Global change drove the evolution of giants
Photo credit: NOAA National Marine Sanctuaries
We live in extraordinary times. The largest animals ever to have lived on Earth happen to be alive today. Blue whales are larger than any mastodon, dinosaur, or mega-toothed shark ever was. They can grow to almost as long as a 737 airplane and weigh about twice as much! And yet, the blue whale is not a freak of nature. They come by their size honestly; they are part of a family of extremely large-bodied filter-feeding whales that includes the gigantic fin whales and Sei whales. Nevertheless, such huge creatures beg the question, why. Why so big? As it happens, this is just the sort of question that studies of evolution can help us answer. Over the summer, while many of us were lounging at the beach contemplating the watery habitat of these massive creatures, scientists announced the results of an investigation into the evolution of body size in baleen whales
Where's the evolution?
Biologists have long marveled at blue whales and wondered when and why they got so darn big. Some suspected that competition with other early whales forced baleen whales into a niche that required a big body. Others hypothesized that the evolution of large sharks caused whales with larger and larger body sizes (those that were too big to eat!) to be favored by natural selection. And of course, it's also possible that there's nothing to explain at all — that body size evolution in whales is just a random walk with some evolutionary changes increasing size and others decreasing size, but no overall trend. In other words, it's possible that, just as a series of coin flips will sometimes generate a string of all heads even if the coin is perfectly fair, blue whales are simply the result of a series of random changes in body size that mainly happened to make them bigger, producing one exceedingly large lineage.
Recently, a team of scientists decided to investigate these ideas further by reconstructing the evolutionary tree of 13 living and 63 extinct baleen whale species. They then estimated the body sizes of all of these different whales, mapped body size onto the tree, and used models of evolutionary change to figure out when and where along the tree's branches changes in body size occurred. The tree below shows their results. The ancestral baleen whale lineages are at the left side of the diagram and were less than 10 meters long, as indicated by the yellow and white coloration of the branches — whereas, many of the descendent whale lineages (shown towards the right of the diagram) were greater than 10 meters long, as indicated by orange and red shading of the branches.
Click on image to enlarge. Figure modified from Proceedings of the Royal Society B
These results suggest that the evolution of large body size was no random walk. Instead, for the first 25 million years of their history, all baleen whale lineages remained small or moderately sized. Then around 4.5 million years ago, many lineages simultaneously started evolving giant sizes. This finding contradicts some prior hypotheses about why baleen whales evolved to be so big. For example, early niche partitioning doesn't make sense as an explanation because, for most of their history, different baleen whale lineages remained around the same size. And since large marine predators existed long before baleen whales got big, we can say that the selection pressure responsible for large whale sizes was probably not predation avoidance.
Instead, the scientists behind this research suspect that the evolution of giant sizes was triggered by several global changes that began about 4.5 million years ago during the Pliocene Epoch. First, the Earth was cooling and entering a series of glacial cycles, in which ice sheets spread towards the equator from the Arctic and Antarctic. Along with these polar ice sheets, came greater temperature differences between the poles and the equator. Such temperature gradients, in turn, led to stronger trade winds, which caused the upwelling of cold, nutrient-rich water from the ocean's bottom to its surface, supporting the krill and planktonic organisms that baleen whales feed upon. At the same time, seasonal run-off from Earth's expanded glaciers poured even more nutrients into the coastal oceans at distinct times of year. In combination, these changes led to a shift in the availability of baleen whales' food sources. The ocean was no longer open for grazing anytime, anywhere. Instead baleen whales had to contend with a feast or famine situation, in which food was superabundant — when you could find it — but it was patchily distributed and fleeting.
In this situation, the researchers argue, efficiency matters. Whales that could consume more food when it was available and migrate over the long distances that separate food patches would have been better able to survive — and both of these abilities are boosted when the animal is large. Lineages with smaller body sizes were more likely to go extinct, and within surviving lineages, larger individuals were favored — resulting in the range of the large to extra-large body sizes we observe in living baleen whales today.
This research highlights the deep and sometimes unexpected connections between Earth's physical systems and living things. Four and a half million years ago, a complex set of changes in Earth systems seem to have set modern baleen whales on an evolutionary trajectory that resulted in majestic animals the size of airplanes. Today, global changes wrought by human pollution, land use, and production of carbon dioxide are also on track to change Earth's biota forever. For example, as increased levels of carbon dioxide cause the planet to warm, the poles will become warmer, ice sheets will shrink, the trade winds will weaken, and resulting upwelling will slow. We don't know for certain exactly how this will affect the survival of whales that depend on the krill and plankton supported by upwelling, but based on examples from Earth's history, we should expect that it will.Primary literature
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