Evolution through mutations

[h6ss]

Hey there, before I start I just want to point out that I'm a statistician and doesn't know much about biology. I've worked with artificial neural networks, which required a minor understanding of some biological concepts, but apart from that, my level of knowledge in biology is very limited.

I was thinking about the theory of evolution and landed on a confusing thought. I will write down a series of assumptions I made which lead to what seems to be a paradox, or just a counter-intuitive conclusion. Now I know it isn't totally true, therefore it implies that some of the assumptions I made are false, and I would just like to know which.

Here are my assumptions :

1. Although diversity can arise through other mechanisms than mutation, mutation remains the most important one.

2. The more evolved (and therefore complex) a species is, the more mutations it requires to evolve again.

3. There are three important types of mutations: beneficial, neutral, or harmful. I have read that most mutations that occur are either neutral or harmful, making the probability of having a good mutation at most 1 in 10,000.

4. Since the large majority of mutations are harmful, evolution help species evolve mechanisms to protect themselves from them. A human cell is less likely to mutate than a less-evolved species' cell.

Which leads to my conclusion :

Evolution requires mutation, but evolution evolves mechanisms to help species protect themselves from mutations. Since the probability of having a benefical mutation is reasonably low, and that the more evolved the species are the more mutations they need to evolve, and that the more evolved they become the less likely their cells are to mutate, then is evolution slowing down?

Thanks!

 

[DaveC426913]

2. The more evolved (and therefore complex) a species is, the more mutations it requires to evolve again.

a] All organisms evolve continually, so there is no "again". It is incremental, small mutations to a simple organism might appear more obvious than small mutations for a more complex organism.
b] This is a circular argument, since it is also the conclusion you draw at the end.

3. There are three important types of mutations: beneficial, neutral, or harmful. I have read that most mutations that occur are either neutral or harmful, making the probability of having a good mutation at most 1 in 10,000.

Those three labels are constructs. Mutations can be both harmful and beneficial simultaneously, and only time can tell if a mutation if the benefit outweighs the harm.
Consider sickle cell anemia, which is a harmful mutation - except that it provides protection from malaria.

 

[Pythagorean]

On the other hand, Dave, can we describe different rates of evolution by morphological or biochemical divergence? I.e., turn the binary variable into a continuous one on which some significant thresholds exist?

For instance, morphologically, humans have diverged significantly from the common ancestor between fish and humans, but fish have not.

 

[DaveC426913]

For instance, morphologically, humans have diverged significantly from the common ancestor between fish and humans, but fish have not.

You sure about that? Or is it humano-centric? I wonder of a fish would see any similarities between itself and its ancestor. Remember, they have not changed to an enterely different eco-system, so their changes might not be so obvious.

A while back on this very board, I suggested that the Coelacanth as an organism that had remained unchanged in 200 million years, was an example of a living fossil, and it was pointed out to me that the modern Coelacanth had undergone 200 million years of evolution as well.

It's not about how many mutations, it's about whether the environment weakly or strongly drives evolution toward changes. Put the Coelacanth in a shallow tidal pool for a few thousand generations, and you can bet it'll evolve as a fast as Darwin's Finches.

 

[Pythagorean]

You sure about that? Or is it humano-centric? I wonder of a fish would see any similarities between itself and its ancestor. Remember, they have not changed to an enterely different eco-system, so their changes might not be so obvious.

A while back on this very board, I suggested that the Coelacanth as an organism that had remained unchanged in 200 million years, was an example of a living fossil, and it was pointed out to me that the modern Coelacanth had undergone 200 million years of evolution as well.

Specifically, I mean morphological differences only. I don't mean to imply fish are the same as the common ancestor, but only that their morphology is similar. You could measure the same thing with say, nervous system function, or the immune system and ask the same question (I don't know what the answers would be, but morphology appears self-evident). I'm not sure how you'd quantify divergence though.

 

[DaveC426913]

Specifically, I mean morphological differences only. I don't mean to imply fish are the same as the common ancestor, but only that their morphology is similar. You could measure the same thing with say, nervous system function, or the immune system and ask the same question (I don't know what the answers would be, but morphology appears self-evident). I'm not sure how you'd quantify divergence though.

Yes, fish have as many mutations, but since they're already well- evolved for their environment, those mutations don't get bred in.In this sense, the mutations are all "harmful", since they would all diverge from an organism that is otherwise ideally-evolved for its environment. Fish that are like their ancestors would be favoured, since it's been working for 200 million years.

 

[Ygggdrasil]

2. The more evolved (and therefore complex) a species is, the more mutations it requires to evolve again.

Why do you think this?

4. Since the large majority of mutations are harmful, evolution help species evolve mechanisms to protect themselves from them. A human cell is less likely to mutate than a less-evolved species' cell.

I'm not sure if this is true. For example, http://www.nature.com/nrg/journal/v8/n8/full/nrg2158.html suggests humans have a higher mutation rate than organisms like insects and worms. Many "simpler" species evolve more quickly than "complex" species just because they reproduce more quickly, not because they necessarily have higher mutation rates.

Yes, evolution can devise ways to protect against mutation, but the mutation rate of an organism is also something that can be optimized by evolution. In the long run, species with very low mutation rates are will be more susceptible to extinction because they cannot generate enough diversity within their populations to deal with changes to the envrionment, so there are evolutionary forces pushing the mutation rate up as well as the evolutionary forces you mentioned that push the mutaiton rate down.

 

[h6ss]

Thank you all for your answers.

a] All organisms evolve continually, so there is no "again". It is incremental, small mutations to a simple organism might appear more obvious than small mutations for a more complex organism.

I felt like the term again would be misleading. The way you put it makes a lot more sense.

Why do you think this?.

I believe evolution of more complex organisms recquire more assets and therefore more mutations, wouldn't it? I'm probably oversimplifying, but aren't there more traits distinguishing us from chimps than traits distinguishing chimps from gorillas, for example?

Yes, evolution can devise ways to protect against mutation, but the mutation rate of an organism is also something that can be optimized by evolution. In the long run, species with very low mutation rates are will be more susceptible to extinction because they cannot generate enough diversity within their populations to deal with changes to the envrionment, so there are evolutionary forces pushing the mutation rate up as well as the evolutionary forces you mentioned that push the mutaiton rate down.

Is it for that reason (obviously not in particular) organisms using sexual reproduction, which provides a bigger and better brewing of genomes, "won" over organisms using asexual reproduction?

 

[Guco]

I believe evolution of more complex organisms recquire more assets and therefore more mutations, wouldn't it? I'm probably oversimplifying, but aren't there more traits distinguishing us from chimps than traits distinguishing chimps from gorillas, for example?

As humans we tend to be very biased towards what we find important as humans. 'Modern' traits like culture and language are very important to us and these traits are related to biological changes in brain structure and size. However, that does not necessarily imply that the amount of genomic changes in humans is significantly greater than in other related animals. From a purely genomic perspective, we are in fact extremely similar to chimps. The differences that do exist are just very obvious and noticeable to us.

Most traits have many different genes (and thus mutations) underlying them, but that does not meant that single traits appear by many mutations happening all at once. Most of those mutations probably had some advantage by themselves and they all 'stacked up' over time. At the same time, single mutations can sometimes have a large difference. For example, human height is influenced by hundreds or even thousands of genes, but at the same time a single genomic mutation might result in a giant or small person (as is the case in many different growth disorders).

Is it for that reason (obviously not in particular) organisms using sexual reproduction, which provides a bigger and better brewing of genomes, "won" over organisms using asexual reproduction?

The vast majority of life on Earth is asexual. And many asexual lifeforms do exchange genomic information. Sexual reproduction just works well for animals and some plants, who are just a portion of total life on earth. One reason it works well is that generates a high amount of individual genomic variety, which reduces the effectiveness of parasites and diseases.

To understand evolution you have to try to interpret nature not through human values or scales.

 

[Ygggdrasil]

Is it for that reason (obviously not in particular) organisms using sexual reproduction, which provides a bigger and better brewing of genomes, "won" over organisms using asexual reproduction?

I'm not quite sure it's correct to say that we "won" over simpler unicellular organisms. There are many more bacteria than animals on Earth and the bacteria are capable of surviving in much more extreme environments. Indeed, the human body carries ten times as many bacterial cells as human cells. Perhaps we are nothing more than glorified incubators for our bacterial masters ;)

Sexual reproduction probably helps weed out deleterious mutations from beneficial mutations, especially in organisms with large genomes that have relatively small population sizes and large generation times. It is interesting to note however, that in certain species such as the bdelloid rotifer, a species that once reproduced sexually has evolved to reproduce purely asexually, suggesting that in some cases asexual reproduction can have benefits over sexual reproduction.