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Getting smaller and diversity

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Getting smaller and diversity

Postby moreno » Tue Feb 08, 2005 11:25 am

Pardon a layman's questions:

1) Are there examples of animals that became smaller over time? evolutionary time that is.

I assume there are, but I never hear about them. If by some chance there are none, why not? If there are few examples, why are there few?

Much as I can see the advantages of getting bigger, smaller creatures are doing rather well out there (as species of course).

2) Evolution is based on mutations being weeded out and or incorperated in the gene pool creating new forms - pardon my lay summation, or correct it necessary to answer me.

I seem to remember seeing a chart with kingdoms and phyla and species over the course of evolutionary history. There was a trend toward species diversity, more forms. But at one of the higher levels of abstraction, phyla kingdoms, something, there did not seem to be an increase. I think there may have even been a decrease.

Why? (is question #2)

Shouldn't diversity increase on all levels?

thanks
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Postby RobJim » Tue Feb 08, 2005 12:58 pm

1) Are there examples of animals that became smaller over time? evolutionary time that is.

Well, all animals began at the microscopic level on the evolutionary scale. So therefore they either got bigger or stayed the same size.

However, if you're asking if a descendent species was ever smaller than some ancestral species, then I think so. Archaeopteryx was the size of a crow, and there are birds smaller than a crow. A crow is a pretty big bird. I am sure there are other examples.

There are reasons why bigger is often better, though. More cells means you can have more cell specialization. Also, bigger means you can fight better, and it's easier to keep warm because the surface area to volume ratio decreases. You can reach up higher to get food and you are harder to kill. Carnivores are able to eat bigger herbivores, and the herbivores get bigger so they are tougher the carnivores. It's like an arms race. Even though smaller animals are numerous by number of resources, remember that one elephant is equivalent in mass (food mass converted to animal mass) to a lot of cockroaches, and much harder to use as food or to step on.

2) Evolution is based on mutations being weeded out and or incorperated in the gene pool creating new forms - pardon my lay summation, or correct it necessary to answer me.

I seem to remember seeing a chart with kingdoms and phyla and species over the course of evolutionary history. There was a trend toward species diversity, more forms. But at one of the higher levels of abstraction, phyla kingdoms, something, there did not seem to be an increase. I think there may have even been a decrease.

Why? (is question #2)


Well, the tree of species diversity and evolution is exactly that, a tree. The only way you can add to the tree is to add more branches on to the end of already existing branches. So, all the greater classifications are defined. The only way to increase diversity is by evolution of previous species.

To create a new phylum, an animal would have to come into being as different from all the current phyla as chordata is from arthropoda. That's a huge difference. Speciation in a small difference. A new phylum could come into being, but it would be a rare even because mutations that give dramatic advantages to a line would have to occur. If it did happen, it would take a long time for humanity to notice the vastly different evolutionary lines the two paths would take. An example might be that the whales might evolve into a phylum seperate from that of other chordates; but the line would have to diverge so far from the current chordates, with specific advantages over all other chordates in some environments, as the chordates diverge from the arthropods.

The hemichordata seem to be the sort of animal that separated chordates from echinoderms. When this (or similar ancestors) evolved from the echinoderms, the phylum chordata was in it's infancy, but if scientists have lived back then they'd probably call it a new species of chordata. It's only when you look back and realize how well that one line did, can you label it a phylum.

I guess when two lines of a species with a major difference diverge evolutionarily and are both immensely successful, and they diverge more and more over time until they are as different from their ancestor phylum as a chordate is from an echinoderm, then scientists can call it a new phylum. This takes time to notice and is just a reclassification based on arbritrary opinions of how differences between species should be organized.
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Postby moreno » Tue Feb 08, 2005 6:37 pm

thanks for the answers. a follow up

1) I agree with the advantages of getting bigger. At the same time many species, most actually, have low body masses. They are doing quite well. It would seem like mutations towards smallness (and more progeny) would be rewarded at least as much as those towards bigness. I wonder why it isn't more frequent: what environmental factors inhibit this trend or if there is some gene level factor prioritizing the acceptance or promotion of increasing mass mutations.
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Postby mith » Tue Feb 08, 2005 8:20 pm

Also note that the current data we have is far from complete. There is a bias toward larger animals such as elephants, I mean it's hard to miss seeing an elephant. But there might be a lot more tiny critters we don't know about out there lurking in the shadows ready to sink their teeth into your fleshy foot.
Living one day at a time;
Enjoying one moment at a time;
Accepting hardships as the pathway to peace;
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Postby RobJim » Wed Feb 09, 2005 10:46 pm

Moreno -

1) I agree with the advantages of getting bigger. At the same time many species, most actually, have low body masses. They are doing quite well. It would seem like mutations towards smallness (and more progeny) would be rewarded at least as much as those towards bigness. I wonder why it isn't more frequent: what environmental factors inhibit this trend or if there is some gene level factor prioritizing the acceptance or promotion of increasing mass mutations.

Your original question seems to ask why animals don't get smaller. Well, in order for that to happen, the animal would have to get big, and then get smaller. If the smaller size is better, why would it get have gotten bigger in the first place?

Getting bigger can help in the same environment or if the environment changes. Getting smaller would only happen if the environment changed.
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Re: Getting smaller and diversity

Postby thank.darwin » Thu Feb 10, 2005 1:36 am

moreno wrote:Pardon a layman's questions:

1) Are there examples of animals that became smaller over time? evolutionary time that is.

I assume there are, but I never hear about them. If by some chance there are none, why not? If there are few examples, why are there few?

Much as I can see the advantages of getting bigger, smaller creatures are doing rather well out there (as species of course).

2) Evolution is based on mutations being weeded out and or incorperated in the gene pool creating new forms - pardon my lay summation, or correct it necessary to answer me.

The answer is simple - In a population of animals, during a shortage of food the small members of the population survive because they require less food - so over short periods of time there can be changes in weight - the same thing can happen on a longer scale of time.

I seem to remember seeing a chart with kingdoms and phyla and species over the course of evolutionary history. There was a trend toward species diversity, more forms. But at one of the higher levels of abstraction, phyla kingdoms, something, there did not seem to be an increase. I think there may have even been a decrease.

Why? (is question #2)

Shouldn't diversity increase on all levels?

thanks
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