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Question simplified and clarified as requested... thanks for the feedback! If humankind evolved on a planet with a consistent, gentle environment, would it negate the need for them to develop variation in skin tone, eye shape, nose shape, etc.?

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  • $\begingroup$ I started to answer this, but I think you're asking a few questions, and I'd like to clarify what they are. Maybe split this up a bit? $\endgroup$ – Ariah Jul 6 '17 at 3:43
  • $\begingroup$ Maybe these are your questions: 1. Whether a human-habitable planet could exist in the absence of extreme weather. 2. Whether extreme weather is a source of evolutionary pressure. 3. Whether or not the absence of that pressure would affect human evolution, and in what ways. $\endgroup$ – Ariah Jul 6 '17 at 3:50
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    $\begingroup$ There was no need for human "races" to evolve. (Please note that the word "race" does not have biological meaning; what are called human "races" are social constructions which vary from culture to culture.) It just happened, mostly, we think, by genetic drift and a dose of sexual selection. They are not related to environment, with the exception that indeed in the tropics light-colored skin would have been selected strongly against. $\endgroup$ – AlexP Jul 6 '17 at 7:30
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    $\begingroup$ This is the most American question I've seen in this forum. I can often tell if they used a real native or just a "black" or "white" person in one of their movies just from their looks. You need to travel around and learn about the world I think. This whole idea of "races" is flawed from the beginning, but regardless you will find that people look a bit different anywhere you go, same climate or not. It's because you will never have a perfect mix of all humans so everyone will look alike as soon as there are some, eg cultural, borders. $\endgroup$ – Raditz_35 Jul 6 '17 at 12:39
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    $\begingroup$ I find it difficult to replace the word race in this context. I realize it's a loaded term, but ethnicity and nationality both reference culture which is not my question. I'm only considering physical traits like differences in melanin. Any suggestions? $\endgroup$ – Moreau Jul 6 '17 at 16:17
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Extreme weather does not really cause evolution into races/species. The cause is reproductive isolation. Isolate groups of your initial population, and they will eventually evolve differences just through random variation, even it the weather &c is the same for all.

Differences in climate can, and often do, determine which variations are selected for, so for instance animals living places with snowy winters may evolve two distinct seasonal color phases, while closely-related ones in warmer climates may stay the same color all year.

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I agree with jamesqf.

I would add that IRL we observe such isolations determined by mountains, valleys, canyonds, deserts, rivers, and dense forest; and these do not have to be particularly remarkable: I recall reading of one instance in which monkeys of one ancestral species are evolving differently on two sides of a river that is barely 30 feet wide. Too far to cross by swinging in the trees available, and too fast to risk wading. Presumably some rare drought a few hundred thousand years ago shrank the river enough to cross. Or froze it, or a flood washed a tree full of monkeys across, or whatever.

Such minor obstacles mean nothing to modern humans; but early human ancestors could be equally flummoxed and result in isolation. It isn't that they can't climb the mountain: Perhaps 9999 years out of 10000, there is no good reason to climb the mountain; water and food are plentiful in the grassland and woods fed by the runoff from the mountain, so life is good, barring extreme climate change, and mountain climbing is dangerous and harsh: A 1000 year drought might send you up there, and on the other side you might find another species isolated for 500 generations, with genetic drift altering their physical appearance (different races) and perhaps making them mutually infertile (different species).

In genetic recombination, not all genes are mix-and-match, and many phenotype features depend on many genes working in concert. The concert can be disrupted if some of those genes from one parent do not fit perfectly with those of the other parent: So (A) has 'abcdef' genes, (B) has 'abgdeh' genes; the 'g' and 'h' are alleles of 'c' and 'f', respectively.

So while the combos 'cf' and 'gh' work fine together, and end up producing the same product of two proteins, the combos 'ch' or 'gf' will clash: what is produced by 'c' won't fit with what is produced by 'h', likewise for 'g' and 'f'.

Thus for these parents (A) and (B), half their children are stillborn, or are spontaneously aborted during development, perhaps even at the egg stage.

Variation like this can arise through genetic drift, and enough such variation can make species that look the same infertile. (Consider how many modern human couples have had difficulty conceiving, or consider how a horse and donkey can mate but produce infertile offspring (mule or hinny)).

This is one mechanism that can cause species to emerge: Isolation can immediately begin the process of genetic drift, which is a random walk, eventually this causes "races" (still mutually fertile), longer isolation will eventually cause speciation (mutually infertile). Along that road another town allows mutual fertility with infertile (or genetically flawed) offspring; the Horse+Donkey = (Mule or Hinny) paradigm; i.e. hybrids are infertile, or have some congenital problem that effectively prevents mating (e.g. a horse born congenitally blind, in the wild, may technically be fertile but never survive to mate.)

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    $\begingroup$ Re "make species that look the same infertile", consider the remarkable similarity between the hummingbird and the hawk moth. Or vice versa, of course. Would an alien biologist suspect that Chihuahuas and German Shepherds are members of the same species? Yet my neighbor's dog is a cross between the two. $\endgroup$ – jamesqf Jul 6 '17 at 18:31
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    $\begingroup$ @jamesqf By "look the same" I obviously mean appear to be the same species, which a hummingbird and a hawkmoth do not; they do not even have the same number of limbs, and are not "remarkably similar" in any respect to a biologist. Yes, an alien biologist with acumen would suspect large dogs and small dogs are of the same species, internally they match perfectly organ for organ. But they would also suspect Horses and Donkeys are the same species: After all, they can actually breed and reproduce. The incompatibility is a subtle genetic issue missed by even very close inspection. $\endgroup$ – Amadeus Jul 6 '17 at 19:31
  • $\begingroup$ Depends on how closely the alien biologist examines them. I'm not talking about dissection, still less about DNA sequencing, but about the creatures observed "in the wild". $\endgroup$ – jamesqf Jul 8 '17 at 4:49
  • $\begingroup$ @jamesqf Okay, we can talk about idiot aliens, I guess, which has nothing to do with the OP question. Or you can read in the spirit in which things are intended: I was talking about genetic drift in isolated populations, which obviously implies a common ancestor, so looks the same means in every visible aspect of their phenotype; with any differences (like height or weight) reasonably attributed to normal genetic variation. But in fact non-visible differences due to genetic drift can make the two groups mutually infertile with one mate from each group, but not within their own group. $\endgroup$ – Amadeus Jul 8 '17 at 23:22
  • $\begingroup$ My point is that it's two sides of the same coin. Genetic differences & evolutionary pressures can produce species whose members look considerably different, or they can produce nearly identical appearance & behavior in species as different as birds & insects. $\endgroup$ – jamesqf Jul 9 '17 at 19:12
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A planet with a constant environment is nigh on impossible. After several attempts to devise a planet with a constant environment they met with failure. Astronomical factors like axial tilt and the shape of the planet's orbit mitigate against environmental constancy. Ditto with surface features like its geography and whatever diversity it would have in its range of biomes.

This consideration alone removes the possibility of a human species shaped by the selective pressures of its environment in a uniform and unchanging manner.

It is noted that other answers favour genetic drift caused by isolation. While this will play a role, it is not most likely cause of ethnic diversity. [For the purposes of this ethnicity refers exclusively to the phenotypic characteristics of human beings with culture excluded.]

On Earth when humans moved into Europe they lost their darker skin tones in order to synthesize vitamin D. As explained above, there is always likely to be geographical locations where environmental selection pressures will play an active role in shaping ethnicity.

However, the main driver of ethnicity will be sexual selection. Where mates choose their partners on the basis of agreeable characteristics. Some of which will involve selecting partners who will be expected to bear the healthiest offspring.

A planet with a constant environment, assuming such a hypothetical construct could exist, but we can pretend it does, would not prevent ethnic diversity arises within a human species. However, despite their ethnic differences, they remain, like here on planet Earth, one species.

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  • $\begingroup$ They remain one species That isn't necessarily true at all! Darwin's whole point was that one species can split into others, and he was right, we see that in its gradual way, one of the most advanced instances right now are horses and donkeys (which can still mate but produce sterile offspring). There is no particular rational reason to believe humans must remain the same species; phenotypical differences could lead to steadily fewer successful matings until it reached zero. $\endgroup$ – Amadeus Jul 7 '17 at 9:34
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    $\begingroup$ @Amadeus Darwin's whole point was that speciation was the result of natural selection. If a population can continue to interbreed they will remain one species. I assumed this hypothetical human species had the same length of an evolutionary history. I wasn't speaking of theirs or our evolutionary futures. I was driving a stake into any dimwitted notions about the nature of race. Something the OP also wanted to avoid. $\endgroup$ – a4android Jul 7 '17 at 9:49
  • $\begingroup$ "Race" is not a dimwitted notion, no more so than "species." "Race" is just a statistical idea that there exists various collection of genes affecting phenotype that are highly correlated and tend to occur together. If you do not believe that is true, you are denying reality, such gene-groups exist, and this is a phenomenon that deserves a name, it is just political correctness that wants to claim there is no such thing as "race". It doesn't have to mean anything in terms of rights or non-phenotype traits, but it is undeniable that such correlated groups exist. $\endgroup$ – Amadeus Jul 7 '17 at 16:14
  • $\begingroup$ @a4android Thanks for the feedback. Dagger in the heart of my original plot line, but actually glad that my planet will support diversity in the dominate species. Just have to find another way to create a species with delicate/conductive enough epidermis to allow for their primary form of communication to be haptic. $\endgroup$ – Moreau Jul 7 '17 at 16:17
  • $\begingroup$ @Moreau Actually, a uniform environment is entirely possible: Amp up the oceans a bit, so your land mass is just the equatorial bulge on your planet, like a thick rubber band on a bowling ball: On earth, land would only be, say, within 300 miles of the equator (the N/S borders can be irregular). Everything else is ocean. All the land is climate-wise so close it would have little evolutionary influence. Earth is already an oblate spheroid (flattened ball); just make your planet a little more flattened, with more ocean and very little tectonic activity (no mountains). $\endgroup$ – Amadeus Jul 7 '17 at 18:45

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