[EDIT] if the genetic mechanism responsible for bringing about evolution is random, and if the environmental favoritism of some heritable traits over others is non-purposeful (like in the example of the black peppered moth during the era of industrial revolution: see here), then one would expect to see some examples of evolution that results in emergence of species the members of which has lower fitness than those of its immediate ancestor species. What I mean by that is that the genetic material in the gametes of some members of some species undergo changes that results in the emergence of a NEW SPECIES the members of which has lower fitness, that can go live in a separate environment.

Are there such examples? if not then why?

I'm not asking here about examples of an offspring having lesser reproductive rate than its parents and yet that offspring belonging into the same species of the parents. Of course that is well known to occur, since many hereditary disorders clearly affects the reproductive rate and survival of the affected individuals. No I'm asking about the emergence of a NEW SPECIES the members of which has lower fitness than those of its immediate ancestor species. The reason why I've mentioned that members belonging to the new species go live in a separate place from that of its immediate ancestor species, is because if they live in the same place, then clearly members of the ancestor species having more fitness would overcome members of the newly emerging species, to result in their extinction by natural selection, that's why I'm asking if it is possible for members of the new species to travel to another environment where it can live, albeit being less fit than members of the ancestor species from which it emerged. I call that backward evolution, though the term is odd, to mean that the direction in that particular case of evolution was towards lesser fitness rather than toward increasing fitness of members of the newly evolved species.

Are there such examples in the history of Evolution.

I'm not asking if the mechanism of such a step in evolution is due to Natural Selection, I'm asking if that evolution step can occur? and if it had occurred? And what are possible mechanisms behind such a step?

With the founder effect, what I'm understanding is that migration is the cause of lessening in genetic variation of the original population, and that this can possibly further lead to emergency of a new less fit species.

What I'm asking about here is a little bit different, I'm asking if the emergence of the new species caused the emerging (less fit) population to migrate as to avoid competition with members of the more fit original species. And because of this separation in place both populations need not compete each other for available resources, since they live at distant places. In other words, if some mechanism caused the birth of some less fit individuals that belongs to a distinct species form its ancestor, and such that those less fit individuals migrated as to avoid competition with the more fit ancestors, and that by this migration they managed to survive and reproduce. So it is not that migration caused at the end the less fitness, as in the founder effect, No its the opposite, its the less fitness that caused the migration.

I hope the question became clear now.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – AliceD
    Commented Mar 7, 2019 at 7:06
  • $\begingroup$ Frankly your edited question is still unclear, although it does add more emphasis on sub-populations traveling to new environments. Please explain how the founder effect (which has been mentioned already) doesn't answer your question. $\endgroup$ Commented Mar 8, 2019 at 12:43
  • $\begingroup$ And your question still has major terminology issues that make it hard to answer (despite all the efforts of commenters to make you clarify what you mean), chiefly your usage of species instead of populations and perhaps your confusion of evolution with natural selection, $\endgroup$ Commented Mar 8, 2019 at 12:52
  • $\begingroup$ @Fizz, thanks about reference to "founder effect", I've updated my question, I hope its understandable now. I'm not familiar with the founder effect, I'm not sure if what I'm asking is an example of it, but from the reference you've linked it appears to be 'change due to migration' while here what I'm asking is "migration due to change", i.e. that some genetic mechanism caused the emergence of individuals that has different genetic material from the original population, such that those individuals belongs to a different species that are less fit than the original population, that caused them $\endgroup$ Commented Mar 8, 2019 at 14:17
  • $\begingroup$ .. to migrate, in order to avoid natural selection, selecting against them in favor of the original species which is more fit then them. $\endgroup$ Commented Mar 8, 2019 at 14:17

1 Answer 1


The question contains a fair number of issues as explained in the comments. I won't go through them here. I will phrase some specific questions which might be of interest to you and very quickly answer them

Can an offspring have a lower fitness than its parent?

Yes, of course. There is a whole load of environmental reasons for that but leaving them aside, the offspring might have inherited some new deleterious mutations

Can the offspring population have a lower mean fitness than a parent population?

Yes. Under constant environment, with an population of infinite size, without migration, without mutations, it would not be possible (unless some fancy linkage disequilibrium and epistasis) as, in such circumstance, the mean fitness of a population increases exactly by the additive genetic variance of the parent population.

However, populations are never infinite. Hence genetic drift is in play and can cause the mean fitness to randomly vary which may cause a drop from any generation to any next generation. Other things such as an environmental change or an extreme mutation rate may also cause the mean fitness to decrease.

Can an offspring have fewer offspring than its parents

Yes. Fitness is an expectation. There is variation from it. Whether or not the fitness of the offspring is higher or lower than the one of the parent, it still remain that there is variation from the expectation.

Can a mutation undo a previous mutation.


Can an entire population back to the exact same stage it was a few generations back.

If by exact same stage, you mean "exact same genome of the entire population", then this would be so fantastically unlikely that I will just say no.


I think you need to post that as an answer explaining why NS will "by definition" always acts to increase the frequency of more fit individuals,

NS is defined as the fitness differential among different genotypes in a populations. The change in allele frequency that results from this fitness differential. Individuals with higher fitness leave more offspring than those with lower fitness leading to a change in allele frequency, alleles associated increase in frequency.

I think you need to post that as an answer explaining why NS will "by definition" always acts to increase the frequency of more fit individuals, so that there cannot be an instance of speciation whereby a less fit species evolved from a more fit species.

You misunderstand the concept of speciation. Your sentences are so non-sensical that it is really hard to be of help to you. Really, you should just have a look at an intro course to evolutionary biology.

I find some difficulty in understanding how NS is "not purposeful", it seems to have a purpose to me, namely "producing more fit species from earlier ones", this is a kind of purpose.

The term "purposeful" is never used in science as it is very much undefined. The term feels to me that to have a purpose you need a conscience or a will. Obviously NS is not a small green creature hidden somewhere with the goal of making life evolve. It is just a natural phenomenon like many others.

You should just follow an intro course to evolutionary biology to understand what the terms natural selection, evolution, mutation, genetic drift and speciation mean.

By then if fossil evidence or laboratory evidence documents an event of what I'm saying here, it means that NS is false. right

I don't understand what you refer to by "what I'm saying here" either.

I highly recommend that you follow a short intro course to evolutionary biology (such as evo101 for example). It will take you a few hours and you will learn a lot. Also, you will be able to ask questions that don't give a headache to whoever is reading it :) Good luck!

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Chris
    Commented Mar 9, 2019 at 14:48

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