How would evolution occur here?

[Speed]

Suppose it turns out that humans with a set of eyes behind their head, or humans with 2 pairs of hands were better than current morphologies.

How would evolution even begin to evolve humans with such features? We don't have any mutations that can produce something remotely close to a second set of eyeballs or limbs.

 

[DaveC426913]

I advise you read up on evolution a bit more. Evolution doesn't go looking for better arrangements.

Evolution in a very small, simplistic nutshell:

Very small changes happen all the time - accidentally and randomly.
Usually the changes have no real effect.
When they do, they most often they cripple or kill the recipient, or merely just put them at a disadvantage in the breeding race. On the whole, they are less likely to breed offspring.
Very occasionally, a recipient will marginally do better in the breeding race. That gives them a marginally better chance of increasing their numbers in future generations.
Now multiply by about 100,000 generations.

As a real world example, frogs have a mutation that, under the right circumstances (in the presence of certain chemicals), produces excess limbs. This almost always is useless or detrimental. But after eons of this, and in combination with other mutations, frogs may one day produce a combination that they can make use of - they may manage to actually survive a little better than their mere four legged counterparts. Their numbers will increase. Mroe millenia, and we have two frog families: four-legged and six-legged.


The key to appreciating evolution lies in grasping the huge numbers involved. Millions of years, thosuands of generations, countless individuals, slow gradual pace (for the most part),

 

[gerben]

Suppose it turns out that humans with a set of eyes behind their head, or humans with 2 pairs of hands were better than current morphologies.

How would evolution even begin to evolve humans with such features? We don't have any mutations that can produce something remotely close to a second set of eyeballs or limbs.

Yes, so I think you can predict pretty safely that something like that will not occur.

The more complex animals that exist nowadays all have four limbs (mammals, birds, reptiles, amphibians, fish). The idea is that this is the case because they all evolved from the same ancestor long ago, a fish which had already the starting points to built legs and arms from (namely a pair of pectoral fins and a pair of pelvic fins). If you want to think about a new species that has more limbs (or more eyes) I think you should start to think about some future species that has as its ancestors something that is still quite primitive today, perhaps after a billion years (via some fish-like stadium, amphibian stadium etc.) it might lead to something human-like with a different anatomy from modern day humans.

 

[Phobos]

Evolution in a very small, simplistic nutshell:

Just a footnote that evolution is not just a testing of random new features. It's also works on existing 'normal' variations within a population. The mutations you referred to essentially add more variations to the whole population.

 

[Phobos]

Suppose it turns out that humans with a set of eyes behind their head, or humans with 2 pairs of hands were better than current morphologies.

How would evolution even begin to evolve humans with such features? We don't have any mutations that can produce something remotely close to a second set of eyeballs or limbs.

In general, new features are developed over vast time periods by modifying existing anatomical features (e.g., if humans became tree-dwellers over millions of years, then perhaps our feet would become more hand-like).

Another way is through the mutation of genes that control the development of embryos. For example, there are genes that control how/when an anatomical feature is developed in an embryo. If that gene were duplicated or modified, it could repeat its commands (saying "do X" twice) or change the timing/duration of the commands it sends (making X appear differently) (where X is some developmental feature).

 

[Janitor]

I have read that some people--and apparently it's not that tiny a fraction of the population--have extra nipples. Granted, a nipple is not as complicated as an eye...

 

[so-crates]

How would evolution even begin to evolve humans with such features? We don't have any mutations that can produce something remotely close to a second set of eyeballs or limbs.

First of all, be careful with your wording - don't anthromorphize complex concepts. Evolution doesn't 'begin' or 'want' anything - it is merely a process by which mutation and natural selection and other mechanism lead to change within species and new species altogether. Alot of people make this mistake, most likely because some popularizations of science play loose with their terminology and perhaps get too philosophical without sticking to essentials, and as a result people misuse terms.

Humans with four eyes "might" have some advantage over humans with two. But since that variation is not available in the population, it is not selected for and therefore not evolved. It would be an incredibly complex mutation for a primate to have eyes in the back of their head, so unlikely that it probably would not happen without some sort of genetic engineering. It would not be so bad if you were talking about, say a fly, but for a primate the difference in genome would be pretty substantial. In a similar vein, when features of an organism become useless (such as appendix in modern humans, eyes in certain blind nocturnal creatures) it is more likely that for the features to become <i>vestigal</i> (shrunken) than to disappear altogether, because it is easier to 'tweak' a gene than to eliminate it altogether. There is still much that is not completely understood about genes, especially with regards to regulation how they affect organism development, but enough is understood to draw simple conclusions.

I remember reading an example about the two-horned vs. the one-horned rhinoceros. Biologists couldn't figure out if two-horns really had a specific advantage over one-horned rhinoceroses. Both seemed to fend off predators equally well. The answer is probably that the effect was random - it just so happened that in one region, a one-horned mutation arose, while in the other region, there was a two-horned mutation. Once this mutation was present, it was selected for.

 

[Speed]

Thanks for your answers all. What I am trying to ask is how evolution produces large changes in a population, where the individuals would be mostly similar to each other. I know some posters have commented on the implausibility of an "evolutionary goal" - however for the sake of this discussion let us assume that a large, complex organ previously unheard of is required in an evolving organism. How would it get it?

In closed populations, the only source of new traits is through mutation. However, four things seem to prevent evolution producing large morphological changes in populations via mutation:
1) Lethality of most mutations;
2) Underrepresentation of mutants that do happen to be fitter than 'normals';
3) The exceedingly tiny probability that a string of mutations would lead to a large, constructive result (such as a 2nd pair of eyes or limbs);
4) The intermediate mutants may be more unfit than the end mutants or even the normals. Suppose a 2nd pair of arms would prove disproportionately beneficial to humans. Precursor humans with vestigial growths in say their ribcage to support this would waste biological energy supporting these growths without the benefits of the 2nd pair of arms whatsoever. In other words, these "semi-mutants" would be worse off than even the normals.

So it would appear, especially with #4, that a gradual change towards the evolutionary goal is not possible.

I don't mind that for something such as this to happen, thousands of generations are required. I would just like to hear an explanation of how a string of mutations can eventually lead to something macroscopically beneficial.

 

[Phobos]

What I am trying to ask is how evolution produces large changes in a population, where the individuals would be mostly similar to each other.

The accumulation of lots of "micro" changes (not necessarily something new, but also via modification of an existing feature).

I know some posters have commented on the implausibility of an "evolutionary goal" - however for the sake of this discussion let us assume that a large, complex organ previously unheard of is required in an evolving organism. How would it get it?

But now we're outside the theory of evolution. Organs are not built for future use. Each "micro" step needs to have a beneficial/neutral quality for current needs.

In closed populations, the only source of new traits is through mutation.

Also through recombination (reshuffled genes through sexual reproduction).

2) Underrepresentation of mutants that do happen to be fitter than 'normals';

Check out "allopatric speciation"...basically the idea that evolutionary changes get a better foothold in an isolated subgroup on the fringe of the main population's range.

3) The exceedingly tiny probability that a string of mutations would lead to a large, constructive result (such as a 2nd pair of eyes or limbs);
4) The intermediate mutants may be more unfit than the end mutants or even the normals.

Again, evolution is for the here and now, not for future planning.

Suppose a 2nd pair of arms would prove disproportionately beneficial to humans. Precursor humans with vestigial growths in say their ribcage to support this would waste biological energy supporting these growths without the benefits of the 2nd pair of arms whatsoever. In other words, these "semi-mutants" would be worse off than even the normals.

A new arm would not be likely evolve that way (piecemeal in a new location). Either the existing arm-creation genes would be doubled via a macromutation or an existing feature would be modified (e.g., legs become arms over time).

It's a common misconception to say that these half-features show that evolution is impossible. But, for example, when animals went from water to land, they did not do it with water-only fins and then wait to evolve. Fish evolved modified fins while still in water (e.g., perhaps as bottom feeders or shallow-water fish) and those types of fish were able to make brief journeys onto land which others could not. Once there, further evolution could occur toward land-limbs. Similarly, birds did not wait in the trees for millions of years until their wings were complete. Instead, their pseudo-wings had some immediate benefit (e.g., balance while running/jumping, or gliding from tree to tree, or for warmth or displays, etc.)

So, the feature starts evolving first for some immediate benefit, then it is used for a slightly different purpose*, then it is modified further for the new situation.

* Consider human hands. Their primary purpose may be for grasping, but they can also be used for swimming, fighting, digging, communicating, building, etc.

 

[Janitor]

A tail is not an eye, but I just happened upon a debate at the site http://www.pbs.org/wgbh/nova/odyssey/debate/index.html
where the claim is made:

Indeed, the most recent issue of Science (Nov-15, page 1082) reported that a single gene controls tunicate tail formation. Mutate it, the tail is lost. Restore it, tail comes back. Just another example of a genetic mechanism producing macroevolutionary change.

I am not necessarily claiming this has much to do with the O.P., so take it for whatever it is worth.

 

[Phobos]

similar to Janitor's post...(but perhaps getting off topic)...
also check out "atavisms" (sudden reappearances of past features due to gene recombinations/mutations)