Lecture 21 macroevolution
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Lecture 21: Macroevolution. Last class: 1) Peramorphosis: add’n of extra stages a) Hypermorphosis : dev’t extended from  to  1.  1. Descendant Ancestor. . - same allometry (relationship of y to x) - early start of y means greater y (not x) at maturity . log y.  1. .

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Lecture 21 macroevolution
Lecture 21: Macroevolution

Last class:

1) Peramorphosis: add’n of extra stages

a) Hypermorphosis: dev’t extended from  to 1

B predisplacement




- same allometry

(relationship of y to x)

- early start of y means

greater y (not x) at maturity

log y


log x

b) Predisplacement:

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

C acceleration
c) Acceleration

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




log y

Larger (or more dev’d)

y (not x) at maturity

log x

2 paedomorphosis
2) Paedomorphosis

  • retention of juvenile features in adult

    A) Progenesis

    B) Neoteny

    C) Postdisplacement

A progenesis




Smaller y, smaller x at maturity vs. ancestor

- Allometry unchanged

- Compare: hypermorphosis

log y

log x

a) Progenesis

  • dev’t stops early

B neoteny




log y

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

log x

b) Neoteny

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

C postdisplacement
c) Postdisplacement

  • y starts growing late rel. to x in descendant vs. ancestor




log y

- same allometry

- late start of y means

smaller y (not x) at maturity


log x

Evolutionary significance of heterochrony
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 heterochrony
Genetic Basis of Heterochrony

Homeotic (Hox) genes:

  • 1st discovered in Drosophila spp.

  • involved in gross alterationsin phenotype

  • Affect develop’t of cuticular structures from imaginal disks

  • in allanimal phyla

  • share # of common


  • e.g. antennapedia

Hox genes
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 d
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

Type of heterochronic process
Type of Heterochronic Process?


vs. Tiger Salamander

  • failure to metamorphose

  • [thyroxine] : can be exp’tally induced

  • external gills in adult (juvenile morphology)

So what s going on
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
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


Wrasse & Angelfish

Skulls of Human, Chimp

& Baboon

Evolution of higher taxa gould
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?


  • 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



- 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 usuallyfatal (no change in rate)