Rapidly Evolving Rodents
That humans can alter the natural environment is well known. We have been hunting, fishing and clearing land for agriculture for tens of thousands of years. More recently humans have gained the ability to drain swamps, dam rivers, level mountains and pave over darn near anything. Environmentalists think this kind of activity is abhorrent, and go so far as to claim that H. sapiens are responsible for a new major wave of extinctions. In general, animal species can either move, go extinct or adapt to human caused environmental changes. Many biologists will tell you that species just can't evolve fast enough to deal with rate of human induced change. As it turns out, this isn't exactly true.
Rapid morphological change in mammals has been infrequently documented. However, recent research has revealed that evolution often occurs on contemporary timescales, often within decades. Well known examples of rapid evolution include such morphological changes as beak length in Florida soapberry bugs, wing length in Galapagos finches, or female lifespan in Trinidadian guppies. In “Contemporary evolution meets conservation biology,” Craig A. Stockwell, Andrew P. Hendry and Michael T. Kinnison link rapid contemporary evolution to the same root causes of the so called extinction crisis.
“Ultimately, contemporary evolution is influenced by complex interactions among population size, genetic variation, the strength of selection, and gene flow, making most management scenarios unique.” they stated in the February 2003 issue of Trends in Ecology & Evolution. “In a world filled with contemporary evolution, conservation efforts that ignore its implications will be less efficient and perhaps even risk prone.”
Recently, a team of scientists used next-generation sequencing technology to conducted a genome scan of nucleotide diversity and differentiation in natural populations of threespine stickleback (Gasterosteus aculeatus). Sticklebacks are a small silver-colored fish, under two inches in length, that are found throughout the Northern Hemisphere in both fresh and saltwater habitats. According to a report in PLoS Genetics by Paul A. Hohenlohe et al., oceanic threespine stickleback have invaded and adapted to freshwater habitats countless times across the northern hemisphere. These freshwater populations have often evolved in similar ways from the ancestral marine stock from which they independently derived.
Example of rapid stickleback evolution.
”Populations of freshwater stickleback arise when new habitats open up and are colonized,” said William A. Cresko, professor of biology, member of the University of Oregon's Center for Ecology and Evolutionary Biology, and a member of the research team. “Alaska has a lot of lakes that have been around only about 10,000 years, formed after glaciers receded. Instead of dying out when they were cut off from saltwater, they evolved very rapidly and in a lot of ways, such as in their bones and armor, the shapes of their jaws, as well as coloration and behavior. When one population no longer recognizes and won't mate with another population, they effectively become a new species, so some of the regions we are identifying may be important for speciation, too.”
Here is documented evidence of evolutionary adaptation to climate change that must have taken place very rapidly, on the order of just a few thousand years, or in a few instances in just a few decades. But stickleback are just tiny fish with a genome about one sixth the size of a human's. Even more exciting evidence of rapid adaptability has surface—this time involving mammals.
The Norway or Brown Rat (rattus norvegicus) is found through the world.
In another recent PLoS article, “Recent and Widespread Rapid Morphological Change in Rodents,” Oliver R. W. Pergams and Joshua J. Lawler report that over the last 100+ years, rapid morphological change in rodents has occurred quite frequently. Prior examples have often been identified within isolated island populations but Pergams and Lawler report that these changes have taken place on the mainland as well as on islands. Their results suggest that these changes may be driven, at least in part, by human population growth and climate change. Here is how they charaterize their work in the article's abstract:
Here we document rapid morphological changes in rodents in 20 of 28 museum series collected on four continents, including 15 of 23 mainland sites. Approximately 17,000 measurements were taken of 1302 rodents. Trends included both increases and decreases in the 15 morphological traits measured, but slightly more trends were towards larger size. Generalized linear models indicated that changes in several of the individual morphological traits were associated with changes in human population density, current temperature gradients, and/or trends in temperature and precipitation.
Some biologists would argue that the rapid changes often observed in such studies are due to phenotypic plasticity—the ability of some species to adjust their physical characteristics in response to their environment. Phenotype is what an organism looks like, its observable physical or biochemical characteristics as determined by both genetic makeup and environmental influences. As University of Toronto evolutionary biologist Anurag A. Agrawal explained in “Phenotypic Plasticity in the Interactions and Evolution of Species” (Science October 12, 2001):
When individuals of two species interact, they can adjust their phenotypes in response to their respective partner, be they antagonists or mutualists. The reciprocal phenotypic change between individuals of interacting species can reflect an evolutionary response to spatial and temporal variation in species interactions and ecologically result in the structuring of food chains. The evolution of adaptive phenotypic plasticity has led to the success of organisms in novel habitats, and potentially contributes to genetic differentiation and speciation. Taken together, phenotypic responses in species interactions represent modifications that can lead to reciprocal change in ecological time, altered community patterns, and expanded evolutionary potential of species.
Evolutionary biologists have been interested in studying the genetic basis of phenotypes, and early work was focused on traits presumed to be unaffected by the environment. Simply defined, evolution involves changes in the predominant genotypes of a local population as a result of natural selection. The change is either a result of selection favoring a recent mutation or a previously rare genotype. Nowadays, it is recognized that phenotypic changes can be influenced by environmental changes without altering an organism's genetic makeup, or genotype. In other words, phenotypic plasticity is a non-genetic adaptation to changing environmental conditions.
Examples of phenotypic plasticity in fish.
Noting that plasticity is notoriously difficult to distinguish from direct genetic evolution, Pergams and Lawler contend that phenotypic plasticity can also be the result of natural selection. “Our results clearly demonstrate rapid morphological change in multiple rodent species from both island and mainland populations.” they conclude. “Furthermore, some of these changes appear to be driven by altered climates and growing human populations.” Regardless of whether you call it evolutionary change or phenotypic plasticity it amounts to the same thing—successful organisms can rapidly adapt to changing environmental conditions, regardless of the source of those changes.
The bottom line according to Pergams and Lawler: “Species that are able to respond quickly to environmental changes, whether through phenotypic plasticity, movement, or evolution will have a higher probability of surviving the rapid human-driven land-use and climate changes projected for the coming centuries.” In other words, the evolutionary race belongs to the adaptable. In this sense, whatever pressure mankind is putting on other creatures is simply a part of the evolutionary winnowing process. Far from being a threat to nature we are doing nature's evolutionary dirtywork.
And far from being H. sapiens' helpless victims, all sorts of lifeforms are busily adapting to environmental changes, both natural and man-made. From an evolutionary point of view, the cause of change does not matter, only the response. One of the reasons for mankind's success is our adaptability. We have spread to practically every continent and every type of ecosystem on Earth.
Rat Evolution. Picture by Alexis Rockman from Future Evolution.
It should, therefore, not be surprising that rats, a species that is found in many of the same places as people, are also highly adaptable. Omnivorous and opportunistic the Norway or brown rat (Rattus norvegicus) is one of the best known and most common rats. Thought to have originated in northern China, this rodent has now spread to all continents, except Antarctica, making it the most successful mammal on the planet after humans.
As I pointed on in my post about Tyrannosaurs rex, when a dominant species eventually answers nature's final curtain call, its extinction makes room for the rise of its successor. Perhaps we should reflect on the fact that the ever adaptable rat is waiting in the evolutionary wings.
Be safe, enjoy the interglacial and stay skeptical.