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I am somewhat new to evolutionary biology, having studied it on my free time as a computer science student. There is one particular thing that has always bothered me for which I have not seen a good treatment, relating to adaptations to the environment with respect to genetic diversity. If it is possible for a population to adapt to rapid environmental changes, and they don't have an adaptation for dealing with change directly (such as a complex brain), it seems to me that every generation must have present within them almost every possible environmental adaptation that the population is capable of expressing (including many irrelevant ones and a few relevant to the particular environmental challenge). Otherwise, it may take too many generations to deal with a change, which may be disastrous for the population.

So my question would be: how does an evolutionary biologist explain the mechanics behind the ability for a population to adapt quickly? Are most environmental changes slow or gradual enough that the population has a few generations to happen upon the mutations that will allow it to survive, and have generally been successful in this regard for 3.5 billion years? Or, are a large majority of possible adaptations present in almost every generation, and just serve no purpose or advantage for most of the population if the provided "benefit" is unneeded (i.e., are effectively neutral)? Or something in between?

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Yes, yes, and yes. And a too quick environmental change can kill a whole population, it happens all the time. (we at least hope so when we're infected with bacteria and give them hell) –  rwst Jul 20 '12 at 14:51
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Great question! A lot of things affect how quickly a population or species can adapt to a new environment, including population size, mutation rate, generation time, standing genetic diversity, and selective pressure.

The diversity of life encompasses practically all combinations of those variables. A bacterial population might very well contain enough diversity to allow a portion of the population to overcome a rapid change. In fact, applying antibiotics to a bacterial population and counting the survivors is a common measure of mutation rates.

On the other hand, organisms with small populations and long generation times will be much less likely to overcome a rapid environmental change. This is why there is so much concern over anthropogenic changes to the environment, including climate change.

It's hard to provide a definitive answer, since "rapid change" is a relative term, and the difficulty of adapting isn't known. Some striking adaptations can be caused by a single base pair mutation, such as in beetles and Monarch butterflies that are insensitive to toxic plant compounds.

It's important to point out that natural selection is based on relative fitness. Therefore, an adaptive mutation will spread because it's likely that carriers will be more fit than all of their neighbors. This does not necessarily mean that individuals without the mutation will die or fail to reproduce, only that those with the mutation will do it better.

Likewise, adaptation is not necessarily caused by the environment changing and wiping out all but a few lucky mutants like in the bacteria example. Instead, the environmental change might make things more difficult, but as long as the population can persist, then mutations will continue to enter the population which could confer a selective advantage against the change. So, no, a population does not (cannot) carry all possible adaptations. A population cannot adapt to an environment it hasn't encountered.

Finally, it's worth pointing out that a species can expand its range through migration. A new, unsuitable environment can act as a migration sink (that is, migrants make it there but fail to establish) for a nearby population. If this happens for long enough, some of the migrants may have a mutation allowing them to establish in the new environment.

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