Search for Evolutionary Trade-Offs Comes Up Empty
Husbands and wives know a lot about trade-offs, but according to Darwinian theory, all living things are in a constant tug-of-war between competing interests. In evolutionary terms, a trade-off is a compromise between competing forces of natural selection. For instance, “Simultaneously obtaining enough food to grow and reproduce while trying not to become someone else’s dinner is a pervasive trade-off faced by many organisms,” explains Mark McPeek (Dartmouth), writing in American Naturalist.1 How does this concept fit in with evolutionary theory?
Trade-offs are central to our conception of how the natural world is organized. Trade-offs shape the choices that individuals make (Sih 1980, 1987; Krebs and Davies 1997), influence evolutionary trajectories and mold genetic diversity (Loeschcke 1987; Rose 1991; Stearns 1992; Roff 2002), and determine which species are able to coexist with one another in the long term (Levin 1970; Tilman and Pacala 1993; Chesson 2000). Trade-offs are presumed to be caused by some genetic or phenotypic trait or traits influencing two fitness components in antagonistic ways. Understanding the mechanisms that cause trade-offs is critical for predicting their consequences (Schoener 1986; Tilman 1987).
So McPeek set out to test the evolutionary trade-off hypothesis. But when he looked for a trade-off among damselflies, specifically his prediction that activity correlates to mortality from predation, he was stumped: he couldn’t find it.
McPeek studied two coexisting species of damselflies that inhabit freshwater lakes. One has larvae that are much more active than the other. The active ones presumably get more food but are more exposed to predation, and suffer higher mortality. What he found, however, is that both species actually obtain the same amount of nutrition, regardless of activity. “However, laboratory studies presented here show that the mechanism assumed by most theoretical and empirical studies to mediate this trade-off, namely activity simultaneously modulating foraging returns and predation risk, does not operate in this system,” he lamented with apparent consternation.
In spite of no difference in the amount of food ingested or assimilated, I. verticalis larvae grew faster than Enallagma larvae because they were better able to physiologically convert assimilated food into their own biomass in the presence of mortality threats. From these studies we understand the phenotypic mechanisms determining the antagonistic patterns of relative growth and survival between these two genera, but why these patterns exist remains unclear.
McPeek lays out his experimental data in exhaustive detail, but in the end, the principle he sought to verify was not found:
If the growth/predation risk trade-off has influenced the evolution of these genera, the walk and production efficiency variables should display positive correlations across species’ phenotypes (i.e., for the “tips”) and in the evolutionary contrasts. The number of walks in the presence of dragonfly predators was correlated across species with the production efficiency and growth rate in the presence of predators, but correlations among the corresponding evolutionary contrasts indicated that these variables have evolved independently; correlations among contrasts for walking and production efficiency/growth variables were all not significant, and they were not even consistent in sign (table 1; fig. 6b).
His ending discussion puzzles over this negative result, and compares it with findings of other studies on evolutionary trade-offs. He really expected the vigor of the one species to exhibit a trade-off:
A trade-off implies that some character or set of characters, either phenotypic or genetic, antagonistically influence two fitness components. As this character (or set) evolves, one fitness component increases while the other decreases, hence the trade-off. Clearly, activity is not that character because activity does not influence growth rate, and they do not evolve in a correlated manner across species (table 1).
A negative result is still a result, and McPeek has to leave it at that: “At present it is difficult to speculate what the underlying character modulating mortality and growth may be to generate the trade-off among the damselflies,” he concludes. “In fact, we must entertain the possibility that this is not a trade-off in the mechanistic sense at all. In other words, no mediating phenotypic or genetic traits may have shaped the evolution of both growth rate and predation risk,” he states with apparent surprise. In fact, evolutionists may have to propose an opposite principle:
Perhaps the direction of causation is also opposite from what we usually assume; these differences between the genera may not have evolved because of selection pressures to allow them to coexist (Abrams 2003), but rather these phenotypic differences may have arisen for other reasons (e.g., drift or past selective agents that no longer influence them), and the fact that these phenotypic differences promote coexistence has allowed the ecology of the system to dynamically capture these two taxa and promote their long-term persistence with their present phenotypes.
This seems to suggest a force for stasis, not evolution. It gets worse; he next points to other studies that show the same thing, such as with tadpoles. We need to find the mechanism for trade-offs, he implores, to understand organizations of species with each other and with other organisms, and to understand ecology. “Such differences in phenotypically mediated community dynamics cannot be correctly discerned or reliably predicted without a thorough understanding of the mechanisms shaping the phenotypes of the interacting species.” So – back to the drawing board.
1Mark A. McPeek, “The Growth/Predation Risk Trade-Off: So What Is the Mechanism?” American Naturalist2004. Vol. 163, pp. E88-E111. � 2004 by The University of Chicago. 0003-0147/2004/16305-40010, Electronically published April 26, 2004.
Notice his suggestion about “past selective agents that no longer influence them” as an explanation for why the evolutionary trade-off was not found. How is a past selective agent that no longer has any influence a testable scientific model? How is it different from a ghost?
You have to feel sorry for Darwinists, hunting in vain for evidence of the mechanistic processes that they hope can explain the world. He couldn’t even find evidence that selection influenced the activity of the damselfly, let alone the damselfly itself, with its wings, muscles, eyes, and countless other engineering marvels (remember what Dickinson taught us about fruit flies? See 12/18/2003 headline for a reminder of the exquisite engineering Darwinism needs to explain).
So another Darwinian principle has been tested and found wanting. Wonderful. Keep up the good work. (See 04/02/2004 headline for another recent example.) At this rate we can just stand back and watch the whole Darwinian edifice implode.