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[help] Question about human evolution - Lakeshore — LiveJournal
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Jay Lake
Date: 2010-10-03 10:19
Subject: [help] Question about human evolution
Security: Public
Tags:help, science
Is 25-50 generations sufficient elapsed time for a human population under significant selection pressure to exhibit significant phenotypical variation?

I'm assuming something like a lost colony scenario, and/or multigenerational space farers. I'm also not looking for 10 foot tall green men or anything like that, but more along the lines of meaningful changes in height, build, coloration, body mass, musculature etc. such that current extreme expressions might become more common or even typical.

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Ulrika
User: akirlu
Date: 2010-10-03 17:30 (UTC)
Subject: (no subject)
Okay, well this is totally not my area of expertise, but I thought that the most recent thinking was that phenotypic variation within a population tends to crop up when that population is not under selection pressure, and that what happens under pressure is selection from among that available crop of variations for those best adapted to the new conditions.
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jackwilliambell
User: jackwilliambell
Date: 2010-10-03 18:15 (UTC)
Subject: (no subject)
What she said. This is one of the underlying principles of the population theorem of evolution: that within a given population there is already significant genetic variation and, if survival conditions change, one or more of those variations will be expressed in greater numbers because they now promote survival.

So, if you have one family that already has the traits you are looking for, within a fairly small number of generations those traits will be spread throughout the population. However, the other (originally more numerous) variations will continue to be expressed from time to time in the population, just in case conditions change again.
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Rovanda
User: rovanda
Date: 2010-10-04 18:03 (UTC)
Subject: (no subject)
Agreed - my understanding is that mutations happen at random.

Mutations that kill the organism before they breed tend to be weeded out of a population unless they're recessive and tied to a non-harmful or beneficial trait.

Mutations that give an organism an advantage in survival and/or breeding are likely to spread quickly as that organism's offspring come to dominate a population.

Mutations that don't help or harm become part of the natural variation of a population. (For instance, I don't have any frontal sinuses, but that doesn't affect my survival in current conditions at all.)

In some ways, the last type are the most interesting, because a mutation that is neutral in one set of environmental conditions may become a benefit or detriment when conditions change. If it's a detriment, the carriers die off, but if it's a benefit, then suddenly the carriers are the only ones who can thrive and the mutation might come to define a population in just a handful of generations. The change in environmental conditions might come because of a new location, climate change, or mass extinction leaving all sorts of niches open for new species to fill.

I've been toying for a while with the idea that populations in higher radiation areas might be more adaptable because of a higher rate of mutation, but the problem is that the rate also needs to be low enough that harmful mutations don't kill off the whole population.
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Panzertron The Ruiner
User: panzertron
Date: 2010-10-03 17:34 (UTC)
Subject: (no subject)
I would think that some of those things would be as dependent on the environment/pressures as they are on genetics. For example, someone born on a high-gravity planet would naturally grow up shorter and stockier than someone from a normal-gravity environment (even in the first high-gravity-born generation). I think adding more generations would only serve to "set" some of these kinds of changes more firmly into the population's development.
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Autopope
User: autopope
Date: 2010-10-03 18:14 (UTC)
Subject: (no subject)
Really?

The only work on high-gee environments I know of is Arthur Hamilton's work on chickens at the Chronic Acceleration Research Laboratory in the 1970s (quoted by Ed Regis in "Great Mambo Chicken and the Human Condition"). Yes, the chickens that grew up in a 2.5G environment were crazy-muscular, had low body fat levels, and were strong. But humans aren't just scaled-up chickens; the square-cube law is going to have an impact.

I'd expect a crazy-high level of joint troubles among humans in such an environment, including fatalities (fall over as an adult, break your neck) and mere crippling injuries (joint capsule blow-outs and spinal discs). Cardiovascular function is also going to put stress on the system -- expect lots of hypertension-induced deaths, with strokes a common failure mode in the mid-30s to mid-40s.

And it's not clear that simple selection is going to help. Such a high-gee society is going to have to breed young -- and accept very high childbed morbidity and mortality and high infant mortality -- but is probably going to have reduced life expectancy at the other end. If folks die young, but only after reaching reproductive status, then the traits leading to early death (joint and cardiovascular weakness) aren't going to be selected out.
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seventorches
User: seventorches
Date: 2010-10-04 03:22 (UTC)
Subject: (no subject)
Then you have to add a condition that rewards long life after reproduction. In fact, strong joints and hearts would tend to do that because there would be more offspring of those people over time. if I lie down in childbed and don't die because I have better survival traits, I am more likely to have more children later on, who will follow and follow. Or am I misunderstanding the concept?
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Autopope
User: autopope
Date: 2010-10-04 13:24 (UTC)
Subject: (no subject)
Yes, you're misunderstanding how selection pressure works. Once you've spawned, the genes have no further use for the elaborate meatbody they construct in order to replicate themselves (over-simplifying Dawkins-era selfish gene theory enormously here). If you breed at age 15-39, the genes that give you a predisposition for dying of a stroke at 40 or having all your joints blow out by 50 are not removed by natural selection.

(There is a loophole available for updated kin-selection theories -- that having grandparents around aids the reproductive fitness of grandchildren in some way, e.g. by helping share the workload of parenting -- but I don't see that helping with this particular category of evolutionary maladaptations unless we posit that only grandparents can successfully carry out bypass surgery, joint replacement, or antihypertension treatment on their grandchildren.)
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Samantha Henderson
User: samhenderson
Date: 2010-10-03 17:35 (UTC)
Subject: (no subject)
I have heard that it took surprisingly few generation for humans moving from Africa to Europe to have a change in pigmentation.
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patches: silly
User: 7patches
Date: 2010-10-03 17:42 (UTC)
Subject: (no subject)
Keyword:silly
It only works if the result is someone who the opposite sex wants to have sex with.

Edited at 2010-10-03 05:42 pm (UTC)
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Brent "Chip" Edwards: knowledge
User: chipuni
Date: 2010-10-03 17:43 (UTC)
Subject: (no subject)
Keyword:knowledge
Because you say "Under significant selection pressure", yes: Look at the differences between people of Iceland and people of Scandinavia.

I'm guessing that you mean "significant phenotypical variation" from the original population... not among each other. There normally wouldn't be any phenotypes that didn't already occur within the original population. But phenotypes that are badly adapted for the new environment would die out in that time.

(Important note: I've only worked in a genetics department for six years, and taken college classes in it. I am not a biologist.)



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shelly_rae: Ishmael
User: shelly_rae
Date: 2010-10-03 17:49 (UTC)
Subject: (no subject)
Keyword:Ishmael
Environmental factors are probably more important for the characteristics you suggest than are genetics. Most humans have the genes to be tall--just look at Asians who settle in the USA within just a few generations their children are as tall as other Americans. IF their diet is Americanized. Much coloring has to do with sun exposure. Muscles due to how much they are used.

True environmental genetic changes take longer. But I believe that 25-5- generations in a limited gene pool would certainly make changes. There's an island of people who mostly have 6 fingers and toes. Limited population and successful breeding of first "mutant".

You'd probably need to look at the genetic markers for diseases like sickle cell to learn more. Or perhaps things like polydactyl cats. :)

That's my brain dump before I go to work. And I'm a medievalist not a geneticist so YMMV.

Anon
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slweippert: Me July 2010
User: slweippert
Date: 2010-10-03 17:50 (UTC)
Subject: (no subject)
Keyword:Me July 2010
I had a short story that I did light research in this area. (by light I mean three separate web sites) I found it would happen faster than 25 generations if tied to survival chances. Unless the variation you're exploring is *not* tied to reproduction chances, i.e. gray eyes. Then the range is closer to what you suggest to percolate through the genome.

DISCLAIMER: I am not an expert at this and someone with a degree in genetics may tell me I'm out of my tree. :)
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Dave O'Neill
User: daveon
Date: 2010-10-03 18:08 (UTC)
Subject: (no subject)
Disclaimer: not my field either.

But the rate of change of certain features in a human population just through diet and other changes can be dramatic and lead to a degree of self selection.

You only have to look at the height of most Western European and Asian populations since the end of WW2. I'm 7 inches taller than both of my parents and 4 more than my brother who was born a decade earlier than I was. I visit college towns in the UK and find that at 6-4 I'm fairly average in height compared to the rest in the room.

You could probably sort out various pigmentation issues pretty quickly in a mixed human population with no concept of miscegenation.
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dinogrl: monkee micky monkey around
User: dinogrl
Date: 2010-10-03 18:20 (UTC)
Subject: (no subject)
Keyword:monkee micky monkey around
Not my obvious area of expertise either, but I took coursework in anthro. What most people have said above. Adaptations occur quickly. Environment is everything. You may want to look at a strange (and somewhat bizarre) book called _Man After Man_ by Dougal Dixon. I went to one of his lectures on dinosaur cladistics (he's as boring as they come), but he has an interesting take (somewhat dated now) as to future generations.

http://en.wikipedia.org/wiki/Man_After_Man:_An_Anthropology_of_the_Future

Edited at 2010-10-03 06:21 pm (UTC)
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martianmooncrab
User: martianmooncrab
Date: 2010-10-03 18:52 (UTC)
Subject: (no subject)
on my ancestors discharge papers from the GAR, he was five foot six, which was average height for a man in the 1860's, most of the folks in my family are about the six foot mark now, which is from better nutrition.
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Kenneth Mark Hoover
User: kmarkhoover
Date: 2010-10-03 19:32 (UTC)
Subject: (no subject)
^This.
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Keikaimalu
User: keikaimalu
Date: 2010-10-03 19:07 (UTC)
Subject: (no subject)
Yes, 25-50 generations are absolutely enough for significant physical changes in a species. Just look at how dog and cat breeds have changed in the last 50 years, due to selective breeding. Traits have become exaggerated to the point of absurdity -- modern-day bulldogs can no longer deliver puppies naturally because of their huge heads; they have to be delivered by C-section.

Not sure it's enough time to develop a separate species that couldn't interbreed with an isolated population from before that 25-50 generations, but physical changes? Hell, yeah. Even 5-10 generations would be enough for some changes.
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miki garrison: egg and sperm
User: mikigarrison
Date: 2010-10-03 19:23 (UTC)
Subject: (no subject)
Keyword:egg and sperm
Possibly, depending on:

1) What you mean by "significant selection pressure" -- do you mean that only 70% of individuals born are able to successfully breed? 50%? 10%?

2) Whether the genes for the traits you are interested in are co-located with those for deleterious traits.

3) How big of a population you're working with -- if it's too small, you can't get the selection pressure tight enough without serious inbreeding.

That said, you can get significant changes in height, body mass, and musculature without genetic changes at all -- environment and behavior really do play key roles there, not only their new environment but the one they are coming from as well. However, with or without genetic changes, all three of those require easy access to lots of food, and protein-rich food at that -- which I would not usually find plausible in a lost colony scenario.
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miki garrison: egg and sperm
User: mikigarrison
Date: 2010-10-03 19:46 (UTC)
Subject: (no subject)
Keyword:egg and sperm
More on the food availability thing:

* Without readily available food (i.e., at least as much access as they previously had to food for a given energy cost, and ideally more), even those who have the genotype that allows for increased body mass, musculature, and height will not end up showing those characteristics in their phenotype. And if they're not expressed, they can't be selected for.

* Because those genotypes are associated with increased total energy needs to reach from birth to breeding, selecting for them can be harmful to the group's survival as a whole unless food access is so easy that it doesn't matter. Anthropologically, there are situations like this where a group selected for traits that were associated with obvious short-term gains (i.e., big guy can bring in bigger game), but it ends up being a net loss for the group as a whole (the increased energy needs of the whole group end up being more than the marginal difference in the food brought in).
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Kenneth Mark Hoover
User: kmarkhoover
Date: 2010-10-03 19:30 (UTC)
Subject: (no subject)
I would think 25-50 generations is more than enough to see significant changes. And the environmental/social pressures impacting the colony don't necessarily have to be unduly harsh, either. Small changes in living conditions may bring about noticeable differences over time.
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V's Herbie: ionic bond
User: vsherbie
Date: 2010-10-03 20:19 (UTC)
Subject: (no subject)
Keyword:ionic bond
Something that is my field! yay!

So, there's really two things going on at once. Thing one, the founder effect, thing two, selective pressure.

On the scale you're talking about, the founder effect would likely have greater influence. If all your space-going folks are chosen for particular traits, as they would have to be, this artificial selection would have a large impact on the genes you have to start with before selection takes place.

As for selective pressure...
In order for a particular trait to take over a population, it has to increase reproductive success. It doesn't matter how strong/smart/whatever a person is if they don't have kids.

This is a pretty good population genetics primer. You'll be especially interested in the last couple sections on selective breeding and small populations.
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threeoutside
User: threeoutside
Date: 2010-10-03 20:40 (UTC)
Subject: (no subject)
Thank you! I kept reading this thinking there was a term I couldn't remember from my population biology course concerning bottlenecks - the founder effect it is! Much would depend on who the original colonists were, their genetic makeup. I'd think. Others have expressed all the other stuff much better than my antique memories of EB could.
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fjm
User: fjm
Date: 2010-10-03 22:49 (UTC)
Subject: (no subject)
The example that comes to my mind of the founder effect is hereditary deafness in, I think, Martha's Vinyard.
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They Didn't Ask Me: avatar
User: dr_phil_physics
Date: 2010-10-03 20:28 (UTC)
Subject: (no subject)
Keyword:avatar
Definitely have to include local environmental conditions, whether planet or space based. An adult moving to a low or high gravity world will make some adaptations, but their children raised in such a world all their live will grow up differently. Ditto for food availability and quality. Being stranded on a high gravity world, for example, will cause settler adults and those children who don't adapt as well to suffer higher casualties, which will help the selection process by weeding out the less desirable traits. Being stranded on a low gravity world might me that descendants could not walk if brought back to Earth, etc.

Better nutrition has meant that there are many more tall Japanese than there used to be -- fast food is rapidly increasing obesity and heart problems in that same population.

I have notes for a low grav world where the local humans are much taller than their parents (or other new colonists) or those from off-world. So the original housing and work modules are used only by "Newsiders" The locals act like they don't care about ever going off-world, and build houses and furniture to fit them.

Just some thoughts.

Dr. Phil
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fledgist
User: fledgist
Date: 2010-10-03 20:59 (UTC)
Subject: (no subject)
You also have to factor in who does the selecting: Do you have polygyny, for example? In that case you'll shrink the gene pool a bit. What kind of economy do you have? That's going to affect reproductive success also..
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cyborgsuzy: canter
User: cyborgsuzy
Date: 2010-10-03 23:19 (UTC)
Subject: (no subject)
Keyword:canter
It's been several years since I took genetics and evolution, but that sounds like plenty of time depending on the starting conditions.

Strong selection pressure could fix extreme phenotypes in a population in even fewer generations if the initial phenotypes you want were popular enough. Especially if you start out with a genetic bottleneck of some kind. Traits like eye/skin/hair color and height are controlled by only a few loci, and follow simple inheritance patterns, so it'd be pretty easy to fix them if, say, you start with a population that is overwhelmingly the same already.

Add to that some kind of really strong selection pressure (that's where you get to get creative, I guess) for extreme height/whatever, and voila.
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cyborgsuzy: canter
User: cyborgsuzy
Date: 2010-10-03 23:20 (UTC)
Subject: (no subject)
Keyword:canter
And now I see that vsherbie said it better, so yeah. That.
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lt260
User: lt260
Date: 2010-10-04 00:01 (UTC)
Subject: (no subject)
Your question has more answers then a cat has lives. You basically can go in any direction you want depending on what changes are required for your story. Space-faring transformations could be seen in just two or three generations. Differences are observed in astronauts that are in Earth orbit for just a few months. Think of what could happen after a few generations (<6). Check out the following videos:

http://www.youtube.com/watch?v=m4XXP73kuAU preventing muscle atrophy

http://www.youtube.com/watch?v=nQ_puPE5Efg preventing bone loss

http://www.youtube.com/watch?v=brMKdXJN_fE sleeping

http://www.youtube.com/watch?v=oASMk_NNvrg sensory overload

http://www.youtube.com/watch?v=WmIVAz5-iEg environmental controls

A lot can change on a generational ship. As for phenotype changes – did you mean actual genotype changes? The genetic make-up of your starting population does not need to be all that different from the ending population to see significant alterations in the physical appearance of the individuals. Becoming green could be as simple as a symbiotic adaptation with very little to no genetic adjustments necessary. If the symbiont leaked glucose into the bloodstream, there would necessarily be modifications in dietary habits, pancreas activity, adipose deposits, and etcetera. Think of all the social changes that would simultaneously occur. Just a few generations and, presto, an “alien” species.

However, if you are considering going from 24 to 25 chromosomes then you are talking about millions of years (more likely tens or even hundreds of millions). Physical appearances in population can be changed dramatically in far less then 25 generations depending on environmental factors.
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