In celebration of the Year of Science’s May theme, sustainability and the environment, this month’s story deals with one of the biggest environmental challenges we face today: climate change. If you follow news coverage of climate change, you’ll be no stranger to the “adapt or die” perspective — the notion that sweeping impacts of climate change are inevitable, and that, to survive, all organisms (whether human, plant, polar bear, or penguin) will be forced to deal with fundamental changes in their environments. But how organisms will handle their new circumstances can be a bit fuzzy. In the popular press, these coping mechanisms — whether a farmer changing the crops she plants, a polar bear eating goose eggs instead of seals, or squirrels breeding earlier in the year — may be lumped together under the term adaptation. For example, just a few months ago, the Associated Press reported on scientists concern that global warming would prompt some California bird species to move north and “wipe out others that are not quick to adapt.” But what, exactly, does it mean to adapt in all of these different contexts?
Where's the evolution?
The word adapt has different meanings in everyday language and in evolutionary biology. In common language, we might say that we adapt to warm weather by wearing light-colored clothes and drinking lots of water. Used in this way, adapting often means changing one’s behavior to suit the circumstances. But in evolutionary biology, the term has a precise — and different — meaning. In evolution, to adapt means to experience natural selection that improves the function of a trait in a particular environment — i.e., to evolve via natural selection. Swapping a dark sweater for a light t-shirt is not adaptation in an evolutionary sense since it involves no evolution at all. The process of evolutionary adaptation is one experienced by whole populations over many generations, not by an individual organism over the course of a sweltering afternoon (or a lifetime, for that matter).
Adaptation or plasticity?
Many recent changes in organisms have been chalked up to climate change. Which of those represent adaptation and which represent phenotypic plasticity? Here are a few examples from each category:
- Canadian squirrels have evolved earlier breeding times. Squirrels with genes for earlier breeding were probably favored because this allows them to take advantage of an earlier spring and hoard more pinecones for winter survival.
- A North American mosquito species has evolved to wait longer before going dormant for the winter. Mosquitoes with genes that cause them to go dormant later were probably favored because it allows the insects to gather more resources during our new, extra-long summers.
- Some plant species around Walden Pond are flowering as much as three weeks earlier than they did 150 years ago. In these species, flowering is partly triggered by temperature, so climate change is the likely cause of this shift.
- Most butterfly species in central California have been taking flight about 24 days sooner in comparison to 30 years ago. When butterfly species mature is closely related to temperature, so climate change is the likely cause of this shift
- Alpine plant species in Austria and Switzerland have changed their range and are now found at higher altitudes than they were 100 or so years ago. Many plant species are restricted to certain areas by ambient temperatures, so climate change has likely allowed these species to move into new habitats.
Evolutionary biology has a special term to describe changes in an individual organism over the course of its lifetime: phenotypic plasticity. That’s a mouthful, but the idea is straightforward. An organism’s phenotype is simply its set of features, and to be plastic means to be moldable or changeable — so phenotypic plasticity just means that an organism’s features can be molded, or influenced to some degree, by its environment. You can think of it as the “nurture” side of the nature/nurture debate. The concept encompasses all sorts of changes to individual organisms, including developmental changes (e.g., an organism reaching a larger body size if it gets good nutrition as a juvenile, but reaching a smaller size on poor nutrition), behavioral changes (e.g., a polar bear eating goose eggs instead of seals, if seals become hard to catch and eggs are plentiful), and physical changes (e.g., a rabbit that grows white fur in the winter and brown fur in the summer). Phenotypic plasticity includes any sort of change to an individual that isn’t caused by changes in its genes.
Telling the difference between evolutionary adaptation and phenotypic plasticity can be tricky because, like adaptations, changes due to plasticity often make a lot of sense in terms of an organism’s survival and reproduction. After all, a polar bear that eats goose eggs when nothing else is available, will probably up its chances of survival. Changes due to phenotypic plasticity are often advantageous for the organism because plasticity itself can evolve by natural selection. The idea here is that, while eating goose eggs is not itself an evolutionary adaptation, the ability to switch to different food sources when the need arises is an adaptation and was favored over the bears’ evolutionary history. A rabbit’s white winter fur is not itself an adaptation, but the physical mechanisms for changing fur colors with the seasons are adaptations. And while putting on a t-shirt is not an adaptation, having the smarts to recognize that it’s hot out and to figure out what to do about it is an important human adaptation. How phenotypically plastic a species is (and in what ways) can evolve over time.
When you hear references in the media to organisms “adapting” to climate change, it’s worth considering what is really meant by this. Are the organisms actually evolving, or are they experiencing changes in behavior or physical traits that can be chalked up to phenotypic plasticity? The difference is important. For one thing, some changes due to plasticity are intentional. We humans will adjust to a warming planet by changing how we live because we are actively trying to make these modifications. Other changes due to plasticity are not intentional at all. A plant species that winds up growing further and further north as the Earth warms is not “trying” to adjust its range. This range shift is the result of environmental and physiological factors that the plant doesn’t control. Most importantly, actual evolutionary adaptations are never intentional. For example, scientists have discovered that, as the climate has warmed in recent decades, Canadian squirrels have evolved shifts in their breeding times that make them more successful in warmer climates. This shift was caused, not by environmental factors, but by changes in the genetic make-up of the population — and so, represents true evolutionary adaptation. The squirrels did not acquire these genetic changes by “trying” or deciding to breed at different times. Their evolution was the simple result of genetic variation and an environment that favored some gene versions (gene versions that affected the timing of breeding) over others. When the term adapt is used to describe all these different sorts of changes — some evolutionary, some not, some intentional, some not — it’s easy to get confused about the mechanism of change being discussed.
Recent research makes it clear that we can expect global warming to impact species in all of these ways. Some organisms will be able to cope because they have the right sort of phenotypic plasticity. So, for example, birds that are able to change their ranges and live where the environment suits them are likely to benefit from being phenotypically plastic. As the climate warms, they will be able to “track” the shifting habitats that are best for their survival. Other species, like the Canadian squirrel, may evolve — i.e., actually adapt — as the Earth continues to warm. But, of course, the biggest and most worrying news is the many species that may fall into neither of these categories, lacking both the plasticity that would allow them to better cope with climate change and the genetic variation that would allow them to evolve in response to climate change. Polar bears may be in this slowly sinking boat. Their long generation times and relatively small population sizes make evolutionary adaptation unlikely. And it’s unclear if they are phenotypically plastic enough to successfully make a living in a new, warmer world. Unless we can mitigate the impact of climate change, many of these species may soon face extinction.
- Bearhop, S., Fiedler, W., Furness, R.W., Votier, S.C., Waldron, S., Newton, J., Bowen, G.J., Berthold, P., and Farnsworth, K. (2005). Assortative mating as a mechanism for rapid evolution of a migratory divide. Science 310 (5747):502. Read it »
- Bradshaw, W. E., and Holzapfel, C. M. (2006). Evolutionary response to rapid climate change. Science 312(5779):1477-1478.
- Forister, M. L., and Shapiro, A. M. (2003). Climatic trends and advancing spring flight of butterflies in lowland California. Global Change Biology 9(7):1130-1135. Read it »
- Grabherr, G., Gottfried, M., and Pauli, H. (1994). Climate effects on mountain plants. Nature 369:448. Read it »
- Nussey, D.H., Postma, E., Gienapp, P., and Visser, M.E. (2005). Selection on heritable phenotypic plasticity in a wild bird population. Science 310(5746):304-306. Read it »
- Réale, D., Berteaux, D. McAdam, A.G., and Boutin, S. (2003). Lifetime selection on heritable life-history traits in a natural population of red squirrels. Evolution 57:2416-2423. Read it »
- Rockwell, R. F., and Gormezano, L. J. (2009). The early bear gets the goose: climate change, polar bears, and lesser snow geese in western Hudson Bay. Polar Biology 32(4):539-547. Read it »
- Willis C. G., Ruhfel, B., Primack, R. B., Miller-Rushing, A. J., and Davis, C. C. (2008). Phylogenetic patterns of species loss in Thoreau's woods are driven by climate change. Proceedings of the National Academy of Sciences USA 105(44):17029-17033. Read it »
- A recent news brief on birds that have shifted their ranges in response to global warming from the Associated Press
- A thorough description of mosquito evolution in response to global warming from the Boston Globe
- A report on the idea that polar bears may cope with global warming by feeding on goose eggs from the American Museum of Natural History
Understanding Evolution resources:
- The basics of how natural selection works to produce adaptations
- Clarification of common misconceptions regarding natural selection and adaptation
- A tutorial on microevolution, including a side trip addressing mosquito evolution in response to global warming
- A news brief on how organisms have evolved in response to global warming
Background information from Understanding Global Change:
- In your own words, explain what an evolutionary adaptation is. Give one example of an evolutionary adaptation not described in the article above.
- In your own words, explain what phenotypic plasticity is. Give one example of phenotypic plasticity not described in the article above.
- Imagine that a friend is concerned about how penguins will fare as the climate continues to warm. He says that he’s worried because “they might not be smart enough to adapt as the temperature keeps going up.” How would you explain the error in his thinking?
- Read this article from the Daily Mail on recent changes in a seal species. What trait(s) has changed? Do you think that this change likely resulted from evolutionary adaptation or phenotypic plasticity, or can you tell from this article? Explain what features of the change in the seal species make you think this.
- Read this article from Reuters on recent changes in a weed species. What trait(s) has changed? Do you think that this change likely resulted from evolutionary adaptation or phenotypic plasticity, or can you tell from this article? Explain what features of the change in the weed species make you think this.
- Read this article from the BBC on recent changes in a bird species. What trait(s) has changed? Do you think that this change likely resulted from evolutionary adaptation or phenotypic plasticity, or can you tell from this article? Explain what features of the change in the bird species make you think this.
- Teach about how environmental changes impact organisms: In this classroom activity for grades 3-5, students observe and conduct an experiment to see whether differences in salinity (the environment) have an effect on the hatching rate and survival of brine shrimp.
- Teach about evolution, climate change, and phenotypic plasticity: This article for grades 9-12 follows scientist Jennifer McElwain as she studies the fossil record in order to learn more about how global warming has affected life on Earth in the past and how it might affect life on Earth in the future. This article comes with a set of discussion questions for use in the classroom.
- Teach about climate change and evolution: In this interview for grades 9-12, UC Berkeley Professor Anthony Barnosky gives the inside scoop on how climate change has affected past speciation of mammals and how it may affect biodiversity in the future.
- Bradshaw, W. E., and Holzapfel, C. M. (2006). Evolutionary response to rapid climate change. Science 312(5779):1477-1478.
- Dearen, J. (2009, February 9). Study: warming climate to hurt Calif bird species. San Francisco Chronicle. Retrieved April 16, 2009 from The San Francisco Chronicle.
- Forister, M. L., and Shapiro, A. M. (2003). Climatic trends and advancing spring flight of butterflies in lowland California. Global Change Biology 9(7):1130-1135.
- Grabherr, G., Gottfried, M., and Pauli, H. (1994). Climate effects on mountain plants. Nature 369:448.
- Rockwell, R. F., and Gormezano, L. J. (2009). The early bear gets the goose: climate change, polar bears, and lesser snow geese in western Hudson Bay. Polar Biology 32(4):539-547.
- Willis C. G., Ruhfel, B., Primack, R. B., Miller-Rushing, A. J., and Davis, C. C. (2008). Phylogenetic patterns of species loss in Thoreau's woods are driven by climate change. Proceedings of the National Academy of Sciences USA 105(44):17029-17033.