Understanding Natural Selection

J0hnSm1th said:

I can see how natural selection would work where a population is under threat of extinction. But how does it shape a population when the threat is not so dire?

For example, in a given population of bacteria, some may have a mutation affording them resistance to antibiotics. When the agent hits, those with the resistance are left standing (so to speak). The resultant population inherits immunity.

Compare that to the evolution of the giraffe. The theory goes that its increasingly long neck allowed it to forage higher - giving a better food supply. I can see how the longer necked giraffes would have an advantage. But unless trees stopped sprouting leaves at height X why would the shorter animals die out? Wouldnt we just see one population with a variety of neck sizes? Even if long necks became common, the shorter necked ones wouldnt disappear because there is nothing to kill them off.

Its possible that, in the past, there may have been extinction events which we cant imagine. Such would explain why species are so varied and discrete instead of having body types across a spectrum. What other things may have led to the extinction of some varieties of animals whilst leaving their cousins unscathed?

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TheInstant said:

I think you're thinking of natural selection as too extreme of a concept. It isn't "all the organisms with the beneficial mutation live and all the organisms without the beneficial mutation die." Although this can happen, it's more common that since those with the beneficial mutation are more likely to live long enough to reproduce, the beneficial mutation will exist with greater frequency in the next generation. And so on.

So in the giraffe example, giraffes with slightly longer necks are more likely to live long enough to reproduce, so the gene for slightly longer necks will become more predominant in the population. Not all of the giraffes with slightly shorter necks have to die before reproducing in order for this to happen.

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J0hnSm1th said:

I can see how natural selection would work where a population is under threat of extinction. But how does it shape a population when the threat is not so dire?

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Chalnoth said:

Humans have slowed the evolution of humans to a near halt, however, due to modern medicine. Basically, there's very little selective pressure at all for humans.

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GoSeminoles! said:

I don't think this isn't true at all. There are plenty of examples of big differences in reproductive success in modern humans. As in much of human history, poor people have far more children than rich people. Muslims have way more children than Christians. How could a paleontologist 1 million years from now examining our fossils or genes be able to tell that the major selection pressures of this age were influenced by economics or religion? Memes, not genes, are the major force now in charge of human evolution.

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J0hnSm1th said:

I can see how natural selection would work where a population is under threat of extinction. But how does it shape a population when the threat is not so dire?

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But unless trees stopped sprouting leaves at height X why would the shorter animals die out? Wouldnt we just see one population with a variety of neck sizes? Even if long necks became common, the shorter necked ones wouldnt disappear because there is nothing to kill them off.

Click to expand...

Such would explain why species are so varied and discrete instead of having body types across a spectrum.

Click to expand...