Heterochrony

[CDarwin]

Does anyone have any particular thoughts on its possible importance or frequency in evolution? I just finished Ontogeny and Phylogeny (old but amazing), and its provoked something of a facsination with the subject. I must say I'm a bit skeptical of his neotenic explanation for human evolution, though.

[PhDP]

Heterochrony is not seen today the same way it was seen then this book was written. Now we understand heterochrony is more "localized", no single heterochronic process accounts for the evolution of most species.

 

Also, while it's pretty certain heterochrony had something to do with our evolution, Gould's radical view that most of our recent evolution could be explained with neoteny is too simplistic.

 

It's a common theme with Gould; he had a lot of good ideas, but his view were often a little too simplistic.

[CDarwin]

That's just what happened with recapitulation, as Gould relates. It started with whole organisms being recapitulated and ended with only one organ or another being recapitulated before the idea was "rejected."

 

I really like Gould's "classification by process." That impressed me a lot in the book, and you can spot fallacies that result from classification by result in other places.

[Mokele]

The prevalence of heterochrony is also dependent upon the taxa: there is a *LOT* of it in amphibians, with numerous independent evolutions of paedomorphic adults, but in mammals, it's a lot rarer.

[PhDP]

True.

 

[...] Our results suggest that sequence heterochrony in embryonic stages has not been a major feature of mammalian evolution. This might be because mammals, and perhaps amniotes in general, develop for an extended time in a protected environment, which could shield the embryos from strong diversifying selection. Our results are also consistent with the view that mammal embryos are subject to special developmental constraints. Therefore, other mechanisms explaining the diversity of extant mammals must be sought.

 

The last part of the abstract of Olaf et al., 2003. Journal of Mammalian Evolution 10(4):335-361.

[lucaspa]

If heterochrony is defined as this: "A heterochronic change is, in general, a change in the rate or timing of development of some cell lines in the body relative to others. A mutation that alters the rate at which a cell line develops relative to other cell lines is a heterochronic mutation. " http://www.blackwellpublishing.com/ridley/a-z/Heterochrony.asp

 

Then it is much more important and prevalent than just neoteny. For instance, the elongation of fingers in bats would be heterochrony. I remember a seminar looking at the wrist bones of birds. A delay in the expression of bone morphogenetic protein for just 6 hours during embryonic development caused one of the normally spheroid wrist bones to elongate to almost the length of the radius. That simply changed the timing of development.

 

This site seems to give several examples that are not neoteny:

http://hoopermuseum.earthsci.carleton.ca//heterochron/intro1.htm

 

The following paper shows how changing the timing of embryonic development leads to the multiple "fingers" in the fins of dolphins:

 

1: Evol Dev. 2002 Nov-Dec;4(6):435-44.

 

Time, pattern, and heterochrony: a study of hyperphalangy in the dolphin embryo

flipper.

 

Richardson MK, Oelschlager HH.

 

Institute of Evolutionary and Ecological Sciences, Leiden University,

Kaiserstraat 63, 2300 RA Leiden, The Netherlands.

richardson@rulsfb.leidenuniv.nl

 

The forelimb of whales and dolphins is a flipper that shows hyperphalangy

(numerous finger bones). Hyperphalangy is also present in marine reptiles,

including ichthyosaurs and plesiosaurs. The developmental basis of

hyper-phalangy is unclear. Kukenthal suggested that phalanx anlagen split into three pieces during cetacean development, thereby multiplying the ancestral number. Alternatively, Holder suggested that apical ectodermal ridge

(AER)-directed limb outgrowth might be prolonged by a timing shift

(heterochrony), leading to terminal addition of extra phalanges. We prepared a series of whole mounted and serially sectioned embryonic flipper buds of the

spotted dolphin Stenella attenuata. This cetacean shows marked hyperphalangy on digits II and III. We confirm previous reports that the proximodistal laying down of phalanges is prolonged in digits II and III. Histology showed that the apical ectoderm was thickened into a cap. There was a weak ridge-like structure in some embryos. The cap or ridge formed part of a bud-like mass that persisted on digits II and III at stages when it had disappeared from other digits. Thus the dolphin differs from other mammals in showing a second period of limb outgrowth during which localized hyperphalangy develops. New phalanges only formed at the tip of the digits. These findings are consistent with a model in which heterochrony leads to the terminal addition of new phalanges. Our results are more easily reconciled with the progress zone model than one in which the AER is involved in the expansion of a prepattern. We suggest that patterning mechanisms with a temporal component (i.e., the "progress zone" mechanism) are potential targets for heterochrony during limb evolution.