Here's a nice example of an evolutionary arms race in action:
BBC: Butterfly shows evolution at work
Scientists say they have seen one of the fastest evolutionary changes ever observed in a species of butterfly. The tropical blue moon butterfly has developed a way of fighting back against parasitic bacteria.
Six years ago, males accounted for just 1% of the blue moon population on two islands in the South Pacific. But by last year, the butterflies had evolved a gene to keep the bacteria in check and male numbers were up to about 40% of the population. Scientists believe the comeback is due to "suppressor" genes that control the Wolbachia bacteria that is passed down from the mother and kills the male embryos before they hatch.
The parasitic bacterium can only be passed on through the female line of its host, so it evolved a mechanism to ensure that only female host embryos survive. But the dearth of male blue moon butterflies created a massive niche that was just crying out to be filled. Any mutation that enabled male butterfly embryos to survive the bacterium's attack would spread quickly through the population. Which is exactly what appears to have happened, and the butterfly sex ratios are rapidly returning to normal.
The development of resistance to the wolbachia bacterium is a lovely example of Natural Selection in action: a single beneficial mutation spreading quickly through a severely depleted population. The alternative was extinction.
With males having dropped to 1% of the population, the blue moon butterfly has just come through an evolutionary bottleneck. Evolutionary bottlenecks are a fascinating subject in their own right. They are a means by which rare, possibly disadvantageous traits can become more common in a population by serendipitously piggybacking on the back of strongly selected beneficial ones. Genes mutate, but it is individuals that are selected. If a particular genetic mutation gives an individual a major selective advantage in an evolutionary bottleneck situation, that individual's genes are likely to become more common in the general population warts and all: the benefits of the highly advantageous mutation might well outweigh the penalties of any disadvantageous ones. Evolutionary bottlenecks lead to in-breeding.
Recent DNA studies indicate that our own hominid line went through an evolutionary bottleneck in its dim and distant past, which left us and our chimpanzee cousins more vulnerable to genetic disease such as cancer. It would be interesting to know what other traits piggybacked on the blue moon butterfly's highly advantageous new wolbachia resistance.