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Global change drove the evolution of giants

September, 2017

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.

Read more about it

Primary literature:

  • Slater, G. J., Goldbogen, J. A., and Pyenson, N. D. (2017). Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics. Proceedings of the Royal Society B. 284: 20170546. read it
News articles:

Understanding Evolution resources:

Discussion and extension questions

  1. In your own words, describe three hypotheses for why baleen whales might have evolved extremely large body sizes.
  2. Use the tree in the article above to answer the following questions:
    1. Which direction does time flow?
    2. What aspect of the diagram indicates evolutionary changes in size?
    3. How is extinction indicated?
  3. Review the process of natural selection. Use the four steps described on that page to explain how whale body size in a single lineage might increase over generations in response to episodic upwelling.
  4. Advanced: Study the evolutionary tree in the article above.  What aspect of this tree suggests that the evolution of large body sizes in whales in not simply the result of random changes?
  5. Advanced: Could hypotheses for the evolution of large size in baleen whales have been investigated without the use of a phylogeny? Explain why or why not.
Related lessons and teaching resources

  • Teach about macroevolution in whales: This article for high school and college students introduces evograms, diagrams that convey information about how a group of organisms and their particular features evolved. Students learn how to read evograms and delve into the evolutionary history of whales, tetrapods, mammals, birds, and humans.
  • Teach basic tree thinking skills: This self-paced tutorial with quizzes from Laura Novick, Kefyn Catley, and Emily Schreiber is appropriate for college students and corrects many common misconceptions about evolutionary trees.
  • Teach about natural selection: In this lesson for the high school and college levels, students become unwitting subjects in a demonstration of natural selection. Students select candies from a bowl and have an opportunity to think about what traits brought about the "survival" of some candies.

References

  • Marlow, J. R., Lange, C. B., Wefer, G., and Rosell-Mele, A. (2000). Upwelling intensification as part of the Pliocene-Pleistocene climate transition. Science. 290: 2288-2291.
  • Slater, G. J., Goldbogen, J. A., and Pyenson, N. D. (2017). Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics. Proceedings of the Royal Society B. 284: 20170546.