Observation and

Etymology: from the Latin “ad” = toward, “aptus” = fitted, appropriate

Three dictionary definitions for adaptation in the biological sense:

1. any alteration in the structure or of an organism or any of its parts that results from and by which the organism becomes better fitted to survive and multiply in its environment

2. a form or structure modified to fit a changed environment

3. the ability of a species to survive in a particular ecological niche, esp. because of alterations of form or behavior brought about through natural selection

In this lab, we will be most interested in the second definition, i.e., in traits that are called . In particular, we will be concerned with how one can demonstrate that a trait is, in fact an adaptation. This is one of the major activities of .

Adaptation as defined in Freeman (2002): Any heritable trait that increases the fitness of an individual with that trait compared to individuals without the trait.

This definition leaves out one feature that is considered key when deciding whether or not a trait is an adaptation: did the feature evolve in response to selection for the function it currently performs?

This issue was discussed most famously in a 1979 paper called “The Spandrels of San Marco and the Panglossian Paradigm: a critique of the adaptationist programme” by Stephen J. Gould and . The message of this paper was that we shouldn’t assume that every feature of an organism is an adaptation, i.e., that it was molded by natural selection for its current function. Gould and Lewontin chide adaptationists for making up too many adaptive stories, akin to Rudyard Kipling’s Just-So Stories (e.g., “How the Leopard Got Its Spots,” “How the Camel Got Its Hump,” etc.). It’s very easy, particularly with a little practice, to look at traits of an organism and come up with stories about why they are adaptive. This is fine as long as the stories can be made into testable hypotheses. The danger lies in making up stories and then accepting them based on their plausibility alone.

Gould and Lewontin referred to the adaptationist program as the “Panglossian Paradigm” in reference to the character of Pangloss in Voltaire’s Candide. Pangloss’s philosophy is “...that things cannot be other than they are, for since everything was made for a purpose, it follows that everything is made for the best purpose. Observe: our noses were made to carry spectacles, so we have spectacles. Legs were clearly intended for breeches, and we wear them.” (Penguin Classics edition, p. 20) Gould and Lewontin accuse people who follow the adaptationist program of making essentially the same mistakes as Pangloss, albeit in a Darwinian context. An adaptationist (to continue the absurd example from Candide) might observe a group of people with glasses, note that the nose is holding up the glasses in each of these individuals, and infer that noses are adaptations for holding up glasses. Nobody would deny the utility of noses for holding up glasses, but this example is trivial enough for it to be clear that noses probably evolved in response to selection for some other function and were later co-opted for eyeglass- holding-up.

The giraffe-neck presents another possible case in which the seemingly obvious function for which a feature is adaptive may not actually be the function for which the feature evolved. Why is the giraffe neck so long? The most oft-cited evolutionary scenario is that it allows giraffes to reach branches not accessible to other animals, thus providing them with a unique food source.

When a feature is secondarily coopted for a function other than the one for which it originally evolved, it has historically been known as a “preadaptation.” This is a somewhat dangerous term because it sounds as if it means that a feature evolved with some goal in mind (e.g., that the giraffe neck became longer in anticipation of future utility for feeding on high leaves.). Still, it is a useful term in that it distinguishes between features that evolved due to natural selection for their current functions, and features that originally evolved for one function and then were coopted for a later function. You should be able to distinguish between a “preadaptation” and an adaptation.

It is also possible that some traits are just not “adaptive,” e.g., they may be by-products of the physical architecture of an organism, or they may be due to developmental or genetic constraints.

Gould and Lewontin use a non-biological architectural example: The spandrels of San Marco. They discuss the of the cathedral of San Marco in Venice and note that there are beautiful mosaics in the triangular spaces that occur where the round dome meets two rounded arches that meet at right angles. An adaptationist might assume that those spaces were put there just so the mosaics would have a spot to live. In fact, these spaces (called spandrels, or, more precisely, “pendentives”), are a necessary by-product of placing a round dome on four rounded arches that occur at right angles to each other. Once the architect decided to build the dome that way, there was no getting around having these spandrels. Three hundred years after the cathedral was built, the mosaics were put in the spandrels. Clearly, the spandrels had no function in and of themselves when the cathedral was built, but somebody later put them to good use.

Gould has written a paper suggesting that the word “spandrel” be used in biology to name features that arise without initial adaptive functions (e.g., those that are architectural by-products of development) but take on new functions later in (Gould 1997).

Gould had earlier proposed, in a paper with Elizabeth Vrba (Gould and Vrba 1982), that we call traits that perform a current adaptive function but arose either for some other function or with no adaptive function at all as “.” Thus both preadaptations and “spandrels” qualify as exaptations. The terminology may seem very nitpicky, but it forces us to think about the details of a trait and how it might have evolved, and it forces us to go beyond the mere creation of plausible stories in the study of adaptation. One example Gould gives of a “biological spandrel” is the umbilicus of a snail shell. A snail shell grows by coiling a tube around a central axis. The space along the axis that necessarily results is known as the umbilicus. The space is a by-product of how the shell is built, but some snails use the umbilicus to brood eggs.

Gould poses the question: is the umbilicus a spandrel in the sense that it arose as a geometric by- product of coiling a tube around an axis and was then coopted by some snail species to be used in egg brooding, or was the design of the shell selectively favored because it provided this handy space for brooding eggs? Gould looks at a phylogeny of gastropods and notes that although all species have an umbilicus, only a few species brood eggs in it, and one can map the apparent origin of this use of the umbilicus on the tree. He concludes that these few independent origins of egg brooding in the umbilicus suggest that the umbilicus space arose as a spandrel that was coopted for egg-brooding in several snail lineages.

This example serves as a transition to the next topic: the comparative method. Given that we want to perform rigorous tests for adaptation rather than just making up plausible stories, how can we do this?

The comparative method seeks to evaluate hypotheses by testing for patterns across species, such as correlations among traits, or correlations between traits and features of the environment. For example, if we consistently see a certain feature associated with a certain environment (e.g., white fur in arctic mammals), then we might ask whether selective pressures in that environment favor that . Or, if we consistently see two traits occurring together (e.g., long, thin beaks in nectar-feeding birds, large testes in bats with large group sizes), we might see if there is some adaptive reason for that association. In the bat example discussed in your textbook, it was hypothesized that it is advantageous for males of bat species that roost in large groups to have relatively large testes because females in larger groups are more likely to have multiple matings, and sperm competition is likely to be more intense as a consequence.

Phylogenies and the Comparative Method: a hypothetical example

Imagine that you look across ten bird species and see that there are eight species in which two traits (e.g., a long beak and a nectar diet) appear together and two species in which the birds have neither trait. This seems like an interesting pattern, and it suggests that the beak shape is an adaptation for feeding on nectar. You might be tempted to perform a standard statistical test of association between beak morphology and diet (e.g., a chi-square test of association).

A problem arises when one considers the assumptions of the statistical tests one would use in this situation. One assumption of standard statistical tests (parametric and nonparametric alike) is that the observations are independent of each other. This is generally true when observations are sampled randomly from a population—the probability of picking one item is independent of the probability of picking another. Another way of thinking about this is in terms of data values—in a sample of independent observations, knowing the value of one observation does not tell you anything about the value of another observation. Does this apply to comparisons across species like the one we’re considering? We need to consider phylogenetic relationships among the species. If the true phylogeny is one large polytomy, then we are justified in saying that the species are phylogenetically independent of each other because this tree indicates that no two species are more closely related to each other than either is to any of the other species in the group. (note that we’re assuming that this is the true phylogeny and not just an indication that we can’t resolve the phylogenetic relationships—you’ll learn more about this in the Phylogeny lab).

In most cases, the phylogeny for a group of species probably does not fit that pattern. There is more likely to be a pattern of dichotomous branching representing the events in the lineage leading from the common ancestor of all the species being considered. If this is the case, then two species that are more closely related to each other are more likely to be similar in phenotype (e.g., in the traits we are comparing across species) than two species that are more distantly related. If there is a hierarchical phylogenetic structure, then the species, and thus the observations on which we want to perform standard statistical tests, are not independent of each other.

The consequence of non-independence is that we have smaller sample sizes than we think we do in our comparisons, because the sample size (actually, the number of degrees of freedom) of a test depends on the number of independent observations in a sample. Recall that you lost a degree of freedom in the chi-square tests of Hardy-Weinberg equilibrium because you estimated your expected values from the data. One loses degrees of freedom in a statistical test as the interdependencies in the data increase, e.g., you lose that first degree of freedom in the chi- square test because you know the total number of observations, and if you know the number of observations in all but one category, you automatically know the number in the last category, because the total has to sum to the total sample size. That number of observations in the last category is thus not independent of the others in the test, so you lose a degree of freedom. The same thing applies when you’re calculating a variance. The variance is the sum of squared deviations from the mean divided by n - 1. n - 1, not n, is the number of degrees of freedom associated with the variance because you estimate the mean in order to calculate the variance. If you know the sample size, n, and you know the estimate of the mean, X-bar, then if you are given the values of all but one observation in the sample, you can calculate the last one—it is not independent of the others.

The same idea applies to these comparisons across species, and this is why it is so important to take phylogeny into account when using the comparative method.

There are a variety of methods for taking phylogeny into account when doing a comparative study of adaptation. The example in your textbook applies to continuous characters (e.g., body weight, testis size, etc.) and uses Joe Felsenstein’s method of phylogenetically independent contrasts (Felsenstein 1985). Although Felsenstein was not the first person to note the importance of considering phylogeny in comparative studies, his 1985 paper was important in awakening evolutionary biologists to this issue.

References:

Felsenstein, J. 1985. Phylogenies and the comparative method. American Naturalist 125: 1-15. Freeman, S. and J.C. Herron. 2001. Evolutionary Analysis. Upper Saddle River, N.J.: Prentice Hall.

Gould, S.J. 1997. The exaptive excellence of spandrels as a term and prototype. Proc. Natl. Acad. Sci. USA 94: 10750-10755

Gould, S.J. and R.C. Lewontin. 1979. The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proc. R. Soc. Lond. B. 205: 581-598.

Gould, S.J. and E.S. Vrba. 1982. : A missing term in the science of form. Paleobiology 8: 4-15.

Pratt, D.M. and V.H. Anderson. 1985. Giraffe social behavior. J. Nat. Hist. 19: 771-781.

Simmons, R.E. and L. Scheepers. 1996. Winning by a neck: in the evolution of the giraffe. American Naturalist 148: 771-786.

Young, T.P. and L.A. Isbell. 1991. Sex differences in giraffe feeding ecology: Energetic and social constraints. Ethology 87: 79-89.

Additional Reading:

Brooks, D.R. and D.H. McLennan. 1991. Phylogeny, Ecology, and Behavior: A Research Program in Comparative Biology. Chicago: University of Chicago Press.

Harvey, P.H. and M.D. Pagel. 1991. The Comparative Method in Evolutionary Biology. Oxford: Oxford University Press.

Martins, E.P. (ed.) 1996. Phylogenies and the Comparative Method in Animal Behavior. Oxford: Oxford University Press.

Pigliucci I, and Kaplan J. 2000. The fall and rise of Dr Pangloss: and the Spandrels paper 20 years later. Trends Ecol. Evol. 15(2):66-70. Lab Exercise

We are going to go into the greenhouse and the surrounding area to examine the plants and look for adaptations, preadaptations, and spandrels. Walk around the area looking at all of the structures of the plants (e.g. stalks, leaves, flowers, tendrils, spines, etc). I want you to find 2 examples of plants exhibiting each of the three traits above. Write down a description of each of the plants and the structure you are looking at and describe why you think it is an adaptation, preadaptation, or spandrel. Describe how you would use the comparative method to test your hypothesis and describe a scientific experiment to test if the object is an adaptation or a spandrel.

Remember, this must be typed and turned in as a MS Word document attached to an e-mail. Your report must be turned in before the start of lab on Friday September 13.