When selection levels collide

If the gene that increases in frequency has a high fitness cost to the individual, the direction of selection can be different at different levels. Suppose that a gene that reduces the speed at which cheetahs can run is one that, through meiotic drive, has a better than 50 percent chance of ending up in the gamete population. Selection at the level of the individual will act to remove such a costly gene, because cheetahs with this gene will be slower and will produce fewer offspring.

Because the fitness of a particular gene can be at odds with the fitness of the individual that carries it, selection at the level of the individual acts to combat the deleterious driver gene. Take, for example, fruit flies.

A particular species of fruit fly has an X-linked driver gene. This driver gene causes males to have only female offspring. Because producing both male and female offspring is the most successful strategy for getting genes into the subsequent generations, female flies are at a disadvantage if they mate with the male flies carrying this driver gene.

Interestingly, the female fruit flies choose mates that have long eye stalks. The connection? The driver gene is associated with a gene for short eye stalk. The female preference for the long eye stalk evolved because the length of the eye stalk correlates to the quality of the mate. Mate with a short-eye-stalk fruit fly, and you end up with only daughters and decreased fitness. But avoid short-eye-stalk fellas, and you get both male and female offspring and increased fitness. (For a down-and-dirty explanation of sex selection, head to Chapter 12.)

Selection at the individual level operates to oppose selection at the level of the driver gene. Females with a preference for males with long eye stalks are less likely to mate with males carrying the driver gene. So even though the gene is in twice as many gametes (males containing the driver gene make only gametes that carry this gene), it doesn't have twice the chance of making it into the next generation because the male fly isn't as likely to be chosen as a mate.

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