Re: Why are there so few vertebrates?

Dear Rita:

Thank you for writing in your question! I hope this gets back to you in time for it to be useful.

That’s a good question! There is a lot more invertebrates then vertebrates, and more invertebrates are being discovered every day. But, like everything in ecology and evolution, there is more then one reason, and probably some reasons we don’t even know about yet. Most likely, once you get thinking about the right types of questions, you will be able to come up with more then I could list here! I will focus on two characteristics of vertebrates and invertebrates, their size and breeding methods. Size, because it changes the scale they live and compete on, and breeding methods because that is how evolution happens, across generations.

The first reason, however, is very broad and pretty simple: Invertebrates evolved first, and thus have had a lot more time to diversify and spread out then the late blooming vertebrates. Evolutionary time produces diversity and specializations, which can result in speciation. As time goes on, more and more species will evolve. Some will go extinct, of course, but more will evolve to take their place. I’m not sure who said it, but “Nature abhors a vacuum.” In ecology this means that when a species occupying one niche, or “spot” in the food chain, goes extinct, then another will evolve to take advantage of whatever the extinct species was living off of. Think if wolves went extinct- there would be a lot more deer running around in Alaska. A species would quickly (evolutionarily quickly, mind you) evolve to take their place. Of course, with the rate that man is making species go extinct, evolutionary time is not fast enough to replace the species we cause to go extinct. They will be eventually, but not for a long, long time.

A principle of speciation is size vs. time. As a generalization, larger animals are more specialized and evolved (not always true, but usually). These “higher evolved” animals take MUCH longer (10-1000x) longer to speciate then small animals, like insects and most invertebrates. This is simply due to the complexity of the animals. In larger animals, most things that change (evolution) are bad. In small insects, something could change that will have a drastic effect on the animal (which could be good or bad). But small insects usually have tons of offspring, so if something goes wrong it doesn’t matter. Plus, more babies mean more opportunities for variability, and thus evolution through natural selection. Larger animals, usually vertebrates, will have fewer young and therefore have evolved for little variability, because they spend so much time rearing and gestating their kids.

Another factor is the size of the animal. Invertebrates are usually small. There are many more niches available on the micro-climate scale. A single lion controls a few square miles of savanna, killing all other creatures above the size of a rat. Thus the population of animals, and the diversity, is kept down. But underneath the lion’s paws are burrows of ants. Two feet away might be a colony of centipedes and termites. Further along might be a bunch of flies buzzing around. There are a lot more resources for insects and invertebrates to exploit, because they need less and operate on a smaller scale.

Obviously there is a discrepancy in species numbers- much more invertebrates then vertebrates. To formulate the broadest evolutionary answer, look at the way species reproduce. Natural selection works on offspring. If you only have a few kids, like most larger animals, then you do not want them to be highly variable. Most change in offspring is bad, big or small. Usually mutations kill the kid. So, larger animals that require bones to support them (vertebrates) have evolved to minimize variation in offspring. Smaller animals, like insects (invertebrates) don’t really worry about that. They have 200,000 eggs at a time. If a few 100 are highly mutated, so be it. This gives a much higher chance of having a beneficial mutation, and so there is a higher chance of one of those mutations to be beneficial, and allow that kid to go off and exploit a new niche, or out compete something else, or whatever- to allow that kid to succeed through natural selection.

The other is size. There is “more available earth” for animals operating on a smaller scale. If you are competing with another insect, you could leave if you are losing, and you would only have to move about 6 inches to find a whole new region to live in. A coyote has to go a long ways to get out of a single wolves territory. There are exceptions- some vertebrates have tons of babies, and some are very small, and visa versa. But this answer is general. Hope this helps.

Look at tol.org (tree of life) for a general outline of the species Also look towards the effect of large vs small brood sizes (numbers of offspring per birth) on natural selection.

You should be able to build a pretty strong case around evolutionary theory.