Evo-devo questions

[zeightyfiv]

Some questions in evolutionary developmental biology that I've formulated and have been puzzling over lately:

• Should defects that do not cause early, catastrophic effects (pleiotropy or otherwise) be expected to cause only milder or delayed negative effects? Is there a definite multimodal distribution? Or a long tail?
   
• What is the meaning of a living fossil? What governs the phenotypic rate of change of a clade? Can it be really said that a clade has not meaningfully changed in a long time? Do such clades genuinely exist, where we think they do? And why haven't they changed when others have?
  • (Examples: cycads, ginkgo, horsetails, welwitschia, horseshoe crabs)
     
• Why do small sub-clades and even singleton species stick around? Shotgun extinction? Unique adaptations? Sheltered niches? Why don't they re-radiate? Why aren't more clades "bushy", rather than, as most large ones seem to be, thin-necked?
  • (Examples: Singletons & sub-clades --"living fossils" above, lycopods. Thin necks -- angiosperms, dicots, birds.)
     
• How do specialized, early-branch-off clades (living fossils or not) maintain their edge, and why haven't others co-opted their niches?
  • (Examples: mosses, sharks, amphibians, conifers)
     
• What governs the rate of growth of phenotypic complexity? Endogenous factors, like infrastructure and metabolism (though granted, the latter may be ecosystem-governed)? Exogenous factors, like ecosystem complexity? Merely a slow and steady ascent?
   
• How do non-interacting phenotypic features evolve concurrently, given the low bandwidth of feedback. Sexual reproduction only? Do the numbers really work out?