In a growing
number of natural systems, evolution has occurred so quickly that scientists
can observe it. Examples of such “rapid” or “contemporary” evolution have been
observed in all major lineages and are driven by natural, sexual, and
artificial selection. When I heard of such studies I knew that this would be the focus of my doctoral studies, which I undertook in a US lab at the forefront of
such work. My first dissertation committee meeting, however, did not go as
planned.
While expressing
my desire to experimentally study rapid evolution in the field, one committee
member (Dr. Derek Roff) said something along the lines of: “We know rapid
evolution occurs and the mechanism causing it so why should we care rapid evolution occurs?” Initially I reacted like many
scientists do when faced with criticism about their area of focus: I assumed
that he didn’t understand the topic. I soon came to realize that he had a valid
point and one that has received much less attention in Evolutionary-Ecology: How does rapid evolution alter our
understanding of other biological processes? Should ecologists,
specifically, care about rapid evolution? This became the focus of my
dissertation.
I discovered
that other biologists had begun investigating how rapid evolution influences
ecological dynamics (especially David Pimentel in the 1960s). Yet the pervasive
notion in ecology was that evolution could be ignored unless one was studying patterns
over millions of years. This idea is expressed most often as a supposed
dichotomy between Ecological and Evolutionary timescales. This separation has
its roots in Darwin’s assertion that evolution is a slow and gradual process.
Rapid evolution, however, is now known to sometimes occur within a few
generations, creating an opportunity for the evolution of populations to change short-term
ecological processes.
To study the
impact of rapid evolution on ecological dynamics, I decided to use experimental
evolution. The basic approach was to create replicated populations that can or
cannot evolve by manipulating genetic variation. Inspired by the studies of David
Pimentel and Nelson Hairston Jr., I wanted to break new ground by taking this experimental
approach into the field. Despite being in a fish evolution lab, I decided to
develop my own study system: green peach aphids.
My focal study
population is found on a small nature reserve in Southern California. I
collected aphids and genetically identified clonal lineages. Using population
growth experiments, I identified clones that differed by up to 17% in
exponential growth rate. I now had the variation required to conduct
experimental evolution.
I first tested
whether rapid evolution impacts ecology under controlled greenhouse conditions.
I created replicate populations of aphids that had either a single aphid clone
(no genetic variation and hence no evolution was possible) and compared these
to populations with two different aphid clones (genetic variation in fitness
and hence evolution was possible). I then let the aphid populations grow on
their mustard host, and counted them twice a week for one month (4-5
generations). An ecologist predicting population growth would simply use the
average exponential growth rate in each population (which is the same on the
first day as any day). However, my results showed that populations with genetic
variation grew up to 34% faster than predicted using mean growth rate!
Genotyping confirmed that this was because evolution occurred in the two-clone
treatments. Aphid clones changed in frequency, increasing the population growth
rate within the time course of population expansion. These results suggest that
to properly predict population dynamics one might need to explicitly account
for evolutionary change.
A more difficult
question to address is whether such effects could occur in nature in the face
of environmental variation and the presence of other species. I thus returned
to the nature reserve and conducted a similar experiment. I created replicated
aphid populations in the field, some of which could evolve and others which could not.
I also covered half the populations with cages whereas the other half were not
caged and thus competitors, predators, and parasitoids could interact with the
aphids. After one month, I found that evolving populations grew significantly
faster, up to 42%, and reached up to 67% higher densities compared to
non-evolving controls, even in the face of environmental variation.
Interestingly, this effect only occurred in the natural uncaged treatments, highlighting that ecological context (e.g. predation and levels of competition)
alters the strength of eco-evolutionary dynamics.
These
experiments showed that rapid evolution can impact population dynamics within
only a few generations – but are these ecological changes feeding back to evolution,
so-called Eco-Evolutionary Dynamics? I tested this idea, in the greenhouse, by
manipulating not only the occurrence of evolution but also initial population
density. Many interesting results were observed. Initial aphid density altered
the rate and outcome of evolution. Density also quantitatively and
qualitatively altered how rapid evolution impacts population growth rate,
sometimes accelerating and sometimes decelerating growth. This experiment showed
that one must account for density, population growth rate, and rapid clonal
evolution to properly predict ecological and evolutionary outcomes.
After many years
of research and lots of counting, I have developed an even greater appreciation
for evolutionary biology. Not only is rapid evolution cool, but it can be
tremendously important and seems to strongly influence ongoing ecological
processes. Changes in exponential growth rates that I have observed could have
large impacts on community and ecosystem processes (but I need to test these
effects more rigorously). Other study systems are starting to quantify how
rapid evolution impacts higher levels of biological organization, and they are
generating very interesting results. This is an exciting time for
evolutionary-ecologists as the integration between ecology and evolution is
leading to new insights and a deeper understanding of the natural world.
Martin M. Turcotte
University of Toronto at Mississauga
Motte-Rimrock Nature Reserve in Perris, CA
Experimental aphid population on an Hirschfeldia incana plant in the field
Turcotte, M.M., Reznick, D.N. & Hare, J.D. Experimental Assessment of Rapid Evolution on Population Dynamics. 2011. Evolutionary Ecology Research, 13, p. 113-131.
Turcotte, M.M., Reznick, D.N. & Hare, J.D. The Impact of Rapid Evolution on Population Dynamics in the Wild: Experimental Test of Eco-Evolutionary Dynamics. Ecology Letters, 14(11), p. 1084-1092. DOI: 10.1111/j.1461-0248.2011.01676.x
Turcotte, M.M., Reznick, D.N. & Hare, J.D. Experimental Test of the Full Eco-Evolutionary Dynamic Cycle Between Rapid Evolution and Population Density. In Review.
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