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It has been said that the human race has basically hit a point where any further evolution is essentially unnecessary: protection from most harmful diseases is provided by medicine, we have very few natural predators, and we are very good at what we do (colonising the planet.)

Is there any sign that human evolution (at least in the first world) is slowing down, or that it has stopped, or are these claims bogus, and we are still evolving at a rapid rate?

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One thing to consider is the theory of punctuated equilibrium, which states that species change is usually very fast in geological time, and the period of change is usually relatively short compared to the lifetime of the species. If this is true (and it's still controversial), then most species would not be in periods of change at any given time, and in particular it would be likely that human evolution would not be producing significant changes. –  David Thornley Apr 7 '11 at 2:00
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There is something fundamentally wrong with the question: it presupposes that evolution has some goal that it tries to reach at a certain speed. It hasn't. Evolution is just the term we use to describe the variety in large populations due to environmental and other changes. It's often only visible in hindsight. In humans, many traits seem to have been selected in recent times and are still in the process of being selected for. Lactose tolerance for instance has been positively selected in the West, but not in Asia. –  Raskolnikov Apr 7 '11 at 11:34
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To jump off of what @David is saying: Your observations are mostly correct, humans have no reason to be changing a lot right now. Probably, we'll never need to physically change ourselves again. At least on the growing horns or scales tip. We solve problems with our brains, not by breeding into them. It is faster that way, selected for, better. However, some cultural shift could create a need. Say if aliens invaded and demanded servitude, killing the strong willed off. Then, being more docile would be selected for. –  DampeS8N Apr 7 '11 at 11:36
    
Depending on one's definition of evolution, you could say that it's speeding up: Note the rapid advances our society as a whole makes in terms of knowledge. –  Lagerbaer Apr 7 '11 at 14:16
    
"protection from most harmful diseases is provided by medicine, we have very few natural predators, and we are very good at what we do (colonising the planet.)" There are many people in developing countries who starve to death, or die from diseases that seem trivial to cure in the developed countries that wont agree with you. –  Ophir Yoktan Apr 8 '11 at 14:59
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3 Answers

up vote 14 down vote accepted

Very short version

It's wrong


There will be only Wikipedia linking here because this stuff is textbook biology. Evolution is an established scientific fact.

It's a complete lack of understanding of how modern evolution works.

Mutation

The best way to understand evolution is by understanding that our genetic characteristics have no physical way to remain static.

Our genetic code mutates via the following mechanisms:

The major sources of such variation are mutation, genetic recombination and gene flow.

  • Mutation occurs because of transcription errors and other radiation or chemically induced mutations happen randomly.

  • Genetic recombination happens as part of the way we reproduce.

  • Gene flow is the mixing of genes that happens when people with very diverse gene sets reproduce -- think about interracial marriage.

The important points to understand here are the following:

  • Changes in our genetic code are random
  • Changes in our genetic code are unavoidable, because the reproduction process is imperfect.

Natural Selection

What happens when there are mutations?

  • In most cases, nothing much - our DNA system is resilient to changes, our "program" is very stable and works anyways as expected.
  • In some cases, everything breaks, and you have genetic disease or death.
  • In some other cases there will be differences.

The important point to note here is that some mutations will be necessarily correlated with having more offspring, and some other will be necessarily correlated with having less. This is called natural selection.

Natural selection is the term in biology for the process by which biologic traits become more or less common in a population due to consistent effects upon the survival or reproduction of their bearers

Favourable mutations will take a large number of generations to propagate across the entire population, and unfavourable mutations will also. This is because the favourable mutation happens rarely, but being favourable, it grows exponentially over time until it saturates the gene pool or becomes less favourable.

The executive summary

Mutations happen all the time, natural selection determines iteratively which mutations spread and which diminish.

See a very good article here for more info.

How does it apply to the claim?

  1. It's impossible to stop mutations. They just happen because entropy always increases.
  2. It's impossible not to select mutations. Some depend on the fact we have freedom to choose sexual partners, for example.
  3. If there were any change in the rate of evolution it would be a change in the rate of mutation, whereas the claim speaks of a change in natural selection. We select differently, but we still select just as much as always*.

A final counterexample

What if a deadly virus were to wipe 90% of our species? Would that "increase the rate of evolution"?

The correct answer is that the rate of mutation is fairly constant, but the selective pressures change constantly. It's a recursive process.


(*) More explanation on the process of natural selection and references to justify the statement

According to Encyclopaedia Britannica (emphasis mine):

Process that results in adaptation of an organism to its environment by means of selectively reproducing changes in its genotype. Variations that increase an organism's chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous variations. As proposed by Charles Darwin, natural selection is the mechanism by which evolution occurs. It may arise from differences in survival, fertility, rate of development, mating success, or any other aspect of the life cycle.
--source

Now, contrast this with the claim:

It has been said that the human race has basically hit a point where any further evolution is essentially unnecessary: protection from most harmful diseases is provided by medicine, we have very few natural predators, and we are very good at what we do (colonising the planet.)

Even if we remove selection through diseases and predators, we still have selection through other means.

How can I claim that the "amount" of selection doesn't change? Let's see what Encyclopaedia Britannica has to say:

Mutation, gene flow, and genetic drift, all of which are random processes, also alter gene abundance. Natural selection moderates the effects of these processes because it multiplies the incidence of beneficial mutations over generations and eliminates harmful ones, since the organisms that carry them leave few or no descendants.
--source

So natural selection is a moderating function that, given a particular genotype, determines the number of offspring.

Since anybody has zero or more children, natural selection always operates.

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*We select differently, but we still select just as much as always. * How do you know? Maybe the standard deviation of the amount of children per woman/man changed. (In the extreme of every woman/man getting the same amount of children natural selection would slow down) Maybe we select based on culture instead of selecting based on genetics. –  Christian Apr 7 '11 at 21:40
    
@Christian: we select because we choose sexual partners, we select because there are diseases and resistances to them, etc. –  Sklivvz Apr 7 '11 at 21:42
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Showing that selection happens isn't sufficient for your claim. You claimed that the amount of selection is the same. –  Christian Apr 7 '11 at 21:44
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If every woman gets 2 children and every man gets 2 children you won't get much natural selection. If most men get no children but those men who do get children get 40 children you have much stronger selection pressures. At the very least you should provide sources for your claims. –  Christian Apr 7 '11 at 23:28
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The idea that the amount of selection is always the same isn't a well established scientific idea. The whole point of sourcing a claim is to show that it's an established scientific idea. A layman without subject experience has no way to know whether your claim that it's a well established scientific idea is true. I do think that this discussion in comments is relevant to other people who want to understand whether the stuff you claim is true and I therefore don't see a reason to have it elsewhere than the comments. –  Christian Apr 8 '11 at 8:31
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First, a few definitions.

Evolution can be defined as the change in "allele frequencies" over time, where "allele frequency" can e.g. be "how often does mutation X occur in this population", or "what fraction of the population has double the copies of gene X".

This evolution can be both adaptive and non-adaptive.

Adaptive changes in allele frequencies are those that respond to selective pressure, i.e. these changes modify the organism such that it responds either better (positive selection) or worse (negative selection) to the environment, and is thus able to have more, or fewer viable offspring, respectively. This change in viable offspring is usually called "natural selection", though the same mechanisms apply for breeding.

Non-adaptive changes in allele frequencies are those that occur regardless of any selective pressure. These include random mutations, genetic drift (i.e. changes in allele frequencies that happen by chance, and are often slightly deleterious), and recombination (the choice of which alleles you get from which grandparent).

When people claim that "evolution is slowing down", they usually mean that there is less adaptive evolution. Before I go into that, let's quickly look at non-adaptive evolution. There would be two ways how non-adaptive evolution could slow down: Either, we are exposed to fewer mutagens, i.e. chemicals or radiation that leads to mutations. This is most likely not the case. Alternatively, non-adaptive evolution could be slowed down if we started reproducing clonally, which hasn't happened yet. In sum, non-adaptive evolution has not slowed down, and may even be speeding up.

The speed of adaptive evolution depends on two factors (see e.g. here): The strength of the positive or negative effect of the allele, and the size of the population. Due to non-adaptive effects, such as mutation, which can revert a beneficial allele modification, or recombination, which can prevent the allele from propagating, there is always some "noise" when passing on alleles to the next generation. Averaged over an entire population, this noise is a lot more important when the population is small than when the population is big. Alleles whose effect is weaker than the noise level will not be selected for, because they'll simply disappear again. Thus, as population size increases, more and more alleles will have a noticeable positive or negative effect, and will thus be selected for or against.

In other words, as the number of humans increases, adaptive evolution should speed up. This prediction has been shown to be true for humans in the last 40k years. However, this study could only look up to some 5-10k year ago, since not enough genome data was available at the time.

So is it possible that since then, speed of adaptive evolution has slowed down? Clearly, the human population size is expanding, so we would expect that this speed-up effect should be still present today. What we can influence thanks to e.g. modern medicine is the selective pressure, i.e. there is less positive and negative selective pressure on some alleles than before. Thus, for certain specific features, adaptive evolution might in fact have slowed down, if the strength of the selection effect has decreased more than the effect of increased population size. At the same time, there are plenty of new environmental challenges for our bodies - for example, it is now quite beneficial to have a genetic makeup that allows you to respond well to mainstream drugs, and, as @Christian has mentioned, there is now the possibility of genetic screening combined with safer abortions that is already changing the male/female ratio.

In conclusion, non-adaptive evolution is possibly speeding up, and adaptive evolution is most likely speeding up as well - it's just still not as fast that people see effects within a handful of generations.

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It may just be me, but "Adaptive changes in allele frequencies are those that respond to selective pressure" sounds suspiciously like Lamarckian inheritance which has received much criticism (current resurgence in interest notwithstanding). Am I reading this wrong? –  horatio Apr 7 '11 at 20:10
    
@horatio: There are changes in alleles that have no effect on the fitness of the individual. Those are the non-adaptive changes. Then, there are changes that do have an effect on the fitness of the individual. If they increase the fitness, there will be positive selection, or positive evolutionary pressure, to keep this allele around. So there is first the genetic make-up that is determined as sperm and egg fuses, second, there is a human formed from that make-up, and if that thusly formed human is comparatively well suited for reproduction, there is positive selection. No Lamarck. –  Jonas Apr 7 '11 at 22:18
    
I read it wrong then –  horatio Apr 8 '11 at 13:42
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The assumption that an absence of natural selection leads to a stop of evolution is false. In the absence of natural selection genetic mutations would still change the gene pool. Genetic drift will still happen.

Evolution doesn't care whether it's "necessary" or not. As long as there are genetic factors that influence the average numbers of children of a woman you will have evolution.

Even in Western society different women have a different number of children. It's hard to imagine that the amount of children that a women gets doesn't get influenced by some personality traits. A lot of personality treats are partly heritable. It's highly unlikely that there won't be genes that influence the amount of children that a woman gets. The same is true for the amount of children that men get.

Embryonic screening will add further evolutionary pressure. In the near future gene therapy might make further changes to our gene pool. We might switch a variety of enzymes against similar enzymes from other species that work more efficiently.

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