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Problem 8: Differences Between Vertebrate Embryos Contradict the Predictions of Common Ancestry

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Editor's note: This is Part 8 of a 10-part series based upon Casey Luskin's chapter, "The Top Ten Scientific Problems with Biological and Chemical Evolution," in the volume More than Myth, edited by Paul Brown and Robert Stackpole (Chartwell Press, 2014). The full chapter can be found online here. Other individual installments can be found here: Problem 1, Problem 2, Problem 3, Problem 4, Problem 5, Problem 6, Problem 7, Problem 9, Problem 10.

Another area where evolutionary biologists claim powerful evidence for common ancestry is the patterns of development of vertebrate embryos. Biology textbooks typically portray the embryos of different groups of vertebrates as starting off development in a highly similar fashion, reflecting their common ancestry.129 However, such claims overstate the degree of similarity between the early stages of vertebrate embryos.

Biologists who investigate these questions have found considerable variability among vertebrate embryos from their earliest stages onward, contradicting what we are told to expect from common ancestry.130 As a paper in Nature stated, "Counter to the expectations of early embryonic conservation, many studies have shown that there is often remarkable divergence between related species both early and late in development."131 Or, as another article in Trends in Ecology and Evolution stated, "despite repeated assertions of the uniformity of early embryos within members of a phylum, development before the phylotypic stage is very varied."132

But most embryologists who acknowledge that vertebrate embyros start development differently will still claim embryos pass through a highly similar stage midway through development, called the "phylotypic" or "pharyngula" stage. These theorists propose an "hourglass model" of development, where it is claimed that similarities between embryos during this midpoint stage provide evidence for common ancestry. One critical biologist explains how this concept is viewed: "It is almost as though the phylotypic stage is regarded as a biological concept for which no proof is needed."133

But when biologists have looked for evidence supporting the existence of a phylotypic or pharyngula stage, they found the data points in the opposite direction. One comprehensive study in Anatomy and Embryology surveyed the characteristics of many vertebrates during this purportedly similar stage, and found that embryos show differences in major traits, including:

  • body size,
  • body plan,
  • growth patterns, and
  • timing of development.134
The researchers conclude that the evidence is "[c]ontrary to the evolutionary hourglass model" and "difficult to reconcile" with the existence of a pharyngula stage.135 Likewise, a paper in Proceedings of the Royal Society of London found the data was "counter to the predictions of the [phylotypic stage]: phenotypic variation between species was highest in the middle of the developmental sequence." It concluded that a "surprising degree of developmental character independence argues against the existence of a phylotypic stage in vertebrates."136

While vertebrate development shows wide variation, evolutionary embryologists seek to force-fit evolutionary interpretations to the data. When every rule is stymied by exceptions, a better way is to simply let the data speak for itself. A non-evolutionary approach to embryology would more easily acknowledge that differences exist between vertebrate embryos at all stages of development, and that vertebrate embryos show some similarities -- but also significant differences -- during the purported phylotypic stage.

References:

[129.] For example, see Colleen Belk and Virginia Borden Maier, Biology: Science for Life, p. 234 (Benjamin Cummings, 2010) ("Similarity among chordate embryos. These diverse organisms appear very similar in the first stages of development (shown in the top row), evidence that they share a common ancestor that developed along the same pathway"); Neil. A. Campbell and Jane B. Reece, Biology, p. 449 (Benjamin Cummings, 7th ed., 2005) ("Anatomical similarities in vertebrate embryos. At some stage in their embryonic development, all vertebrates have a tail located posterior to the anus, as well as pharyngeal (throat) pouches. Descent from a common ancestor can explain such similarities"); Holt Science & Technology, Life Science, p. 183 (Holt, Rinehart, and Winston, 2001) ("Early in development, the human embryos and the embryos of all other vertebrates are similar. These early similarities are evidence that all vertebrates share a common ancestor. ... They embryos of different vertebrates are very similar during the earliest stages of development").

[130.] For example, one paper states "Recent workers have shown that early development can vary quite extensively, even within closely related species, such as sea urchins, amphibians, and vertebrates in general. By early development, I refer to those stages from fertilization through neurolation (gastrulation for such taxa as sea urchins, which do not undergo neurulation). Elinson (1987) has shown how such early stages as initial cleavages and gastrula can vary quite extensively across vertebrates." Andres Collazo, "Developmental Variation, Homology, and the Pharyngula Stage," Systematic Biology, 49 (2000): 3 (internal citations omitted). Another paper states, "According to recent models, not only is the putative conserved stage followed by divergence, but it is preceded by variation at earlier stages, including gastrulation and neurulation. This is seen for example in squamata, where variations in patterns of gastrulation and neurulation may be followed by a rather similar somite stage. Thus the relationship between evolution and development has come to be modelled as an 'evolutionary hourglass.'" Michael K. Richardson et al., "There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development," Anatomy and Embryology, 196:91-106 (1997) (internal citations omitted).

[131.] Kalinka et al., "Gene expression divergence recapitulates the developmental hourglass model," Nature, 468:811 (December 9, 2010) (internal citations removed).

[132.] Brian K. Hall, "Phylotypic stage or phantom: is there a highly conserved embryonic stage in vertebrates?," Trends in Ecology and Evolution, 12(12): 461-463 (December, 1997).

[133.] Michael K. Richardson et al., "There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development," Anatomy and Embryology, 196:91-106 (1997).

[134.] Michael K. Richardson et al., "There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development," Anatomy and Embryology, 196:91-106 (1997). See also Steven Poe and Marvalee H. Wake, "Quantitative Tests of General Models for the Evolution of Development," The American Naturalist, 164 (September, 2004): 415-422; Michael K. Richardson, "Heterochrony and the Phylotypic Period," Developmental Biology, 172 (1995): 412-421; Olaf R. P. Bininda-Emonds, Jonathan E. Jeffery, and Michael K. Richardson, "Inverting the hourglass: quantitative evidence against the phylotypic stage in vertebrate development," Proceedings of the Royal Society of London, B, 270 (2003): 341-346;

[135.] Michael K. Richardson et al., "There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development," Anatomy and Embryology, 196:91-106 (1997).

[136.] Olaf R. P. Bininda-Emonds, Jonathan E. Jeffery, and Michael K. Richardson, "Inverting the hourglass: quantitative evidence against the phylotypic stage in vertebrate development," Proceedings of the Royal Society of London, B, 270:341-346 (2003) (emphases added). See also Steven Poe and Marvalee H. Wake, "Quantitative Tests of General Models for the Evolution of Development," The American Naturalist, 164 (3):415-422 (September 2004).

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