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Misconceptions of Evolution Part III February 12, 2009

Posted by Ian in Science.
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Part three of my ongoing series discussing common misconceptions of evolution.  It starts here, and see part I and part II.


 

4.  Mutations drive variation

Mutation is often given the driving seat in discussions about evolution, noting that new features only come about through mutation.  While this is certainly a part of the picture, it neglects the more important source of variation which is that animals of a particular species do not all have the same DNA to start with.  In fact there can be rather large phenotypic differences between individuals of exactly the same species.  This means for example that if a species moves to a colder climate it probably does not require mutations to develop longer coats in response – the potential for this is already in the existing gene pool even if its expression has been suppressed over time due to a warmer climate. 

In fact it is this very reason that we have sexual reproduction as opposed to simple cell division – because it facilitates lots of new combinations of genes and gives the evolutionary process so much more fuel to work with.  Also combining different genes can lead to innovative solutions to problems that are simply new combinations of existing genes but are able to serve some function over and above what the genes originally coded for in the parents. 

Mutations are one off events that have to survive quite a while before they make an impact on the overall species (the organism with the mutation has to survive and then pass on this mutation) whereas natural variation among species members can reoccur several times.


5.  Species Are Uniform

Related to item four is the notion that species are a discrete entity.  We tend to think of a species evolving as a whole entity and then suddenly splitting into two species as if it was a spontaneous event.  However reality is way more fuzzy than that because individuals in a species are still highly varied.  The typical view looks something like:

speciation_classic

Here we see a simple line delineating the original species and the two species that it splits into.  While this is a useful image in principle, reality is never this black and white.  The real story would look something more like this:

speciation_real

Here we see a wide variety of variation (each dot might represent an individual) and we see that as they split off there is a messy period where interbreeding still works sometimes and that the change suddenly seems a whole lot less drastic.  The change is also a lot more gradual with the change beginning earlier with a slight increase in overall variation leading to speciation after.

These two points are crucial because they highlight that random chance, while important, is not the only force at work here and that mutations are only part of the picture.

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1. Dace - February 21, 2009

“While this is certainly a part of the picture, it neglects the more important source of variation which is that animals of a particular species do not all have the same DNA to start with. In fact there can be rather large phenotypic differences between individuals of exactly the same species. This means for example that if a species moves to a colder climate it probably does not require mutations to develop longer coats in response – the potential for this is already in the existing gene pool even if its expression has been suppressed over time due to a warmer climate. ”

Do you mind saying a bit more about this Ian? It’s not quite clear. I gather that what you’re saying is that, even if we ignore future mutations, the natural variations that exist within a species will often include some genes that provide adaptive advantages for their bearers, and these will come to predominate, so that the species as a whole ‘adapts’ to the new environment.
Or, alternatively, you might be saying that genes can be switched on or off by environmental influences, and that we could therefore have adaptation in individuals without the selection of genes. I think you mean the former, although the latter seems to me plausible – I guess it depends on what you mean by genes being ‘suppressed’.

Also, I have some queries about speciation. If species are defined in terms of interbreedibility, what is that stops members of different species from interbreeding? Is it something at the genetic level, or is it a kind of physical incompatibility, or a lack of sexual attractiveness amongst widely varying individuals?
And if it is at the genetic level, are we talking about a reduced probability of conception according to a standard distribution (the more the difference, the lower probability of conception, or something), or would we be talking of some discrete limit on conception?

2. Ian - February 23, 2009

Good questions 🙂 My responses based on my understanding are below although I should note I’m no expert lol.

Do you mind saying a bit more about this Ian? It’s not quite clear. I gather that what you’re saying is that, even if we ignore future mutations, the natural variations that exist within a species will often include some genes that provide adaptive advantages for their bearers, and these will come to predominate, so that the species as a whole ‘adapts’ to the new environment.

Pretty much. If you consider how varied people are you can see there is a lot of fodder for differential survival if the circumstances were right.

Or, alternatively, you might be saying that genes can be switched on or off by environmental influences, and that we could therefore have adaptation in individuals without the selection of genes.

Not really – everything comes down to the genes since that is the only mechanism for passing on traits to descendants. Changes in individuals that can’t be passed on are meaningless to evolution.

I think you mean the former, although the latter seems to me plausible – I guess it depends on what you mean by genes being ’suppressed’.

What I mean by ‘suppressed’ is that there are dominant and recessive genes. In a population the dominant gene may become so prominent that the recessive one is rarely expressed in the phenotype but it is still there so if conditions changed, those rare exceptions would lead to a reduction on the distribution of the dominant gene and the recessive gene would start being expressed more frequently.

Also, I have some queries about speciation. If species are defined in terms of interbreedibility, what is that stops members of different species from interbreeding? Is it something at the genetic level, or is it a kind of physical incompatibility, or a lack of sexual attractiveness amongst widely varying individuals?

All three to varying degrees I think, although the second two lead back to genetics in the end. Genetically things like different genes or fused chromosomes (e.g. chimps versus humans) would reduce the odds of fertile offspring.

Physical incompatibility such as a chihuahua and a great dane (same species) would prevent interbreeding and ensure divergence which would probably lead to an inability to breed over time genetically (as both species would diverge along different paths quite quickly).

Sexual selection would lead to differentiation much quicker as well, eventually leading to genetic changes that prevented breeding.

And if it is at the genetic level, are we talking about a reduced probability of conception according to a standard distribution (the more the difference, the lower probability of conception, or something), or would we be talking of some discrete limit on conception?

Each change is minor so the former would have to be the case. A discrete change would be impossible to spread because the first individual with that change would have no-one to breed with. This is one important thing that pops out of number 5 above.

3. Dace - February 24, 2009

Aha! Very good :).

Thanks Ian.

4. Heraclides - March 3, 2009

[off-topic, then on a bit later…]

My posts to Thinking Matters are being held up under moderation (it’s a long story), but I agree about the point re “proofs” and wrote something in passing about this.

I should probably let you say more there, as reading your blog you seem comfortable with the background!

Now getting on topic…

Nice illustration of speciation, by the way. I have to admit at first glance I picked it to be a blurry micrograph! 🙂

everything comes down to the genes since that is the only mechanism for passing on traits to descendants. Changes in individuals that can’t be passed on are meaningless to evolution.

I’ll be nasty and complicate this by saying that, at least in a very limited number of cases, epigenetic changes can influence the next generation or two, giving a means to short-term responses to environmental changes. Longer term changes would require the gene be altered, as you say though (but I’m sure someone will debate me on that…)

Incompatibility for breeding can occur at genetic and molecular levels too. It’s the subject of a lot of reason of late, as Ian will no doubt know. Long story short, genome structure is one of the things being examined.

5. Dr. Johnson C. Philip - March 3, 2009

Ian, it is good to see your Blog. I hope to be reading your posts on a regular basis!

Johnson C. Philip
India

6. Ian - March 4, 2009

I’ll be nasty and complicate this by saying that, at least in a very limited number of cases, epigenetic changes can influence the next generation or two, giving a means to short-term responses to environmental changes. Longer term changes would require the gene be altered, as you say though (but I’m sure someone will debate me on that…)

I wouldn’t call it nasty lol. Anyway I still think ultimately the change needs to be genetic but genes that can be expressed multiple ways (such as epigenetic changes) would surely be selected ahead of less flexible genes so in that sense natural selection is a step ahead of us 🙂

Incompatibility for breeding can occur at genetic and molecular levels too. It’s the subject of a lot of reason of late, as Ian will no doubt know. Long story short, genome structure is one of the things being examined.

As far as I know there are two basic levels of breeding incompatibility – physical inability (including sexual selection) and genetic inability. Physical inability prevents the egg getting fertilised in the first place while genetic inability prevents a fertilised egg from producing viable offspring. Often it is both.

7. Heraclides - March 4, 2009

Ian:

Anyway I still think ultimately the change needs to be genetic Oh, I agree. I was trying to say as much, just adding the wrinkle that for a small number of generations epigenetics pulls different trick that affects survival, etc. 🙂

As far as I know there are two basic levels of breeding incompatibility Sure. When referring to genome structure, something slightly different is at play. It’s not the genes and what they encode, but the compatibility of the genome structures, which bring in the so-called non-coding elements, and the molecular make-up of the genomes e.g. different centromere positions, teleomere structures, ploidy issues, number of chromosomes and other genome “compatibility” issues. Still “genetic”, but not in the sense of “genes”. Anyway I should stop waffling and work…

re post 5, I can’t but help think about starting a wager to see how he takes to try quote-mine you! In discussions over the way, he has not addressed any of the science, other than to quote-mine things to try make them look like agreeing with him own, erm, “position”.


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