Parallel evolution

Parallel evolution is the similar development of a trait in two distinct species that are not closely related but that share a similar ancestral trait, evolving in comparable ways under similar evolutionary pressures. It differs from convergent evolution, in which unrelated lineages independently evolve a similar trait with no shared ancestral basis for it; in parallel evolution the trait already existed in a shared ancestor, and the two descendant lineages simply develop it further along similar lines, though scientists note that telling the two patterns apart in practice is often difficult because it requires knowing whether the ancestor really carried the trait in the first place. Marsupial and placental mammals, which diverged roughly 100 million years ago, offer a striking example: evolving independently on separate continents after the extinction of the dinosaurs, the two groups nonetheless produced strikingly similar animals in response to similar ecological opportunities, including comparable sabre-toothed predators, wolf-like hunters and mole-like burrowers.