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I was reading the Mutation theory of De Vries; there I encountered this following statements

Mutations are discontinuous, random & non-directional.This is in contrast to Darwinism where variations are continuous & unidirectional.

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[Hugo deVries] believed that it mutation that causes evolution and not the minor variations (heritable) that Darwin talked about. Mutations are random and directionless while Darwinian variations are small and directional. Evolution for Darwin was gradual while deVries believed mutation caused speciation and hence called it saltation (single step large mutation).

Could anyone please explain what direction means in the two statements above?

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  • $\begingroup$ Can you please indicate your sources? $\endgroup$
    – Remi.b
    Commented Aug 24, 2017 at 14:56

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(I give an explanation to the statements, please understand that almost all evolutionary changes are mutational in the basis).

Let's say the greater height is advantageous in one area, while in other area the smaller height is advantageous. In the former area, the direction of phenotypic changes will be to the individual with higher height, while in the latter the direction will be to smallest one. The evolution "seeks" example to follow, so you "can" find which one was an example to move towards to (who is the reference point).

In the case of mutations the is no apparent direction. These chaotic changes can be advantageous, or may be not - one cannot find a particular direction towards some "example" / reference model.

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  • $\begingroup$ Hi, sir. Does that mean mutation is not influenced by Natural Selection? $\endgroup$
    – user10379
    Commented Nov 22, 2015 at 16:50
  • $\begingroup$ Natural Selection is not a physical object, it is artificial term which describes the general biological changes in species. The process of "natural" selection is naturally-"passive" in the terms of causality. Mutations in turn are physical changes that take place in genes, they can be beneficial, detrimental or neutral, they are influenced by multiple factors and the result of all of these influences over time can give us "Natural Selection" picture. $\endgroup$
    – Ilan
    Commented Nov 22, 2015 at 17:32
  • $\begingroup$ Thanks; +1 ! But could you tell what continuous means in the quoted line above? $\endgroup$
    – user10379
    Commented Nov 22, 2015 at 17:34
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    $\begingroup$ If you will work hard in ten years you will earn 1 billion dollars - it is continuous process, you can explain it in understandable terms. If you win 1 billion in a lottery today - it is discontinuous, abrupt change, which actually have no any other explanation other than chance. Good luck %) $\endgroup$
    – Ilan
    Commented Nov 22, 2015 at 17:42
  • $\begingroup$ "In the case of mutations there is no apparent direction." The following paper published on August 25, 2015 claims that there are indeed directed mutations. Evidence for Retromutagenesis as a Mechanism for Adaptive Mutation in Escherichia coli journals.plos.org/plosgenetics/article?id=10.1371/… $\endgroup$
    – Makoto Kato
    Commented Dec 7, 2015 at 5:47
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It simply means that mutations happen and do not, in and of themselves drive evolution.

Mutations do not necessarily need to be advantageous or deleterious, they just happen and exist within the population at some low background. However when you combine mutation with environment and Natural Selection, then certain characteristics are favored while others are not. If there is a change in the environment, then what was once advantageous could become neutral or disadvantageous, while mutants that were always in the background end up with a competitive advantage. Also note that mutations need to occur in the germ-line in order to have any effect on the population. Mutations in somatic tissue cannot be passed from one generation to the next.

Evolution by Natural Selection (Darwinism) in contrast moves forward in one direction. Once change is selected for and speciation takes place, there is no turning back. The new species can not revert back to the old one. It moves in one direction, based on the conditions in the environment and individuals in the population capable of adapting to change should it arise. If the mutation has given those individuals a reproductive advantage within that environment, then their progeny will inherit the trait while the progeny of other may be less fit and reproduce less or not at all, and Evolution moves forward.

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  • $\begingroup$ "It simply means that mutations happen and do not, in and of themselves drive evolution." The following paper published on August 25, 2015 claims that there are indeed directed mutations. Evidence for Retromutagenesis as a Mechanism for Adaptive Mutation in Escherichia coli journals.plos.org/plosgenetics/article?id=10.1371/… $\endgroup$
    – Makoto Kato
    Commented Dec 7, 2015 at 5:46
  • $\begingroup$ One poorly written paper with no significant data does not overturn a theory. You can always design an experiment to produce the results that you want. And nothing evolved. They didn't have a new species, just bacteria that were able to survive selection. They did not test to see if the bacteria that "reverted" were diploids a functional gene on a F Plasmid. Also the bacteria have a AmpR marker, but they do not once mention that they used antibiotics in their medium. With the small number of "revertants," those could have just as easily have been contaminants. $\endgroup$
    – AMR
    Commented Dec 7, 2015 at 7:48
  • $\begingroup$ And nobody said that you cannot have revertants. There are a good number of examples of gene conversion that occurs from yeast through to mammals. Look up Yeast Mating Type Switching. $\endgroup$
    – AMR
    Commented Dec 7, 2015 at 7:48
  • $\begingroup$ "One poorly written paper with no significant data does not overturn a theory." Would you explain in detail why you think the paper was poorly written? Also would you elaborate on why you think it has no significant data? $\endgroup$
    – Makoto Kato
    Commented Dec 7, 2015 at 15:07
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    $\begingroup$ If you do not know how to read a research paper critically and determine its significance, then maybe you should stop vandalizing the site with posts that add nothing to the discourse. $\endgroup$
    – AMR
    Commented Dec 7, 2015 at 20:33
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Directional vs directionless

What is the meaning of directionless in this context

First note that the term directionless is not used (or not often at least) in the scientific literature.

I will explain the term "directionless" by talking about mutations first. Consider a given phenotype, say height. What directionless means in this context is that new mutations aren't more likely to increase than to decrease the phenotype.

Are mutations really directionless

It makes things easier to explain to someone the very basics of evolutionary processes to tell that mutations are directionless. It is however not necessarily true. When mutations are not directionless, we talk about mutation bias (see for example Eyre-Walker 1998, Harr and Schloterrer 2000 or Shah et al. 2010 ).

An obvious trait on which mutations tend to be bias is fitness. A mutation is much more likely to be deleterious than beneficial.

Is selection directional

Yes, if you talk about fitness. Selection select variants yielding that are associated to higher fitness. Note however, that if you look at, say, height, some environments may select for taller individuals, while other environments may select for shorter environments. As such selection is no "directional" on any phenotype.

Mutations and selection

Something is potentially misleading in your quote

It is true that Darwin did not quite grasp the saltatory nature of mutations, however the text puts in opposition the concept of natural selection and the one of mutation which is can be very misleading to a layman. I would need the entire text to really give you a better opinion on. Please always indicate your source!

Mutation and natural selection

In short, mutations (whether biased or not) is the ultimate source of variation on which selection can act. Selection increase the frequency of the variants (called alleles) that are associate to higher fitness. Through selection a few alleles end up to very high frequency up to the point that all individuals carry the same allele. This cause an increase in the mean fitness of the population but also removes variation (as deleterious alleles are washed out). Mutations introduce new variation that selection can select on.

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