Charles Darwin’s idea that an unguided natural process led to all the beauty and diversity of the world, including its Undeniable appearance of design, guides scientific thinking to this day. But what if his signature mechanism — natural selection — cannot be measured? Without measurement, a theory reduces to anecdote. A recent paper in the Proceedings of the National Academy of Sciences threatens to do that, at least in regard to “the evolution of human body form.” The implications go far beyond human physiology.
Consider limb length. Say you want to deduce how natural selection has affected the dimensions of the femur bone. The authors point out that one cannot measure directional selection on one bone without taking into account how all the other bones are affected.
Human morphological variation is thought to have been partially shaped by natural selection associated with environmental factors like climate. Patterns of variation in body form correspond with latitude, but evolutionary processes that yielded this variation are not yet established. Examining the traits used in these studies (e.g., limb lengths) independently ignores their genetic covariation, which affects their responses to evolutionary forces. To address this relationship, we estimated the directional selection necessary to evolve correlated traits reflecting body shape across latitudes and examined trait-specific responses. Although most traits appear to be under directional selection, their response is constrained by between-trait covariance. This finding suggests that trait differences among human groups may not directly reflect the forces of selection that shaped them. [Emphasis added.]
Let that sink in for a moment. (And set aside the astonishing confession that “evolutionary processes” that yield variation “are not yet established” even 157 years after Darwin published his Origin.) How would a scientist ever know that a trait is under directional selection except by looking at it, and comparing it with other similar traits in other individuals or species? The Abstract of the paper elaborates on the problem. Covariation, they show, can overwhelm the evidence for directional selection and confuse it with genetic drift. (Keep in mind that we are looking for a way to measure natural selection.)
Variation in body form among human groups is structured by a blend of natural selection driven by local climatic conditions and random genetic drift. However, attempts to test ecogeographic hypotheses have not distinguished between adaptive traits (i.e., those that evolved as a result of selection) and those that evolved as a correlated response to selection on other traits (i.e., nonadaptive traits), complicating our understanding of the relationship between climate and morphological distinctions among populations. Here, we use evolutionary quantitative methods to test if traits previously identified as supporting ecogeographic hypotheses were actually adaptive by estimating the force of selection on individual traits needed to drive among-group differentiation. Our results show that not all associations between trait means and latitude were caused by selection acting directly on each individual trait…. Additionally, in contradiction to ecogeographic hypotheses, the humerus was under directional selection for longer values by latitude. Responses to directional selection in the tibia and radius induced a nonadaptive correlated response in the humerus that overwhelmed its own trait-specific response to selection. This result emphasizes that mean differences between groups are not good indicators of which traits are adaptations in the absence of information aboutcovariation among characteristics.
A Darwin advocate might complain that these three authors only studied three bones in the human anatomy (femur, tibia, and humerus) among a finite number of human individuals in a finite number of population groups. They certainly can’t draw conclusions about natural selection in general, supported (as it is assumed) by overwhelming evidence. Indeed, the authors do show epistemic modesty:
An important point to recognize here is that we do not exhaustively cover all dimensions of body form, some of which might be more directly tied to fitness differences than the ones we do consider. It is entirely possible that the strong selection gradients that we estimate from comparisons of groups in sub-Saharan Africa and those in the arctic are driven by selection on some unmeasured aspectof body form (or physiology) genetically correlated with both humeral length and overall form. Although this is certainly the case, our analyses cannot account for characteristics we did not measure.
Game over? No. They believe their findings support general principles about measuring natural selection that apply anywhere.
Nonetheless, our analyses considerably narrow the space of possible models that could be used (e.g., models that go beyond mean trait comparison) and gives a guide as to which traits we might consider in the future. Studies of humerus-to-body size allometry suggests a possible direction in which this study might be taken.
They deduced that while femur length might have contributed to fitness, it caused covariant changes to the humerus that decreased fitness. You can’t vary one trait without mucking up the works elsewhere. A wider axle on a car might sound great until you realize the brakes won’t reach the rims. A bigger brain sounds advantageous, but not if it can’t pass through the birth canal. Everything is tied together.
Lest one think this problem is restricted to studies on human body form, consider this principle: measuring selection on one trait in isolation can be misleading.
As we show, the covariation of traits within an evolutionary framework affects the capacity of those traits to respond to directional selection. A multivariate approach avails us of the ability to apprehend otherwise cryptic evolutionary processes.
Aha! So while they recognize that selection is hard to measure, those “cryptic evolutionary processes,” sneaky as they are, can be apprehended with multivariate analysis. Yet notice this admission in the conclusion: natural selection can select against fitness!
Directional selection has contributed to the differentiation of the tibia, radius, femoral head, and biiliac breadth among human groups from different ecogeographic contexts. In some respects, these results accord well with previous ecogeographic studies. More importantly, however, we show that differences in trait averages among groups are not good indicators of how traits responded to natural selection. Notably, because selection acted on correlated traits, the humerus shows an evolutionary response to directional selection opposite to its influence on fitness. Patterns of mean difference among groups are not necessarily an accurate reflection of the processes that created that variation.
We’re not trying to turn these authors into anti-evolutionists. Surely they believe in natural selection. The question is, How is selection to be measured?If directional selection can make a trait respond opposite to fitness, all bets are off. The evidence becomes anecdotal. A creative theorist can make any trait a product of directional selection, and any other trait a product of genetic drift. Notice how they strongly affirm selection in one sentence, only to undermine it in the next.
We show strong evidence for directional selection on many of the traits, which are hypothesized to vary among humans as a result of climatic adaptation under the ecogeographic model. However, the magnitude and orientation of directional selection across the traits often does not match the pattern and magnitude of morphological change. Changes in trait means among regions (Table 1) do not mean selection was acting directly on those traits, orthat the force of selection was the same as the direction of difference in means.
How, then, can they know what selection did? How can they measure it? They can’t. They just said that the force of selection can act contrary to observed morphological traits. What necessary connection is there, then, between selection and traits?
Because trait averages are not good indicators of natural selection, these authors have pulled the rug out from traditional ways of measuring fitness. Notice the general principle in their final sentence:
By focusing only on realized differences between groups,traditional adaptationist ways of studying ecogeographic variationhave left the ways in which natural selection acted undescribed.Natural selection’s effect on the humerus would not be apparent from a standard adaptationist perspective focused on explaining evolution using group differences on a trait-by-trait basis.
That makes you wonder what other facts of nature are being overlooked (“not… apparent”) by “traditional adaptationist ways” of looking at living things. This paper sounds like a sermon from Darwin insiders telling the choir they have been singing off key. But then, they provide no “A 440” tuning fork to get them back on key. The result is atonality, confusion, dissonance.