CONCEPTS OF ADAPTATION
Intro
Adaptation is a key concept in ecological anthropology, yet there is much disagreement about what it means
This lecture aims to clarify this rather complex and confusing concept
Begin by considering adaptation as distinct type of explanation
Then outline principles of natural selection that provide theoretical underpinnings for much of biological ecology
Finally, examine distinction between evolutionary & phenotypic adaptation, as well as controversy over levels of selection
Next lecture takes up problems specific to human adaptation, including roles of decision-making or intentionality/agency, and of culture
Types of Explanation
Attributes of living things (including ecological, behavioral, and social aspects) can be analyzed in two distinct but complementary ways:
1) in terms of proximate or immediate causes -- how a particular phenomenon works (= "How?" questions)
2) in terms of ultimate or historical causes -- why a particular phenomenon exists & takes a particular form (= "Why?" questions)
Thus, proximate analysis of an ecological system would explain how it functions (e.g., How does adoption of agriculture affect the health, demography, settlement pattern, and various other features of a non-agricultural population and the environment it utilizes?)
An ultimate analysis would try to answer a different set of questions: (e.g., Why did agriculture arise in or spread to population X? Why do some populations fail to adopt agriculture even when they could? Why does agriculture differ from one time or place to another?)
For human social behavior, proximate explanation generally = intentional analysis; ultimate explanation = evolutionary analysis, emphasizing either genetic or cultural adaptation
I'll return in next lecture to how these two types of explanation can be integrated or reconciled, but for now let's concentrate on evolutionary explanations
In most times and places, "why" questions have been answered in teleological manner: that is, by postulating a purpose for things being the way they are (and hence, by implication -- and often explicitly -- a purposeful being who created & designed things)
In other words, most belief systems have assumed that the structure of the world is a product of intentional action on the part of supernatural beings of one kind or another
Natural science is of course different, in that it rejects the teleological tradition and seeks mechanistic or naturalistic explanations
Just as physicists are unwilling to say earthquakes happen because the gods are shaking with anger, or water flows downhill because it "seeks its own level," ecologists aren't content with viewing the ecological phenomena as the products of intentional design (God's plan, wisdom of "Mother Nature," etc.)
Yet there is the undeniable truth that ecosystems, or at least the organisms that inhabit them, are highly structured, interact in often intricate ways, and seem designed to fit the environments they inhabit
To explain these aspects of biological design without invoking an intentional designer, biologists since 1859 have increasingly relied on Darwin's theory of natural selection
Darwin was certainly not the first naturalist to document the adaptive fit of organisms to their environments, or to propose that contemporary species were outcomes of an evolutionary process
But he was first to propose a thoroughly naturalistic explanation for these features -- to answer "why" questions with an evolutionary and mechanistic account that required no purpose or intentional creator
(Many people of course were and are unhappy with Darwin's attempt to banish purpose from biological design; it is not my goal to convert anyone from religious belief system to naturalistic one, but rather to help you grasp the logic of natural selection because of its fundamental role in contemporary biology and ecology)
Darwin's fundamental achievement, then, was to provide a causal theory for the evolution (via natural selection) of adaptive design
Way he did this was really quite simple, though still misunderstood by today's average college graduate
Natural Selection
Darwin's theory of natural selection is a simple but profound and general argument, and is based on only 3 basic assumptions (observations or empirical generalizations) from which evolution by natural selection is deduced
The 3 assumptions are:
1) individuals vary
2) some of this variation is heritable (transmitted to offspring)
3) some heritable variation affects individuals' chances of reproducing
Given these 3 conditions, adaptation through natural selection is inevitable
To see why , consider each of the 3 conditions in more detail:
1) individual variation (in morphology and behavior) is all around us, and Darwin (+ many others since) have thoroughly documented this
2) heritability of variation is rarely 100%, but all natural selection requires is that there be a significant degree of heritability, such that some variation is transmitted to offspring
3) differential reproduction is due to overproduction of young, competition for resources, and fact that some variants are better fitted to local env. conditions
(Notice that factor 3 highlights the ecological context in which evolution by natural selection takes place)
Darwin realized that given these 3 factors, as individual variants underwent struggle for survival, fitter variants would replace the less fit; he termed this the principle of natural selection
Darwin adopted this phrase by analogy with "artificial selection" (plant and animal breeding carried out by people over the last 10,000 years), but this was ironic choice since explanatory power of argument comes from substituting natural, mechanistic, blind process of differential reproduction of heritable variants for goal-directed choice exercised by human or supernatural designer
Since specific qualities favored by natural selection will be a function of environment as well as existing features of the population, there is no general rule we can use to say what traits will be adaptive; it all depends on particular environment & population
For example, in environments with high incidence of malaria-bearing mosquitos, abnormal hemoglobin molecules that reduce ability of malaria to parasitize red blood cells is adaptive (fitness-enhancing), and various forms of abnormal hemoglobin have in fact evolved repeatedly in different human populations subject to endemic malaria
On the other hand, if environment changes, old adaptive fit will probably worsen, and selection may favor once rare variants that are now better adapted than the common form to these changed conditions
For example: if malaria disappears, abnormal hemoglobin no longer has any advantage, and in fact produces some cost (sickle-cell anemia), so selection will favor its elimination from gene pool
Theory of natural selection tells us that any system that exhibits variation, heritability, and differential reproduction will produce creatures "designed" to fit their environments, as well as evol. of new species due to env. changes or colonization of new habitats; specialization of competing species to divide up scarce resources; evolution of better techniques of both predators and prey; and host of other ecological phenomena
Theory of natural selection thus answers "Why?" questions about living things by proposing that most design features are adaptations that spread and persist because they increased reproductive success of their bearers, by improving the fit between organism and environment
Darwinian concept of adaptation has proven to be powerful tool for understanding biological diversity, but its application to ecological phenomena requires some additional concepts
Adaptation: Evolutionary vs. Phenotypic
First, it is useful to distinguish two meanings of adaptation, evolutionary vs. phenotypic
Evolutionary adaptation is trans-generational; it occurs via differential reproduction of more adaptive variants (i.e., via natural selection); an example might be the evolution of lighter skin color in areas of low sunlight (and of darker skin in areas of high-intensity sunlight)
Phenotypic adaptation involves various sorts of processes (physiological adjustments such as acclimatization; learning; decision-making); these occur within lifetime of a single individual, rather than across generations; an example might be tanning of lighter skin upon exposure to strong sunlight
Does phenotypic adaptation occur through a process of natural selection? Clearly not
But does it have something to do with natural selection? Definitely yes!
The link is this: the biological characteristics of individual that produce phenotypic adaptation are themselves usually products of evolutionary adaptation
In other words, the capacity of any individual organism to make adaptive adjustments arose by natural selection acting on the ancestors of that individual
For example, tanning is a process of phenotypic adaptation -- a way for lighter-skinned individuals to adapt (phenotypically) to intense sunlight by building up protective pigmentation in their skin
But the capacity to tan is an evolutionary adaptation, presumably evolved genetically by our ancestors via a process of natural selection
This takes us back to our distinction btwn "how?" vs. "why?" questions:
• To know how tanning works, we study it as a phenotypic adaptation
• To know why capacity for tanning exists, we study it as an evolutionary adaptation (for which natural selection is the ultimate cause)
Hence, even though any given instance of phenotypic adaptation is not strictly Darwinian, it can be analyzed in Darwinian terms if we have reason to believe it is due to a mechanism or characteristic that is itself a product of evolutionary adaptation
If this logic is correct, then even elaborate behavioral responses to particular conditions (such as varying one's generosity according to expectations that the recipient will reciprocate) might usefully be analyzed in adaptationist terms
Second important clarification of adaptation concept has to do with what is called "levels of selection problem"
Key issue here is, What level of biological organization (gene, cell, organ, individual, group, population, species, ecosystem, biosphere...) are adaptations generally designed to benefit?
Darwin had no workable mechanism of inheritance, but the discovery of genes as the primary mechanism of inheritance in organic evolution had many profound consequences for evolutionary theory, including a clarification of the levels of selection issue
In particular, linkage of natural selection theory to Mendelian inheritance provided answer to query "survival of the fittest what?"
The ultimate answer is, the fittest gene or gene combination; but of course genes express their fitness (i.e., relative rates of replication) thru their phenotypic effects
But what kind of phenotypic effects will be favored by natural selection?
In past, biologists often spoke of evolved traits as serving "the good of the species," or even the stability of ecosystems; but there is no basis in evol. biology for supposing that natural selection favors any trait for that reason
That's because selection acts via differential fitness of heritable variants, and this process works much more rapidly and effectively on variants that affect individual survival and reproduction than those that affect populations, species, or higher-level categories
Thus, in most cases individual-level selection outpaces any higher-level selection
To see why, consider example of population regulation discussed by Irons (pp 11-12)
If population increases beyond the sustainable limits, it will overharvest its food, and perhaps crash to low levels or even go extinct
So wouldn't natural selection favor populations that limit size to sustainable level?
Well, population growth is function of individual reproduction; so this really means individuals would have to evolve mechanisms to produce fewer offspring than they could currently support so that in the long run resources weren't overexploited
Suppose we start with a population composed of such reproductively-restrained types, nicely balanced below the long-term sustainable population size
(Starting here of course begs the question of how the population evolved to be composed of restrained types; but since such a starting point is adaptive for the population, it makes the subsequent story even more convincing in exposing the weakness of group-selection arguments)
Then allow a mutant to arise that raises 3 offspring instead of just the replacement number of 2 (2 offspring = "replacement" if each offspring has 2 parents, and there is no mortality before reproduction -- the last assumption being unrealistic, but affecting only the numerical details and not the logic of the argument)
If there is not enough food or some other limiting resource to successfully raise all 3 offspring, then natural selection will not favor the higher (mutant) reproductive rate; but if 3 offspring can be successfully raised, and this mutant prolific strategy is heritable (think of some behaviorally stereotyped organism like an insect, not humans!), then it will begin to increase in the population
Similarly, the prolific strategy can spread by gene flow from adjacent populations
Even though in long run the prolific strategy may doom the population to overexploit its resource base and crash, it is still favored by individual-level selection; and this will overwhelm group-level selection in all but exceptional circumstances (involving frequent group extinction, extremely low rates of gene flow between groups, etc.)
Although this stylized example represents the case where individual fitness and population survival are at odds, it is important to note that traits favored by individual-level selection may often have the incidental effect ("byproduct") of favoring population or species survival (e.g., if giving birth to fewer offspring than physiological maximum allows one to raise more surviving offspring total, as noted above)
For example, many plant species have evolved chemicals that are toxic to their predators, and this keeps them from being eaten up; this has the incidental effect of reducing the chance that the species will become extinct from over-predation, but that is not why this feature evolved -- rather, it evolved because mutants who had this toxicity trait survived better and passed on this trait to more offspring than those who lacked the trait
(If you don't find that example convincing, note that many plants also produce chemicals that poison seedlings of their own species, which aids in competing for scarce growing sites; these clearly can't be designed to maximize the species survival or well-being!)
It's even less likely that natural selection can produce adaptations to benefit higher levels of organization (sets of species, ecosystems, or the biosphere as a whole, as in some versions of the "Gaia hypothesis")
Here again, we must be careful to distinguish fortuitous consequences (incidental effects) of individual adaptation from features selected because they benefit higher levels
For example, predators may end up taking weaker members of a prey population because they are easier to catch, which will incidentally increase the average healthiness of the prey population
But this isn't part of any evolutionary design to "cleanse" the prey species and maintain its "health"; rather, it is simply a consequence of natural selection having "designed" predators to catch their prey as efficiently as they can (see later lecture & readings on "foraging strategies"), and of prey being designed to escape as best they can (what some evolutionary ecologists have called an "arms race" between predators and prey)
Even more contrary to evolutionary theory is notion that ecosystem or biosphere stability could be an evolutionary adaptation; this would require a large pool of ecosystems or biospheres which exhibit differential survival and reproduction and among which natural selection or some other process could "choose" to produce the most stable form -- and all occurring at a rate high enough to counteract lower-level selection that might work at cross purposes to ecosystem or biosphere stability
In sum, adaptations designed to benefit species or ecosystems per se are theoretically unlikely, and those proposed to date are better explained in terms of evolutionary benefits to individuals (or the incidental effects of same)
What about human ecology and behavior?
Social scientists (incl. many anthropologists) have a long history of explaining social features as functioning to ensure survival and smooth functioning of the group or society
Ecological anthropology has often exemplified this tendency, with many analyses claiming that human ecological adaptations can be explained as designed to ensure group survival, prevent overpopulation, maintain ecological stability, etc. (see examples near end of Vayda & Rappaport article)
As discussed in the Irons reading, Darwinian theory suggests this may be a mistaken assumption, and the attempt by some to justify this assumption by appeal to theories of group selection in evolutionary biology (e.g., Rappaport's reliance on the group-selection arguments of biologist Wynne-Edwards) exemplifies the hazards of not having a good current understanding of the source discipline from which one is borrowing
However, human societies possess certain features that prevent us from simply applying Darwinian theory as we would to any other species
In particular, need to consider the roles of culture and decision-making in human ecological adaptation -- the topic of the next lecture & class session