On the number of limbs.

[elvex1]

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? )

[Luxorien]

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.

[elvex1]

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!!! Do you have more information about why it wouldn't produce a viable mammal?

[JackBean]

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.

[Darby]

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.

[wildfunguy]

Shouldn't we expect things like insects and spiders to be more diverse since they reproduce rapidly and in such vast numbers?

[JackBean]

And I thought that invertebra overpopulate vertebra

[Darby]

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