Do changes in the physical environment contribute to the evolution of mammals?
Fossils of Orohippus, an ancestor of today’s horse dating from about 52-45 million years ago, have been found in in Wyoming and Oregon. Florida Museum of Natural History.
Barnosky: I think they do, but it depends on what scale of change you are talking about. There are many different scales of environmental change. Climate change is an example, Climate changes happen from year to year and decade to decade, over hundreds of years, over thousands of years, in hundred thousand year cycles, and over millions of years. In the work we have been doing it seems that the ones that affect mammal evolution at the population scale are probably very large time scales like the Pleistocene glacial and interglacial cycles, 1.8 million to 10,000 years ago. The mid-Miocene climate optimum, sustained for millions of years roughly 18 million to 14 million years ago, had very pronounced speciation in mammal groups.
How does climate change provide conditions for speciation?
Barnosky: Basically, when climate changes, species are trying to follow the climate that they are adapted for, so they move around the landscape to stay in the same climate space. When they do that, populations get left behind. These populations might get isolated enough to spur morphological (that is, physical) or genetic changes. You might get a species or population trapped in a place where climate is changing, which would induce a selective force to make them change or go extinct.
What do fossils tell us about mammalian changes through space and time?
Barnosky: One thing they tell us is that as you go through the past 70 million years, there are many times at which we get a changeover from one set of species to a new set of species. Those changeovers are called “faunal turnovers,” and the things that contribute to them are both the evolution of new species and the immigration of species from other areas. These turnovers occur regularly throughout time. Mammal fossils also provide clues to the details about their history as they were affected by climate change and evolutionary factors.
The history of the planet can be divided into time periods that are defined by the mammalian species alive during these periods. Today we can sequence DNA from many fossil bones, providing us a glimpse into genetic changes that have occurred over time. Other tools include molecular clocks—determining when species diverged using DNA sequence differences—and morphometrics—measuring changes in physical features.
Can fossils help predict potential biotic effects of global warming?
Barnosky: Fossils are one of the best ways to predict potential effects of today’s global warming. When we are looking at a change that’s happening within a human lifetime, the first thing we have to figure out is the answer to “Is this within the natural range of variability of what had happened when humans weren’t around?” The only way we can do that is to use the fossil records to trace communities back through time. In other words, delimit the bounds of what is normal and compare what is happening today with what is normal in the absence of humans. So that is one way they are essential. The other way to approach this is to study how these changes actually play out over a very long time scale so we can find analogous changes in the fossil record.
Humans are changing climate at unprecedented rates. What possible effect can this have on mammalian evolution?
Barnosky: I think basically most of the information we have about mammal evolution is that the speed at which it takes place is a little bit constrained. How fast can we have genetic mutations and how long can we maintain populations in isolation? These things play out over hundreds of thousands of years on the time scale. The kinds of environmental changes that humans have initiated have been playing out only over a hundred-year scale. I think the bottom line is that humans have changed evolution. The kind of selective pressures that we have put on mammals are so strong there is not going to be time to regenerate biodiversity. Mammal speciation does not occur that fast.
How can scientists arrive at various future scenario predictions about climate change effects on mammal species?
Barnosky: There are a couple of ways you arrive at predictions. One is to basically have models of how climate is going to change. We are beginning to understand the physiological and other constraints on various species relative to climate. Knowing that climate will change by x amount we can surmise that it will affect a species in a certain way. Another way to predict scenarios is to look at the fossil record. The most useful kinds of fossil record for this purpose would be where we can actually take an ecosystem today, obtain fossil deposits to trace history back through thousands and maybe tens of thousands of years, and then observe how those communities have changed and see whether there are more changes at times when there was climate change, and whether those are analogous to today’s fast rates.
When population rises or declines during climate change does genetic diversity rise or decline?
Barnosky: It depends very much on the life history of the animal—what the animal did to survive. If you are an animal who does not disperse very much, like gophers, you would expect that as populations decline, genetic diversity would decline, because the fewer individuals you would have, the fewer genes you have in the gene pool. However, if you are a big disperser, like the voles, then what can be found by some of the studies is that as populations decline, dispersers from other areas join these populations, and that actually increases genetic diversity.
In Earth’s history, were there many mammalian speciation events, and if so, was climate a factor in all of them?
Barnosky: There were many different speciation events, certainly just by virtue of the fact that we have about four thousand mammal species on Earth today. Each one of those is a speciation event, and for each one of those, there were almost certainly many more species that evolved but went extinct without making it into the fossil record. Are those speciation events clustered in terms of climate change? My guess is that at certain kinds of climate changes there were clusters of speciation events. What’s important is how long the climate change lasted. The climate change 15 million years ago in the western United States is probably a good example of mammal speciation.
What have you learned through your research?
Barnosky: Probably the most important thing is that you have to pay a lot of attention to the scale of climate change and the scale of evolutionary change when you are talking at the population level and species level. Different magnitudes of naturally occurring climate change are key. When you do that you find that the short, less intense climate changes, that is a couple of degrees C, that are not maintained for more than hundred or a few thousand years will stimulate changes in populations but not necessarily stimulate speciation events. In order to get a speciation event, you have to move to a new climate state and stay there for many hundreds of thousands to millions of years.
It’s important to note, however, that there is an ongoing debate in evolutionary biology about the influence of climatic changes on evolutionary processes. The “Red Queen” hypothesis claims that climatic change is less important than biotic interactions in causing evolutionary change. The “Court Jester” models see climatic change as a very important force in speciation.
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