Sunday, 23 September 2012

Rapid Evolution Makes Creationists Crabby

Blue mussel (Mytlius edulis)
Here's a nice example of evolution occurring not over the millions of years that we normally expect with a slow accumulation of small changes over time but in as short as fifteen years. And it not quite as straightforward as we might expect either.

It involves our old friend, the evolutionary arms race, this time involving the American East Coast population of the blue mussel (Mytlius edulis) and an invasive species of aggressive Asian crab (Hemigrapsus sanguineus).

In common with other bivalve molluscs, mussels close their shells at a hint of danger and so predators have had to evolve a strategy for opening them. One of these predators is the green crab (Carcinus maenas) which has adopted the strategy of simply breaking the shell of the blue mussel with its pincers.

In response to that strategy, the blue mussel has evolved a neat trick. The obvious way would be to evolve a thicker shell but thicker shells are a drain on the mussels' resources and are not always needed because green crab are not always in sufficient numbers to pose a serious enough threat to invest in a permanently thick shell, so the blue mussel has evolved the ability to detect a chemical in the water given off by green crabs, and only thickens it's shell when the population reaches a critical threshold number.

Neat, eh?

Asian or Japanese shore crab, Hemigrapsus sanguineus
But, in 1988 the Asian or Japanese shore crab was reported in New Jersey into the Atlantic and by 1991 had started to invade the southern parts of the US East Coast where it found a population of blue mussels which had no effective defence against them because they were unable to detect their presence. The Asian crab quickly spread north as far as southern Maine, but no further.

However, fifteen years later, in 2006, two researchers, Aaron S. Freeman and James E. Byers, made an amazing discovery, published in Science. They found that blue mussels had already evolved the ability to detect a different chemical produced by the Asian crab and were now using the same defence strategy as they did for the native green crab.

By contrast, the 'control group' of blue mussels from the northern shores of Maine showed no such ability.


Invasive species may precipitate evolutionary change in invaded communities. In southern New England (USA) the invasive Asian shore crab, Hemigrapsus sanguineus, preys on mussels (Mytlius edulis), but the crab has not yet invaded northern New England. We show that southern New England mussels express inducible shell thickening when exposed to waterborne cues from Hemigrapsus, whereas naïve northern mussel populations do not respond. Yet, both populations thicken their shells in response to a long-established crab, Carcinus maenas. Our findings are consistent with the rapid evolution of an inducible morphological response to Hemigrapsus within 15 years of its introduction.

Now, if you're a creationist and have had the courage to read this far, you've almost certainly been looking for an excuse to reject this examples as an example of evolution, or otherwise pour scorn on the idea. You've probably by now decided your best tactic is to dismiss it as an 'example of microevolution' but not of 'macroevolution' and are going to rely in the creationist mantra, 'macroevolution is impossible'.

So, just suppose whatever genetic change involved in this evolution had inhibited the ability of the carrier to breed with non-carriers and produce fertile offspring. We would now be classifying the evolved blue mussels as a different species which appeared to have replaced the non-evolved species along the coast of New England, with the more 'primitive' form maintaining a toe-hold in northern Maine.

What would have been impossible about that?

In fact, this scenario is not at all unlikely given what we already know of the distribution of Mytlius edulis and the hybridisation and subspecies we know about.

Systematics and distribution

The Mytilus edulis complex

Systematically blue mussels consist of a group of (at least) three closely related taxa of mussels, known as the Mytilus edulis complex. Collectively they occupy both coasts of the North Atlantic (including the Mediterranean) and of the North Pacific in temperate to polar waters, as well as coasts of similar nature in the Southern Hemisphere. The distribution of the component taxa has been recently modified as a result of human activity (invasive species). The taxa can hybridise with each other, if present at the same locality.
  • Mytilus edulis sensu stricto: Native to the North Atlantic.
    • Mytilus edulis platensis ( = Mytilus chilensis), the Chilean mussel: Temperate waters in the Southern Hemisphere.
  • Mytilus galloprovincialis, the Mediterranean mussel: Native in the Mediterranean, the Black Sea and Western Europe. Introduced in the temperate North Pacific, South Africa and elsewhere in the Southern Hemisphere. A distinct lineage native to the Southern Hemisphere also exists.
  • Mytilus trossulus: North Pacific, northern parts of the North Atlantic, Baltic Sea.

Mytilus edulis, strict sense

The Atlantic blue mussel is native on the North American Atlantic coast, but is found intermixed with M. trossulus north of Maine. In Europe It is found from the French Atlantic coast northwards to Novaya Zemlya and Iceland, but not in the Baltic Sea. In France and in the British Isles, it makes hybrid zones with M. galloprovincialis, and also is sometimes intermixed with M. trossulus.

A genetically distinct lineage of M. edulis is present in the Southern Hemisphere, and has been attributed to subspecies Mytilus edulis platensis. This includes the Chilean mussel.

So, which creationist of those who have got this far is going to volunteer to explain how this is not an example of rapid evolution over a space of some fifteen years?

Further reading: Understanding Evolution
Musseling in on evolution

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