Lecture 21: Macroevolution

Last class:1) Peramorphosis: add’n of extra stages

a) Hypermorphosis: dev’t extended from to 1

b) Predisplacement:

y starts growing early rel. to x in descendent vs. ancestor

log x

log

y

1

1

- same allometry (relationship of y to x)- early start of y meansgreater y (not x) at maturity

Descendant

Ancestor

c) Acceleration

• faster growth of y rel. to x in descendent vs. ancestor

log x

log

y

1

Larger (or more dev’d) y (not x) at maturity

Descendant

Ancestor

2) Paedomorphosis

• retention of juvenile features in adultA) ProgenesisB) Neoteny C) Postdisplacement

a) Progenesis • dev’t stops early

log x

log

y

1

Smaller y, smaller x at maturity vs. ancestor- Allometry unchanged- Compare: hypermorphosis

Ancestor

Descendant

b) Neoteny

• slower rate of growth of y rel. to x in descendant vs ancestor

log x

log

y

1

Ancestor

Descendant

- Smaller or less developed y rel. to x at maturity

c) Postdisplacement• y starts growing late rel. to x in descendant vs.

ancestor

log x

log

y

1

1

Ancestor

Descendant- same allometry- late start of y meanssmaller y (not x) at maturity

Paedomorphosis Peramorphosisunderdevelopment overdevelopment

Rate Slower Faster(Neoteny) (Acceleration)

Onset Time Later Earlier(Postdisplacement) (Predisplacement)

Offset Time Earlier Later(Progenesis) (Hypermorphism)

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  

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.

Genetic Basis of HeterochronyHomeotic (Hox) genes:• 1st 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

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

Hox Genes cont’d3. contain highly-conserved 180 bp region

- involved in bindingHox genes are regulators - control timing and

expression of other genese.g. Ubx (ultrabithorax) in Drosophila:

controls expression of 85 - 170 genes

Type of Heterochronic Process?

Axolotl

vs. Tiger Salamander

• failure to metamorphose • [thyroxine] : can be exp’tally induced• external gills in adult (juvenile morphology)

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

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

Hatchetfish

Wrasse & Angelfish

Skulls of Human, Chimp & Baboon

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?

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

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)