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?
Evolution is not slowing down, nor it can.
Suppose there's a lot of selection pressure on height: tall people make more children. Thanks to evolution, people will get taller on average.
Suppose there's no selection pressure on height: tall and short people make the same number of children. Thanks to evolution, the variety of heights will increase.
In both cases, evolution is at work. It does not slow down.
There will be only Wikipedia linking here because this stuff is textbook biology. Evolution is an established scientific fact.
It's a lack of understanding of how modern evolution works.
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:
What happens when there are mutations?
The important point to note here is that some mutations will be necessarily correlated with having more offspring, 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.
Mutations keep on happening as they are intrinsic in the process of reproduction. The gene pool "evolves" by allowing variety in the directions not suppressed by natural selection, and enforcing uniformity in the directions where natural selection is active. In either case, evolution is at work.
See a very good article here for more info.
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.
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.
