SenseiOtho, you mention how species tend to appear in the fossil record and stay around barely changed for a long time before disappearing again. You also mention how in the Cambrian there was a rapid increase in diversity of body plans the likes of which we have not seen since. The answers to both of these queries are actually part of a larger fact about evolution.
There is a saying that I've heard thrown about in biological circles, and that is "evolution proceeds despite natural selection".
As you rightly pointed out there are a very large number of mechanisms to minimise the amount of mutations that creep into the gene pool. This is because environments tend to change only slowly over time and so when an organism is optimally suited to its environment its best interests are actually to stay the same rather than continue to change. It's no longer adaption if the animal is already adapted. Any mutations are only likely to be detrimental.
When the organism's environment does change, however, it must adapt as well. These mechanisms to control mutations still operate of course, but they can never filter out everything - if they did the species would ultimately die out. This is where natural selection kicks in again. In a stable environment it tends to favour organisms that stay the same if they are already optimally adapted. When the environment changes natural selection favours those organisms who are slightly better at survival. It's the most basic and simple principle of evolution. And you have to keep in mind even a changing environment takes thousands if not millions of years to change, and while mutations - especially beneficial ones - tend to be very rare after the "weeding out" process, any species is represented by a large number of reproducting individuals over many many generations. The sheer numbers will ensue that beneficial mutations will find their way into the gene pool eventually.Evolution takes a long time. And this whole process of stops and starts is something called "punctuated equilibrium". A lot of creationists seem to think it's a hasty quick-fix invented by evolutionists to save their theory, but nothing could be farther from the truth. Evolution would actually be harder to swallow without it. It would be like insisting that Moses and his followers when crossing the Sinai desert packed up all their tents every morning, carried them a few metres then set them up again, thereby crossing the desert in 40 years. (I didn't make that up. Thank Richard Dawkins!)
Now, the mechanisms for gene regulation described are probably also a large part of the reason for the cambrian explosion - or rather the lack of them.
As Kyu has already pointed out, complex life did not suddenly pop into existence at the beginning of the Cambrian. There was the Ediacaran before that, and through the fossil record and some educated guesswork we can make a perfectly reasonable line of descendants from single celled organisms to all the variety of multicellular body plans that developed in the Cambrian. There are no completely unexplainable gaps (though there are some gaps without enough solid evidence to tell us with explanation is the right one).
But that's not what I want to talk about. The reason we don't find the kind of massive diversification any time after the Cambrian is probably at least partly due to the lack of these gene control mechanisms found in todays organisms. (I say probably because we obviously don't have a copy of any genomes from organisms back then. It's an educated deduction, but that doesn't mean I won't accept a new theory if it can explain things better)
Gene regulation and mutation supression are, like every aspect of biology, evolved things. Multicellular life was still relatively new in the Cambrian and these functions simply hadn't arisen yet - and not just because they didn't have time to. Also because they weren't necessary back then. With multicellular life only right out of the package, the Earth was overflowing with untapped ecological niches and environments. It would not have been in an organism's best interest to stay the same for too long if it had the opportunity to exploit some resource not yet exploited by other organisms.
The other important factor would have been the genome of organisms back then. Nowadays genes are immeasurably complex things - any part of our body will have a number of different genes controlling it, and every gene controls a number of different parts of our body. It can be safe to assume that things would'nt have been quite as complex in the Cambrian, and as a result a single gene mutation could have had a much greater effect on an organism's basic morphology.
One might also be able to assume that organisms, being much simpler, would have had a somewhat lower chance of muations being detrimental. Without the complex interrelationship between the different organs and structures in a body an organism can afford to have farily drastic mutations. And this is true even if it makes the organism less effective overall, provided they can explot an untapped niche in the process. They can work on improving their effectiveness later.
Okay, I think I've covered everything I wanted to cover...
There is a saying that I've heard thrown about in biological circles, and that is "evolution proceeds despite natural selection".
As you rightly pointed out there are a very large number of mechanisms to minimise the amount of mutations that creep into the gene pool. This is because environments tend to change only slowly over time and so when an organism is optimally suited to its environment its best interests are actually to stay the same rather than continue to change. It's no longer adaption if the animal is already adapted. Any mutations are only likely to be detrimental.
When the organism's environment does change, however, it must adapt as well. These mechanisms to control mutations still operate of course, but they can never filter out everything - if they did the species would ultimately die out. This is where natural selection kicks in again. In a stable environment it tends to favour organisms that stay the same if they are already optimally adapted. When the environment changes natural selection favours those organisms who are slightly better at survival. It's the most basic and simple principle of evolution. And you have to keep in mind even a changing environment takes thousands if not millions of years to change, and while mutations - especially beneficial ones - tend to be very rare after the "weeding out" process, any species is represented by a large number of reproducting individuals over many many generations. The sheer numbers will ensue that beneficial mutations will find their way into the gene pool eventually.Evolution takes a long time. And this whole process of stops and starts is something called "punctuated equilibrium". A lot of creationists seem to think it's a hasty quick-fix invented by evolutionists to save their theory, but nothing could be farther from the truth. Evolution would actually be harder to swallow without it. It would be like insisting that Moses and his followers when crossing the Sinai desert packed up all their tents every morning, carried them a few metres then set them up again, thereby crossing the desert in 40 years. (I didn't make that up. Thank Richard Dawkins!)
Now, the mechanisms for gene regulation described are probably also a large part of the reason for the cambrian explosion - or rather the lack of them.
As Kyu has already pointed out, complex life did not suddenly pop into existence at the beginning of the Cambrian. There was the Ediacaran before that, and through the fossil record and some educated guesswork we can make a perfectly reasonable line of descendants from single celled organisms to all the variety of multicellular body plans that developed in the Cambrian. There are no completely unexplainable gaps (though there are some gaps without enough solid evidence to tell us with explanation is the right one).
But that's not what I want to talk about. The reason we don't find the kind of massive diversification any time after the Cambrian is probably at least partly due to the lack of these gene control mechanisms found in todays organisms. (I say probably because we obviously don't have a copy of any genomes from organisms back then. It's an educated deduction, but that doesn't mean I won't accept a new theory if it can explain things better)
Gene regulation and mutation supression are, like every aspect of biology, evolved things. Multicellular life was still relatively new in the Cambrian and these functions simply hadn't arisen yet - and not just because they didn't have time to. Also because they weren't necessary back then. With multicellular life only right out of the package, the Earth was overflowing with untapped ecological niches and environments. It would not have been in an organism's best interest to stay the same for too long if it had the opportunity to exploit some resource not yet exploited by other organisms.
The other important factor would have been the genome of organisms back then. Nowadays genes are immeasurably complex things - any part of our body will have a number of different genes controlling it, and every gene controls a number of different parts of our body. It can be safe to assume that things would'nt have been quite as complex in the Cambrian, and as a result a single gene mutation could have had a much greater effect on an organism's basic morphology.
One might also be able to assume that organisms, being much simpler, would have had a somewhat lower chance of muations being detrimental. Without the complex interrelationship between the different organs and structures in a body an organism can afford to have farily drastic mutations. And this is true even if it makes the organism less effective overall, provided they can explot an untapped niche in the process. They can work on improving their effectiveness later.
Okay, I think I've covered everything I wanted to cover...
Too weird to live, and too rare to die.
