21.4 Changes in Developmental Pathways Underlie Evolutionary Change differentiation happens from differential gene expression that happens with signals that begin a domino of regulatory transcription factors if even one of these things don’t happen, they embryo could die, if it is just modified in one way, they could develop in a different way these cascades and regulatory transcription factors are the basis of what denton? evolutionary change evo-devo = evolutionary-developmental biology what that studies is how changes in developmental genes had led to new phenotypes the example they give is the limb loss of snakes they say how relatives have limbs, and their ancestors had four functional legs with feet and toes but how now most don’t develop any sort of limbs, but boas and pythons do have small pelvic bones, a femur, and a claw so they ask how did they loose their legs? Researchers can know this because the regulatory cascades for limbs are pretty well understood They give the example of the hox genes in chicken embryos: hox 6 and hox 8 are
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