I just want to understand the concept of natural selection and its relation to evolution.

Evolution by natural selection occurs when we have hereditary trait(s) that causes an effect on reproduction rate of a population, either towards the positive, and thus deemed as beneficial or towards the negative and thus labeled as harmful.

Evolution by natural selection would result in keeping the beneficial trait(s), and wiping out the harmful ones.

The idea according to Darwin is that the slightest harmful effect a hereditary trait(s) would bring to a population then with time it would result in extinction of the population with that trait(s). [On the origin of Species, p.78, chapter: natural selection: "On the other hand we may feel sure that any variation in the least degree injurious would be rigidly destroyed"].

Therefore natural selection could be viewed to result ultimately in washing out of injurious traits, and only promoting beneficial traits. So natural selection can be the cause of extinction of some genotypes, and ultimately of some species.

In this sense natural selection can be considered as a creative natural tool, like a rubber wiping bad intermediate solutions, and only keeping the correct steps, thereby facilitating reaching into a solution.

Now suppose that we have a population of living subjects with varying heritable traits on an Island, and those had equal fitness over a very long period of time, so they were pretty much adaptive to their environment. Now suppose a volcano erupted or a big comet stroked that island that ended all life in it. Lets suppose that the genotypes of that population is not present outside of that Island. So these genotypes became extinct.

Here this event doesn't have the same functional genre that natural selection had, it actually didn't act in a differential manner promoting those genotypes with better fitness and wiping out those with lower fitness. No it actually was NOT fitness dependent at all, it simply squashed all life forms without regards to whatever feature was brought up by the different genotypes, all were simply extinguished.

Now can we label this event of indiscriminative extinction to be the work of natural selection also? Or we just label it as inadvertent environmental factor?

If the latter, then are those inadvertent environmental factors part of the mechanism for evolution?

Let me give another example:

The beetles stochastic example

Suppose we have a population of brown and green beetles, now suppose that we divide the place they live in into two territories A,B, now suppose that the movement of beetles across those territories is random, so at some moment of time all brown beetles happen to be in territory A, other times they have different distribution, and as said the movement pattern is totally random, i.e. there is no significant difference in the pattern of movement of both genotypes of beetles across territory A and B. Now suppose by the act of randomness a moment came where all brown beetles were located at Territory A, while some of the green beetles were in territory A and others in territory B, and suddenly came a big comet and hit territory A at that time that caused all the brown beetles to die, and also caused all green beetles on territory A also to die, but since territory B was not hit, the green beetles in B remained a life.

Now a moment of evolution did occur, since a change in the heritable characteristics of the beetle population did occur. But that change was merely accidental, there was no role attributable to fitness of the green beetles that resulted in their survival, it was a mere random event that caused all of the brown beetles to die. In some sense it's a kind of a stochastic environmental factor that caused that extinction. Fitness of the green beetles didn't have any role in that case of differential survival.

This can be called selection, it is environmental kind of selection, BUT it is NOT the outcome of differential fitness of the genotypes in that environment.

Now this moment in evolution is it described as an event of evolution by natural selection?

More broadly speaking, is it the case that any point of change in the heritable characteristics of a population (i.e. evolution) that is merely due to an environmental factor [like the above example], would qualify as a case of evolution by natural selection? or only those points where the resultant change is attributable to fitness differential that is genotype determined?

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    $\begingroup$ FYI, you misunderstand the quote: "On the other hand we may feel sure that any variation in the least degree injurious would be rigidly destroyed". This does not suggest that any deleterious variation would result in extinction of a species. It's a restatement of what selection is. $\endgroup$
    – De Novo
    Mar 21, 2019 at 1:14
  • $\begingroup$ if you read the whole chapter of natural selection in Darwin's book "on the origins of species" then you'ed definitely get the impression I'm speaking about. In the struggle for life, Darwin highly adheres to Malthus principle that resources are not enough that ultimately even the slightest disadvantage a heritable trait would bring with it then this would be enough [over sufficiently long period of time] to cause extinction of that trait. Any variation that causes negative shits in balance in the struggle for life, would cause extension. That's what I mean here, and what Darwin meant. $\endgroup$ Mar 21, 2019 at 10:50
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    $\begingroup$ DO NOT read Darwin expecting to understand the concept of natural selection and its relation to evolution. Darwin wrote over 150 years ago, and is entirely irrelevant to modern evolutionary theory. You are not educating yourself; you are simply confusing yourself, and wasting everyone's time by expecting people to explain obsolete, irrelevant concepts as if they're currently important. Darwin was a great thinker and set some foundations for modern evolution, but he is now important only as history, not science. $\endgroup$
    – iayork
    Mar 21, 2019 at 17:45
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    $\begingroup$ You can find good courses on-line, such as the one from Yale: oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-3 (Look at the next lecture on drift as well) $\endgroup$ Mar 22, 2019 at 7:03

3 Answers 3


The idea according to Darwin is that the slightest harmful effect a hereditary trait(s) would bring to a population then with time it would result in extinction of the population with that trait(s).

Citation? I certainly don't think any modern biologist would claim that a single deleterious allele will doom a population.

Therefore natural selection could be viewed to result ultimately in washing out of injurious traits,

In real life, there is no infinity far away "ultimate" point to be reached. What's deleterious today might be beneficial if the environment changes. And the environment is always changing.

Now can we label this event of indiscriminative extinction to be the work of natural selection also?

I don't see why you would, since survival was not influenced by genetics.

If the latter, then are those inadvertent environmental factors part of the mechanism for evolution?

Well, if it kills every single organism, I don't think we can say that population is evolving any more. But bottlenecks randomly and radically changing allele frequencies are hardly an unknown mechanism for evolution.

  • $\begingroup$ I've given the citation, according to Darwin, he believed in Malthus rule about geometric growth of populations with arithmetical growth in supply of food that eliminating that abundance. So over time any slight injurious trait would result in extinction of that genotype, due to that struggle for life, since by Malthus law its impossible for all of them to remain extant. $\endgroup$ Mar 20, 2019 at 20:20
  • $\begingroup$ +1, I think a key issue here, which you point out, is that there is no inherently "good" or "positive" trait, the value of the trait depends on the environment, which is changing. $\endgroup$
    – De Novo
    Mar 20, 2019 at 21:40
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    $\begingroup$ Simply citing Maltheus does not mean that Darwin really believed that a single deleterious variation would destroy a species. The strength of Origin of Species was all the work Darwin had put into observing variety across a number of different organisms. I don't think he ever said "Well, these barnacles are going to be extinct soon, because I observed this undesirable trait in one of them" $\endgroup$
    – swbarnes2
    Mar 20, 2019 at 22:26
  • $\begingroup$ @swbarnes2, I've given a clear reference to Darwin, and the tone of his speech clearly imparts what I've said here, "the least injurious"a trait would be then this would cause extinction, but I did NOT say "soon". I didn't cite Malthus, I've cited Darwin account and Darwin's own claims about malthus principle in relation to the struggle for life, that's not my citation, that Darwin's citation himself, read his chapter on natural selection very carefully. – $\endgroup$ Mar 21, 2019 at 12:00
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    $\begingroup$ You are missing many points. Darwin is almost entirely irrelevant to modern evolutionary theory and has been since the modern evolutionary synthesis in the 1940s (and even more so since Kimura and other modes of evolution), so arguing about what Darwin said is a waste of time. It's like claiming Egyptian hieroglyphs disprove internal combustion. Further, trying to map Darwin's arguments onto modern genetics is silly; Darwin had no clue about genetics, and entirely misunderstood heredity. Your reading of Darwin is wrong, too, but I won't bother explaining why since it's so irrelevant. $\endgroup$
    – iayork
    Mar 21, 2019 at 13:04

Natural selection is an important part of evolution, but not the only part

Evolution is described as changes in heritable characteristics over time: you can look at changes in allele frequencies and call that evolution.

Genetic drift, for example, describes genetic changes that are caused by random sampling rather than selection. A natural disaster (if we assume it acts only to kill off some individuals, rather than changing the environment), could hypothetically contribute to genetic drift.

Consider some population of a species of birds on a group of islands. The birds can fly between islands, so they interbreed but not at the same rate (i.e., they are more likely to breed within their island than between islands). Therefore, if we look at allele frequencies, they are not identical. They will be especially likely to drift apart if they aren't especially important to survival (an example in humans could be something like eye color).

One day, all the birds on one of the islands die by some disaster. If we look at allele frequencies across the whole population across all the islands, those frequencies just suddenly changed: any alleles that were more common in the doomed island were just reduced. Evolution just happened (heritable characteristics just changed), with no selection.

These effectively random changes can be quite important evolutionary forces over the long run, especially when population bottlenecks occur, in which case a small population survives rather than is wiped out.

  • $\begingroup$ I'll concentrate on your sentence .., with NO selection. So the above is not an example of natural selection. Is that correct? $\endgroup$ Mar 21, 2019 at 6:27
  • $\begingroup$ @BryanKrause on closer reading, I disagree. If the birds suddenly died due to stochastic effects, this is not natural selection. If the birds on one island suddenly died because of a natural event, this is natural selection. The fact that the event is a catastrophe or sudden does not make it any different from the rare visit of a particular predator, a change in tide patterns, extinction of some food resource, or the sudden presence of antibiotics, complement, or antibodies. $\endgroup$
    – De Novo
    Mar 21, 2019 at 7:44
  • $\begingroup$ @DeNovo It's only natural selection if a heritable phenotype is involved in the process, by my view. An island that is randomly destroyed by disaster is in fact stochastic. If some survivers remained because of protective heritable phenotypes then that would be natural selection. I'm curious how RemiB would weigh in. $\endgroup$
    – Bryan Krause
    Mar 21, 2019 at 12:58
  • $\begingroup$ @BryanKrause I would agree that if the phenotype wasn't involved in the selection process it wouldn't be natural selection, but I can't imagine a case where that would be true $\endgroup$
    – De Novo
    Mar 21, 2019 at 15:52
  • $\begingroup$ @DeNovo I agree it's a bit of an artificial scenario, but read the OP's scenario: by random chance, individuals of different phenotypes are in two regions. One region is destroyed at random by a natural disaster. Maybe a hurricane floods an island and kills every member of a species on that island but not other islands. Allele frequencies are changed subsequently by this random event, which did not target one region or the other based on anything related to the heritable phenotypes of the individuals affected. $\endgroup$
    – Bryan Krause
    Mar 21, 2019 at 15:58

Is a change in allele frequency resulting from a catastrophe evolution by natural selection?

The catastrophe itself is an example of a change in environment that brings with it a change in selective pressure. The events you describe (a volcano eruption, or a comet strike) would select for existing variation in the population. Heritable traits that are more common in those surviving the event would have been selected for. This represents natural selection.

Consider the description of natural selection from the UC Berkeley Understanding Evolution Course. We need the following things: variation, differential reproduction, and heredity.

Provided there were some variants that are able to survival, and their survival is the result of some heritable trait, this certainly fits the bill.

What if nothing survived?

This is highly implausible. I'm not aware of any event where this has occurred. Volcanic eruptions are colonized by microorganisms, and the K-T extinction certainly benefited us. If you are focused on, e.g., a specific population that happens to not have enough variation to survive the catastrophe, then that particular population has become extinct. This becomes more plausible the smaller and more homogeneous the population is.

To be honest, I'm not sure if complete loss of a heritable trait that made the population susceptible to the catastrophe (e.g., limited flight range in a bird population that was entirely unable to escape an eruption) would be considered natural selection in the event that the entire population was lost. From a framework of a larger population of birds that included that trait in some birds not on the island in question, I suspect it would, despite the fact that the birds without the trait may not have been exposed to the selective pressure. I would argue that, because there is a decrease in the frequency of a trait in the population due to selection against that trait. Perhaps I should ask a question about it. I welcome any comments or edit suggestions on this point.

Changes that are not due to natural selection

As discussed in a chat with @BryanKruase, changes in allele frequencies that are associated with, but not on the causal pathway to failure to reproduce, would not be considered natural selection. So, selection of birds with a longer flight range, provided that flight range is heritable, would be considered natural selection. If those birds also happened to have more green feathers, an increase in the frequency of that trait would not be due to natural selection. One need not invoke a rapid change in environment (seen from one perspective as a catastrophe) in order to see both types of changes (the trait under selection as well as those associated with that trait). From my perspective, that may have obscured the question. As Bryan pointed out, one should be careful not to overestimate natural selection as the principal cause of evolution.

Your green/brown beetle example would be an example of a change in a heritable trait that is not evolution due to natural selection. Here a random event led to all the brown beetles failing to reproduce, but the brown trait itself was not selected for or against. Speaking of the trait, then, and accepting your hypothetical, we can say that the change in this trait was not due to natural selection.

A few additional comments on your question:

On reading Darwin: I'm glad you're carefully reading Darwin. It's been a while since I've done that. I would recommend you not get too caught up in whether something fits with his description. That was the state of things 160 years ago.

On an "inadvertent catastrophe": this phrase is a little confusing. Your examples are natural phenomena. I'm not sure you mean to distinguish your examples from purposeful changes, e.g., for an experiment.

On changes "merely due to an environmental factor": selective pressures in natural selection are always due to an environmental factor of some sort or another.

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    $\begingroup$ The event killed ALL the living population in the island, where is the selection? $\endgroup$ Mar 21, 2019 at 6:18
  • $\begingroup$ @ZuhairAl-Johar if that happened, that's the selection. I'm not sure what is unclear there? No traits allowed for survival under that selective pressure. $\endgroup$
    – De Novo
    Mar 21, 2019 at 7:03
  • $\begingroup$ evolution by natural selection occurs when we have variation in heritable traits that cause differential fitness that would result in extinction of the less fit genotype. In other words "natural selection" is a function of differential fitness. But here the catastrophe caused extinction was not a result of variation in heritable traits, definitely this point in evolutionary history (i.e. the catastrophe causing this massive indiscriminate extinction) is not the result of what Darwin was speaking about in his chapter "natural selection". $\endgroup$ Mar 21, 2019 at 7:11
  • $\begingroup$ Its true that the catastrophe had resulted in a change in the heritable characteristic of the total living population on earth, since that would lack the ones that became extinct, and a moment of evolution had occurred yes, but that was not due to evolution by natural selection, i.e. that change in hereditary characteristics didn't result from a change in fitness of completing genotypes. It was a blind force having no direction at all. The natural disaster didn't select for anything that it acted upon, it destroyed them all. $\endgroup$ Mar 21, 2019 at 7:18
  • $\begingroup$ @ZuhairAl-Johar the catastrophe is the environment, not the selection. In a hypothetical and extremely implausible case that has never occurred (100% of life everywhere is abolished), absolutely everything was selected against because there wasn't a variant that allowed survival in that environment. In any other case, something is selected for. $\endgroup$
    – De Novo
    Mar 21, 2019 at 7:18

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