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Further Reading

SCIENCE: Operation ID

Heads or Tales

How Evolutionary Theory "Predicts" What It Finds

by Casey Luskin

Article originally appeared in
Salvo 30

In Salvo 29, we saw that biology textbooks often overstate the degree of similarity between the earliest stages of vertebrate embryonic development when making the case for common ancestry. Some evolutionary biologists acknowledge that vertebrate embryos begin development very differently, but still claim that embryos pass through a highly similar stage midway through development, called the "pharyngula" (or also the "phylotypic" or "tailbud") stage.

These theorists propose an "hourglass model" of development, where it is claimed that similarities between embryos during this convergent midpoint stage provide evidence for common ancestry (see below).

This pharyngula stage of development is viewed by some as powerful evidence for evolution. New Atheist and developmental biologist PZ Myers named his popular blog "Pharyngula" and gave it the tagline, "Evolution, development, and random biological ejaculations from a godless liberal." There, he has argued that "vertebrate embryos at the phylotypic or pharyngula stage do show substantial similarities to one another that are evidence of common descent. That's simply a fact."1

But is it a fact that this pharyngula (or phylotypic, or tailbud) stage exists?

No Proof Needed

In 1997, a team of embryologists decided to investigate this question. In a groundbreaking study published in the journal Anatomy and Embryology, they lamented that "it is almost as though the phylotypic stage is regarded as a biological concept for which no proof is needed."2 When they looked for proof of a phylotypic or pharyngula stage, they found that the data points went in the opposite direction.

After taking high-quality photographs of vertebrate embryos during this purportedly similar stage, they surveyed the embryos' characteristics, and found that they showed differences in major traits, including: (1) body size, (2) body plan, (3) growth patterns, and (4) timing of development.

The researchers reported: "Contrary to recent claims that all vertebrate embryos pass through a stage when they are the same size, we find a greater than 10-fold variation in greatest length at the tailbud stage." They concluded that the evidence is "contrary to the evolutionary hourglass model," and that it shows "considerable variability—and evolutionary lability—of the tailbud stage, the purported phylotypic stage of vertebrates." In their view, this "wide variation in morphology among vertebrate embryos is difficult to reconcile with the idea of a phylogenetically-conserved tailbud stage."3

These patterns hold true not just for physical characteristics of development, but also for important properties of the genome. A 2013 article in PLoS Genetics found that genomic properties related to DNA sequence, gene age, gene family size, and gene expression did not match the convergent patterns predicted by the phylotypic stage. As the paper concluded, "Several features do not show any significant pattern over embryonic development, often in contradiction to previous reports."4

Likewise, a study in Proceedings of the Royal Society of London found that the data was "counter to the predictions of the phylotypic stage," since "phenotypic variation between species was highest in the middle of the developmental sequence." It concluded: "[A] surprising degree of developmental character independence argues against the existence of a phylotypic stage in vertebrates."5

Predict Whatever You Find

A few years ago, I cited much of this evidence in response to PZ Myers,6 who had called an undergraduate biology student a "flaming moron" who didn't "know word one about basic biological concepts"7 for challenging some of PZ's embryological arguments for evolution. PZ responded to me in a similar manner, calling me "stupid," "embarrassing," and a "cacophony of inanity." In case the message wasn't clear, he suggested: "Go back to third grade, Casey. You are a very silly, ignorant fellow."8

Rather than making me upset, PZ's rhetoric encouraged me. He never even attempted to contradict any of the evidence I raised that challenged the existence of the pharyngula stage. If he had an evidence-based rebuttal, surely we would have heard that instead of large amounts of name-calling.

But PZ did make a few substantive points in response to the student and me. He conceded that early vertebrate embryos can "vary greatly" and that "there is wide variation in the status of the embryo."9 But he wasn't going to let those facts challenge his theory, as he countered: "I wish I could get that one thought into these guys' heads: evolutionary theory predicts differences as well as similarities."10

That's intriguing, I thought. Earlier we saw PZ cite the "substantial similarities" between vertebrate embryos in the pharyngula stage as "evidence of common descent." But later, when forced to admit the "wide variation" among embryos, PZ tells us that "evolutionary theory predicts differences" too. Perhaps that's true, but then how can he cite the "similarities" among embryos in the pharyngula stage as evidence for common ancestry?

In reality, according to PZ, evolutionary theory predicts whatever it finds. Such logic might help save his theory from falsification in light of all the differences between vertebrate embryos across many stages of development, but it doesn't help construct a robust theory that makes testable predictions. As the old adage says, "the theory that explains anything really explains nothing."

What's Left of Common Ancestry?

In Salvo 25, we discussed how the case for common descent is often said to be "cumulative," based upon multiple lines of evidence ranging from biogeography, to fossils, to DNA, to anatomy, and lastly, to embryology. How is the theory faring?

In biogeography, evolutionists resort to unlikely and speculative explanations about animal migration—theorizing, for instance, that a species must have rafted across vast oceans—in order for common descent to account for their peculiar locations around the globe. With regard to paleontology, we saw that the continuous branching pattern predicted by common ancestry is not found, and that the fossil record is instead dominated by the abrupt explosion of new life forms.

In Salvo 27 and 28, we saw that within the realms of DNA and anatomy, the tree of life is in tatters, and that evolutionary biology predicts that similarity is the result of common inheritance, except when it isn't. Now, we've seen something very similar within embryology. Evolutionary biology predicts that similarities will exist between vertebrate embryos, except when we find differences, and then it predicts those, too.

Much data contradicts the sometimes-made predictions of common descent, but what, if anything, does evolutionary biology actually predict?

As PZ Myers has shown us, common descent seems to predict whatever is expedient. If there's any clear pattern here, it's this: the data often fail to fit the predictions of common descent, but when that happens, proponents of common descent don't get worried. They simply change their predictions. 


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