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For example,

See the following evolutionary graph:

enter image description here

$A$ is an original species.

$B$ and $C$ evolved from $A$ but reproductive isolation.

$D$ and $D'$ are almost the same, so they are not reproductive isolation.

My question is that is this phenomenon to be possible?

If it is possible, is there any example of such species in nature?

Thanks.

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The are many allopolyploid plant species, that is, polyploid species with chromosomes derived from two or more diverged taxa. For example, common wheat, Triticum aestivum, is an allohexaploid carrying chromosomes from two separate ancestor species, einkorn wheat Triticum urartu, and a goatgrass, Aegilops speltoides. (Note than two ancestor species are not even in the same genus.) In general, the taxonomy of wheat is an unsettled mess, with three different ancestral species (Triticum urartu, Aegilops speltoides and Aegilops tauschii, multiple genome duplication and hybridization events, and disagreement between traditional morphological taxonomy and molecular taxonomy. (Note how the table in the Wikipedia article needs to have separate columns for species recognized by traditionalist morphology-based phylogeny, and species recognized by ultra-modern molecular phylogeny.)

In the genus Brassica of cruciferous vegetables we find a well-known relationship between six species, three ancestral diploid species, and three derived allotetraploid species. Note that all six species are important crops:

The triangle of U

The so-called triangle of U showing the relationship between three ancestral and three allotetraploid species in the genus Brassica. Brassica nigra is the black mustard; Brassica oleracea is cabbage, broccoli, cauliflower and so on; Brassica rapa is turnips; Brassica juncea is the Indian mustard; Brassica napus is rapeseed and rutabaga or Swedish turnips; and Brassica carinata is the Ethiopian mustard. As for U himself, he was a Korean botanist, Woo Jang-choon, who, during most of his life, lived under the rule of Imperial Japan and used the Japanified name Nagaharu U. Diagram by Adenosine, Nashville Monkey et al., available on Wikimedia under the Creative Commons Attribution-Share Alike 2.5 Generic license.

In animals, examples of species carrying genes originating in more than one ancestral species are rarer but they do exist.

One of the best studied cases is the genus Panthera of big cats, where four out of five extant species (the tiger, the leopard, the lion and the jaguar) have been found to carry genes originating in other species of the genus; for a nice diagram see Jordana Cepelewicz, "Interspecies Hybrids Play a Vital Role in Evolution", in Quanta Magazine, 24 August 2017.

Henrique V. Figueiró, Gang Li, Fernanda J. Trindade, et al. writing in Science Advances, have the following to say in their article "Genome-wide signatures of complex introgression and adaptive evolution in the big cats" (Science Advances, 19 Jul 2017, Vol. 3, no. 7, e1700299, DOI: 10.1126/sciadv.1700299):

The great cats of the genus Panthera comprise a recent radiation whose evolutionary history is poorly understood. [...] We observed pervasive genealogical discordance across Panthera genomes, caused by both incomplete lineage sorting and complex patterns of historical interspecific hybridization. We found [in jaguars] at least two genes (DOCK3 and COL4A5, both related to optic nerve development) bearing significant signatures of interspecies introgression and within-species positive selection. These findings indicate that post-speciation admixture has contributed genetic material that facilitated the adaptive evolution of big cat lineages.

The genus Panthera is a remarkable group to investigate these issues because it comprises five big cat species that arose from a recent and rapid diversification process. Understanding the history of their unique features [...] depends on resolving the underlying phylogeny of the Panthera clade, a task that has been notoriously difficult to accomplish. Recent analyses have indicated that genealogical discordance caused by both incomplete lineage sorting (ILS) and post-speciation admixture has contributed to produce such a complex system.

The lion lineage exhibited the most widespread signatures of ancient admixture, likely due to its broad historical range throughout much of the Holarctic region, overlapping with several congeneric species. These results considerably expand the recent evidence for hybridization between the snow leopard and the lion + leopard ancestor [...] and reveal a much more complex history of post-speciation admixture in this group than was previously appreciated.

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    $\begingroup$ ILS is caused by variation in a common ancestor that hasn't fixed in two descendant lineages. If the ancestor had variants A and B for a given locus, and those variants happen to persist in the daughters lineages, then individuals with variant A in one daughter are more closely related to individuals with variant A from the other daughter than they are to individuals in their own lineage that happen to have variant B (at least at that locus). While this can cause phylogenetic trees for individual loci to conflict with the "species tree", it is distinct from introgression and hybridization. $\endgroup$ – cereal_killer Aug 27 '20 at 1:45
  • $\begingroup$ In your parenthetical listing of Panthera species, you have tiger twice. Judging from the rest of the answer, one of those should be lion, right? $\endgroup$ – BThompson Aug 27 '20 at 12:57
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    $\begingroup$ @BThompson: Thank you, yes, it was the lion. (The once which does not have admixture from other species is the snow leopard, Panthera uncia.) $\endgroup$ – AlexP Aug 27 '20 at 14:04
  • $\begingroup$ To be clear, I am saying that "...incomplete lineage sorting, where two species continue to exchange genetic material for some time after the speciation event..." is inaccurate. In ILS the conflicting genetic material existed in the ancestor. ILS occurs even in cases where the nascent daughter species exchange no migrants. Specific to the question, ILS does not result in an individual with genetic material from two ancestors. We generally call exchange of genetic material after speciation "introgression". $\endgroup$ – cereal_killer Aug 27 '20 at 15:37
  • $\begingroup$ @cereal_killer: Will change accordingly. $\endgroup$ – AlexP Aug 27 '20 at 15:58
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Yes, as long as the genetic distance between B and C isn't too large. The result is a hybrid species, or a multi-species hybrid complex[*].

Fertile hybrids that can generate a new species are not common among mammals, but hybridization is reasonably common in fish and amphibians, and happens all the time in plants.

[*] For example, there is a weird group of Australian carp which form a hybrid species complex between an all-male line and an all-female line which each reproduce semi-clonally, requiring sex with a host species to reproduce but displacing the host chromosomes; a disexual population which serves as the host for the aforementioned sexual parasitism; and hybrids of the three.

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  • $\begingroup$ I just realized that there may be human Intervention in evolution... $\endgroup$ – chansey Aug 26 '20 at 18:44
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    $\begingroup$ Maybe they're not common among mammals, but they exist. A notable example is the coywolf, which is a fertile hybrid of coyote and wolf that has formed self-sustaining populations in eastern North America: en.wikipedia.org/wiki/Coywolf $\endgroup$ – jamesqf Aug 26 '20 at 19:08
  • $\begingroup$ @jamesqf I think you may misunderstood what I mean. I don’t mean hybridization, but natural selection. $\endgroup$ – chansey Aug 26 '20 at 19:18
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    $\begingroup$ @chansey: But hybridization (and subsequent evolution of the self-reproducing population) IS the only way to have two different species be the ancestors of a new species. Otherwise you're talking about convergent evolution. It's conceivable that crocodile descendants might somehow evolve to look like parrots, and fill the same ecological niches, but they wouldn't BE parrots, any more than say dolphins are ichthyosaurs, or birds are pterosaurs. $\endgroup$ – jamesqf Aug 26 '20 at 23:31
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    $\begingroup$ @chansey: For completeness, note that this is limited to multicellular life. Bacteria & other microbes have things like horizontal gene transfer going on, which makes the definition of species rather imprecise. For instance, the shared genome of Prochlorococcus: sciencemag.org/news/2017/03/… $\endgroup$ – jamesqf Aug 27 '20 at 16:49
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Homo sapiens is an example of such a species. People (with non-African ancestry) have genes from Neanderthals and Denisovans. There is some debate about whether those are separate species or sub-species of Homo sapiens, but they were almost certainly reproductively isolated from our main line of descent, apart from the cross-breeding events that got us their genes.

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    $\begingroup$ Given that species is a fuzzy concept, the distinction between species and sub-species is also bound to be fuzzy: en.wikipedia.org/wiki/Species_concept $\endgroup$ – ThomasW Aug 28 '20 at 1:02
  • $\begingroup$ There's also the Old World and New World populations. The question doesn't specify "different species", just reproductive isolation, and the Old World and New World populations were likely reproductively isolated for thousands of years. $\endgroup$ – Acccumulation Aug 29 '20 at 4:21
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Parallel evolution:

As opposed to convergent evolution, involving similar features coming from very diverse species(like compound eyes in octopi and humans), parallel evolution is similar traits arising independently from similar but distinct species. Species A may have lived in the plains, and B moved to the mountains and became nimble at climbing rocks. C moved to the coast, and became good swimmers. Once B and C are sufficiently different/incompatible as to be different species, they are separate. This is divergent evolution. Now let's suppose a disease kills off A. B and C move to fill the niche left on the plains, and each evolves similar traits to survive the plains. Only now the two species may be incompatible with each other due to other reasons (chromosomal rearrangements, mismatched mating seasons, whatever). The resulting two species, D and D', might look identical but be in competition with each other for resources (namely the plains).

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  • $\begingroup$ Thanks. Is there any example of such species in nature? $\endgroup$ – chansey Aug 26 '20 at 22:24
  • $\begingroup$ @chansey: You mean cryptic species, that is, two or more different species that look so much alike that zoologists didn't even know that they were distinct species? There are many of them. How many exactly we don't know, because they are cryptic; but more and more have been discovered in the last 50 years or so. (Just one example: the lined forest falcon Micrastur gilvicollis and the plumbeous forest falcon Micrastur plumbeus.) $\endgroup$ – AlexP Aug 27 '20 at 16:42
  • $\begingroup$ @AlexP "There are many of them", but they cannot interbreed. Another interesting phenomenon is ring species, although the gene flow occurs between neighbouring populations of a species, but at the ends of the "ring" the populations don't interbreed. From mathematical view, it is not elegant. $\endgroup$ – chansey Aug 27 '20 at 19:30
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If B and C remained genetically compatible but simply didn’t reproduce, such as if they were geographically separated, then they would be two species.

If D and D’ later met and did successfully reproduce, then they have merged back into a single species.

However, if both sides evolved enough for D and D’ to be considered separate species from B and C, most likely due to different selection pressures on each side of the divide, then it seems unlikely that they would remain genetically compatible or even try to reproduce when they met.

I’m not aware of any examples, but it’s theoretically possible if the separation wasn’t very long. Most likely, we would retroactively declare B, C and D/D’ to have been subspecies of A all along. This may be, for example, what happened with Neanderthals and Cro Magnons merging.

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  • $\begingroup$ "However, if both sides evolved enough for D and D’ to be considered separate species from B and C, most likely due to different selection pressures on each side of the divide, then it seems unlikely that they would remain genetically compatible or even try to reproduce when they met." I mean that B and C are already different species, but due to selection pressures, B and C "return" to the same species which is not A. $\endgroup$ – chansey Aug 26 '20 at 18:59
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    $\begingroup$ Consider brown bears and polar bears. They prefer to mate with their own, but at the border, they’ll occasionally interbreed, so the two “species” have remained genetically compatible. If all the snow melts, polar bears may merge back into the brown bears and be a single species again. If so, were they ever really two species in the first place? $\endgroup$ – StephenS Aug 26 '20 at 18:59
  • $\begingroup$ I mean that crocodiles and parrots may have the same ancestor, but is it possible at some point in the future that crocodiles will evolve to parrots? Or parrots will evolve to crocodiles? Or they will evolve to the 3rd species which compatible with both crocodiles and parrots? Due to selection pressures. $\endgroup$ – chansey Aug 26 '20 at 19:12
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    $\begingroup$ Wolves, dogs, and coyotes can intermix. In fact most of the coyotes near where I live are a result of this mix, which may be a good example. $\endgroup$ – workerjoe Aug 26 '20 at 19:12
  • $\begingroup$ @chansey Parrots and crocodiles? No way. They have diverged way too far genetically to ever come back together. There are people who breed hybrid canids or hybrid felids, and nature occasionally does it herself, but hybrid offspring may not be viable even within the same genus, much less further away. $\endgroup$ – StephenS Aug 26 '20 at 19:25
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Lichen is an example of a hybrid organism, made from algae and fungi. It has many distinct features, breads as a single organism and can grow where neither of its components could.

Other evolutionary examples are the origin of chloroplasts, the origin of mitochondria and the origin of nucleus.

The DNA does not need to fuse immediately together, while in case of chloroplast and mitochondria many genes were finally relocated into nucleus, with only tiny genomes remaining inside the former symbionts.

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  • $\begingroup$ The lichen is a good example of a 'higher' lifeform developing from a merger of unrelated lifeforms. Most of the other answers deal with closely related species that haven't drifted apart enough to be non-fertile with each other yet. $\endgroup$ – Michael Richardson Aug 28 '20 at 19:42

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