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February 18, 2005

Evolution Inaction

Something has been niggling at me since the Redback post, which I realize was just about the most popular thing I've written.

As the Knick-Knack salesman oh-so-delicately put it..."What's with all the goddam nature?"

Or words to that effect, at least. Can't help it. As I said before, week-to-week different things get caught in the mental filter. Bad things happen when I try to adjust it, so no I longer do that. Those who find feel the goddam nature does not resonate well should bide their time. I'll return to a diet of ill-informed political commentary and tit links soon enough. As the Good Book almost says, "Man doth not blog on bugs alone, but by every word that proceedeth out of the mouth of the Lord."

However, none of the above is part of the niggle. This passage, from Warrick Angus of the Taronga Zoo, is.

Interestingly, I have found different colour morphs of redback spiders living within a meter of each other in the same habitat. This suggests that perhaps the variation is genetic rather than environmental.

Back in the day, when I was the summer camp Nature Man, one of things we did was to find one of the common garden spiders that inhabited the camp and restrict it, as best we could, to a diet of katydids, the bright green relatives of grasshoppers and crickets. After a week or so, the web silk produced by the spider took on a greenish tinge.

It was pretty quick change, as these things go, but it was an environmental one, rather than a genetic one. Genetic changes are produced in a species as the result of natural selection, and take quite a bit longer. One of the most famous examples of this is the English Pepper Moth, which took on a darker coloration over time as soot produced by British factories began to discolor the trunks of the trees upon which the moth dwelled. The contrast between the dark bark and the lighter moths allowed it to be more easily spotted by the birds that preyed upon them, making it less likely on the whole that the light-colored moths would survive long enough to breed. Darker moths, on the other hand, were more likely to blend in with the new environment, and thus more likely to pass on the genes that made them that color. There's been some controversy over that example of natural selection, but it is still accepted as valid by those with hands on experience with the moth.

So, with the example of the Peppered Moth in mind, what selection pressures could exist in the Redback's environment such that they would produce a less distinctly patterned morph of the species?

To put it bluntly, what eats Redbacks?

Now, not only must this theoretical species eat Redbacks, it must hunt them by sight, else there would be no selection pressure driving the observed color change. Furthermore, has there been a change in the Redback's environment--like the one observed in the Peppered Moth's environment--that would lead to differential predation rates between the black and brown color morphs of the Redback?

Answering the first question is easy. Yes, there is, or rather are, a number of potential species that both hunt and eat Redback spiders, though they don't do so at the same time--The Hunting Wasps, a number of which are found in Australia. Some hunt by smell, still others by sight.

Adult Spider Wasps typically live off nectar, when they live off anything at all. The females of the species occupy themselves by constructing nests, either by hollowing out holes in vegetation, or by laboriously building them out of mud. Once the nest is constructed, they go out in search of spiders with which to fill them, for spiders are what the larvae of the Spider Wasps' live upon in their days of carefree youth.

But not just any spider. It depends on the species of wasp. Some even hunt Tarantulas. Once she locates a suitable spider, the female wasp stings it in a very specific location, so that the spider is alive, but paralyzed. She then drags it back to the nest, lays an egg on the spider, and departs to find another. When the egg hatches, the wasp larva burrows into its still-breathing host, and eats it from the inside out.

It's very much like Alien, yes.

I don't know of a Hunting Wasp species that preys on Redbacks specifically, but mud-dauber wasps prey on the Redback's American cousin, the Black Widow, so there's no reason to think an ecological equivalent does not exist in Australia.

Mind you, it does not have to be a hunting wasp that is pressuring the Redbacks.
Daddy Long Legs and White-tailed spiders also hunt them.

Which is all well and good, but is there an ecological condition in New South Wales--the area where Warrick is finding his brown Redbacks, that put the black redbacks at a disadvantage when it comes to being hunted?

The answer to which is....I don't know. It pains me to say it, but the number of possible conditions is some great that one would really need to be in country in order to formulate a hypothesis.

I don't suppose anyone would like to fly myself, SW, Ngnat and Scotty M Down Under for a month of fieldwork?

I thought as much. Suppose I'll have to wait until Warrick write his thesis.

Update: Supposedly there is a video of a tarantula hawk stalking its prey here, but I am unable to make it work.

Posted by Bigwig at February 18, 2005 02:00 PM | TrackBack
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