1. Lecture 21: Macroevolution Last class: 1) Peramorphosis: add’n of extra stages a) Hypermorphosis: dev’t extended from  to  1

2. b) Predisplacement: y starts growing early rel. to x in descendent vs. ancestor log x log y   11 11 - same allometry (relationship of y to x) - early start of y means greater y (not x) at maturity Descendant Ancestor

3. c) Acceleration faster growth of y rel. to x in descendent vs. ancestor log x log y   11 Larger (or more dev’d) y (not x) at maturity Descendant Ancestor

4. 2) Paedomorphosis retention of juvenile features in adult A) Progenesis B) Neoteny C) Postdisplacement

5. a) Progenesis dev’t stops early log x log y   11 Smaller y, smaller x at maturity vs. ancestor - Allometry unchanged - Compare: hypermorphosis Ancestor Descendant

6. b) Neoteny slower rate of growth of y rel. to x in descendant vs ancestor log x log y   11 Ancestor Descendant - Smaller or less developed y rel. to x at maturity

7. c) Postdisplacement y starts growing late rel. to x in descendant vs. ancestor log x log y   11 11 Ancestor Descendant - same allometry - late start of y means smaller y (not x) at maturity

9. Evolutionary Significance of Heterochrony? 1. Large changes in phenotypes easily accomplished - mutations at one or several loci may be involved 2. Likely important in speciation -gene pools w diff. heterochronic mutations  repro. isol’n

10. 3. May release lineages from phylogenetic constraints - e.g. paedomorphosis: descendant no longer passes through the same develop’l stages as ancestor - can “free” the sp. from the constraint imposed by that structure - only affects existing structures.

11. Genetic Basis of Heterochrony Homeotic (Hox) genes: 1 st discovered in Drosophila spp. involved in gross alterations in phenotype Affect develop’t of cuticular structures from imaginal disks in all animal phyla share # of common characteristics e.g. antennapedia

12. Hox Genes 1. organized in gene complexes - probably involves gene duplication 2. spatial & temporal collinearity: - 3' end expressed anterior; 5' end expressed posterior - 3' end expressed earlier in dev’t than 5' end

13. Hox Genes cont’d 3. contain highly-conserved 180 bp region - involved in binding Hox genes are regulators - control timing and expression of other genes e.g. Ubx (ultrabithorax) in Drosophila: controls expression of 85 - 170 genes

14. Type of Heterochronic Process? Axolotl vs. Tiger Salamander failure to metamorphose [thyroxine] : can be exp’tally induced external gills in adult (juvenile morphology)

15. So what’s going on? not postdisplacement : age at maturity ≈ other salamanders not progenesis : body size at maturity ≈ other salamanders (progenesis  tiny adult) Neoteny: somatic dev’t slows & is overtaken by normal sexual maturity  giant juvenile

16. D’Arcy Thompson early 20th century comparative anatomist “On Growth & Form”: transformation grids: explain changes in shape & determine allometric growth measurements made & plotted on rectangular coordinates same measurements made in a related organism or a different stage in dev’t shown as deformations of grid system now : partial warp analysis

17. Hatchetfish Wrasse & Angelfish Skulls of Human, Chimp & Baboon

18. Evolution of Higher Taxa (Gould) new groups often arise from neotenic or progenetic ancestors e.g. flightless birds e.g. insects: from larval form of millipede- like ancestor? e.g. chordates  larval cond’n of tunicates?

19. Saltationists distinctive features of higher taxa arise through “systemic mutation” (complete reorganization) Argument: - few intermediates among higher taxa - little selective advantage to incipient structures - results in dramatic, discontinuous effects

20. Neodarwinists Counter-argument: - characters of higher taxa evolve mosaically - many intermediate forms e.g. Archaeopteryx, Lepidoptera - early stages of complex structures selectively advantageous - mutations with disruptive pleiotropic effects usually fatal (no change in rate)