, and comparison with those found in shell middens, shows that, over the last 7000 years, and especially over the last 1,500 years, the size at which this conch reaches sexual maturity has become smaller and smaller, coinciding with humans selecting the largest during foraging in the lagoons in which they live. There has been a clear advantage to the genes of this species to mature earlier whilst still relatively small, the larger and later maturing ones having been selected for by humans and so removed from the genepool, while the smaller and earlier maturing ones were left to reproduce.
The result of this is that this conch now contains only 66% of the meat that its recent forebears had and is considerably smaller.
Abstract
Intensive size-selective harvesting can drive evolution of sexual maturity at smaller body size. Conversely, prehistoric, low-intensity subsistence harvesting is not considered an effective agent of size-selective evolution. Uniting archaeological, palaeontological and contemporary material, we show that size at sexual maturity in the edible conch Strombus pugilis declined significantly from pre-human (approx. 7 ka) to prehistoric times (approx. 1 ka) and again to the present day. Size at maturity also fell from early- to late-prehistoric periods, synchronous with an increase in harvesting intensity as other resources became depleted. A consequence of declining size at maturity is that early prehistoric harvesters would have received two-thirds more meat per conch than contemporary harvesters. After exploring the potential effects of selection biases, demographic shifts, environmental change and habitat alteration, these observations collectively implicate prehistoric subsistence harvesting as an agent of size-selective evolution with long-term detrimental consequences. We observe that contemporary populations that are protected from harvesting are slightly larger at maturity, suggesting that halting or even reversing thousands of years of size-selective evolution may be possible.
An almost perfect example of evolution by natural selection resulting in a change of allele frequency and a consequent difference in the phenotype and how this was driven by an environmental change with no change or mutation required in the genome of the species. Simply natural selection acting on existing variability within the species.
Note also that, now the species is protected in certain areas, so removing this environmental pressure, the species is showing signs of evolving back to its former size at maturity again.
Now, if any creationists can explain why this isn't evolution by natural selection, or why removing the later-maturing, larger, specimens and so favouring the earlier-maturing, smaller specimens in the competition for resources wouldn't result in future generations being smaller and maturing more early, or why this wouldn't increase those alleles of genes causing earlier maturity at a smaller size in the species gene pool, then please feel free to do so here.
"Intelligent Design" creationist might also like to explain why their imaginary benevolent designer has been actively working to make it harder for humans to find food.
Note: attacking creationist parodies of evolution won't do. What you need to do is to address the real, scientific definition of evolution - i.e., a change in allele frequency over time. We already know the infantile parodies of evolution creationists normally attack are false and don't agree with science. That's what they were designed for.
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