Evolution to complex forms: why?

One thing about evolution I’ve never quite grasped - why did complexity (multicellular organisms, etc.) evolve. The basic goal of life (in a non-anthromorphic sense) is to pass on genes. But, by and large, the more complex an organism is, the fewer offspring it has, which is contrary to this idea. There are a hell of a lot more bacteria out there (and therefore more packets of bacteria genes surrounded by a bacteria) than there are of mammals, for example.
I’m obviously missing something. What is it?

Thanks,

Sua

The first life-forms were about as simple as possible. The vast majority of life on the planet has remained that simple. Only a minority of species have become more complex than the bacteria. Why? Because at some point in the past they found a competitive advantage in doing so. Evolution has no long-term goal other than survival and reproduction.

This is perhaps best explained through r and K-selection.

As to the idea that the more complex an organism is the fewer offspring it has, well, this is just off the top of my head, but maybe more complex organisms are more durable and likely to survive themselves, so there’s less need for a huge number of offspring to maintain the species. Also, the larger an animal the more food it needs, and the fewer competitors for the available food supply, the more likely it will survive long enough to reproduce.

I wouldn’t even go that far. There’s no “goal” of the evolutionary process to create creatures that reproduce or survive. That’s like saying it’s a goal of mathematics for there to be numbers higher than a billion. It’s just that it works that way. Some single-celled organisms became multi-cellular and some of them happened to survive.

–Cliffy

Here in the U.S., the politicians are evolving into stupider and stupider forms. So it goes both ways.:rolleyes:

As stated previously, evolution has no goals. The goals of organisms are, generally speaking, to survive and to reproduce. In some cases, the goal is simply to survive long enough to reproduce.

In a very general sense, complex forms arise because they are being built upon previous forms. Viewed this way, complexity is simply an accumulation of minor tweaks made over several millenia.

As for the “why”, one could look at the evolution of complexity as an effort to keep all one’s eggs in one basket (so to speak). Instead of producing many offspring, each of which have slight variations in relatively few traits between themselves, and spewing them forth in the hopes that some of them will survive long enough to themselves reproduce, a complex form provides multiple traits which can vary, any one of which might confer some reproductive and/or survival advantage over its brethren. So, rather than variation being seen in multiple offpsring, variation can be found in the slight tweaking of multiple “parts”. Both methods (many, relatively simple offspring; or few, relatively complex offspring) are viable strategies, as explained by Colibri’s r / K selection link.

It is a bootstrapping system.

As AndrewL pointed out, among the very first life forms some level of complexity provided an advantage over the simpler neighbors. Based on that complexity, some of the second order forms gained a small advantage over their neigbors with a similar incremental increase in complexity. At some point the forms reached a level where any adaptation in the arms races (who will eat whom?) required some increase in complexity.

This is probably why we see continuing “arms races” leading to a top predator pyramid, often leveled by a catstrophe that destroys the most complex forms. However, since we have never had a catastrophe that actually sent us back to eukaryotes, the “top” survivors of each crash are already geared toward building upon complexity, again.

Exploitation of habitat. If there are available resources, something will come along to use them. Single-celled organisms aren’t geared towards using every available niche, so if a multicellular organism arises that can use untapped resources (inluding eating the unicellular ones), it can thrive with a minumum of competition.

Not every complex organism has few offspring. Houseflies, fish, frogs are some that have hundreds or thousands of eggs per brood–there are numerous strategies available.

Evolution is undirected and full of accidents. For example, what happens if a single-celled organism splits, but for some reason the process fizzles out in the middle, causing it to have twice as many chromosomes as before? Or even the two resultant cells doesn’t separate and stick together? Suppose that’s caused by a genetic mutation. Now this slightly bigger clump could have a survival advantage of being too big to be eaten by predators.

You should take into account not just the number of offspring, but the number of copies of DNA. Consider how many cells (trillions?) there are in the human body, all with more or less identical genomes. At some point in evolution, sibling cells found it more advantageous to cooperate, and thereby increase their chances for survival as a unit, rather than compete.

I don’t believe that complex organisms have any sort of inherent “advantage” – they have some advantages at some times.

Complex organisms are simply the result of increased variability. It’s really hard to get less complex than bacteria, so virtually all variations will be either toward the same amount or (in a tiny number of cases) slightly more complexity.

4 billion years later, bacterium still rule the earth. The rest of us are just hosts.