Vernon asked,
>In my reply I asked him to suggest how the alleged transition
fish-amphibian could possibly have taken place, for in my mind it defied all logic. In particular, in the early stages of the assumed process, I failed to see what possible selection advantage would accrue from the encumbering of normal fin activity with the growth of incipient legs and feet (together with the necessary internal adjustments). My experience over the years has been that evolutionists assure me that it must have happened. They are far more comfortable discussing later assumed developments in which leg and foot function obviously become significant factors in creature survival.
>I also requested evidence of the fact that such transitions have been found in the fossil record.
>To date, neither request has been met. Am I therefore correct in inferring that this essential stage in the evolutionary enterprise is to be taken on trust?<
No; actually the lack of reply results from my being busy moving, with limited computer access even when I have had free time.
It it true that a foot is less suited for rapid swimming than a fin, as evidenced by the development of paddles in many lineages of secondarily aquatic tetrapods, in addition to the standard fish condition. However, feet are better for crawling along the bottom and are good for a lurk and lunge mode of predation (i.e., sit still until prey comes within reach). Modern newts and other aquatic salamanders survive well in the water despite having legs and feet rather than fins. Some fish with a lurk and lunge lifestyle, e.g. batfish, have leglike pectoral fins. The earliest amphibians appear to have been in freshwater, which tends to have fewer large open expanses and more shallow, obstructed areas than the ocean, making fast swimming less advantageous.
Also remember that there were no advanced tetrapods to compete with. As long as the modified limbs did not prevent survival, they could persist, allowing for further modification by mutations and selective pressures.
I am not quite sure what you mean by "encumbering normal fin activity with the growth of incipient legs...". In particular, the evolutionary understanding is that the fins evolve into legs and feet. This does encumber the function of the fins as fins. However, I have seen drawings by a young earth advocate, purportedly representing what transitional forms ought to look like, which featured a fish with frog legs stuck on the sides. Not only are frog legs highly modified from those of primitive amphibians, but also there were no organisms with separate fins and legs at the same time. Rather, the transition is from paddle-like fins to more leggy fins to finny legs to legs adequate for terrestrial life.
As to the evidence of this taking place, the modern coelacanths and various extinct sarcopterygian fish have paddle-like fins with bones similar to the leg bones of amphibians. (Other sarcopterygian fish have reduced the limbs, including some modern lungfish.) A recently discovered specimen from Pennsylvania has both fin bones and foot bones in a single limb. I do not have the exact reference at hand, though I can find it if needed. There was an article in Science or Nature; information and photos were also on the web site of the Academy of Natural Sciences of Philadelphia last I checked, about a year ago. The feet of the earliest known amphibians have some unusual features, thought to reflect their relatively early place in the fin-foot transition, including extra toes (more than 5). Many other similarities between the skeletons of the earliest amphibians and the sarcopterygian fish also indirectly support a fin-foot transition, as do the molecular similarities betwee!
n the modern tetrapods and the sarcopterygians.
A different line of evidence comes from studies of molecular developmental biology. Similar genes are used in making fins in fish and legs in tetrapods. I do not know of any study on the living sarcopterygians for these genes.
I apologize for taking so long to reply; there are even older messages awaiting a response.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droigate Spa
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