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First off, let's get definitions out of the way:

A "race" here is defined as stable, heritable, phenotypic, clearly visual distinction between large demographic groups living in different geographic areas - but all of whom actually belong to the same biological species (e.g. can freely and effortlessly interbreed; and have fairly minimal genetic differences where you would have trouble easily distinguishing members of each "race" from genome alone).

For an apex intelligent species that lives on multiple continents of a planet that is fairly earth-like geologically and planetologically (e.g. seasons, climate variations etc.. are similar), how likely would it be that they would evolve into distinct and easily distinguishable "races", based on the current understanding of evolutionary biology?

By "How likely", I mean is it on the scale of "practically inevitable barring special circumstances, due to these and those evolutionary biology rules", or "Homo S. is this way thought pretty unlikely chain of random environmental coincidences that on average are unlikely to happen elsewhere"


Background: the question arises when creating First Contact setting, and deciding if the alien species would be utterly bewildered by the fact that some Homo S. members look to be completely different color.

An example of this would be Eric Flint's Ishtar in "Mother of Demons", where the issue was solved by the fact that the aliens were chromatophoric and thus the skin color changed for each individual based on emotions.

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  • $\begingroup$ Not an answer to the main question, but thinking of whether aliens will be surprised by colour differences, many animals change colour with age several times during their lifetime, so even an alien species without races may have considerable colour variation. $\endgroup$ – trichoplax Oct 6 '14 at 21:00
  • $\begingroup$ One thing it would help to specify is how much variation naturally arises in members of the parent population. If there's naturally a lot of variety, then you can trivially get races out of any kind of population bottleneck or founder effect. If not, then you're relying on gradual adaptation to differing environments. $\endgroup$ – octern Oct 7 '14 at 6:35
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    $\begingroup$ Also, thank you for providing a detailed, concrete definition of what you mean by "race." You've allowed us to have the interesting parts of the discussion without the tedious, politically/culturally fraught ones. $\endgroup$ – octern Oct 7 '14 at 6:36
  • $\begingroup$ @octern - YW. I observed enough questions on the topic on Skeptics.SE to know what works and what doesn't :) $\endgroup$ – user4239 Oct 7 '14 at 11:51
  • $\begingroup$ @octern - that rate of variation is what I'm after, really. $\endgroup$ – user4239 Oct 7 '14 at 11:52
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The so-callled "races" of humanity are just minor adaptations to local conditions. If a sentient species spreads all across the world in primitive times, then will evolve variations to adapt to local climate and other conditions.

People who live on or near the equator have black skin and hair to prevent UV damage from sunlight and usually smaller lithe builds for dumping heat. People who in arctic or subarctic regions develop heat trapping translucent skin which also promotes Vitamin-D photosynthesis. They have larger thicker builds to retain heat. Pygmies(M'Butu) are adapted to living in dense tropical rainforest in rough terrain. Most tropical rain forest dwelling people are short. Peoples who live at altitude are barrel chested with much larger lungs. Polynesians are adapted for swimming with a layer of insulating fat under the skin but none in their rock hard muscles underneath. People from the Eurasian steps or descended from them, as well as the Masai in Africa, carry genes to allow them keep digesting lactose as adults, 80% of humanity do not carry that gene and largely past the age of 5, must drink fermented milk.

The variation occur fairly rapidly. Modern humans are only 50,000-100,000 years old and all the variations we see today around in the time when the first modern humans migrated out of Africa (which still retains 80% of human genetic diversity.) IIRC, the lactose gene is believe to be around 8,000-10,000 years old.

If wanted a homogenous species, you need to keep them in a homogenus environment for most there evolutionary history.

Humans migrating into new environments drives the adoptions of variations. The way to create a relatively homogenous looking alien population is to have them evolve to sentience within a single but large and uniform regional biome where the selection pressures will be the same resulting in individual that look the same. As biomes are primarily defined by latitude, which defines climate. A single biome that fairly narrow follows the same latitude for a long distance, will have the same climate and exert the same selection pressures over it's length.

Imagine a planet in an ice age with a large but isolated continent like Eurasia whose Northern areas behind a line of mountains, are covered in a kilometer thick ice sheets leaving only a 500km strip of coastal plain in the south that runs uninterrupted along the line of the equator nearly perfectly for 10km or more. The biome would be uniform because the climate would be uniform for laying in the same latitude. Plants and animals that evolved on one end could easily migrate to the other without any adaptation. So could sentient life forms.

(This true of the central Eurasian Steppes. Plant and animal species vary little from Pacific to the Black Sea.)

All the sentients would have the same markers of adapting to the equatorial zone e.g. if they were humans, everybody would have black skin and hair of the same shade. Since migration of peoples and trade would be easy, genes would flow up and down the coast rapidally, preventing geographical isolation needed to produce variations.

If the sentient species had developed to a sea faring civilization before the ice age ended, then at the end of the ice age, they would spread all over the world but it might be just a few hundred years or so before they became space fairing. Not really enough time for significant variations to arise and in any case, the sea travel would swap to many genes around from place to place to provide the geographical isolation necessary for visible variations to arise.

Such a world would have an overall lack of diversity and relatively few species of all kinds because it started out with just the one biome. When those aliens visited worlds with dozens or hundreds of biomes, they would be startled by the riot of variations of all species.

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  • $\begingroup$ Your answer doesn't take into account genetic drift. Any populations that are isolated from each other will experience genetic drift even if their environments are identical. This is because which alleles are passed down is an inherently random process. As long as there are reproductively isolated populations genetic drift will cause them to differ. $\endgroup$ – Mike Nichols Jul 14 '15 at 20:24
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    $\begingroup$ My hypothetical biome prevents drift by maintaining continuous genetic communication across its entire east-west axis. If the biome is large and contagious, there is little or no reproductive isolation and therefore little drift. Again, the species of the Eurasian steppe clearly show this. Conversely, in a place like New Guinea where mountain valleys running north-south chop the island into isolated pockets. The island contains a staggering diversity of subspecies and human languages, often in less than 5 miles as the crow flies. $\endgroup$ – TechZen Jul 15 '15 at 16:01
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While skin color may not differentiate race in their case (although I'd assume it would amongst other characteristics that also change between races for us), I'd say it's nearly impossible for them to not have different races in the dominant species.

Does it ever happen?

I argue from example here, since I'm not an evolutionary biologist or a biologist at all. How many species do not have races? Of all the large mammals I can think of, there isn't any that has one variation, while many are genetically compatible - some produce sterile offspring but it's usually when the distance is so large they can be considered different species (see Liger and other cat hybrids - Ligers where considered to be sterile, but other hybrids are not). The same applies from reptiles to insects to fish etc.

What's different about sapient species?

With sapient species it's even less likely. Once the species is competent enough, it's likely groups, early in its maturation, will migrate, while others will stay put - all of this due to resource scarcity or the need to move to more resource-rich land (or water). At that point it's exactly the same race and species - yet by the time resources are not a problem anymore, the phenotype has time to change, but the species as a whole doesn't. Hence, you get races in the same species, due to the speed of advancement for a sapient species (especially considering that sexual selection is less likely to produce incompatible offspring or radical changes - by the time sapience comes around, its possession and the capacity for it become strong selection drives).

Now I'm making some unfounded assumptions here (obviously) but I think they're warranted considering that current science on these matters hasn't studied other sapient species.

edit - I'd like to add that, for an earth-like planet, we have to ask, what's the likelihood of the planet developing life that isn't indistinguishable from ours?

edit - As an addendum, per request of the OP, I'm adding examples of species that have arisen as a result from interbreeding between species that have recently diverged, without being done so by humans. The following are examples of hybrid speciation

  • the Clymene dolphin, which coexists with its parent species in the same habitat and is the only settled case of speciation. Being a mammal however gives us at least a good indicator that this can happen and has happened. Being a mammal is important because most cases of speciation are encountered in insects and fish.

  • the Red wolf who's status as a species is unsettled, but is a likely case of hybrid speciation.

  • the Anas genus; lots of species in the genus are believed to have arisen from hybrids produced by other species closely related.

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  • $\begingroup$ +1, but examples of interbreedable species differentiation that arose through natural selection would significantly improve the answer. $\endgroup$ – user4239 Oct 5 '14 at 17:07
  • $\begingroup$ @DVK You mean species arriving from different evolutionary paths to a state where they can produce fertile offspring? $\endgroup$ – mechalynx Oct 5 '14 at 17:16
  • $\begingroup$ backwards. Species that started 100% identical, speciated to the point of clear regional phenotypal differences naturally but easily interbreed and are the same species genetic-difference wise. $\endgroup$ – user4239 Oct 5 '14 at 17:25
  • $\begingroup$ @DVK Ah ok, because the way I read it the first time, I got chills just thinking of how hard it would be to get an example of that xD I'll find some and update as soon as I can. $\endgroup$ – mechalynx Oct 5 '14 at 17:28
  • $\begingroup$ @DVK Not sure, but do these count?: Anas & Red Wolf - this section explains why: Hybrid Speciation. If I understand correctly, those two are examples of species that diverged in the past and are capable of interbreeding enough to create a new species through hybridization. $\endgroup$ – mechalynx Oct 5 '14 at 21:53
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Let me start of with quoting this excellent answer regarding phenotypical differentiation in humans vs animals:

compare this:

enter image description here

to this:

enter image description here

or this:

enter image description here


Or this:

enter image description here

to this:

enter image description here
This is phenotypic variation:

enter image description here


And of course, you can't beat the birds of paradise when it comes to variation (though, strictly speaking, these are different species):

enter image description here

Assuming an evolutionary model for the creation of a world it seems quite likely thus to see strong variation in an alien species, variation that could even be more diverse than the human variation we see and it seems quite unlikely that an alien species originating from a similar world geologically would thus be faced by the differences seen by humans.

What could explain aliens that are surprised?

A world where there is just a single continent possibly, as in that case chances for differentiation are far smaller. Still that would be a world that is geologically different from ours, but at least it gives some space for story telling whilst the world itself can be quite similar.

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    $\begingroup$ Picture wall crits you for 9999 damage: You die $\endgroup$ – mechalynx Oct 5 '14 at 16:39
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    $\begingroup$ While very impressive and pretty, this is extremely misleading. 100% of non-human pictures represent either (1) distinct species (birds) or (2) Human-driven selection specifically for given phenotype, and mostly NOT geographically localized as far as selection pressures {a smaller question is, are all dog breeds interbreedable? I don't know anything about dogs so I'll assume "yes"}. Do you have any examples of globe-spanning single species with similar variations that are stable across geographies? $\endgroup$ – user4239 Oct 5 '14 at 17:01
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    $\begingroup$ @DavidMulder I like example-based answers but it would really help clarify it if you added a summary of the linked question (to the extent that it is relevant of course) and if you grouped pictures to differentiate phenotype variation between species and breeds. I think DVK's objections pretty much boil down to, the answer isn't clear enough. $\endgroup$ – mechalynx Oct 5 '14 at 17:21
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    $\begingroup$ @DVK - Dogs are considered a "ring species". Defined as species in which distant variation cannot interbreed directly but that they can breed with nearer variation and eventually work to the other end. "Horse shoe species" would probably be a better term. Dogs qualify because you can't breed a Great Dane to a teacup Chiwawa, even in the lab, but you can breed with intermediate breeds. Birds of Paradise are also considered a ring species as species can usually breed with the species in the next valley over but not those from the other end of the island. $\endgroup$ – TechZen Jul 15 '15 at 18:36
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    $\begingroup$ @DavidMulder - I am fixing broken image links as per the Meta post here. This answer just happens to have been corrected before the post it quoted. This isn't misrepresentation, it's fixing - no content has changed. $\endgroup$ – user10945 Mar 13 '17 at 12:11
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Generally yes, for a nomadic apex predator species on a single Earth-like planet prior to global civilization.

Geographic differences in phenotype rely on a consistent environmental stressor.

  • If the planet has no significant geographical differences (climate, predation, diet, etc) then the stressors remain consistent and so does the species (pre-genetic engineering).
  • If a species went from relatively immobile within a single locale (say, an unadventurous turtle race) to global mobility in short order then local differences would only manifest if the species then fell back into neolithic civilization for hundreds of millennia.
  • If a species does move into different geographies, each clade will need to remain in that environment for tens or hundreds of millenia (depending selection pressure) with reduced gene flow from other clades. This tends to suggest, as with Humans, that the initial migration to each region is very small and then locally multiplies, so that future (small) migrations are incorporated into the local genepool.

For a globalized species, any residual racial difference remains until the global genepool blends together or personalized body modification from genetic engineering renders the point moot.

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Any form of prolonged reproductive isolation will cause divergence. In the short-term (relatively speaking) it will form races and subspecies, and in the long term it will form distinct species. Humans evolved from a common ancestor and spread across the entirety of the planet from there. Once we became geographically isolated each population began to diverge, in part due to different environments and different selective pressures and in part due to random chance in the form of genetic drift.

Any species will form races if there is sufficient reproductive isolation between populations. This could be geographical as we see in humans, but for aliens you could also explain reproductive isolation using caste systems or other non-mixing factions. The extent to which these races are different depends on just how isolated they are, how long they have been isolated, and how quickly they are diverging.

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For a different perspective, consider the Earth in the year 10000. 300 generations or so has passed since today and in every generation, people has moved to different parts of the Earth until the connection between location and genes has been lost. The variation will still be there, but it will be spread out all over the place. Nobody will be talking about races anymore. Historians will know about them, but most people will not.

Now, send these people to a planet like today's Earth. They will be surprised at a) Most people with certain skin colour will be collected in a certain place. and b) these primitive people think that skin colour is important.

Optionally, you could throw in a period where gengineering your kids was the norm. Almost everybody make their kids look like whatever the fashion was, and this change would be inheritable. A few generations later everybody looks the same. In this case they would also be surprised at the variation of the primitives.

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It is extremely unlikely that a species could develop without 'racial' variation, provided that it has spread across its whole planet, for the reasons others have stated. However, it is very possible that once a civilization goes global, these variations will blend and effectively disappear due to interbreeding.

We are still living in a world with many geographical, cultural, and national boundaries, and these boundaries tend to isolate populations and keep the human species segregated into distinct groups. But it is quite common, when humans of different races live together, that they tend to 'blend' together after a few generations, provided they are not kept distinct through social constructs. As travel becomes easier, people become more tolerant of differences, and nations are forced to work together, these boundaries are fading, and (provided that civilization is heading in a direction of eventual world unity) are likely to continue to do so in the time to come.

It is not unlikely that by the time a species has developed to a spacefaring level, it will already have blurred together into a single race. Of course, this only applies if the species is still mostly confined to a single planet; isolated populations on different planets are likely to diverge even further. So I would say this species has achieved world unity several generations back, but is still relatively new to space.

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