Humans drive evolution of conch size

March 18, 2014, Smithsonian Tropical Research Institute
Prehistoric fighting conch Strombus puglis (L) and modern shells of the same species (R) show how the shellfish has decreased in size over time. Credit: Smithsonian Tropical Research Institute/Courtesy of Aaron O'Dea

The first humans to pluck a Caribbean fighting conch from the shallow lagoons of Panama's Bocas del Toro were in for a good meal. Smithsonian scientists found that 7,000 years ago, this common marine shellfish contained 66 percent more meat than its descendants do today. Because of persistent harvesting of the largest conchs, it became advantageous for the animal to mature at a smaller size, resulting in evolutionary change.

Human-driven evolution of wild animals, sometimes referred to as "unnatural selection," has only previously been documented under scenarios of high-intensity harvesting, like industrialized fishing. "These are the first evidence that low-intensity harvesting has been sufficient to drive evolution," said lead author Aaron O'Dea of the Smithsonian Tropical Research Institute. "The reason may be because the conch has been subjected to harvesting for a long period of time." Published March 19 in Proceedings of the Royal Society B, the findings are based on a comparison of mature shell sizes prior to human settlement, from shells excavated from human trash heaps representing various points in the last few thousand years and from modern sites.

As a juvenile, the fighting conch Strombus pugilis lives hidden in the muddy sediments of lagoons. It emerges to compete for mates when it reaches , but only after it has thickened up its outer lip as a protection from predators. By observing the size of shells and the thickness of lips in fossil, archeological and modern conchs the researchers found that size at sexual maturity declined during the past 1,500 years in concert with human harvesting.

The study brought together ecologists, paleontologists and archeologists to expose the effects of long-term subsistence harvesting on an important marine resource. Co-authors include Marian Lynne Shaffer, at the time an undergraduate student of the University of Wisconsin-Green Bay, and archeologist Thomas Wake of UCLA's Cotsen Institute of Archeology.

Smithsonian Tropical Research Institute scientist Aaron O'Dea pilots a boat in Panama's Bocas del Toro in this file photo. Credit: Smithsonian Tropical Research Institute/Sean Mattson

The team suggests that declining yields may not be the only detrimental effects of an to mature at smaller size. The ability to reproduce, the quality of offspring and other vital traits can be damaged by size-selective evolution. Further study is required to learn the extent to which the fitness of S. pugilis has decreased because of long-term size-selective evolution.

"There is a glimmer of hope that the evolutionary trend toward smaller size can be halted or reversed," said O'Dea, drawing attention to the fact that modern sites that are protected from harvesting have the largest conchs. "Marine protected areas not only serve to protect biodiversity, they can also help maintain genetic diversity. This study shows that such genetic diversity is critical to sustain value of marine resources for the millions of humans that rely upon subsistence harvesting around the world."

Explore further: Chronic harvesting threatens tropical tree

More information: Evidence of size-selective evolution in the Fighting Conch from prehistoric subsistence harvesting, rspb.royalsocietypublishing.or … .1098/rspb.2014.0159

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2.3 / 5 (3) Mar 19, 2014
" became advantageous for the animal to mature at a smaller size.."
This implies a conscious decision to change size, which is totally false. The allele for larger size is selectively removed from the population by humans leaving ever more of the allele coding for a smaller size in the population, producing the effect without any decision making process in the organism.
1 / 5 (3) Mar 19, 2014
Ecological variations that include the presence or absence of other organisms of different sizes contribute to ecological adaptations via conserved molecular mechanisms in species from microbes to man.

Genetic predispositions are epigenetically effected. Thus, the question arises from this report: What was the cause of the evolutionary change? Did the evolutionary change "just happen" or were conserved molecular mechanisms involved, such as those that link the epigenetic landscape to the physical landscape of DNA in the organized genomes of species from microbes to man?

"Human-driven evolution" sounds a lot like "Mutation-driven evolution" in the context of industrial melanism and predatory birds that ate different moths. Does eating the largest of the conch populations induce mutations that result in smaller size, or do mutations automagically cause size differences like they automagically caused the color differences in the moths?
3.4 / 5 (5) Mar 19, 2014
"It became advantageous" is something I've read many times but I never took it to imply a conscious decision.
1 / 5 (3) Mar 20, 2014
"It became advantageous" is something I've read many times but I never took it to imply a conscious decision.

Agreed. It's typically a way for evolutionary theorists to simply say they have no idea of what facilitates species diversity, which they attribute to mutations -- because that's what's always been done by other theorists.
5 / 5 (3) Mar 20, 2014
Thus, the question arises from this report: What was the cause of the evolutionary change?

It's rather simple: Those that had a mutation that made them grow smaller were selected for - quite literally (by the simple mechanism of those being too large being selected 'against'... by bein selected by humans for consumption).
The small ones got to pass on their mutation - the others didn't. End of story.

Mutation and selection. How hard is that to understand?

I think you need to hop over to wikipedia and read up on what epigenetics is. From what you write it's pretty clear that you're confusing the term with something else.

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