Egg-Shape Evolution Theory Cracks Under Pressure
Touted as an evolutionary explanation for bird egg shapes, a new hypothesis celebrating natural selection falls like Humpty Dumpty under a gentle breeze of questioning.
I think that, if required on pain of death to name instantly the most perfect thing in the universe, I should risk my fate on a bird’s egg. —Thomas Wentworth Higginson, 1862
Natural selection is the hero of a paper in Science Magazine about bird eggs, and Phys.org was sure to make that clear in its write-up. “How eggs got their shapes: Adaptations for flight may have driven egg-shape variety in birds,” the bold headline announces. Read further in this article classified under evolution, and you see that a Professor of Organismic and Evolutionary Biology from Harvard used an evolutionary framework to determine the “implications of egg shape in an evolutionary and ecological setting.” This egg story is drenched in evolutionary seasoning, to make sure the reader won’t miss the taste.
And if that isn’t enough evolutionary flavor, Claire N. Spottiswoode in Science Magazine marinates the egg story in Darwin’s special brand Natural Selection Sauce. She says,
- The selection pressure that best explains its evolution comes from the characteristic we most associate with birds: flight.
- This hypothesis predicts that species under strong selection for flight-related adaptations—such as migrants and aerial insectivores—should have elliptical or asymmetric rather than spherical eggs.
- The authors use an index of aerodynamic wing shape as a proxy for such selection, and find that this is by far the best predictor of egg shape.
- Why, then, are there no hot-air balloon–shaped eggs? Not only do they appear developmentally hard to produce, but perhaps they offer no obvious selective advantage over a spherical egg: They are still inconveniently wide, with little increase in volume.
- Egg collecting is now deeply unfashionable and rightly illegal. But from its heyday in the late 19th to the mid-20th century, it has bequeathed to us data that can yield wonderful evolutionary insights, as Stoddard et al.‘s study underlines.
Evolution. Evolution. Evolution. Got it? Eggs evolved by natural selection, and Science shows how. Darwin’s theory explains “an old mystery in natural history,” Phys.org says. No smart person should ever doubt evolution again. Look how useful evolutionary theory is to science! Spottiswoode says,
Every bird egg serves the same function: to protect and nourish the offspring within while it grows from two cells to a fully formed chick. Yet this identical function is served by a striking diversity of egg shapes. Explanations for both the origin and function of this diversity have remained little more than anecdotal. On page 1249 of this issue, Stoddard et al. marry biophysics and ecology to provide a general theory that explains how and why diverse egg shapes arose. Based on a mathematical model, the authors predict that simple changes in the forces experienced by the shell membrane as the egg develops in the female’s oviduct are sufficient to generate the observed egg-shape diversity across all birds. The selection pressure that best explains its evolution comes from the characteristic we most associate with birds: flight.
One certainly can’t fault the scientific rigor of Stoddard’s team. They accessed a museum collection of almost 50,000 eggs from 1,400 bird species. They proposed a hypothesis. They used math and built a chart with ellipticity on one access and asymmetry on the other axis. They compared the positions of eggs on this chart with the flight behaviors of the species. They made predictions that were fulfilled, and covered anomalies with auxiliary explanations. They showed how their hypothesis succeeds over the “anecdotal” proposals of others. Evolution wins again!
In an eggshell, the explanation goes like this: the demands of flight create selection pressure on egg shape. Spottiswoode dispenses with old theories about clutch size, the need to prevent rolling off cliffs, and other “intriguing but ultimately parochial hypotheses” to lead into the new-and-improved idea hatched by Stoddard’s team:
Instead they find consistent support for a simple hypothesis. Birds are streamlined for flight. Perhaps streamlined birds need narrower eggs to negotiate their narrower pelvis, and because the only way to fit a chick into a narrower egg is to make the egg longer, elliptical or asymmetric eggs result. This hypothesis predicts that species under strong selection for flight-related adaptations—such as migrants and aerial insectivores—should have elliptical or asymmetric rather than spherical eggs. The authors use an index of aerodynamic wing shape as a proxy for such selection, and find that this is by far the best predictor of egg shape. Swifts that live almost all of their lives on the wing have elliptical eggs. Sandpipers that traverse the globe have elliptical, asymmetric eggs. Puffbirds and trogons of tropical forests that may rarely leave their territories tend to have relatively spherical eggs. So, too, do flightless ostriches, but not penguins—perhaps because they must be streamlined to “fly” underwater. Within specific taxonomic groups, additional correlations suggest that other demands, such as clutch size, do further modulate egg shape, but none applies generally across all birds. Why, then, are there no hot-air balloon–shaped eggs? Not only do they appear developmentally hard to produce, but perhaps they offer no obvious selective advantage over a spherical egg: They are still inconveniently wide, with little increase in volume.
Can the evolutionary answer stand up to a gentle whiff of questioning? A running theme at CEH is that natural selection is a vacuous concept masquerading as a scientific explanation. By failing to provide real concrete predictions that are testable, natural selection reduces to the Stuff Happens Law—the opposite of explanation. Whatever happens, “it evolved,” so that the explanation becomes a just-so story. Are these scientists and reporters playing make-believe again? Or have they really demonstrated the value of Natural Selection theory for science? Let’s think about it.
We should note first that egg shapes are examples of microevolution. Getting a chick to develop in 21 days that can hatch and fly is the big issue for evolution; egg shape and size seem very minor by comparison. We might compare the phenomenon to a magician pulling a rabbit out of a hat. Changing the shape of the hat or the size of the rabbit doesn’t matter as much as being able to do the trick itself. Furthermore, the evolutionary story fails to rule out creation or intelligent design, because advocates of those positions are perfectly happy to see variation in egg shape for different species, and are willing to admit some degree of change over time. So far, then, we don’t see the evolutionary story deserving of privileged status.
The biggest piece of evidence they adduce is the chart showing a correlation between flight behavior and egg shape. It’s an interesting pattern. Correlation, however, does not imply causation. Does the egg shape drive the female bird’s oviduct, or does the oviduct drive the egg shape? If aerodynamic efficiency makes natural selection drive egg shape, one would think it would also drive everything else about the bird, like beak shape and mass. But beaks among strong flyers vary all over the map (consider pelicans, hummingbirds, and Arctic terns). Furthermore, for good reason, females only lay eggs when they are not flying. And what about the males, who don’t have an oviduct? It’s not exactly clear why natural selection would have any influence on egg shape. How do they know the differences are not due to genetic drift or some other non-Darwinian mechanism?
More importantly, the scientists, and Ms Spottiswoode and the Phys.org reporter, fail to apply neo-Darwinian theory correctly. They do not identify any mutation in egg shape genes that consistently appears and gets selected when a flyer needs an elliptical egg to survive and produce offspring. That should be the case if natural selection is a law of nature superior to the Stuff Happens Law. They fail to show how every other member of the population died out, such that only the individuals possessing the mutation survived to lay eggs. The explanation, in fact, sounds Lamarckian (inheritance of acquired characteristics) – certainly no less anecdotal than the preceding hypotheses. A look through the main paper reveals the authors admitting that in some respects, the preceding hypotheses made predictions about egg shape that work just as well as theirs. Wobbling between multiple conflicting variables, their flight-adaptation hypothesis reduces to speculation with a very weak empirical basis.
Adding to the trouble, their phylogenetic analysis fails to find a consistent ancestry connecting flight ability to egg shape, leaving them scrambling for auxiliary hypotheses like convergence and parallel evolution. Watch the perhapsimaybecouldness index rise like a stiff breeze, threatening the stability of their hatched hypothesis:
We do not suggest that a female’s flight behavior during the egg formation period directly affects egg formation, nor do we suggest that egg shape so strongly influences the flight abilities of female birds during their egg-laying period that selection has produced an aerodynamic egg. Rather, we propose that general adaptations for strong flight select for a constrained, muscular, streamlined body plan in both males and females, giving rise in the latter, directly or indirectly, to asymmetric and/or elliptical eggs. The precise physiological mechanisms by which morphological adaptations for flight might affect egg shape are unknown. However, the answer most likely lies in the two parameters highlighted by our biophysical model: egg membrane thickness variations and the differential pressure applied across the membrane, both of which are potentially shaped by selection for a streamlined body plan.
Humpty Dumpty just fell. Wasn’t natural selection Darwin’s famous ‘mechanism’ to explain everything in biology? They just said the mechanisms are ‘unknown’, and only ‘might’ affect egg shape. They just said natural selection might work ‘directly or indirectly’—well, which is it? Clearly they do not know. Selection might have been ‘giving rise to’ (miracle words) “asymmetric and/or elliptical eggs,” implying that whatever influence natural selection had was general, non-specific and ambiguous. Is this an explanation, or something they find ‘most likely’ and ‘potentially’ explanatory? That’s only an opinion—a preference. Readers should make up their own minds about the strength of the evidence, not kowtow to the authors’ bluffing about the success of their hypothesis.
In short, their hypothesis crashes to the ground right during the Darwin celebration, leaving a scrambled mess of just-so storytelling behind. Spottiswoode concludes that Stoddard’s team has not shown natural selection to play a causative role, and except in a “satisfyingly general” way (i.e., a storytelling way), has really explained very little at all:
Stoddard et al. conclude that variation in egg shape at a broad scale is best explained by variation in the need to fly. But although satisfyingly general, this discovery will be far from the final word. A bird’s egg was famously described by abolitionist Thomas Wentworth Higginson as “the most perfect thing in the universe” [3/09/17], but this apparent perfection is most likely the sum of multiple ecological, structural, and developmental compromises. It remains unclear why egg shapes tend toward being spherical in the absence of strong selection for powered flight. Do asymmetry and ellipticity carry costs, such as making an egg easier to break into or harder to break out of? And why has natural selection solved the streamlining problem with elongate and symmetric eggs in some species, and elongate and asymmetric eggs in others—that is, what best explains the variation along the x axis of the figure? Did elongate eggs repeatedly evolve in concert with narrower pelvises, and do their shell membranes vary in thickness and composition in the way that Stoddard et al.‘s model predicts? Their paper opens up a rich seam for researchers to explore.
The authors end with nothing left but futureware , in hope that all the king’s horses and all the king’s men can pick up the pieces and build a better evolutionary story.
Our macroevolutionary analyses suggest that birds adapted for high-powered flight may maximize egg size by increasing egg asymmetry and/or ellipticity, while maintaining a streamlined body plan. Moving forward, it will be important to determine how the developmental process of egg shaping is coupled, in terms of physiology and genetics, with evolutionary constraints associated with flight strength and efficiency.
The very thing they promised to explain—egg shape by natural selection (‘selection pressure’)—they now say has to be be explained in the future—egg shaping by natural selection (‘evolutionary constraints’). If natural selection theory has this much trouble with something as simple as egg shape, how can it explain flight itself, where multiple adaptations must appear simultaneously to keep the bird airborne? (See the Illustra film, Flight: The Genius of Birds.)
Exercise: Here’s another paper in PNAS that purports to show how natural selection explains symbiotic relationships. It looks very impressive, with lots of math and jargon. But does it really succeed in proving the explanatory power of Darwinian theory? Or is it more like the glitzy ballroom on the Titanic hiding a flawed engine room unable to sustain impact by the iceberg of pointed questions? Look for evidence of high PCI (perhapsimaybecouldness index), exceptions to rules, storytelling, speculation, and fudging of parameters to obtain desired conclusions.