We often use the phrase “when pigs fly” to describe something that is extremely unlikely to happen. But why is it so crazy that pigs could ever evolve wings? In fact, why didn’t they evolve wings?
Maybe pigs never evolved wings because they couldn’t. That is, there are developmental constrains on the basic body plan of pigs that prevent them from evolving wings.
This concept of developmental constraints occurs frequently in evolutionary and developmental biology. Certain stages of development are non-negotiable; if development deviates too far from a given program, there are serious consequences for the organism.
On the other hand, there must be some breathing room in developmental programs so that organisms can evolve and adapt.
So, how has nature struck a balance between the need to stick closely to a developmental plan, but also allow for noise and fluctuation so that adaptation can occur?
Comparative studies of morphological data have led to an hourglass model of development in which the most constrained stages occur in mid-development when the basic body plan of an organism is established (called the phylotypic stage), whereas early and late stages are less constrained. This theory has been most recently supported by molecular studies that show gene expression patterns are most conserved during mid-development.
In a huge collaborative effort, Wang and colleagues1 sequenced the genome and transciptome of two turtle species, the soft-shell turtle and the green sea turtle. They were able to answer long standing questions about the evolutionary origins of turtles (they’re a sister group to crocodilians and birds), gain insight into the molecular mechanisms of unique turtle characteristics (they might live so long because of a gene with a role in antioxidative stress) and into how a turtle builds its shell (co-option of Wnt signaling normally used in limb bud formation).
But perhaps most pertinent here is their comparative analysis of turtle embryo development. The authors’ previous studies made broad comparisons among vertebrates, sampling from different sub-taxa (for example, frogs vs. mouse) and they found that in this case, the most conserved stage was the vertebrate phylotypic stage. In their present study, the authors asked what the most conserved stage of development is if the two organisms are both vertebrates and amniotes (a subtaxa of vertebrates). Would it be the vertebrate or the amniote phylotypic period?
They compared gene expression in all developmental stages of the soft-shell turtle to all stages of the chicken embryo and found that the stage with the most shared gene expression corresponded to the vertebrate phylotypic stage, not the amniote. They also found that turtle-specific expression of 223 genes begins after establishing the basic vertebrate body plan.
The authors’ findings suggest that, in the case of vertebrates, evolution is constrained by the developmental establishment of the vertebrate body plan, but that later developmental stages were fair game for natural selection to act on, ultimately ending up in morphological novelties like the turtle shell. It would be interesting for the authors to expand their comparisons to other amniotes with unique morphological features to see if this pattern holds true.
- Want et al (2013). “The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan.” Nature Genetics 45:6. doi:10.1038/ng.2615
Irie, N. and S. Kuratani (2011). “Comparative transcriptome analysis reveals vertebrate phylotypic period during organogenesis.” Nature Communications 2. doi: 10.1038/ncomms124