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The aggressive behavior could have existed first and the reduction in birth rate could have been a trait later.
In what way does aggression help matters? I don't see that it has any impact on the price of tea in China.
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The reduction in birth rate could have helped a group of ancestors of all these rodents survive geographically isolated from the rest of their species.
Isolated groups have even less space at their disposal. This would make the problem worse because they'd overpopulate more quickly. Invoking evoluck, shorter cycles would wipe the trait out that much faster.

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The trait in question must exist from the very beginning. It cannot arise, compete, and survive in a population that does not possess it. It's very nature is to not compete, and not pass on to future generations.
You act as if the reduction in birth rate is a complete stopping of the birth of all lemmings.
Self-regulation isn't restricted to lemmings. It's been observed in tons of animals and plants. Some are known to slow down, and some are known to stop reproduction altogether.

But even a slowdown is a "competitive disadvantage". Natural selection is supposed to quickly and efficiently rid the population of such specimens.

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http://www.talkreason.org/articles/Falsify.cfm

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