How is evolution possible in contemporary humans?

Question

I would like to know if evolution is continuing to happen in modern humans, assuming things like existence of the nuclear family structure, fidelity to one partner, etc. It seems to me the answer would be NO because evolution depends on differential reproductive rates, but in the modern world, all male humans have roughly 2.5 (or whatever the number) kids. Add in the process of culturally modified selection pressure, and it seems to me that even an "unfit" male would end up having a couple of offspring. The fittest male (or female) is no better off than his or her contemporaries because of this "leveling" effect.

However, the impression I get from the popular science media is that scientists think evolution is continuing to happen. I would like to know what the actual scientific consensus is, and why. Thanks.

Answer 1

It is certainly not true that "all male humans have roughly 2.5 (or whatever the number) kids". First of all, male and female humans have exactly the same reproductive rate. For obvious reasons, every time a male has offspring, a female must have had also. Last I checked neither male nor female humans are capable of parthenogenesis (certain popular religious beliefs notwithstanding).

Second, let's assume that the 2.5 number is correct. That would be the average number of children per couple. That does not mean that all couples will have 2.5, or even that most couples will have 2.5. It just means that the average will be 2.5. If, for example you have one couple with 6 children, one with 2 and two with 1, the average will be (6+2+1+1)/(1+1+2)= 2.5.

On to the main point. What does selection mean? In its simplest form, that the individual most likely to survive (the famous "fittest") is also most likely to reproduce. This is a very simple concept, the longer you live the higher your chances of managing to have offspring. If you die two weeks after birth it is going to be hard to manage to reproduce yourself. This has not changed.

So, what does "fitness" mean? It can mean many things. If you are a warm blooded creature at the beginning of an ice age for example, it could mean being better at regulating your temperature than your peers. If you are a 21st century human, it could mean being funnier on twitter than your peers. The two are not fundamentally different. They can both be selected for or against. As long as one mate is chosen over another, selection is happening and the "fittest" (in each particular context) is most often selected.

Add in the process of culturally modified selection pressure, and it seems to me that even an "unfit" male would end up having a couple of offspring. The fittest male (or female) is no better off than his or her contemporaries because of this "leveling" effect.

"Culturally modified selection pressure", as you call it, is still selection pressure. Cultural factors can change what it means to be "the fittest" but there is no objective gold standard of "fitness". While it may be true that in modern human society, different characteristics are selected for than was the case with early Homo sapiens, this does not mean that "evolution is not occurring". On the contrary, it is occurring but perhaps it is moving in a new direction. In fact, this is essentially a circular argument. By definition, "fittest" means most likely to survive and reproduce. It does not mean strongest or fastest or prettiest. It just means whoever is better at reproducing. If that happens to be those individuals who are best at square dancing, then it is they who are the fittest.

Take the example of a modern human with diabetes. Medicine allows diabetics to lead fully productive and largely normal lives. So, perhaps diabetes is no longer a selective criterion. This does not mean that the diabetic cannot be selected for or against based on their fitness on other scales.

Whatever the selective pressure, whatever it may be that defines a "good mate", if selection is present then so is evolution. The only way to remove a species from the process of selection would be to have all (or none) individuals of each and every generation reproducing at the same rate. This is clearly not the case with humans. Surely not everyone around you has, or will have, children? There you go, selection!

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UPDATE:

In answer to your comment, yes indeed, in order for a selective pressure to make itself felt and affect phenotype (at the species level), it needs to be constant across several generations. However, even the absence of selective pressure affects evolution. As others have mentioned below, active selection is not the only mechanism of evolution.

Your main question however seems to be the following: If modern society (medicine etc) allows individuals that would not survive in the wild to reproduce, how does that affect evolution? The main points in my answer, and all others here, are:

1. Even if we accept that modern humans have removed themselves from the purely "biological fitness"-based selection pressure (an assumption I am not at all sure is true), and assuming that this removal is constant enough over many generations (again unclear), even if all this is true, evolution is most certainly still occurring. It may even be faster since genotypes that would not survive in the wild persist in the gene pool, thereby increasing its diversity.

2. As you point out in your comment below, for such social pressure to make itself felt, it needs to be constant across many generations. We are probably not there yet.

3. Most importantly, as I said above, there is no such thing as an absolute biological fitness. When the ecosystem changes, so does the definition of fitness. Modern humanity's ecosystem, our habitat, is intimately connected with our culture and society. If an individual is better at reproducing in that context, then that individual is more fit.

Answer 2

Evolution is defined as the change in the allele frequency of a population through time. In other words a change in the genetic diversity of the population through time. This change can be the result of 4 processes, known as the mechanisms of evolution:

1) gene flow - this is the movement of individuals into or out of the population.

2) genetic drift - this is the removal of individuals due to random events (such as accidents).

3) mutation - this is the spontaneous change in the DNA, which instantly increases genetic diversity.

4) selection (natural, sexual or artificial) - this is the increase in the frequency of alleles (traits) that confer a fitness advantage (i.e., those with a given trait will leave more offspring). Most adaptive changes in gene frequency are the result of selection.

Even if one presumes that your analysis is correct and natural selection is no longer operating on modern humans (although this is suspect, as is pointed out by other answers), evolution would still be occurring due to mechanisms 1 - 3. If we go further and consider the Earth's human population as a whole (eliminating gene flow as a mechanism) you would still have evolution via genetic drift and mutation.

In other words, yes, evolution is not only possible but inevitable in modern human populations.

Answer 3

In the US, different races have significantly different birth rates. Obviously race has a genetic component, though I'm guessing that the racial differences are mostly due to cultural and economic reasons. Still in this sense, the US population is evolving: alleles associated with some populations (African Americans, Hispanics) are becoming more common, while those associated to other populations (white non-hispanic, etc) are becoming less common. So the US is definitely evolving to become "less white". Note that this is true even though the overall birth rate reported in the study is not much above the minimal replacement rate of 2.0.

Another example: religiosity is associated with increased fertility, and there is some evidence that religiosity has a genetic component. In that sense, the population may be evolving to be more religious over time.

Similarly, there are genetic factors to obesity, which has a negative effect on fertility. These genes may even have been actively selected for in the past (the thrifty genome hypothesis), but are probably being selected against now.

We are currently undergoing one of the biggest shifts in human fertility in the history of our species: in the past century, we've developed technologies which allow people to more carefully choose when to become pregnant (condoms, abortions, hormone birth control). Any genetic factor which has an impact on use of birth control could lead to evolutionary changes: this could range anywhere from ethnicity associated with economic or religious status (poverty, Catholicism), impulse control, etc.

This is all somewhat speculative (except for the first example), but it's certainly possible for evolution to occur in modern human society, and it likely is occurring.

Answer 4

Darwin used the term “descent with modification” to describe evolution. That means a change between generations in the characteristics, or traits, of a population. It is a process which occurs by four processes - mutation, migration, drift, and selection. The conditions for evolution are i) a population must be able to reproduce, resulting in a descendant population, ii) offspring must tend to resemble their parents and this occurs because traits are determined by heritable information, e.g. DNA, and iii) that heritable information must have the potential to change from one generation to the next. It does not require selection.

Here is a paper about evolution and adaptation in humans:

"Despite popular misconceptions, natural selection does operate in modern human populations. New studies even show that changes associated with modernization are deeply reshaping selection pressures and, perhaps, bits of our biological nature."

Selection is still occurring in humans too, leading to further adaptation (a subprocess of evolution) (e.g. here and here). Many people naively think and suggest that selection has been halted in humans, the fact is, it hasn't. We still have variance in reproductive success based upon the genes individuals possess, e.g. some people die from genetic diseases before reproduction.

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Here are some illustrations describing the mechanisms of evolution. Note that drift and selection both cause variance in reproductive success among individuals in a population, for drift the variance is independent of the genes, while for selection it is a tendency for the carriers of one allele to have higher reproductive output than another.

Mutation:

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Migration:

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Drift:

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Selection:

Answer 5

Modern humans are continuing to evolve. Let me give you an example.

Before cattle were domesticated over 99% of all adults were lactose-INtolerant (i.e., they did not produce the enzyme needed to break down lactose (lactase) and could not digest it, i.e. got diarrhoea if they drank milk). All infants produce this enzyme, but in lactose-intolerant people the gene that codes the enzyme responsible for breaking down lactose becomes suppressed after the age of 1 or so. Before domestication of cattle, lactose-intolerant people were by far the majority in any human population. Why? Because if you never consume milk, constantly producing an unneeded enzyme is a huge waste of protein, especially in conditions of limited food supply. With the domestication of cattle, people who could utilize lactose suddenly had a huge advantage - they had another source of protein and calcium. They quickly became bigger and stronger than people who could not consume dairy products in adulthood. Thus, people in Europe and Africa (where cattle were domesticated) quickly became lactose-tolerant, unlike people in Asia, where to this day most people are lactose-intolerant. The evolutionary pressure at that time was huge - people in Europe and Africa went from mostly lactose-intolerant to mostly lactose-tolerant in 10 000 years or so. Interestingly enough, in this respect cats evolved in parallel to humans and now cats in Europe are predominantly lactose-tolerant, whereas cats in Asia are mostly lactose-intolerant.

Of course, this is not the only change that has occurred within the last 10 000 years and it surely is not the last one that will take place. It is just an illustrative (and hopefully interesting) example of evolution and natural selection in modern humans. In what direction humans will evolve in the future is very speculative.

As requested, a couple of references: http://evolution.berkeley.edu/evolibrary/news/070401_lactose

http://www.smithsonianmag.com/arts-culture/lactose-tolerance-and-human-evolution-56187902/?no-ist

Answer 6

Evolution is possible for the same reasons it has always been possible (and I'll let the other answers' details comment on the mechanics, as they have done already...

I would like to offer a *slightly more philosophical point of view on the term, however. I think that we have arrived at a point in our evolution where our prefrontal cortex is impeding our biologically driven urges, somewhat (which would have us evolving in similar ways to all the creatures less aware of the consequence of their actions).

I think it's impossible to say to what extent our evolution will now be driven by conscious choices we make about what traits we value vs. the traits that our biology drives us towards. Certainly innate biological drivers are incredibly powerful, but as is evidenced by our desire to promote our intellect, and our progressive urge to help even the sickly and mentally challenged to get the most out of life (up to and including procreating), rather than to evaluate our potential partners purely based on physical traits and our brute strength, these purely biological imperatives are governing our reproductive choices less and less.

There is also much (reasonable) speculation as to what our new found understanding of genetics will allow us to artificially do to ourselves in the near future.

I think it's important to consider both these factors when speculating about how we're evolving, but I think it's not even slightly controversial to suggest that we're definitely evolving in a different way now than we were 500k years ago.