Freethought & Rationalism Archive

Blinn · 06-21-2002, 01:56 PM

lpetrich posted the following yesterday in the "hollow beliefs vs. truth" thread. Thought I'd follow up in a new thread with what I found.

Quote:

Originally posted by lpetrich:
<strong>I was searching for some discussion of the evolution of the frog's tadpole phase when I came across <a href="http://www.users.bigpond.com/rdoolan/amphrep.html" target="_blank">this creationist page</a>, which flatly denied that amphibians can evolve into reptiles.

However, some of the supposedly impossible steps can be seed in living amphibians, notably frogs sensu lato. These may be divided into frogs sensu stricto, which are not very well adapted to dryness, and toads, which are better-adapted. What helps toads is their thicker skin, which makes them more reptile-like.

Also, while most frogs go through a tadpole phase, some species of frogs are direct-developing, hatching from their eggs in their adult form. This has happened several times; some species with this feature are closely related to species that still feature indirect, through-tadpole development. Direct-developing frogs lay larger eggs than is average for frogs, which allows longer development and makes it more difficult for water to escape. And inside the egg, direct-developing frog embryos have some tadpole features. All these features are essentially reptile-like; reptile, bird, and mammal embryos still have tadpole-like features like gill pouches and associated circulation.

What I had been looking for was some discussion of the question of whether early amphibians had also had an aquatic tadpole-like phase, and whether frogs' tadpole phase is a vestige of that. But I couldn't find anything, even though I suspect that both are correct.</strong>

From <a href="http://www.amazon.com/exec/obidos/tg/stores/detail/-/books/0226557634/glance/ref=lib_rd_TFCV/104-9951678-8131127" target="_blank">Tadpoles: The Biology of Anuran Larvae</a>

(I left out most of the citations to save myself some typing)

This chapter is "THE ANURAN TADPOLE Evolution and Maintenance" Reid N. Harris

p. 288

Quote:

The origin of a free-living larval stage can be explained with the phylogenetic null hypothesis, i.e., anurans evolved from ancestors that had a complex life cycle. The ancestral larval to postlarval size versus shape trajectory is thought to have included a minor metamorphosis�I define a minor metamorphosis as minimal variation between many traits in the larva and post-larva. Compared with frogs, salamanders have a minor metamorphosis�A minor metamorphosis seems to have occurred in the dissorophids� the presumed ancestors to modern amphibians, but the question of anuran origin is still unresolved.

p.290

Quote:

Is there any evidence that direct development did or did not facilitate the evolution of divergent life stages in anurans? Can direct development be considered primitive in frogs? I argue that if direct development is primitive, then an analysis of Leiopelma, which retains many primitive traits, or a fossil ancestor should reveal a larval size-shape trajectory that retains primitive, perhaps salamander-like traits. If direct development is derived, then Leiopelma and other species with direct development should have a larval size-shape trajectory that retains some trace of a tadpolelike trajectory. Most workers have considered a complex life cycle to the [sic] primitive in amphibians� The argument for primitiveness is that most anurans have complex life cycles and that most fish and salamanders also have complex life cycles. However, commonness of a trait cannot be used to establish a character polarity. For example, most mammals are placentals, but it is generally concluded that the placenta is a derived character. The life cycles of the coelacanth or lungfishes are not helpful in determining what life cycle is primitive in frogs. The crossopterygian Latimeria has direct development and the lungfishes have complex life cycles.

Fossil evidence does not seem conclusive in determining whether a complex life cycle is primitive in frogs. Bolt (1977) suggested that the Dissorophoidea is ancestral to the three modern groups of Lissamphibia. The evidence includes similarities in the pedicellate teeth of both groups. Adult and juvenile remains of the dissorophids have been found. Bolt proposed a paedomorphic origin of the modern amphibians based on their similarity to juvenile dissorophids. It cannot be concluded that dissorophids had a free-living larval stage because larval remains have not been found. Protoanurans are known from the fossil record, but larval stages of these transitional forms are not known.

He goes on to say that further analysis is needed in the form of a series of complete size-shape trajectories for a variety of traits in Leiopelma, and that if it �turns out to have a derived direct development, the hypothesis that direct development can facilitate a phenotypic divergence between life stages remains testable should suitable fossil material be found.�

So there ya' go... we don't know.

Blinn · 06-29-2002, 12:08 PM

*bump*

I posted this a while back for lpetrich and I think he missed it, so here it is again, for what it's worth.

liquid · 06-29-2002, 12:47 PM

If you are interested in ading further examples of this sort of change, look up axolotl/salamanders.

lpetrich · 06-29-2002, 01:43 PM

So the big question is whether the frog's specialized tadpole stage is ancestral or not.

One interesting feature of tadpoles is their outward leglessness; as the tadpole becomes an adult frog, it grows legs. This is something unlike the state of fish and salamanders, which have their side limbs for most of their lives.

Thus, the tadpole phase includes slowed-down leg growth, with the tadpole-to-frog change featuring speeded-up leg growth.

Blinn · 06-29-2002, 02:19 PM

<a href="http://www.npb.ucdavis.edu/winter2002/128/recommended_readings_set2/Elinson_direct_dev_review.pdf" target="_blank">Frogs without polliwogs: Evolution of anuran direct development</a>

I found this article a while back but haven't gotten around to reading it yet.

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06-29-2002, 12:08 PM	#2
Blinn Veteran Member Join Date: Aug 2001 Location: Ohio, USA Posts: 1,162	bump I posted this a while back for lpetrich and I think he missed it, so here it is again, for what it's worth.

06-29-2002, 12:47 PM	#3
liquid Veteran Member Join Date: Jan 2001 Location: UK Posts: 1,440	If you are interested in ading further examples of this sort of change, look up axolotl/salamanders.

06-29-2002, 01:43 PM	#4
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	So the big question is whether the frog's specialized tadpole stage is ancestral or not. One interesting feature of tadpoles is their outward leglessness; as the tadpole becomes an adult frog, it grows legs. This is something unlike the state of fish and salamanders, which have their side limbs for most of their lives. Thus, the tadpole phase includes slowed-down leg growth, with the tadpole-to-frog change featuring speeded-up leg growth.

06-29-2002, 02:19 PM	#5
Blinn Veteran Member Join Date: Aug 2001 Location: Ohio, USA Posts: 1,162	<a href="http://www.npb.ucdavis.edu/winter2002/128/recommended_readings_set2/Elinson_direct_dev_review.pdf" target="_blank">Frogs without polliwogs: Evolution of anuran direct development</a> I found this article a while back but haven't gotten around to reading it yet.

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