What does the Comparative Method Reveal About Adaptation?

The use of the independent contrast method in comparative tests is studied. It is assumed that: (i) the traits under investigation are subject to natural selection; (ii) closely related species are similar because they share many characteristics of their niche, inherited from a common ancestor; and (iii) the current adaptive significance of the traits is the focus of investigation. The main objection to the use of species values in this case is that third variables which are shared by closely related species confound the relationship between the focal traits. In this paper, I argue that third variables are largely not controlled by the contrast methods, which are designed to estimate correlated evolution. To the extent that third variables also show correlated evolution, the true relationship among the traits of interest will remain obscured. Although the independent contrast method does not resolve the influence of third traits it does, in principle, provide a greater resolution than the use of the species mean values. However, its validity depends on the applicability of an evolutionary model which has a substantial stochastic component. To illustrate the consequences of relaxing this assumption I consider an alternative model on an adaptive radiation, where species come to fill a fixed niche space. Under this model, the expected value for the contrast correlation differs from that for the species correlation. The two correlations differ because contrasts reflect the historical pattern of diversification among species, whereas the species values describe the present–day relationships among the species. If the latter is of interest, I suggest that assessing significance based on the species correlations can be justified, providing that attention is paid to the role of potentially confounding third traits. Often, differences between contrast and species correlations may be biologically informative, reflecting changes in correlations between traits as an adaptive radiation proceeds; contrasts may be particularly useful as a means of investigating past history, rather than current utility of traits.

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“…This criticism is not addressed here, but it has been considered in depth elsewhere (e.g. Gould & Lewontin 1979 ;Leroi et al 1994).…”

Section: Discussionmentioning

confidence: 98%

Correlated evolution and independent contrasts

Price

1997

Phil. Trans. R. Soc. Lond. B

313

281

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“…44). Intraspecific analyses are particularly important for discerning relationships among traits due to differences in resource allocation (for example, allocation trade-offs) from other potential sources of covariation among traits (for example, genetic effects, independent selection).…”

Section: Discussionmentioning

confidence: 99%

Female monopolization mediates the relationship between pre- and postcopulatory sexual traits

et al. 2014

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Theory predicts a trade-off between investments in precopulatory (ornaments and armaments) and postcopulatory (testes and ejaculates) sexual traits due to the costs associated with their growth and maintenance within the finite energy resources available. Empirical studies, however, have revealed considerable inconsistency in the strength and direction of relationships among these sexual traits. Ambiguity may result from variance in the marginal benefits gained by increasing investments in either pre-or postcopulatory sexual traits. Here, in a broad comparative study, we test the prediction that the relationship between pre-and postcopulatory sexual traits differs among taxa relative to the importance of male-male contest competition within them. We find that covariance between pre-and postcopulatory sexual traits gradually shifts from strongly positive to strongly negative with increasing male-male contest competition. Thus, our findings reveal a potentially unifying explanation for the oftentimes inconsistent relationships in the strength and direction of covariance among sexual traits.

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“…Recognizing how adaptationist explanations are structured in actual practice helps give clarity to problems that have plagued biology, such as debates over tautology/circularity, and resolve false conflicts, such as the mutual scorn that often characterizes the adherents of the comparative, population/ quantitative genetics, and optimality approaches (e.g. Calow 1987;Leroi et al 1994;cf. Zimmermann, 1983p.…”

Section: Resultsmentioning

confidence: 99%

“…If a given pattern has an adaptive cause then by definition at some time in the past, not just the observable present or moments captured in fossil traces, heritable variation with fitness consequences was present (Leroi et al 1994;Forber and Griffith 2011). Selection on this variation is assumed to have led to the pattern observed today ( Figure 3B).…”

Section: Optimality Modelingmentioning

confidence: 99%

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How to Study Adaptation (and Why To Do It That Way)

Olson¹,

Arroyo-Santos²

2015

The Quarterly Review of Biology

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Some adaptationist explanations are regarded as maximally solid and others fanciful just-so stories. Just-so stories are explanations based on very little evidence. Lack of evidence leads to circular-sounding reasoning: "this trait was shaped by selection in unseen ancestral populations and this selection must have occurred because the trait is present." Well-supported adaptationist explanations include evidence that is not only abundant but selected from comparative, populational, and optimality perspectives, the three adaptationist subdisciplines. Each subdiscipline obtains its broad relevance in evolutionary biology via assumptions that can only be tested with the methods of the other subdisciplines. However, even in the best-supported explanations, assumptions regarding variation, heritability, and fitness in unseen ancestral populations are always present. These assumptions are accepted given how well they would explain the data if they were true. This means that some degree of "circularity" is present in all evolutionary explanations. Evolutionary explanation corresponds not to a deductive structure, as biologists usually assert, but instead to ones such as abduction or induction. With these structures in mind, we show the way to a healthier view of "circularity" in evolutionary biology, and why integration across the comparative, populational, and optimality approaches is necessary.

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What does the Comparative Method Reveal About Adaptation?

Cited by 202 publications

References 103 publications

Correlated evolution and independent contrasts

Correlated evolution and independent contrasts

Female monopolization mediates the relationship between pre- and postcopulatory sexual traits

How to Study Adaptation (and Why To Do It That Way)

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