The Evolutionary Psychology FAQ

Edward H. Hagen, Institute for Theoretical Biology, Berlin

What is the EEA? (detailed answer)

In order to understand the precise definition of the EEA, we must understand the definition of a selection pressure. Many of the misconceptions about the EEA can be avoided by adhering closely to the precise definition of the EEA derived from the theory of natural selection. As noted above, the EEA is the set of all selection pressures faced by an organism's ancestors over 'recent' evolutionary time (i.e., over approximately the last 1000-10,000 generations). To understand what a selection pressure is, we must understand how a mutation spreads in a population. It must alter the phenotype in some way that enhances reproduction (ignoring drift and other similar processes for the moment). As emphasized elsewhere in this FAQ, reproduction is an enormously complex process; that it happens at all is a near miracle. Reproduction involves a vast number of physical processes that must proceed correctly if reproduction is to be successful. Given the design of an organism, given all the physical transformations that have to take place in order to reproduce, and given ALL the environmental conditions that the organism may encounter with some non-zero probability during its life, there is a (relatively) small set of *potential* transformations of the environment--where the term environment may include aspects of the organism itself--that will enhance rather than impede reproduction. These potential reproduction enhancing transformations are called selection pressures. Stated another way, selection pressures are those aspects of the environment that can have a notable impact on the reproduction of members of a particular species over evolutionary time. The EEA of any species is the set of all features of the environment that could have had some impact on the reproduction of members of this species over recent evolutionary time.

For example, let's assume that an herbivore regularly ingests a particular plant toxin, and that this toxin has a detrimental effect on sperm quality. Let's also assume that there are enzymes that can neutralize this toxin, but that the herbivore cannot produce these enzymes. The fact that the plant toxin can be neutralized by an enzyme is an example of a *potential* transformation that could facilitate the reproduction of the herbivore (because it would result in improved sperm quality). Thus, the plant toxin is a selection pressure and is therefore an aspect of the EEA of the herbivore. Should a mutation arise that produces a toxin neutralizing enzyme, this mutation will spread in the population. After many generations, all herbivores of this particular species will now be able to neutralize this plant toxin. If the plant goes extinct, the herbivores will still be able to produce the detoxifying enzyme (for many generations, at least), and this particular toxin is still considered an aspect of the species' EEA. Alternatively, if no mutation ever arose to produce a detoxifying enzyme, this plant toxin was still a feature of the species EEA. It was a selection pressure, even though no adaptation evolved to neutralize it.

On the other hand, if a different toxic plant also grew in the same area as the first toxic plant, but the herbivores never ate that plant, then the second plant and its toxin are not considered part of the species' EEA. The second plant and its toxin were never a selection pressure--they had no impact on the herbivores' reproduction, and no transformation of the second toxin would facilitate herbivore reproduction. So, in use, the EEA refers not only to transformations of the environment that were necessary for reproduction, but also transformations that could have *potentially* facilitated reproduction. It does *not* refer to aspects of the past that could not impact reproduction in any way.

Notice that for a mutation to spread to fixation (i.e., to the entire population), it must transform the environment in a reproduction facilitating way for many generations (1000, say). This means that the mutation must interact, via the phenotype, with a recurrent aspect of the environment--an aspect that the organism and its descendants are likely to encounter with some significant probability over each of their lifetimes. For example, in the case of the plant toxin, it is not necessary that every individual herbivore regularly ingested the toxin; it is only necessary that, over evolutionary time, members of this species encountered the toxin frequently enough that those who could neutralize it would have had, on average, a reproductive advantage over those who couldn't.

Once a mutation has reached fixation, it must continue to experience a selection pressure (stabilizing selection) in order to remain in the genome; otherwise it will tend to be eliminated by subsequent mutations. Eyes evolved long before humans appeared, but if sunlight was not a part of the human EEA, we would have lost our visual capabilities, as have certain species of cave-dwelling fish. In the case of the plant toxin, if that particular plant went extinct, the ability to produce the neutralizing enzyme would degrade over time due to random mutations of the gene that produced the enzyme--there were be no selection pressure against organisms that could no longer neutralize the toxin, because the toxin is no longer part of the environment. Thus, even if an adaptation evolved in an ancestor species (as eyes did in an ancient ancestor of humans), the selection pressures that maintained eyes over recent evolutionary time are considered part of the human EEA. Stabilizing selection pressures are part of the EEA.

It is worth noting that the organism is part of its own EEA. For example, the heart creates a pressure differential in the circulatory system; this pressure differential then results in a nutrient rich liquid (blood) being circulated to other tissues. Thus blood, arteries, and veins were essential features of the EEA of the heart (and thus of all organisms with hearts, including humans).

The definition of the EEA as the set of all selection pressures acting over recent evolutionary time has some notable implications. First of all, selection pressures are adaptation specific. The selection pressures acting on visual abilities are (in general) not the same as those acting on toxin neutralizing abilities. Thus, the evolutionary history of vision will (again, in general) not be the same as the evolutionary history of toxin neutralization. For example, one adaptation (like vision) may have a much longer evolutionary history than another (like the ability to neutralize a specific toxin). Another implication is that species can be adapted to a variety of mutually exclusive environmental conditions e.g., day and night, hot and cold, feast and famine, high population densities, low population densities, male biased sex ratios, female biased sex ratios, lots of predators, few predators, etc., so the EEA definitely does not refer to a fixed or static time or place.

Perhaps the most important implication is the following: organisms possess functional traits because those traits were selected for over evolutionary time. This means that those traits reliably performed their functions in past environments, and may or may not properly perform them in current environments. Thus, the EEA refers to those aspects of past environments to which an organism is adapted. Any organism can possess adaptations which no longer serve any reproductive function, and may even impede reproduction.

A couple of quick examples will illustrate many of the foregoing points. We have lungs because oxygen existed in our atmosphere *in the past,* not because oxygen exists in the immediate present. Should oxygen somehow disappear from the atmosphere, we would still have lungs, they just wouldn't work (and we would quickly go extinct). Fortunately, the current environment strongly resembles the EEA in this regard. As another example, Richard Coss has done work on physiological and psychological ground squirrel adaptations to rattlesnake predation. He shows quite convincingly that these ground squirrels retain protective adaptations even when they haven't faced rattlesnake predation pressure for very long periods, but that these adaptations are increasingly degraded the longer the squirrels have been in rattlesnake free environments.

Some aspects of the modern environment do diverge quite radically from the human EEA. Two examples: 1) automobiles kill far more people today than do spiders or snakes, but people seem to be far more averse to spiders and snakes than they are to automobiles. Why? Because in the EEA, spiders and snakes were a serious threat, whereas automobiles didn't exist. Thus, it was possible for us to evolve an innate aversion to spiders and snakes, but not to automobiles. 2) Safe and highly effective birth control is a modern invention. For most of human history, women were pregnant or lactating for much of their adult lives. Interestingly, women in natural fertility populations (modern populations that don't have access to modern methods of birth control) appear to have a much lower rate of reproductive cancers than do women in populations with easy access to modern birth control. It has been argued that early and frequent pregnancies may prevent reproductive cancers. Because modern forms of birth control did not exist in the EEA, there was no selection pressure against the reproductive cancers that may accompany their use.

If the current environment of a particular species fails to resemble the species' EEA in too many ways, then the species will go extinct. Since the human species is clearly not going extinct, the common complaint that evolutionary psychology views humans as currently living in an entirely novel environment is clearly false. Most aspects of the modern environment closely resemble the human EEA. Hearts, lungs, eyes, language, pain, locomotion, memory, the immune system, pregnancy, etc., all work as advertised--excellent evidence that the modern environment does not radically diverge from the EEA.

Much research in evolutionary psychology proceeds as follows. First, identify a plausible selection pressure (often called a reproductive problem), like predation, for example. Second, hypothesize a cognitive solution to this problem, e.g., the ability to detect and avoid predators. Finally, devise and conduct experiments to see if humans in all populations have a specialized ability to detect predators and then avoid them. If they do, this implies that the human species has evolved psychological mechanisms for detecting and avoiding predators. Click here for more on this particular example.

Copyright 1999-2002 Edward H. Hagen