When discussion turns to the rapid evolution of resistance in response to toxic chemicals, inevitably someone has to ask, “So, what about us? Can we evolve our way out of this mess?” Years ago, one might have responded with a smart remark about microbes and mosquitoes inheriting the earth. Now, as we enter a new age of genomics, transcriptomics, metabolomics and all sorts of other ‘omics, the way we think about genetics, heritability and evolution is undergoing its own rapid evolution, the notion that human populations can undergo evolution as a consequence of our own folly no longer seems so far-fetched.
Still it’s a complicated question. And so we might begin with a few more manageable questions: 1) are humans still evolving 2) if we are, how rapidly can we evolve and 3) what kind of threat would be required to elicit rapid evolution in humans?
To some degree, human culture, technology and behavior, have enabled us to sidestep natural selection, likely altering our evolutionary trajectory. Even so, according to evolutionary biologist Stephen Stearns and colleagues, humans have not yet become a static, unchanging species. “[T]raits in many human populations,” they write “are subject to natural selection, and have the genetic potential to respond to it…” in other words the pieces for evolution to occur in humans are in place (see Measuring selection in contemporary human populations by Stephen Stearns et al.,) At the very least, certain human traits (age at first birth, height, and menopause for example) remain susceptible to selection, which means that despite our advances we have not yet become a “post-evolutionary” species. This is a good thing because for a species to no longer evolve, its differential survival and reproductive success must no longer be under genetic control – an unsettling prospect given how well these interactions have served life for the past 3.5 billion years or so. Yet before we celebrate, we ought to consider the second question, how rapidly could we evolve?
For species facing imminent decline as a result of environmental change (physical, chemical or ecological), rapid change requires a rapid response if a species is to persist. The old evolve or die scenario. But as evolutionary biologists Andrew Hendry and Michael Kinnison point out in their 1999 perspective, The Pace of Modern Life: measuring rates of contemporary evolution, “…claims of rapid evolution, mean little without specifying what ‘rapid’ actually means.” So just how rapid is rapid?
A few examples might help. We often hear about rapid evolution in pathogenic microbes. With generation times as short as 20 minutes (not to mention all sorts of DNA swapping that goes on – a topic well beyond this post) it’s no surprise that disease causing bugs like staph and gonorrhea have evolved adaptations to almost every chemical we throw at them. And though mosquitos don’t reproduce every twenty minutes, these notoriously rapidly evolving pests mature within a week and may lay thousands of eggs within a few weeks. It’s essentially the same with lice, bedbugs, cockroaches and other common pests upon whom we waged chemical warfare. So how does this play out for less fecund species with longer generation times?
Studies over the past few years conclude that estuarine killifish and Atlantic tomcod have adapted to PCBs and similar chemicals over the course of fifty years or less. Yet even for these little fishes, fifty years means roughly 50 generations (or fewer) and hundreds if not thousands of progeny per generation, depending on the species. Fifty generations for us spans roughly 1,200 years, and when it comes to offspring, despite our talent for overpopulating the planet, we are no match for more fecund early maturing species whether rapidly evolving mosquitoes, mice or minnows.
Yet there are some more reproductively “frugal” species, which have also shown evidence of rapid evolution. Darwin’s finches mature between less than a year and four years of age and lay anywhere from a few to dozens of eggs each year, (again, depending on the species.) Yet, when faced with drought and changes in available foods, differences in body and beak size can evolve within a few generations, which is astoundingly rapid.
All of the above are easily categorized under “contemporary” evolution occurring not only within a few centuries (see Hendry and Kinnison’s paper for more), but within a single human life span. So, how competitive are humans in this evolutionary race?
One common example of recent evolution is lactose tolerance in pastoral populations, an evolutionary shift which enabled adults to benefit from a new food source rich in fats and proteins. But recent does not equate with rapid or contemporary, and although the trait evolved within the last 10,000 years (recent for humans) it is unclear how many generations were required for tolerance to spread throughout a population. So it seems that the answer to the second question is, rapid is relative, and relatively speaking, we are marathoners rather than sprinters. In part, a consequence of our reproductive strategies combined withour cultural and technological know-how.
That said, rapid advances in genetics, biology, biochemistry, technology and associated fields are reshaping the science of evolution. We now understand that based solely on our genes, humans are not so different from mice, we know that there are myriad small genetic differences spread across the human population, and that we know very little about the control and expression of genes which must contribute in large part to the differences between mice and humans. One groundbreaking development when it comes to understanding gene expression is epigenetics – a swiftly developing field focused on understanding biochemical and heritable alterations of gene expression. Though too large a topic to tackle here (see The Epigenetics Revolution by Nessa Carey and Epigenetic synthesis: a need for a new paradigm for evolution in a contaminated world by David Crews and Andrea Gore) in light of the pending epigenetics revolution, dismissing rapid or even contemporary evolution in humans may yet be premature.
So, we are evolving. How rapidly evolution can occur in humans is, as of yet, unclear, as is whether or not we have the capacity to evolve resistance in response to toxic chemicals. But even if we could evolve as rapidly as a killifish, or finch or mosquito, can we envision a scenario where we’d be exposed to conditions which are either persistent enough or are strong enough, or sufficiently wide-spread (if we think about epigenetic changes as discussed below) to force the hand of evolution within a few generations? Which brings us to the third and perhaps most speculative question, what kind of environmental pressure would lead to rapid evolution in humans?
The consequences of not adapting to pressures exerted on microbial populations by antibiotics, insecticides on mosquitos and food scarcity for birds are clear. What pressures might act upon human populations? Food scarcity? New disease patterns associated with climates change or antibiotic resistance? New dietary staples like refined sugars? Or caffeine? (A role for caffeine in human evolution has already been suggested in a few studies.) And will these pressures be sustained long enough or will they be strong enough to influence the course of human evolution?
Writes evolutionary biologist Jonathan Pritchard, in his Scientific American essay How we are evolving “…our data suggest that the classic natural selection scenario, in which a single beneficial mutation spreads like wildfire through a population, has actually occurred relatively rarely in humans in the past 60,000 years. Rather this mechanism of evolutionary change usually seems to require consistent environmental pressures over tens of thousands of years—an uncommon situation once our ancestors started globe-trotting and the pace of technological innovation began accelerating.”
Perhaps. But as the study of epigenetics blossoms, we might soon consider the influence of chemicals like BPA and others that cause epigenetic changes. Could we evolve rapidly as a result of chemically induced alterations in gene expression?After all, not all evolutionary change is necessarily positive or beneficial. Depending on how evolution is defined change may refer to gene frequency or to alterations in the frequency of expressed genes, or both.
We have learned to curtail the potential for sustained multi-generational exposures to any one industrial contaminant (PCBs for example were banned in the 1970s, and production and release of other halogenated organic chemicals if not restricted, are at least under scrutiny), yet there are tens of thousands of chemical contaminants and we are continually producing and releasing new products. We also know that some chemicals, not to mention many other modern day stresses can cause heritable epigenetic change within a single generation which can persist for three generations or more (see Crews et al., Epigentic transgenrational inheritance of altered stress responses and Why Fathers Really Matter, by Judith Shulevitz.)
We can hope that humans are unlikely to encounter the persistent inter-generational pressures necessary to evolve resistance to chemicals like PCBs à la estuarine killifish, but it is also possible that we may experience a far more insidious kind of exposure to chemicals which cause epigenetic changes. How these chemicals and stresses will impact human evolution is anybody’s guess.
We might also hope that the more things change, the more they stay the same. As Russell Powell discusses in his paper The Future of Human Evolution, nature has provided life with defenses against randomly drifting into oblivion, emphasizing that “…there is every indication that the vast majority of human biological functions are under strong stabilizing selection, which on a proper conception of evolution means that they continue to evolve.”
Perhaps if epigenetic changes caused by modern day chemicals and stress are a growing threat, stabilizing selection will prevail, although given our track record, chemicals causing epigenetic changes may outpace stabilizing processes unless those too have an epigenetic component. Or perhaps microbes and mosquitoes really will, some day in the far off future, inherit the earth.
Note: this blog post has been submitted to the National Evolutionary Synthesis Center best Evolution Themed blog contest (see a description here.) Anyone can enter…though that just means more competition for rapidly evolving blogs such as this one!