First lets consider that a species is a group of individuals that can reproduce by breeding fertile offsprings.

It's usually said that evolution takes a very long time. Lets say that 1000 generations are required for species A to evolve into species B.

Which species would generation 999 belong to, A or B ?
If A then it means that there was no evolution in 999 generations and that the entire evolution is going to happen in a single generation from 999 to 1000.
If B then which specie does generation 998 belong to ? etc etc

In the end it seems to me that if there is evolution it can only be instantaneous, but that requires many improbable things, one of them being that the spermatozoids and ovules of species A contain DNA of species B !

How does that work ?

EDIT this is not a duplicate I'm not asking what a species is I'm asking what is the genetic process of evolution, how do we move up in the evolution tree.

  • $\begingroup$ In short, the concept of species is a subjective one and is not objectively defined. You should have a look at How could humans have interbred with Neanderthals if we're a different species?. While your post is not directly a duplicate of the linked post, your question(s) are being answered in the accepted answer of this other post. I am therefore voting to close as duplicate. If you think your question has not been answered in this other post, please let us know $\endgroup$
    – Remi.b
    Jan 8, 2018 at 6:16
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    $\begingroup$ Btw, the singular of "species" is "species", not "specie". $\endgroup$
    – Remi.b
    Jan 8, 2018 at 16:18
  • $\begingroup$ The observed minimum within animals is three generations. ---- While the question is marked as a duplicate, the answers differ between "How many generations does it take for one specie to become a new one?" and different from "How could humans have interbred with Neanderthals if we're a different species?"; ------ For the former the answer would be: The observed minimum within animals is: Three generations. Lamichaney et al. Science, 2018: Rapid hybrid speciation in Darwin’s finches $\endgroup$
    – tsttst
    Jan 19, 2018 at 1:00
  • $\begingroup$ @tsttst there are plenty of cases of hybrid speciation (single generation) i plants especially. $\endgroup$
    – Remi.b
    Jan 19, 2018 at 10:01
  • $\begingroup$ @Remi.b good point; and there is also at least one synthetic specification in Drosophila; intended to stay close to natural evolution in animals and thus close to the previously linked question - which has an excellent answer about general themes of evolution $\endgroup$
    – tsttst
    Jan 19, 2018 at 17:03

2 Answers 2


Your question is like asking about the color spectrum. A wavelength of 530nm is green; a wavelength of 580nm is yellow. So let's say it takes 60nm of wavelength change to go from green to yellow. What color does a wavelength of 579nm have, green or yellow? If it's green, then does the color shift in a single nm difference of wavelengths? If it's yellow, what color is 578nm, etc?

The answer in this case is, 579nm is yellow; so is 578nm. 570nm is more ambiguous; some people might say it's yellow, others think it's greenish-yellow, in some contexts it might look clearly green. And so on along the spectrum; the reality is that there is no hard boundary between "green" and "yellow". There are colors that are clearly green, colors that are clearly yellow, and there is a difference between those two, but there are also plenty of intermediates that are a bit like green and a bit like yellow, and those intermediates are what you go through when you go from green to yellow. There is no definite point everybody can agree on where a single shift happens.

It is the same thing with species. A species is a group of organisms that interbreed to produce fertile offspring, but "interbreeding to produce fertile offspring" isn't a on/off thing. Organisms are more or less fertile. What if organisms from two groups interbreed and produce offspring, and only half of those offspring are fertile? Or they have half as many offspring as couples within the two groups have?

The reason species are an important concept, and can be distinguished visually as being like each other and unlike members of other species, is that within a species there is widespread sharing of genes over the generation, as individuals breed with each other, but they don't share genes with other species. Meaning species will evolve as a unit; changes that happen within a species will spread between individuals of that species, but won't spread outside of it.

But because sharing genes isn't an on/off thing, you could have 100% of genes being shared from one population to another (then they're clearly the same species), or 0% (then they're clearly not), or 90%, or 70%, or 10%... That's where you find the fuzzy boundaries. The races, strains, varieties, sub-species, all those different words to describe populations that are different but aren't outright different species. And the debates on whether different groups are different species or not in the first place, in the cases where it's really ambiguous.

And the same thing is true of evolutionary change in general. You don't have a mammalian carnivore that isn't a cat, and then bam! a cat. You have a mammalian carnivore that isn't a cat (or dog, or weasel... but kind of like any of those in various ways. Like generic carnivore-ish), then a mammalian carnivore that's a bit more like a cat or a dog (and another one next to it that's a bit more like a weasel), then a mammalian carnivore that's even more like a cat (and others that are even more like a dog, and even more like a weasel), then a mammalian carnivore that's clearly feline but isn't a cat, or a lion, or a tiger; it's a different species of feline that doesn't exist today, and also looks more generic-feline-ish than any modern feline. Then there's a feline that's more like a cat (and others around it that are more like a lion, and a tiger), but also clearly isn't a modern cat. Then there are felines that are really like cats but a bit different from modern ones and it's hard to tell, maybe we should extend the definition of "cat" to include them? And then bam! a cat. But that cat isn't clearly a Jungle Cat, or a European Wildcat, or a domestic cat, or any modern species; it's its own "archaic cat" species. And then there are cats that are more like Jungle cats, or more like domestic house cats...

Also don't confuse the creation of new species with evolutionary change in general. Over the long term they are related, because once two groups are different species then they will change through evolution in independent directions, and they will end up looking different from each other as well as from their common ancestor. But over the short term you can have speciation with very little morphological or genetic change (you just need two groups that used to interbreed, to no longer do so), or morphological/genetic change without speciation (if all the change happens within one group).

Both speciation and genetic/morphological change can happen within a single generation, but usually happen over much longer timescales. But there is no hard rule for how long that is; as far as speciation goes, if groups aren't prevented from interbreeding in some way then they will keep breeding with each other and can stay a single species indefinitely (whether individual from generation 10,000 could interbreed with individual from generation 1 is a different question, but it's not like it's a situation that happens). As far as evolutionary change goes, natural selection means species change to adapt to their environment. Once they're adapted to it, if it happens their environment never changes then natural selection will push them to stay the same as they are, again indefinitely. In the real world most environments change over time so organisms do change more often than not, but just to illustrate that the timelines are a product of circumstance, not an intrinsic feature of evolution itself.

  • $\begingroup$ Ring species are a great example of how fertility can vary even within a single species, and just how vague the concept of species is. Penguins are another a great example since some species can successfully crossbreed in the wild. $\endgroup$
    – John
    Mar 7, 2018 at 3:26

Species is a hard concept to really pin down but your definition is fine for animals and most plants. If the only distinction we make is their reproductive success, and we know g1000 is the only generation that is part of species B and it is unable to reproduce with g999 then something must have happened when g1000 was born that makes all members of g1000 unable to successfully reproduce with all members of g999. The most likely scenario is geographic or temporal isolation, they must have migrated into another area or their mating season went out of sync with g999. In that case the real speciation happened in one generation, from g999 to g1000.

If on the other hand we know g1000 is species B and g1 is species A, but we haven't decided if g2 through g999 are part of either A or B then the possiblity exists that g2-999 are intermediate populations. Using the temporal isolation example again imagine g1 has mating season from Feb-07 to Feb-16, g100 has mating season Feb-08 to Feb-17 and every hundred generations the mating season shifts one day, then g1000 has mating season Feb-17 to Feb-26. This means there is no way for g1000 to reproduce with g1 simply because they never have sex but g1000 can reproduce with g100 and up. This is the closest explanation we have for speciation, any obvious differences between species come only after many generations of isolation.

Regarding DNA, all species are related and all related species share a lot of their DNA. The closer their relation, the more DNA they share. Even humans and grass both have the genes that allow us to get energy from sugar. So yes, all cells of species B will contain DNA from species A, there is just a tiny difference that is just big enough to keep them from breeding. This could be mating season differences, but also incompatible genitals or one of the species has become allergic to the other, or anything else you can think of.

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    $\begingroup$ Welcome. Can you add sources to allow other users to background read on your answer? $\endgroup$
    – AliceD
    Mar 6, 2018 at 21:38

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