# Evolution question: what doesn't \_everything\_ have \? **URL:** \ **Category:** Factual Questions\ **Created:** [October 2, 2005, 12:27am UTC](https://boards.straightdope.com/t/evolution-question-what-doesnt-_everything_-have-whatever/324397 "2005-10-02T00:27:54Z")\ **Posts on this page:** 2\ **Page:** 2
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [October 2, 2005, 9:53pm UTC](https://boards.straightdope.com/t/evolution-question-what-doesnt-_everything_-have-whatever/324397/21 "2005-10-02T21:53:24Z")
> [@ianzin](#): > > (b) in any given species we can study, we notice traits which _seem_ to have helped the species to survive _but_ this is not a perfect and failsafe correlation. It is not a perfect correlation because we have no control group. We cannot assess the relative survivability of the exact same species plus or minus one specific, named trait. If we could, we might find some surprising results. We might find that while we imagine trait X confers a survival advantage, the same species _lacking_ trait X does even better. A control group of sorts is found in island faunas. For example, birds on predator-free islands often lose the power of flight while closely related species on the mainland retain it. Similarly insects and plants may lose traits that protect them from predators or herbivores in places where the latter are absent. Of course not all the other features of the environment are exactly equivalent; but this still indicates that certain features appear to survival benefit in some circumstances but lose it in others. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [October 2, 2005, 11:16pm UTC](https://boards.straightdope.com/t/evolution-question-what-doesnt-_everything_-have-whatever/324397/22 "2005-10-02T23:16:04Z")
> [@Der Trihs](#): > > I didn’t say they would destroy themselves, unless they are on an island. Sorry, that how I interpreted it. I f you simply meant that they will eat and breed themsleves into a famine form which they will rapidly recover and settle down to a stable relationship, that’s true. But in that case I don’t; know why you have to postulate a sci-fi monster. That’s true of all predators, even fairly incompetent ones like rats. The only reason rats exist as they are now is because they or their ancestors have reached equilibrium with the environment. When rats have reached isolate islands they behaved exactly like your sci-fi monster. This behaviour has nothing to do with the predator being less efficient than it can be evolutionarily. Island rats don’t become less efficient predators than mainland rats. In fact often exactly the opposite is true. > [@](#): > > They will never prosper the way a more moderate species will; compare the common cold or even AIDS to Ebola. Ebola never spreads for long, because it kills so fast; it’s the disease version of that sci-fi monster I imagined. It tends to kill an entire village, then it stops because it destroyed it’s own resources. But it doesn’t. Ebola still exists. This simply the normal behaviour of a predator in a naïve environment. In this case the naive environment is the human body, but you will see the same thing when you give a wolf pack access to a pen full of sheep. Nonetheless wolves and the ebola virus still continue to exist despite occasionally slaughtering naïve prey. This isn’t a case of species evolving to be moderate, it’s case of a species settling into equilibrium. That equilibrium might be reached when all the sheep are killed and the species is forced to return to other food sources, or it might be reached when the prey species adapts. Either way the predator won’t become more moderate and less efficient at obtaining prey, it can’t do that. Every individual predator has to garner as much prey as it needs to reproduce maximally. They can’t afford to save for the future because their closest relatives won’t be saving and will simply use the prey instead. It’s the tragedy of the commons on evolutionary terms. [Previous page](https://boards.straightdope.com/t/evolution-question-what-doesnt-_everything_-have-whatever/324397.md?page=1)