In the past 250 years, various forms of honeysuckle have been introduced to the Northeastern states. In the late 1990s, researchers led by Bruce McPheron of Pennsylvania State University discovered that this invasive honeysuckle was infested by a particular fruit fly species they called the Lonicera fly. When they analyzed DNA to determine its relationship to others, they were stunned to find that it was a hybrid of two closely related flies, the blueberry maggot and the snowberry maggot.

In laboratory experiments, the researchers found that the Lonicera hybrid preferred its honeysuckle host plant over its parent species’ host plants and that each parent species preferred its own host plant over the other’s. However, both parents also accepted honeysuckle. The researchers suggest that since the two parental species were thus more likely to encounter each other on honeysuckle in the wild, the newly invasive weed served as a catalyst for matings between the species and the formation of the hybrid species that now prefers honeysuckle.

The sunflower and Lonicera fly examples raise the question of whether hybridization between species has been more frequent than biologists once assumed. The most provocative report of possible hybridization came from the recent analysis of more than 60 percent of the Neanderthal genome sequence, which raised the specter of our ancestors commingling their genes with a long-diverged cousin.

Analyses of the overall genetic distance between Neanderthals and modern humans reveal that our DNA is 99.84 percent identical to that of Neanderthals. This small divergence indicates that the two lines split off from each other about 270,000 to 440,000 years ago. The fossil evidence shows that Neanderthals were restricted to Europe and Asia, whereas Homo sapiens originated in Africa. Various kinds of evidence indicate that modern humans migrated out of Africa and reached the Middle East more than 100,000 years ago and Europe by about 45,000 years ago, and would have or could have encountered Neanderthals for some time in each locale. The crucial question for paleontology, archaeology, and paleogenetics has been what transpired between the two species. To put it a little more crudely, did we date them or kill them, or perhaps both?