Washington, May 30 (ANI): Using molecular and computational techniques, UCLA evolutionary biologists and a colleague, have reported how in the last 35 million years-since the ancestor of all living whales appeared-have fast whales changed their shape and body size.
Analysing the evolutionary tempo of modern whale species, the researchers have provided the first test of an old idea about why whales show such rich diversity.
"Whales represent the most spectacularly successful invasion of oceans by a mammalian lineage. They are often at the top of the food chain and are major players in whatever ecosystem they are in. They are the biggest animals that have ever lived. Cetaceans (which include whales, as well as dolphins and porpoises) are the mammals that can go to the deepest depths in the oceans," said Dr. Michael Alfaro.
"Biologists have debated whether some key evolutionary feature early in their history allowed whales to rapidly expand in number and form," Alfaro said. "Sonar, large brains, baleen (a structure found in the largest species for filtering small animals from sea water) and complex sociality have all been suggested as triggers for a diversification, or radiation, of this group that has been assumed to be rapid.
However, the tempo - the actual rate of the unfolding of the cetacean radiation - has never been critically examined before. Our study is the first to test the idea that evolution in early whales was explosively fast," he added.
One explanation for whale diversity is simply that they have been accumulating species and evolving differences in shape as a function of time.
The more time that goes by, the more cetacean species one would expect, and the more variation in body size one would expect to see in them.
"Instead, what we found is that very early in their history, whales went their separate ways from the standpoint of size, and probably ecology," said Alfaro.
"This pattern provides some support for the explosive radiation hypothesis. It is consistent with the idea that some key traits opened up new ways of being 'whale-like' to the earliest ancestors of modern cetaceans, and that these ancestors evolved to fill them. Once these forms became established, they remained," he added.
Species diversification and variations in body size were established early in the evolution of whales, reported the researchers.
Large whales, small whales and medium-sized whales all appeared early in the history of whales, with the large whales eating mostly plankton, small whales eating fish and medium-sized whales eating squid.
"Those differences were probably in place by 25 million years ago at the latest, and for many millions of years, they have not changed very much. It's as if whales split things up at the beginning and went their separate ways. The distribution of whale body size and diet still corresponds to these early splits," said the study's lead author, Dr. Graham Slater, a National Science Foundation-funded UCLA postdoctoral scholar in Alfaro's laboratory.
"The shape of variation that we see in modern whales today is the result of partitioning of body sizes early on in their history. Whatever conditions allowed modern whales to persist allowed them to evolve into unique, disparate modes of life, and those niches largely have been maintained throughout most of their history," said Alfaro.
"We could have found that the main whale lineages over time each experimented with being large, small and medium-sized and that all the dietary forms appeared throughout their evolution, or that whales started out medium-sized and the largest and smallest ones appeared more recently - but the data show none of that. Instead, we find that the differences today were apparent very early on," he added.
Killer whales are an exception, having become larger over the last 10 million years, said the researchers.
Killer whales are unusual in that they eat mammals, including other whales.
"If we look at rates of body-size evolution throughout the whale family tree, the rate of body-size evolution in the killer whale is the fastest. It came from the size of a dolphin you would see at SeaWorld about 10 million years ago and grew substantially," said Slater.
The study was published in the early online edition of Proceedings of the Royal Society B. (ANI)