Natural Selection Not at Work

The principle of natural selection is straightforward: some organisms are better adapted to their environment than others. They survive (or, more generally, they reproduce more frequently); others don’t (reproduce less frequently). The premise of natural selection is that differences in organisms are what allow some creatures to survive better than others. For example, short-haired dogs living above the Arctic Circle will not survive (and reproduce) as well as long-haired dogs. The basis for these differences is usually genetic.

A new study by University of Rochester biologist John Jaenike sheds light on an unusual case of natural selection, however, occurring in the fly Drosophila neotestacea. As with many organisms (including humans), some D. neotestacea enjoy a symbiotic relationship with bacteria. In this case, one type of bacteria protects the flies from a parasitic worm that may otherwise leave female flies infertile.

Jaenike’s research showed that natural selection in the flies has been furthered not by differences in the flies themselves, but in whether they carry the bacterium, known as Spiroplasma. If they do, the females are ten or more times more likely to remain fertile if attacked by parasites. And because the bacteria are passed down from a mother fly to its offspring, they are “inherited” in a manner similar to genes.

“Normally, you’d think that a species would evolve resistance using its own genes, but in this case they evolved resistance by spread of this symbiont,” Jaenike said. But as in previous examples of natural selection that are labeled “evolution,” we find no increase in genetic information—either in the flies or in the bacteria. Thus, natural selection working in D. neotestacea is not the same process as—and so is not evidence for—the “evolution” allegedly responsible for the diversity of life we see around us.