In Evolution, Nice Spiders Finish Last

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To an arachnophobe they may seem universally creepy, but spiders can actually be nice. One strange species of spider, Anelosimus studiosus, consists of individuals of two distinct personalities: docile and aggressive. And new research finds that, in this species at least, nice guys finish last.


A. studiosus is found in both North and South America, where it builds big communal webs housing approximately 40 female spiders. Other spider species are similarly social, but A. studiosus is the only one known to have two distinct personality types—one nice and one not so nice. 

Docile spiders tend to stick closer to home when starting new colonies, and they rarely fight predators or hunt for prey. Aggressive-type spiders, on the other hand, are always looking for a fight. And when it comes time to leave the web, they venture out much further than the docile spiders. Each spider has one personality type or the other, and they pass that personality type on to their offspring. With mixed parents, baby A. studiosus spiders will still end up with one disposition or the other, not somewhere in between.

The question for biologists, then, is why would two such distinct personalities evolve and persist? To answer that, behavioral ecologist Jonathan Pruitt collected some wild female spiders in Tennessee and brought them into his lab. Here, Pruitt did some spider matchmaking. He divvied up the spiders into pairs—either two aggressive spiders, two docile spiders, or one of each.

Written By: Breanna Draxler
continue to source article at blogs.discovermagazine.com

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  1. The conclusion of the article is:

    …When predators were around, lineages founded by docile spiders…were eight times more likely to go extinct. These spiders spent their time reproducing instead of defending their webs, and so they were eaten by invaders or their prey was stolen from their webs.

    In the end, colonies started by docile pairs had entirely died out, and only a quarter of those started by mixed pairs had survived. However three-quarters of the all-aggressive colonies were still alive.

    In those colonies where invaders were removed, the docile spiders did just fine. Thus the presence of competitors is essentially driving the docile spiders into…an evolutionary dead end): make babies or protect the nest, but not both. Aggressive spiders, on the other hand, seem able to manage both demands at once. This ability of one species to drive another (?) into dead-end strategies has long been hypothesized, but never demonstrated in the wild before now.

    Is this confused? The predators could be endangering the docile subpopulation but not the species. Unless there’s something carried by the dociles that ends up in the aggressives, say through the mixed matings, that helps to keep the aggressives going; say they might come to abandon nesting/protection completely<

    So which is more of an advantage in the end: the docile spider’s ability to reproduce or the aggressive spider’s ability to survive?…Pruitt says that either extreme can lead to extinction, so a mix is probably the best place to be.

    Huh? I don’t understand. It seemed like there was no advantage to being docile except in the case where no predators were present which is an artificial condition. They could get by if the pressure from predation was very low. If the experimental colonies were isolated, then lack of mixing would produce homogeneous population, put that’s artificial. So, is that why under wild conditions the docile characteristic is retained? Or is the extinction of the characteristic proceeding at an extremely low rate?<

    Under what real life situation is being aggressive disadvantageous?

  2. This article’s ending has a very confused understanding of evolution. Evolution does not achieve a trait ratio in the population that minimises the extinction probability; it achieves a trait ratio in the population that is stable to the effects of natural selection. They are liable to be in completely different places. I wonder whether the original research contained any comments as silly as the ending to this article.

    This experiment doesn’t prove as much as the article thinks, again because of a misunderstanding of evolution. The fact that the likelihood of descended-from-a-pair populations quickly going extinct correlates with the phenotypes of the pair in question doesn’t answer the question of what natural selection does within that population to the trait ratio as time passes. A real population doesn’t pass through a 2-spider bottleneck, and its trait ratio is subject to natural selection, random drift and occasional mutations. Theoretically, if a species is split into many very small populations (so-called demes), as time passes the surviving demes would mostly be those with an initially highly aggressive ancestry. But that says nothing regarding whether docility would decline over time, either in demes or the species as a whole.

  3. From what I understand of evolution, in nature it finds its own equilibria, whereas the conditions here were manipulated.

    I think I may lack sufficient understanding of natural selection to comment!

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