A lot of these questions builds on discussions i'm having with people and the internet! e.g. 'understanding evolution', berkeley, and very general review articles.

Evolution can be described as 'descent with modification'; a change that builds up in a population over time. The changes involved are random mutations to the DNA that is not directed. I also imagine that that change is 'established' in the population before the next change occurs, or is the process continually/simultaneously happening e.g. branching of the phylogeny (i simply dont know about the extent variation in DNA within a species... in terms of appearance all animal species seem very specific i.e. a blue footed booby is a blue footed booby!)?

What is the smallest 'unit' of modification that needs to occur for this to be 'inherited' by the whole population? In discussion with some people they say that the smallest possible change can occur for gradual 'macroevolution'. Are small changes (that have no effect on phenotype) able to be part of this process? It makes more sense to me that they are never propagated because they wont be particularly favrouable i.e. or even a mutation that may lead to a favourable mutation after a combination of mutations would not be enough to lead to macroevolution (or speciation (i understand that speciation requires many another factors of course e.g. reproductive isolation, etc. but i think the point stands)), is this correct?! How do i understand this?

Furthermore, all the examples that i've been reading about small incremental mutations leading to better chances of survival (etc.) are, frustratingly, examples of major changes in phenotype/appearance or examples of already-existing variation (e.g. colours of beetles and moths, bacterial resistance to antibiotics). Is there evidence/examples of not-specifically-advantageous mutations that cause macroevolution. If yes, how is this explained to happen if every time a small mutation may require reproductive isolution (for example), and what is the evidence? If not, then every small mutation MUST have some positive advantage for that animal to survive - are there examples/evidence of this, and how would this practically happen?

(as a post-preface... i don't have a background in biology but am reading a lot about it. I have a strong background in physics/chemistry. My post-grad research is heavily lab based. I'm looking for something resembling control experiments (despite the obvious difficulties). Too many people talk to me in terms of 'plausibility arguments', it currently frustrates me! There was an interesting paper by Bromham "Testing hypotheses in macroevolution" that i read, but it doesn't go into detail about what i'm looking for: http://www.sciencedirect.com/science/ar … 8115001065 )

Hi Bruno,
Quite alot of questions - I'll try and tackle at least some!

>evolution can be described as 'descent with modification'; a change that builds up in a population over time. The changes involved are random mutations to the DNA that is not directed. I also imagine that that change is 'established' in the population before the next change occurs,

Not so. Populations harbour a lot of genetic variance arising ultimately from mutations. Mutations are arising all the time and ultimately proceed to fixation (the mutant allele becomes the only variant in the population) or loss (it disappears from the population). Selection plays a big role here but so does random chance. At any one time there will be lots of mutations at lots of genes at all sorts of different frequencies in a population. So it's definitely not a sequential process at all!

>Are small changes (that have no effect on phenotype) able to be part of this process?

Absolutely, much genetic evolution has no detectable effect on phenotype and may be considered neutral. However, also bear in mind that a mutation may impact some aspect of phenotype that we haven't measured (e.g. change the rate of activity of some enzyme in an obscure metabolic pathway). Also bear in mind that small effects on measured phenotypes may not always be detectable in practice.

Macroevolution is what you "see" when you look for differences at high level taxa (e.g. between species). So almost by definition you do not see "macroevolutionary" change in a population in the way you imply. Rather - lots of small changes accumulate over time so that when you compare lineages that diverged quite a while ago they are now so different as to be viewed as different species/genera/families etc. In fact, the terms micro- and macroevolution are not really used by biologists (but are focussed on a lot by ID/Creationists) but to the extent they are useful (debatable) then the key point (for me) is that microevolution is a process (testable, predictable, observable, measurable over human-relevant time frames) and "macroevolution" is the outcome of that process running for longer times frames (e.g. thousands of generations or more) which are not typically observable for humans (unless you have v short lived organisms to observe).

So don't look for a mutation causing macroevolution. With a few arguable exceptions (which I won't get into here) macroevolution is just the accumulation of microevolutionary change.

>then every small mutation MUST have some positive advantage for that animal to survive -

This does not follow logically to me. Many mutations will have no effect on survival but in general (exceptions being in very small populations) deleterious mutations are unlikely to be fixed in a population due to being selected against. Note that selection acts differently according to local conditions, so a mutation that is advantageous in one population (and ultimately fixed as a result) of a given organisms could be disadvantageous in another (where it is selected out). So whther a mutation is beneficial, deleterious, or neutral may depend on local context a lot.

Last edited by Alistair Wilson (17th May 2016 18:37:15)