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On the number of limbs.

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On the number of limbs.

Postby elvex1 » Mon Feb 04, 2013 6:21 pm

I've been wondering, what does limit the number of limbs?.
For example, nearly all vertebrates are cuadrupedal or have vestigial limbs that add up to 4 limbs, why is it that there are no, let's say, hexapedal ( it would be possible for a, let's say, 4 legs bird? )
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Postby Luxorien » Mon Feb 04, 2013 10:46 pm

Interesting question. I don't have a complete answer, but my guess would be either 1) extra limbs are no great advantage and cost energy or, 2) contingency (that is, things just happened to fall out that way). Gould is famous for having said that if we could "rewind the tape of life" and play it again, we'd get much different results.

Once the basic body plans were established in the Cambrian period, it would have been very difficult for taxa to change their body plan once it was established. Screwing with Hox genes in a complex mammal would probably not result in a viable organism, but screwing with Hox genes in an insect? That can work. My guess would be this is why we see more variation in the number of limbs among arthropods.
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Re:

Postby elvex1 » Tue Feb 05, 2013 3:20 am

Luxorien wrote:Interesting question. I don't have a complete answer, but my guess would be either 1) extra limbs are no great advantage and cost energy or, 2) contingency (that is, things just happened to fall out that way). Gould is famous for having said that if we could "rewind the tape of life" and play it again, we'd get much different results.

Once the basic body plans were established in the Cambrian period, it would have been very difficult for taxa to change their body plan once it was established. Screwing with Hox genes in a complex mammal would probably not result in a viable organism, but screwing with Hox genes in an insect? That can work. My guess would be this is why we see more variation in the number of limbs among arthropods.


That's very interesting!!! :D Do you have more information about why it wouldn't produce a viable mammal?
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Postby JackBean » Tue Feb 05, 2013 4:34 pm

Because it's not so easy to gain or lose limbs. Even aquatic mammals which seem to have no limbs, have still rudimentary legs. In other words, it is easier to change something than to gain or lose it.
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Postby Darby » Tue Feb 05, 2013 7:46 pm

Four-finned ancestors, not too surprisingly, leads to four-limbed descendants.

Arthropods have been a bit more flexible, but the general trend seems to have been toward leg loss, as in insects. Plus wings that did not start as legs.
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Postby wildfunguy » Sun Feb 17, 2013 2:35 am

Shouldn't we expect things like insects and spiders to be more diverse since they reproduce rapidly and in such vast numbers?
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Postby JackBean » Sun Feb 17, 2013 9:34 am

And I thought that invertebra overpopulate vertebra
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Postby Darby » Mon Feb 18, 2013 11:38 pm

Insects and spiders are more diverse because there are more potential niches for them.
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