It is astounding that despite diverse environments, all these varied birds underwent a similar or convergent evolution of being rendered flightless. Before we address why the birds would no longer want to soar up in the skies, it is imperative to analyse the genesis of why birds fly in the first place. Flight mechanism inherently comes with incredible benefits primarily for escaping predators, hunting and travelling long distances or migrating for better climatic conditions and food.
Flying also comes with high costs, consuming huge amounts of energy thereby limiting the body size and weight of the bird. A bird which does not fly conserves energy enabling them to survive on scarcer and also less nutritious food.
Changes in the environment, either decreased pressure for flying or increased pressure to develop other traits, made the birds lose their distinctive trait of flying. Steamer ducks found in South American coastal environments are a point in case. The climate is pleasant throughout the year, food reasonably abundant and there is not much fear of predators. Amid little selective pressure to fly, these birds have lost the flying ability over time. For penguins there was an immense pressure to be better swimmers, which provided a greater advantage in terms of food over flying, with the two abilities requiring a tradeoff.
Probably nature cannot allow you to be both a great flier and a swimmer! Likewise, with the extinction of dinosaurs about 65 million years ago, many birds like ostrich and emu facing no challenge from predators made land their permanent base.
These birds preferred to nest and feed on the ground and therefore, had a natural disposition to be earth-bound. So changes can add up in those bits of DNA as animals evolve. And they may have changed a lot from species to species.
Scott Edwards and his colleagues got around that problem by decoding the genetic instruction books, or genomes , of 11 bird species. Edwards is an evolutionary biologist at Harvard University in Cambridge, Mass.
Eight of the species were flightless birds. The researchers then compared these genomes to already completed genomes from other birds.
They also included 25 species of flying birds. Among these, 2, had accumulated more mutations than expected in ratites — but not in other birds. That high number of ratite mutations shows that those bits of bossy DNA are changing faster than other parts of their genomes. That might mean the bossy bits have lost their original functions.
The researchers were able to figure out when the rate of mutations had sped up — in other words, when evolution happened fastest. Those times could have been when the bossy DNA stopped doing its job and birds lost their ability to fly. It may even have happened as many as five times. Those regulatory DNA bits tended to be close to genes that help make limbs, such as wings and legs.
That hints that they might tweak gene activity to make smaller wings. The team tested how well one such bossy DNA bit could turn on a gene in chicken wings when chicks were still inside their eggs. That piece of bossy DNA is called an enhancer.
The team tried one version of the enhancer from elegant-crested tinamous, a species that can fly. That enhancer turned on the gene. That suggests changes in that enhancer turned off its role in wing development. And that might have contributed to rheas becoming flightless, the scientists conclude. New research suggests that some of those differences are linked to changes to regulatory DNA.
Lily Lu. One current hypothesis for why ratites, except tinamous, are flightless is that the ancestor of all the species had lost the ability fly, and tinamous later regained it.
Rather, the ancestor of ratites probably could fly and tinamous retained that ability, while related birds lost the ability, mostly because of changes in regulatory DNA, he says. Aside from in the ancestor of birds, flight has evolved only a few times: in pterosaurs, in bats, and perhaps a couple of times in insects, Edwards says.
Birds have lost flight multiple times. There are no known examples of regaining flight once it has been lost, he says. The researchers also found that more than protein-coding genes were evolving — building up mutations — faster than expected in flightless ratites, but those genes tended to be related to metabolism rather than shrinking wings.
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