Convergent Evolution: When a Good Idea Moves Between Species

Evolution doesn't do things on purpose; it's not sitting at a big desk in a corner office somewhere making haphazard decisions about which animals lay eggs or get pouches on their tummies. Evolution is the process of organisms changing over the course of many generations to suit the conditions under which they live. And some traits, like flying, are particularly useful – it can help you catch prey or avoid predators and easily move to new food sources and ecological niches – so it's evolved separately in different groups of animals several times. However, flying doesn't look the same across the groups. For instance, bats developed a membrane between their abdomen, arms and fingers to catch air, while birds sprouted feathers along a finger-fused forelimb, which means bats can maneuver their wings separately while birds' have to move together. Flying insects just fashioned wings out of their exoskeletons.

So, convergent evolution can tell us a lot about what kinds of adaptations work to help species survive all the trials and tribulations they might face in a particular type of environment – what ecologists call a biome. For instance, in North America the kangaroo rat lives in the Sonoran Desert where it spends the scorching days in a cool, dry burrow and the cool desert nights collecting seeds, vegetation and the occasional insect if they can get it. Everybody in the desert wants to eat them – coyotes, bobcats, rattlesnakes, owls – but the kangaroo rat is fast and agile with powerful back legs and extremely sensitive hearing, all of which helps it survive a hard-scrabble, bottom-of-the-food-chain desert biome lifestyle. And though the kangaroo rat doesn't have an enviable life, it's effective: Two other rodents on Earth – the Australian hopping mouse in the Australian outback and a little jumping rodent called the jerboa native to the deserts of North Africa, Asia and the Middle East – evolved separately, and yet incredibly similarly.

But how does convergent evolution happen? This is a trickier question, and the development of genetic tools over the past 20 years has been helpful in picking it apart. In a 2019 study published in Science, one group of researchers at Harvard University have looked at the development of flightlessness in birds – a trait in birds that has evolved several times over – and exactly how evolution pulled it off in penguins the same way it did in ostriches.

Flightless birds, or ratites, can't fly for a couple of reasons: Somewhere along their lineage, they have lost their keel – the bone that runs perpendicular to the breastbone on flying birds that the pectoral muscles attach to – and they have reduced forelimbs, ranging from nearly absent in the kiwi to still obvious but reduced in size in the ostrich.

However, there are many ways that particular convergent traits can evolve.

"Before genomics, one could use developmental tools to figure out if the same or different developmental mechanisms seemed to be involved in convergent phenotypes, but the idea of levels of convergence – same mutation, same gene, or same pathway – has developed in large part because it is possible to look in the genome for these things now," says Tim Sackton, director of Bioinformatics at Harvard. "In the ratites, for example, we were able to show that the same regions of the genome that control where and when certain genes are expressed are repeatedly evolving in flightless birds, but this doesn't seem to involve the same nucleotide mutations."

And yes, where some traits converge from completely different corners of the living world, the opposite is also true: divergent evolution is the process by which groups from one species or organism begin to develop different traits, hereby splitting into separate species. This often happens when populations of a species are separated geographically, and over time they adapt to the conditions of their new spot, whether it's increased predation pressures or abiotic factors like a change in climate.

One famous example of divergent evolution was found by Charles Darwin in his travels to the Galápagos Islands in 1836. "Darwin's finches," as they're now known, were a group on tanagers (not true finches) that lived on different islands in the archipelago – the main difference between them being the shape of their beaks, which changed over the generations due to the particular foods available to the birds on the different islands.