Host Switch Hypothesis

Evolution is the process by which organisms develop unique derived traits. Evolved traits that decrease an organism’s survival rate are selected against in a population. Traits that increase an organism’s survival rate, on the other hand, are often selected for, meaning that those traits will appear more frequently in the genetic codes of members in a population. This process of selection can take several forms, one of them called sexual selection. Sexual selection occurs when one member of a particular species selects a mate with more favorable traits than other members of their species. An example of sexual selection would be the evolution of “hairlessness” in modern humans. Modern humans do not have a single coat of undiversified hair, unlike

gorillas and the ancestors of humans and chimpanzees) (Reed, 2007). The ‘recent host switch’ hypothesis, would explain the genetic similarities between the lice acquired by modern humans and the lice acquired by gorillas, chimps, and other primates (Reed, 2007). Because of the evidence of these genetic similarities and differences in Pediculus and Pthirus, the evolutionary divergence of humans, gorillas, and chimps appears to be directly correlated with the evolutionary divergence of different species of lice (Reed, 2007). This evidence allows us to conclude that in order for modern human head lice to have evolved from its ancestral form, humans first had to evolve into a less hairy, less neanderthalian versions of themselves. The evolved trait for hairlessness, then, must have occurred through some form of natural selection. Who would have thought that head lice, the natural enemy and nightmare of second-graders and their parents, could be the key to unlocking the--quite literal--naked