July 18, 2023 | David F. Coppedge

Body Size Change Is Not Evolution

Evolutionists keep looking for laws to make
Darwinism scientific. There are too many exceptions.


—If body size is evolution, everybody evolves when they grow from baby to adult.—

In our 15 Sept 2008 article “Looking for Laws to Make Darwinism Scientific,” we discussed Bergmann’s Rule. This rule (no relation to CEH author Jerry Bergman) claims that animals grow larger bodies in cold climates. It makes sense at a physical level; a larger body loses heat more slowly because the surface-to-volume ratio is lower. But the rule works except when it doesn’t; there are too many exceptions. And it’s not evolution anyway. A larger body still has all the same organs, tissues and cells, and the same genetic code.

Another “rule” with too many exceptions to call it a scientific law is Allen’s Rule: animals grow shorter limbs in cold climates. Now, three evolutionists admit that both of these rules have problems, but maybe they become more empirically justified when combined. They try their hypothesis out on birds.

Complementarity in Allen’s and Bergmann’s rules among birds (Baldwin, Garcia-Porta and Botero, Nature Communications, 15 July 2023).

Biologists have long noted that endotherms tend to have larger bodies (Bergmann’s rule) and shorter appendages (Allen’s rule) in colder environments. Nevertheless, many taxonomic groups appear not to conform to these ‘rules’, and general explanations for these frequent exceptions are currently lacking. Here we note that by combining complementary changes in body and extremity size, lineages could theoretically respond to thermal gradients with smaller changes in either trait than those predicted by either Bergmann’s or Allen’s rule alone. To test this idea, we leverage geographic, ecological, phylogenetic, and morphological data on 6,974 non-migratory terrestrial bird species, and show that stronger family-wide changes in bill size over thermal gradients are correlated with more muted changes in body size. Additionally, we show that most bird families exhibit weak but appropriately directed changes in both traits, supporting the notion of complementarity in Bergmann’s and Allen’s rules. Finally, we show that the few families that exhibit significant gradients in either bill or body size, tend to be more speciose, widely distributed, or ecologically constrained. Our findings validate Bergmann’s and Allen’s logic and remind us that body and bill size are simply convenient proxies for their true quantity of interest: the surface-to-volume ratio.

Sounds evolutionary, but a bigger parrot is still a parrot. Where is the evidence for speciation by the Darwinian staples of mutation and natural selection? Whatever is going on, it is very slight, the authors say.

Overall, our study shows that geographic bill and body size gradients complement each other in terrestrial, non-migratory birds to enable smaller than expected morphological responses to temperature change. Additionally, it shows that either one of these typically weak geographic gradients is more likely to be detected as significant if statistical power is large, or if change in the other trait is constrained by ecology. These findings highlight the multivariate nature of selection, and remind us that body size and extremity size are simply convenient proxies for what both Bergmann and Allen envisioned as the likely target of selection from ambient temperatures: an individual’s surface-to-volume ratio.

The authors think that Bergmann’s rule and Allen’s rule work together or against each other. If a bird gets a much bigger bill, it will not evolve as big a body size. And if another bird grows fatter, its bill size will not evolve as much. But if the bill evolves along with the body size, both differences will be slight. The evolutionists point to this phenomenon as highlighting the “multivariate nature of selection” (which means “stuff happens in multiple ways”). Then they claim that the target of natural selection is surface-to-volume ratio.

Under this theory, a fat man who doesn’t shiver as much on a ski trip has evolved. Is he a new species? Has macroevolution occurred?

Circular Reasoning

The authors use the e-word evolution ten times, but nowhere do they prove that Bergmann’s rule or Allen’s rule or the combination of the two causes macroevolution.  That’s the real issue in trying to dredge evolution out of these so-called rules: did a family of birds evolve from a different family of birds, or from pre-birds? Even creationists accept minor changes in body size and beak size. To call this evolution muddies the waters, making readers think that natural selection has been demonstrated by a scientific law of nature.

The paper also assumes that families with many species evolved by a Darwinian process. It’s a circular argument to look at existing species, assume that they evolved by Darwinian evolution, and then use the number of species in each family as evidence that Darwinian evolution occurred. Do they point to mutations that led to major changes? No. Do they point to specific genes that were selected? No. Rather than doing a truly unbiased sampling, they tossed out many groups that did not conform to the pattern they were seeking.

Simply put, our analyses strongly suggest that most avian lineages have expanded their thermal ranges by evolving small and complementary changes in bill and body size rather than evolving pronounced changes in only one of these traits. Our findings are therefore consistent with both Allen’s and Bergmann’s observations and help explain why independent evaluations of these rules often fail to reach statistical significance….

Indeed, some of their references dating back decades declare that Bergmann’s rule and Allen’s rule are false: e.g., “Bergmann’s rule is invalid” (1987). Other papers measured different traits than beak size and weight to guess whether the “rules” were rules at all.

—Do scientific papers about evolution really give solid evidence for it?—

Guessing Game

Reading this paper suggests that the number of unknowns outnumber the knowns, and the number of assumptions outweigh the certainties. Their phylogenetic diagram showed conformity to Bergmann’s rule and Allen’s rule varying all over the map, with no clear trend. Here are just a few of the factors that had to be excluded, modified, or assumed to come up with the “potential correlation between bill and body size changes” they tried to measure.

  • Whether feather density or covering is more important for adaptation to cold than beak size or body weight
  • Whether models of beak volume correspond to real beaks on real birds
  • Whether bill area functions in thermoregulation as much as presumed
  • Whether beak size is compensated in cold climates by other factors, such as blood vessel constriction
  • Whether geographic range, family size or other factors are more important
  • Whether the traits measured vary between sexes
  • Whether the trends (if any) are visible in the fossil record
  • Whether Bayesian analysis was appropriate for the situation
  • Whether “findings may be affected by the phylogenetic non-independence of the variables included in the PCA” (principal component analysis)
  • Whether the regression analysis is able “to account for independent uncertainty in our observations of the predictor and response variables”
  • Whether different models would yield contradictory results
  • Whether there is a real signal in overwhelming noise
  • Whether variations measured were reflected in the genes
  • Whether minute changes can be detected by statistical methods
  • Whether any alleged trends follow in other animals: mammals, reptiles, and amphibians
  • Whether the few bird families that appear to show a trend are sufficient to overcome many that do not

This paragraph is particularly telling:

Having observed that most families do not exhibit pronounced morphological changes in bill or body size across thermal gradients, we now focus our attention on the few that do (Fig. 4, Supplementary Fig. 9). Specifically, we now investigate whether particular family characteristics determine the strength of morphological changes across thermal gradients, or the likelihood that we can detect such changes with traditional statistical methods. One possibility is that conformance to Allen’s or Bergmann’s rule is simply easier to detect when either statistical power or effect sizes are large. For example, even weak gradients could be more easily distinguished from noise at larger sample sizes (i.e., in larger families). Similarly, interspecific differences could be more pronounced (and hence easier to detect) in families that cover wider temperature gradients or in families with bigger bodies or bigger beaks (because larger lineages need larger size changes to achieve a given percent difference in surface-to-volume ratio). Alternatively, major changes in one trait could be more likely to be observed when changes in the other trait are difficult to achieve. For example, small-bodied lineages might already be near a lower limit for body size and may therefore need to adjust their surface-to-volume ratio through changes in bill size alone. Similarly, large-billed lineages may find it easier to evolve pronounced changes in body size because further enlargement of their beaks would require additional reinforcements of the skull, enlargement of jaw muscles, and other potentially costly features. Given that bill morphology is critical to foraging success and tends to experience strong multivariate selection, it is also plausible that bill specialization constrains the evolution of bill size and thereby favors a more pronounced evolution of changes in body size across thermal gradients.

Additional uncertainties not addressed in the paper could cloud the issue. For instance, is beak size age dependent? If so, who decides the right age for measuring the ‘official’ beak size of a species? Do other traits, like leg length, diet, basal metabolic rate, or other things affect thermoregulation more than the beak does? Did the authors think of everything that could affect their conclusions? Look at these decisions they had to make for species that did not fit the trend they sought:

Through this model, we found that 81 out of 107 taxonomic families (i.e., 76%) do not exhibit significant conformance with either Bergmann’s or Allen’s rules (Fig. 3b, Table 1), where conformance is defined as a credible interval for a slope estimate that exhibits the expected direction and does not include zero. Intriguingly, the two families that exhibited conformance to both rules in this analysis did not show significant variation in absolute bill size over thermal gradients (Supplementary Fig. 2). Thus, the apparent increase in relative bill size observed in these families was driven not by Allen’s expectation of enlarged bills, but rather by a relative reduction in body size (i.e., an indirect effect of Bergmann’s rule). This observation led us to re-evaluate the appropriateness of our proxy for estimating how appendages contribute to thermal adaptation.

Possibility Thinking

In addition to the possibility of other unknowns not considered by the authors, the high number of hedging words degrades the epistemic value of the paper, including any trends they allege. These words are like a heavy dose of pepper seasoning the paper, making one sneeze at the conclusions: could (12x), potential (12x), proxy or proxies (10x) may (5x), possibility (2x), probability (4x), methodological choices (1x), uncertainty (6x), credible (5x; who decides?), realistic, scenario, and estimate (30x).

In conclusion, it’s hard to gauge any scientific value from this paper, despite the large number of bird species included and the apparent rigor of the mathematical modeling done. Garbage in; garbage out: or as we say, Darwin in, Darwin out (DIDO).

An unbiased reader might suspect other motivations for the paper, such as job security, pressure to publish or perish, or busy work to use up funds. One thing the paper certainly did not establish: evidence for evolution, despite repeated uses of the word and 25 uses of the term phylogenetic.

Evolutionists desperately wish to make their work look scientific. Sorry; this paper failed miserably. Some reasons: (1) the uncertainties outnumbered the certainties. (2) The hedging words used diminished confidence. (3) It had nothing to do with Darwinian evolution anyway! The size changes in bodies and beaks were too minor to convince an unbeliever in Darwinian evolution that mutations and the Stuff Happens Law are capable of innovating major changes like wings and eyes.

Minor changes within families is not doubted even by young-earth creationists. Darwin skeptics need to practice critical thinking when facing the volume of papers about “evolution.” It can seem intimidating to see so many papers alleging that evolution explains the world around us because the evidence is overwhelming. But where is it? As shown in this example, it is laughably absent. Where’s the beef?

Creationists are not at all bothered by changes in body size or in beak size within families of birds or other animals. In fact, research going on at the Institute for Creation Research examines the evidence for pre-programmed adaptability (example article, 30 June 2023). There are known cases where species adapt not due to Darwinian evolution, but to sensors within the organism that trigger epigenetic responses. This speaks of intelligent engineering for robustness so that a species does not go extinct with every slight change in the environment. If the Creator planned for species to fill the earth, as stated in Genesis 1, then adaptation is evidence of forethought and engineering proficiency in the design of organisms.





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