Comparing Explanations for "Trade-offs" in Darwinian Theory and ID
Darwinists and theorists of intelligent design refer to "trade-offs" in living systems, but they explain them in ways that are radically different, and tellingly so. Theodore Garland is an evolutionary biologist at the University of California, Riverside. His "Quick Guide" to "Trade-offs" in Current Biology provides an opportunity to compare the explanatory power of ID and Darwinism. First of all, what is a trade-off?
In engineering and economics, trade-offs are familiar enough (e.g., money spent on rent is not available to buy food). In biology, a trade-off exists when one trait cannot increase without a decrease in another (or vice versa). Such a situation can be caused by a number of physical and biological mechanisms. One type of mechanism is described by the so-called 'Y-model', which states that for a given amount of resource (e.g., energy, space, time), it is impossible to increase two traits at once. A commonly cited example is a trade-off between the size and number of eggs that, for example, a fish, bird or turtle can produce in a given clutch. Depending on the organism, this trade-off can be caused by a limitation in the amount of energy available, the amount of time available to produce eggs or the amount of space available to hold eggs (e.g., inside the shell of a turtle). Similarly, time spent foraging may be time wasted with respect to finding a mate. Trade-offs also occur when characteristics that enhance one aspect of performance necessarily decrease another type of performance. (Emphasis added.)
Notice that Garland has illustrated biological trade-offs with designed ones (engineering and economics). He quotes Charles Darwin on the point that animals must evolve as "integrated wholes":
The whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified. This is a very important subject, most imperfectly understood.
So what have Darwinian evolutionists learned about this in the past 155 years, since "Biologists have made major advances since then"? One advance, Garland says, is the theory of trade-offs: "Indeed, the concept of trade-offs underpins much of the research in evolutionary organismal biology, physiology, behavioral ecology, and functional morphology, to name just a few fields."
Having set up trade-off theory as a foundation of progress in evolutionary understanding, what does he say it has produced, specifically? We can dismiss his appeals to design as rhetorical misdirection:
Having survived a decade of frigid winters in Wisconsin, I like to use the example of gloves versus mittens. Gloves are good for making snowballs and getting keys out of your pocket, but they do not keep your hands nearly as warm as mittens do. Moreover, you must remove the mittens to get the keys. Returning to biology, limbs can be 'designed' for speed, through lengthening and thinning of bone, but this will often reduce strength and make them more likely to break when in use. Hence, a predator that evolves to be a fast runner may have to trade-off its ability to subdue large or strong prey (e.g., cheetah versus lion).
He can't help but say "designed," even if using scare quotes. Face it; a cheetah's limbs look designed for speed! By using "design" words and examples, Garland has not yet restricted his explanation to mutation and selection. Notice that he has just attributed purpose and planning to the cheetah! The "predator" is the subject of the verb "may have to trade-off its ability" to subdue large or strong prey.
As he goes on to describe more examples, and the ubiquity of trade-offs in nature, he keeps evolutionary theory in the shadows. For all the reader knows, the trade-offs could have been designed.
When push comes to shove, instead of giving a scientific explanation consistent with neo-Darwinism, Garland gives excuses. He says it's too hard a problem:
In some cases, expected trade-offs based on mathematical models or on basic biological principles are not found. This may occur because nature has more 'degrees of freedom' than assumed by simple conceptualizations that predict trade-offs. For one example, aside from changes in fiber-type composition, muscles can evolve to be larger, positions of origins and insertions can shift, legs can become longer, and gaits can evolve (including bipedality). As another example, animals may be able to acquire and process more food (e.g., by altering their preferred prey type), thus allowing them to secure more energy and increase both number and size of offspring.
Saying something "can evolve" is not the same as saying it "did evolve." Garland waffles, saying evolution does things this way sometimes, and that way sometimes, but there's no way to know. While his references to mathematical models are helpful, there's nothing about them that is strictly Darwinian. In fact, they can sometimes be counter-Darwinian:
Although it is easiest to conceive of and recognize trade-offs between only two traits, organisms comprise an almost infinite number of 'traits', and trade-offs may appear only when we include multiple traits in an analysis. The Y-model can be expanded to include multiple traits at multiple levels of biological organization. Speed and stamina might not trade-off in some group of organisms (perhaps even showing a positive relationship), but a composite measure of locomotor performance abilities might be negatively related to one or more aspects of the life history (e.g., growth rate, age at first reproduction, fecundity). Similarly, a physiological or biomechanical trade-off -- even if it affects physical fitness (e.g., locomotor abilities) -- does not necessarily indicate any trade-off with Darwinian fitness (lifetime reproductive success). Of course, a small effect on a performance trait (e.g., a 2% reduction in speed associated with a 2% increase in stamina) could, for some organisms under some ecological circumstances, make the difference between eating and being eaten.
Garland ends with more cases where Darwinian evolution might not be the explanation for observed trade-offs:
A negative relationship alone does not prove that two traits necessarily trade-off in a functional or evolutionary sense. Rather, it is possible that natural selection simply never favored the evolution of species that have high (or low) values for both traits. Whether a trade-off (or evolutionary constraint) necessarily occurs can be tested by selection experiments and experimental evolution with tractable model organisms, by phenotypic engineering (such as hormone manipulations), by direct molecular-genetic manipulations, by a search for organisms that break the rules, or by developing a thorough understanding of how organisms work. Finally, it is worth noting that many sexually selected traits, such as the exaggerated tail feathers of male peacocks, may benefit the ability to obtain mates but hinder escape from predators, reduce foraging ability or increase the energetic cost of locomotion. These situations can also be viewed as trade-offs.
A reader looking for a specific tie-in of an observed trade-off to a Darwinian explanation will be disappointed. Merely stating that an animal (like the peacock with its outrageous tail) exhibits trade-offs doesn't explain how mutation and selection produced it. In support of Darwinian evolution, Garland provides nothing more than a list of exceptions and excuses. Remarkably, he expects intelligently designed "manipulations" involving "phenotypic engineering" to provide support for an explanation based on unguided processes.
The one time he mentions selection leading to trade-offs is in his concluding paragraph about constraints:
Constraints can be defined as anything, internal or external to an organism, that limits the production of new phenotypes. For example, if the circulating levels of a hormone change, then any cell that has receptors for that hormone is likely to be affected. Thus, selection favoring increased aggressive or agonistic behavior may have adverse consequences for parental behavior. This example should make clear that, in biology, the concepts of trade-offs and constraints are often closely related.
That one case, though, is mentioned as a possibility, not a demonstration. From this article, we gain the distinct impression that neo-Darwinism is not helpful to understanding trade-offs.
ID and Trade-offs
The theory of intelligent design looks at trade-offs much the same in observational terms, but very differently in explanatory terms. Discussions of trade-offs generally come up in responses to criticisms of bad design (dysteleology) in nature. Design theorists explain that ID does not imply that every trait must be perfect or ideal. Rather, design must be evaluated holistically. To infer design for the human body does not require that we have the visual acuity of an eagle or the speed of a cheetah. Suboptimal design does not falsify intelligent design.
Paul Nelson has used the analogy of a laptop. Nobody would argue that a laptop is not designed. But given the design goal of a lightweight, portable computer, that goal constrains the individual parts. A laptop can't afford a heavy disk drive or a giant screen; all the parts must contribute to the overall goals of portability, small size and light weight. A desktop computer, with different design goals, will have different trade-offs (e.g., less portability). So will a motorcycle compared with a racing car.
Observationally, therefore, design theorists would agree that trade-offs are ubiquitous in biology. They would deny, however, that the trade-offs arise by unguided natural processes, except maybe to accentuate designed trade-offs over time. Instead, they would say that the functional performance of the whole animal in its niche provides evidence for design, even if specific traits are not the best of what's possible. A sloth provides as much evidence for design as a cheetah.
Both Darwinian evolutionists and design theorists recognize the ubiquity of trade-offs in biology. Both can evaluate them in terms of constraints. Both agree on the necessity of evaluating trade-offs holistically, viewing organisms as "integrated wholes." But when it comes to explaining their origins, the two ways of thinking part company.
An evolutionary biologist struggles to avoid design terms: engineering, economics, biological organization. He never ties trade-offs to random mutations. He almost personifies selection, saying that if an animal evolves to be fast, it "may have to trade off" its ability to be strong. And he can't help but use design analogies, like mittens versus gloves.
Design terms and analogies, however, come naturally to intelligent design theory. Since our uniform experience with objects exhibiting trade-offs, whether mittens versus gloves, or laptop versus desktop computers, is that they proceed from intelligent causes, it's only natural that trade-offs in biology reflect origin by design for functional constraints.