Hox genes are found in both insects and vertebrates, with conserved collinear organization and similar functions in body plan patterning. What does this conservation imply about the common ancestor of insects and vertebrates?
AInsects evolved from vertebrates recently
BThe common ancestor of all bilaterians (living over 500 million years ago) already possessed a Hox gene cluster with collinear expression, and this toolkit has been inherited and modified in all descendant lineages
CHox genes evolved independently in insects and vertebrates through convergent evolution
DThe conservation is coincidental and has no evolutionary significance
The conservation of Hox genes across all bilaterian phyla — with the same collinear organization, similar expression patterns, and even functional interchangeability in some cases — is far too detailed to have evolved independently. This is deep homology: the common ancestor (Urbilateria) had a Hox cluster that patterned its AP axis, and this system was inherited by all descendant lineages. The dramatic morphological differences between insects and vertebrates were achieved by modifying the regulation and downstream targets of these conserved genes, not by inventing new patterning systems. This is the foundational insight of evo-devo.
Question 2 True / False
Evo-devo predicts that morphological evolution occurs primarily through mutations in protein-coding sequences of developmental genes.
TTrue
FFalse
Answer: False
Evo-devo's key prediction is the opposite: morphological evolution occurs primarily through changes in cis-regulatory elements (enhancers, promoters) that control when and where developmental genes are expressed, not through changes in the proteins themselves. Coding mutations in toolkit genes tend to be pleiotropic (affecting many tissues simultaneously) and are usually deleterious or lethal. Regulatory mutations can alter gene expression in one tissue or at one developmental time without affecting other functions of the same gene. This modularity of cis-regulatory elements makes them the preferred substrate for morphological evolution — they allow fine-tuned changes to specific structures without collateral damage.
Question 3 Short Answer
Explain how a cis-regulatory mutation could produce a novel morphological trait without altering any protein-coding gene.
Think about your answer, then reveal below.
Model answer: A mutation in an enhancer element could drive expression of an existing gene in a new location or at a new time during development, creating a novel structure from the existing developmental toolkit. For example, the evolution of wing spots in Drosophila species involves gain or loss of enhancer elements that drive expression of pigmentation genes specifically in wing regions — the pigmentation enzymes are unchanged, but their spatial deployment is novel. Similarly, the loss of pelvic spines in sticklebacks involves deletion of a pelvic-specific enhancer for Pitx1 (a limb development gene) — the protein is unchanged and functions normally in other tissues, but its pelvic expression is lost, causing pelvic reduction. These examples show that new morphologies can arise from regulatory rewiring of an existing genetic toolkit.
Sean Carroll, Neil Shubin, and others have documented numerous cases where morphological changes map to cis-regulatory changes rather than coding mutations. This has shifted the search for the genetic basis of morphological evolution from coding sequences to the vast non-coding regulatory genome — the 'dark matter' of evo-devo.