Does Interspecific Competition Mean Anything in an Evolutionary Context?

The first thing I think when I read the title to this post is, "Man, that's a long friggen title, and it's not at all catchy." The next thing I think is, "I thought I said I wasn't going to write about ecology." Well, I actually wrote:

"I am in no way, shape, or form an organismal biologist nor am I an ecologist, and if you catch me out of my element, by all means, please set me straight."

So, I invite you to put me in place if I'm talking out of my ass. But why would I write about an ecological concept like interspecific competition? This actually stems from a course that I am TAing this semester, entitled "Populations and Communities". The class is a sort of primer for ecology and evolution, introducing the students to behavioral ecology, population ecology, population genetics, and community ecology, among the general ecology and evolution topics. For one of the laboratory experiments, the students study plant populations grown with and without another species present to examine the effects of interspecific competition (it is designed as an application of the Lotka-Volterra model). This triggered a thought in my mind: "What does interspecific competition mean in an evolutionary sense?" My mental meanderings can be found below the fold . . .

Natural selection, one of our favorite evolutionary forces, operates on individuals -- certain genotypes are more fit than others and leave more offspring who, in turn, leave more offspring ad infinitum. As a self proclaimed "reductionist", I like to think of selection operating on the actual genotypes or, even better, the alleles (with the genotype merely part of the environment). The reductionist within me (tiny little fellow) likes to think of it as selection operating on nucleotides, and the rest of the genome becomes part of the environment. So, I asked myself, where can between species competition fit into this framework? What difference does it make if an individual, or an allele, or a nucleotide must compete with a member of its own species or one from a different species?

Before I can let my mind wander any further down this road, I must define what I mean by interspecific competition. For the purpose of this discussion, we will consider two seperate species that are diverged enough such that there is not dispute that they are different species. These two species occupy the same (or very similar) ecological niches. By definition, two species cannot occupy the same niche in the same space at the same time -- one must shift its niche, move to a different location, or go extinct. I will assume that the two species recently came into contact, and that they occupy extremely similar niches.

The one difference that stands out to me between intra- and inter-specific competition is that in the former there is a single gene pool, whereas in the latter there are two separate gene pools. If we assume random mating within each species, we should expect a decrease in population size if there is interspecific competition. Population size plays an important role in evolution. Smaller populations experience more stochastic fluctuations in allele frequencies; this is analogous to flipping a fair coin (no matter how many times you flip it, you expect it come up heads half of the time, and tails the other half, but the more times you flip it, the higher the probability that the frequency of heads will be 0.50). Population size also affects the ability of natural selection to fix beneficial mutations and purge deleterious ones. The strength of selection is proportional to the fitness effect conferred by a particular allele (s) and the population size (N). An allele with drastically beneficial or deleterious effects will be fixed or lost regardless of population size, but those alleles that are considered "weakly deleterious" or "weakly beneficial" will only experience significant selective pressures in larger populations. This means that deleterious alleles are more likely to remain at appreciable frequencies and beneficial mutations have a lower probability of fixation in smaller populations.

At this point, I guess we can hazard a guess at the original question: yes, inter-specific competition does have an evolutionary meaning because it reduces population sizes. We would expect that a species involved in a fight for limited resources with another species will have a lower population size than one not in such a competitive environment. The end result would be more fluctuations in allele frequencies due to drift. Additionally, these populations would have more mildly deleterious alleles segregating in their gene pool, and a beneficial mutation will have a harder time reaching fixation.

Let us dwell on natural selection for bit. With a new competitor in their midst, each species faces a new pressure . . . or do they? If we think of selection operating on individuals, it should be no different if an individual is competing with its own species or another species. An individual (or new mutation) will be under selective pressures to exploit a new niche in either case, so does this selective pressure differ under interspecific competition? The one situation I can think of in which it would, is selection for a larger effective population size. The effective population size (the size of an idealized Wright-Fisher population that models our actual population) is affected by census population size, mating patterns (whether they are random or not), ancestral population sizes, and other demographic causes. To maximize its effective population size, a population could undergo random mating, but this would come at a cost to those individuals who are reaping the benefits of non-random mating (ie, the dudes who are getting all of the chicks). This makes me think switching the mating scheme is probably unlikely (does anyone know of an example where this has happened?). Alternatively, if the species explored a new niche, it could increase its census population size which would lead to an increase in effective population size. This pressure to explore a new niche would be greater under interspecific competition, as compared to intraspecific competition, because the initial population size would be smaller in the former scenario. Here is where my limited knowledge of ecology forces me to pose the question, do populations that are competing with other species explore more niches than populations without such competition?

I have thought about how the genetic relatedness of individuals in the population affects interspecific competition, but I couldn't figure out how to work it in to my framework. For example, what role does kin-selection play in the relative effects of intra- and inter-specific competition? Is interspecific competition fundamentally different from intraspecific competition because in the latter alleles/genotypes are competing with more closely related alleles/genotypes? Any suggestions would be welcome. Also, please point out any egregious mistakes or erroneous assumptions on my part.