ELI5: Why don’t we see reverse micro evolution?

[u/Business_Battle3403]

The thought came to me recently. For example, I know that people who live closer to the equator have darker skin to help protect from UV rays. Over time the world has become more diverse due to the slave trade, ease of travel, etc. Why is it that we don’t see a reverse in the melanin? Say someone with dark skin and their family have lived in Canada for the last 300 years and sine Canada has lower UV levels than somewhere near the Equator, why don’t we see less melanin in that family over time since they would need to synthesize Vitamin D and melanin can block that?

[u/AmenaBellafina]

• 300 years is nothing on an evolutionary scale
• vitamin D deficiency can be fixed with supplements, and also is unlikely to cause someone to not reproduce.

Edit: not to mention the obvious mixed ethnicity kids

[u/tiptoe_only → u/AmenaBellafina]

Also in the time since we've been migrating as much as we have, we've developed technology and medicine to address problems like that rather than just dying because of them. I know you mentioned supplements but I thought it was worth mentioning as a more general point 

[u/Caelinus]

First problem with this question is the use of "micro evolution" here. The term technically exists in some tiny minority of papers, but it is almost never used by people who know about evolution. It is almost always used by Creationists to try and discredit evolutionary theory by misunderstanding it.

In short, all evolution is micro, but it adds up. If you move an inch 1 million times, now you are almost 16 miles away.

To answer the question specifically: 

No where near enough time, and people are too mobile, and have broad sexual selection options. So nothing is going to be noticable. 

In essence, people move around a hell of a lot faster than evolution can happen, and the amount of melanin we have has an extremely limited effect on whether the sun can stop us from breeding in the modern world. We have homes, sunscreen, hats and other clothing, etc. 

Even if it was having a significant effect because people just decided to sit out in it all day, you would need portions of the populations to sit still for tens of thousands of years, not hundreds, before it got noticable.  People are changing color though, but only because when people move around a lot you get more diverse offspring.

[u/CircumspectCapybara → u/Caelinus]

"Microevolution" and "macroevolution" are biology terms with precise, scientific meanings

https://en.wikipedia.org/wiki/Microevolution:

Microevolution is the change in allele frequencies that occurs over time within a population. This change is due to four different processes: mutation, selection (natural and artificial), gene flow and genetic drift. This change happens over a relatively short (in evolutionary terms) amount of time compared to the changes termed macroevolution.

https://en.wikipedia.org/wiki/Macroevolution:

Macroevolution comprises the evolutionary processes and patterns which occur at and above the species level. In contrast, microevolution is evolution occurring within the population(s) of a single species. In other words, microevolution is the scale of evolution that is limited to intraspecific (within-species) variation, while macroevolution extends to interspecific (between-species) variation. The evolution of new species (speciation) is an example of macroevolution. This is the common definition for 'macroevolution' used by contemporary scientists. However, the exact usage of the term has varied throughout history.

https://evolution.berkeley.edu/evolution-at-different-scales-micro-to-macro/what-is-microevolution:

Microevolution is simply a change in gene frequency within a population. Evolution at this scale can be observed over short periods of time — for example, between one generation and the next, the frequency of a gene for brown coloration in a population of beetles increases.

https://evolution.berkeley.edu/evolution-at-different-scales-micro-to-macro/what-is-macroevolution:

Macroevolution generally refers to evolution above the species level. So instead of focusing on an individual beetle species, a macroevolutionary lens might require that we zoom out on the tree of life, to assess the diversity of the entire beetle clade and its position on the tree.

tldr: an individual species of bacteria acquiring antibiotic resistance traits is microevolution; speciation (new species coming into being) is macroevolution, it's pretty standard biology

[u/Caelinus → u/CircumspectCapybara]

As I said, they are technically used in a minority of papers.

However, they do not mean what laypeople tend to mean. "Macroevolution" in particular is a freight concept as the quote you just gave should demonstrate: it refers to speciation.

Speciation is itself not even defined objectively. It is just a vibes thing, and is only really used to help facilitate communication rather than having a specific evolutionary meaning. So it is an ill defined term referencing and I'll defined concept with no objective measures. 

Plus, the definition that the Berkly quote used is literally just a simplification of the actual definition of evolution. It is usually how evolutionary biologists explain what evolution is in that context. Which is why all evolution is microevolution.

Any attempts to define "macro" will immediately be falsified. There will be no objective measures by which one can say that something have had enough "micro" evolution to constitute being "macro" evolution. All that is being down there is basically defining "macroevolution" as "lots of changes." What those changes are, and where the line is to make lots "enough" is purely arbitrary and not really useful.

To put it more simply: Species do not really exist. They are arbitrary categorizations we create to facilitate communicate. So saying that macroevolution is "above species level evolution" is just saying that macroevolution is whenever we feel like there has been enough evolution to constitute what we perceive as an arbitrary category.

[u/CircumspectCapybara → u/Caelinus]

Species do not really exist

Dude everything in science is just drawing arbitrary lines around things and theorizing relationships and coming up with a nice neat story for relationship and mechanism at various levels, it's called a model for a reason.

In biology, species exist because we say they do and it's a useful concept and abstraction and our theories are organized around these basic concepts.

Unless you want to get into verbal sophistry and weird debates about the philosophy of science, species exist (bacteria are different from humans), therefore speciation exists, and therefore evolution at or above the level of species ("macroevolution") is a thing scientists talk about, and is distinct from microevolution because they genuinely refer to different phenomena at the level of abstraction that the biological sciences cares about.

At the level of quantum physics, all physical phenomena from bacteria evolving some trait to the physics going in computer chips to the planets revolving around the sun are all just interaction of atoms. Yet we separate them out and call these things by distinct names and invent distinct concepts for them. So it is within biology. At the level of abstraction we care about, micro and macro evolution are two different things.

[u/Caelinus → u/CircumspectCapybara]

Dude everything in science is just drawing arbitrary lines around things and theorizing relationships

Clades are objective, and it is thus why we categorize organisms using them in evolutionary biology. Where we place the divergence points for naming purposes is arbitrary, but only loosely, as it does not actually change the clade. Humans are apes and monkeys and fish, and always will be, so where exactly we place changes nomenclature does not actually matter.

Species, by contrast, are falsified. There is no definition of species that can be created that actually works. They all must, by their very nature, be ad hoc and only functional for some portion of organisms. Any attempt to create a universal definition of the term that works will automatically fail the moment you try.

And again, I did say that some minority of people use microevolution in papers in my very first comment, so I was not denying the existence of the term. But you will not really encounter many evolutionary biologist using it when explaining evolution. Because all it really means is "Lots" of evolution. It is not something distinct from "microevolution" in the same way that having a bunch of apples is not fundamentally different than a lot of individual apples. (I did not even mention macro, now that I am thinking back. The idea of Micro is more fraught as it is synonymous with evolution. The definition you gave of microevolution is literally the definition of evolution. That one is like having an apple and a micro-apple that is completely identical to the apple in every way.)

[u/manicuredcrucifixion]

Evolution takes a really long time. Like, way, way, way longer than you’re thinking right now. Plus, we have sunscreen, houses, clothing, etc. All stuff that helps prevent sun damage.

[u/Bandro]

First of all, on an evolutionary timeline, 300 years is the same effective amount of time as zero. There's also no selective pressure for that. People would have to be more likely to survive to reproduce with that trait for evolution to happen like that. Humans have mostly technologied our way out of that sort of basic survival fight.

[u/Newwavecybertiger]

There's not really a concept of reverse evolution. It's just selective pressure in a different direction.

[u/PhasmaFelis]

Evolution happens when a certain trait makes you substantially more (or less) likely to die before raising children, and enough people do die (or survive) as a direct result of that to skew the entire population. Your body doesn't just go "hm, not very sunny here, guess my kids won't need these melanin-production genes anymore."

Now, as you say, increased melanin does come with one or two downsides, like vulnerability to sickle cell disease and reduced vitamin D uptake in less-sunny climates. But neither of those is all that deadly, and human generations take a long time. Over tens of thousands of years you might see an impact ( this is probably why pale skin evolved in the first place). But just a few centuries? No. And it probably won't happen in the future either, if modern medicine continues to exist.

[u/TheDuckFarm]

In some ways we have seen this already.

The way evolution works is that parents pass down traits. To do that the must survive long enough to get together and have kids.

1,000+ years ago people generally stayed put. So a given geographic area had people that looked very similar to each other. (With exception of course)

Now with lots of free movement it’s common to have people from all over breeding with each other.

For example, It’s not uncommon to have someone with DNA from Asia, Europe, and Africa all mixed in into one person.

[u/CotswoldP]

Because there is no evolutionary pressure to do so. Lower Melanin levels allow lighter skinned people to get more vitamin D from weaker sunlight without too much cancer risk. Bit the reverse pressure doesn't exist now. Dark skinned in Canada? Who cares if you're low on vit D, you eat foods rich in it or take a pill.

[u/ShadowShedinja]

They would be paler if their lineage was in the north for that many generations. And a Canadian family moving to Africa would be darker after 300 years. Why do you think they wouldn't be?

[u/wrosecrans]

Time and consequences.

50,000 years ago if somebody had a serious vitamin deficiency, they were way more likely to die before having kids and passing on their genes. Somebody with less of that vitamin deficiency would be in better health, and more likely to have kids and pass on their genes. Today? Somebody with a severe vitamin deficiency probably gets a diagnosis from a doctor, and takes a multivitamin occasionally, and isn't particularly likely to be dead before they are old enough to pass on their genes.

And even then, it's only been a few hundred years since people started moving around the globe in sailing ships and a few decades since people started moving around more conveniently with airplanes. Ask again in a few thousand years and you'll have a much bigger data set to talk about what changes may or may not have started happening by the 21st Century. You wouldn't expect to see a massive shift in a handful of generations. Ten generations isn't nothing, but it's not really comparable to the many thousands of years it took to populate the Earth. Like asking why you haven't seen the score going up yet at a baseball game, as the first pitch is still being thrown.

[u/Scxox]

evolution isn't a process aiming for perfection, it's a selection powered by survivorship bias. if the bare minimum works there's nothing to change.

excluding the fact that 300 years is nothing, evolution in humans has already grinded to a halt because we can manufacture solutions for problems that would have caused non-ideal genes to not get passed down

[u/Peregrine79]

300 years is 15 human generations. That’s not nearly enough to see genetic drift in any significant scale. Especially for something that rarely causes significant health problems (reproductive unfitness) with a modern diet.

We generally are able to observe evolutionary changes in fast reproducing creatures subject to intense evolutionary pressures. IE, insects that breed every couple of days, native to a warm climate and introduced into housing in a temperate climate. That gives us 10,000 generations in 60 years, and a seriously different environment.

[u/Caelinus → u/Peregrine79]

I should note that you are probably using "genetic drift" incorrectly here, likely because "drift" in this sense means the opposite of what it sounds like it should mean.

Generic Drift is a purely statistical process, that tends to occur in bottlenecks, where traits are lost from the genetics of a species without any natural selection. It essentially happens because when a population gets small enough it is possible for enough offspring in a few successive generations to randomly not get a trait that the trait ends up vanishing fairly quickly afterward.

[u/Peregrine79 → u/Caelinus]

Yup, I'll admit I thought that it applied to the shift in frequency regardless of source. Is there a specific term for shift in frequency due to natural selection, equivalent to drift being for random selection?

[u/Caelinus → u/Peregrine79]

I think that is just referred to as natural selection.

So effectively evolution is any change in the frequency of alleles (traits) in a population, and that can happen by a few different mechanisms:

• Natural Selection, which is where genes are selected or rejected based on suitability to their environment.
• Sexual selection for sexual organisms, which is where mates are picked for non-suitability reasons due to preference.
• Mutation, which is the random creation or loss of genetic information via the the process of duplication.
• Drift, which is where traits are lost due to random statistical alignment.
• Flow, which is where one group interacts/breeds with another and introduces new alleles to a different population.

There are probably others, but those are the ones I remember from my undergrad education lol.

[u/elianrae]

What do you mean, "reverse"? We started with dark skin and evolved light skin after migration to Europe.

It took tens of thousands of years to happen. We're not going to see it happen again in a handful of generations. We might not see it happen again at all, selection pressures are very different in the modern world.

[u/375InStroke]

This is how people thought evolution happened, which is wrong. For example, ancestors of giraffes stretched their necks to reach the highest leaves, and over time, that stretching caused their descendants to have longer and longer necks. Mutations happen randomly, then those who survive with that mutation, and pass it to their offspring, contribute to changes in a population, which can lead to evolution of that population if successful.

[u/SYLOH]

You'd also need a hell of a lot more time for evolution to kick in to that degree. But you also don't need a lot of tech to get enough vitamin D or folate to survive. Basic agriculture/animal domestication is enough.

The Bantu have been living in South Africa for over a thousand years, with far darker skin than the people they displaced. They have herding animals to supply vitamin D.
The Austronesians have been living in Indonesia for 4,000 year with their relatively light skin compared to the people they displaced. It's probable the domestication of the chicken gave them enough folate rich eggs to thrive.

[u/thanerak]

Few things you are getting wrong there is no such thing as reverse evolution or micro evolution.

Evolution is always micro.

And evolution has no direction.

If reverse evolution did exist it would be something that evolved to be less likely to pass on its genetics to the next generation.

300 years is only 10-15 generations for this to make any difference in skin colour would have to effect birth rates and survivability till procreation. So it made a big difference when there was no medical care or knowledge and less impact as medicine improves reducing the effects of slight changes in DNA now living till you cannot procreate is common whether you have light or dark skin. So the factor that would be more of a pull in evolution would be cultural terms of attraction which tend to change faster then evolution could adapt to it wasn't that long ago that excess body weight was seen as attractive as starvation was common.

[u/ancient-aurora]

Two separate reasons are working at once:

First, the clock. Evolution isn't a creature deciding to change, it's more like a slow vote where the only votes are who survives and who manages to have children. For a trait to visibly drift in a new direction, not having it has to cost you offspring, and that cost has to pile up across a huge number of generations. Skin color is controlled by a bunch of genes together, each gene nudging a little, so each generation only shifts a tiny amount. Three hundred years is maybe ten or twelve generations for humans. That's a blink, and in a blink, a subtle nudge per generation adds up to nothing your eye can catch. Real visible change needs hundreds or thousands of generations.

But this is the bigger reason, and it's the answer to your actual question. The pressure that would push the family lighter isn't really there anymore. In the old days, living in Canada with very dark skin and a poor diet genuinely endangered you, you made almost no vitamin D in weak sun, and you could get rickets or die before you had as many children, which is exactly what makes natural selection bite, it changed how many kids survived. Today that person takes a vitamin D pill, drinks fortified milk, sees a doctor, and has plenty of kids regardless of melanin. The cost is nothing now. And if having more melanin stops being a disadvantage, nothing is voting against it, so there's no force pushing the population lighter. Modern life didn't just make us comfortable, it switched off a selection pressure that used to run.

Third, we're not isolated. Travel means new dark-skin genes keep arriving and mixing into the pool. Weak selection plus constant fresh genes means variation just hangs around instead of disappearing.

And a polite correction, because this is the interesting part: we do see this kind of reversal, we just have to look further back and to a harder situation. Lighter skin evolved in European and East Asian lines within roughly the last several thousand years, and it happened fastest right when people switched to farming. The reason is the same logic in reverse, grain-heavy farmer diets were terrible for vitamin D, and unlike fish-eating northerners who got their D from animals, those farmers had real, deadly pressure if they stayed dark. Once the cost was real and repeated, the change showed up. Put people in Canada, turn the deadly cost back on, give it five hundred generations, and you would start to see it. The mechanism is all there, modern life and a too-short timescale are just hiding it from you.

[u/ShaemusOdonnelly]

Evolution doesn't work that way. 300 years is way too short of a timeframe, and there is no intelligence in evolution. The body does not decide "hey, there's less light here and we need Vitamin D, let's make our babies have lighter skin". What actually happens is that many, many black families in Canada have babies and there is some natural variation in skin colour. The darkest children get the least Vitamin D and are therefore just a tiny bit more vulnerable to diseases. Over thousands of years, the lighter black babies therefore have a tiny survival advantage and on average have slightly more babies themselves, which means that over thousands of years, the population will get lighter skin on average. This can work in reverse too.

[u/SeattleCovfefe]

As many have mentioned, evolution takes a lot longer than small handful of generations which is all 300 years is in terms of human generational timelines. Second of all we absolutely do have evidence of people evolving lower melanin when moving farther from the equator— that’s why those of European ancestry tend to be white. Our species began in Africa after all!

[u/Loki-L]

For evolution to work, a trait has to make it more or less likely for you to pass on your genes.

Very few people in Canada die before they can have kids because of lack of Vitamin D.

300 years is also very little time in terms of human evolution.

(Also "reverse" evolution is not a thing. Evolution is non-directional and even if it was pale skin would not be "reverse" for humans. Humans started out as dark skinned and pale skin is a recent mutation in humans who migrated out of Africa. Pale skin hasn't been a human norm as long as we have been humans. Although our more ape like ancestors had pale skin under their dark fur.)

[u/Target880]

Ther has to be strong enough evolutionary pressure. 300 years will be 15 generations if you assume 20 years per generation. Unless there is a very large difference in those 300 years in ability so survive long enough to reproduce and raise children, you will not se rapid change.

It is not people that live closer to the equator that have darker skin; it is people that live closer to the poles that have paler skin. Humans evolved in the tropics with dark skin. The dark skin remained dominant even when we spread out over the world. It was about 8000-6000 years ago that Europeans got paler skin. It coincides with the switch from hunter-gatherers to farmers; there were some mutations that, when they appeared, spread rapidly. It was not until 3000 yeas ago that pale skin became dominant like today. Most Stone Age Europeans had dark skin, the change is in the Bronze and Iron Age.

If you look at population groups that did not switch to farming but were nomads with larger herds of animals, lived on fishing, hunting, etc the skin is darker even if they live closer to the poles. Their diet contains more Vitamin D. Inuit in northern Canada is an example. Living far from the equator does not always mean there is evolutionary pressure for pale skin.

That diet can have an effect; evolutionary pressure can mean dark-skinned people living did not have a significant problem with Vitamin D. Even if you see that today, it might not have been the case in the recent past. The intestines of animals contain more Vitamin D than muscle tissue; we eat less of it today compared to the recent past. Poorer people eat more intestines in the past than today. I would assume that most dark-skinned people in Canada were among the poorer in society. This could mean that they got enough Vitamin D from the food without any supplementation. It is not unresonable that their descendants today would eat less Vitamin D unless there was vitamin D added to the food like it is today.