Cope's Rule: can microevolution explain it?

Body size has been a fascinating subject in science for quite some time. Why were some dinosaurs so big? Why are some organisms so small? What's the optimal size for individuals in a given population? Some of this obsession with size is due to something called Cope's rule. As with all 'rules' it is not a rule, but an idea proposed by Edward Drinker Cope. Funnily enough, he didn't coin the term himself, but he did write about the concept at the end of the 19th century. Basically, Cope's rule is that within a given lineage, body size will increase with time.

Example of Cope's rule where within the lineage that includes today's horses, shows body size getting larger through time. During the Eocene, the ancestors of modern day horses were 10 times smaller.

But before I get into Cope's Rule, I do have to talk about Cope himself. Cope is perhaps most famous (besides having a rule named for him) for being part of the "Bone Wars" where Cope and another paleontologist, Othniel Charles Marsh basically duked it out in person, in literature, and even in nomenclature. For instance, Marsh named one dinosaur Mosasaurus copeanus (divide the species name in half and look at them separately). Cope named one dinosaur Anisonchus cophater and wrote to his assistant “Osborn, it's no use looking up the Greek derivation of cophater, because it is not classic in origin. It is derived from the union of two English words, Cope and hater, for I have named it in honor of the number of Cope-haters who surround me.”

Anyways, back to Cope's rule. Cope's rule has been of great interest to researchers, resulting in a large number of papers published in all the glamour journals.

Just a handful of the many papers published about Cope's rule.

Generally, Cope's rule is considered on the macroevolutionary scale. Ok, that's cool, but what could be driving this phenomena? Is there, perhaps, a microevolutionary reason underlying Cope's rule? This is a logical thought as there are lots of individual fitness enhancing reasons to be larger. Larger individuals have been found to have better performance, more dominance, increased mating, and high fecundity. Furthermore, larger individuals can better tolerate short term environmental changes, avoid predators, and extract nutrients. So it would appear that being bigger is better.

And if you look at compilations of selection estimates, this is what has been found. Selection for body size appears to be more often positive, meaning there is increased fitness if the body size is larger.

Figure modified from Kingsolver and Diamond 2011. The red line shows that selection estimates tend to be positive more often, especially when compared with other phenotypic traits. For more information see Kingsolver and Pfennig 2004.

So, if there is positive selection on body size, then this could provide microevolutionary support for explaining the underlying mechanisms of Cope's Rule. Cool! But are populations responding to this positive selection for body size? Populations should be responding to this positive selection by getting larger through time at microevolutionary time scales, and lucky for us, we could actually try to see if this was happening.

These days, there are lots of reviews and studies focusing on contemporary or "rapid" evolution. So much so, that we've assembled a database (that is still continually being added to!) of phenotypic changes. We took studies that had allochronic data: studies of the same population that quantified phenotypes at least twice in time. From, these, we can calculate a rate of change. So, if the rate of change is different from zero, it means the population phenotype is changing through time, and the sign (positive or negative) will tell us the direction of change. In the case of body size, this would mean a positive rate of change indicates the population is getting larger.

So, we calculated rates of change for body size, and just visually, we can see that it seems more of the rates are negative as opposed to positive. This means that populations appear to be getting smaller through time, not larger!

Frequency histogram of Darwin numerators, one of the two types of rates we calculated.

Well, why might this be? Perhaps there is an explanation, such as human disturbances. The database is quite extensive, looking all sorts of populations that have been subject to varying levels of human intrusions. For example, we know that size selective harvesting (think of the cod fishery) can select against larger body size and populations will get smaller. So, we looked at only undisturbed populations, and still, the median rates of change were negative (Darwins = -1087).

Hmm, well, if we run simple, or fancy tests on the data, the results indicate that body size does not appear to be increasing in contemporary populations. If you want to know more about the fancy tests, you can read the article...

So what's happening? If there is positive selection for body size, why are contemporary populations not getting larger? Well, first there are the selection estimates themselves. Selection estimates are an important metric for us evolutionary biologists, but they are not perfect. Selection estimates can be affected by small sample sizes, unmeasured confounding variables, spatiotemporal variation, and imperfect fitness surrogates. Also, there can be a disconnect between selection estimates and actual phenotypic change, and this can happen for various reasons such as countergradient environmental changes, environmental covariance between traits and fitness, and covariance between non heritable traits and fitness. In other words, selection estimates are an important and useful tool, but they might not be the best way to assess microevolutionary trends. Still, our results indicate that body size is not getting larger, and this might be because larger body size does not increase individual fitness as clearly as we thought. Being big means you have to grow faster, and this can lead to increased foraging risk, increased mortality, and structural problems.

Looking at actual phenotypic changes through time in contemporary  populations does not appear to provide a microevolutionary mechanism underlying Cope's rule. However, this isn't really surprising given that there are many differences in micro- and macroevolutionary time scales, and selection can act at different levels (individual vs. population vs. species) and we here focused more on individual level selection. Furthermore, while there is evidence for Cope's rule in the literature, there are also many examples where evidence for Cope's rule have not been found. Perhaps the time has come to retire the rule, and instead, focus on untangling the underlying mechanisms of macroevolutionary trends without names and rules.

Look! We're published!
onlinelibrary.wiley.com/doi/10.1111/evo.12653/abstract