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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.

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Time rate of evolution is multiple orders slower than time rate of our sociology science perception. If nuclear war will happen, which is probable, then entire humanity appear to last for 30000 years. This is zero in evolution scale. So, evolution DOES continue, but it goes so slow that we cant see it. –  Suzan Cioc Oct 27 '12 at 19:18
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I am tempted to say that "contemporary" (rather ambiguous time-frame) humans are under evolutionary pressure in much the same ways as we were in posterity. Humans still have sex which is the major mechanism of evolutionary diversity, which creates genetic diversity. This diversity is causes selective pressure in social, physiological and intellectual domains. All I would argue that "modern society" has done for us, when compared to ancient humans, is change the weight placed on us by each selective pressure. –  leonardo Oct 29 '12 at 19:58
    
This might be a perverse theory, but what if what we identify today as "development disorders" like autism, is actually evolution responding to modern information age pressures –  Gabriel Fair Feb 11 '13 at 1:34
    
@GabrielFair you should read David Brin's Existence it suggests exactly that! –  terdon Mar 30 '13 at 17:26
    
No I haven't. I will add Existence to the top of my reading list. While we are on the topic have you heard of any research or peer reviewed studies referencing a theory like this one? –  Gabriel Fair Apr 1 '13 at 1:28

5 Answers 5

up vote 17 down vote accepted

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, lets 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 are both selection. 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!


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.

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Thanks for the answer. A follow-up: It's true that not all couples have the same number of children, but it seems like whether or not they have more children than average is no longer strongly dependent on biological fitness. Therefore, given that reproductive rates are fickle from one generation to the next (due to cultural factors), the signal would not have time to develop, right? It seems to me that there should be a persistent multi-generational selection pressure for the phenotype to develop, which is missing in the modern context. Maybe evolution is occurring, but significantly slower? –  Joebevo Oct 25 '12 at 4:38
    
Very thorough answer. +1 –  LanceLafontaine Oct 26 '12 at 16:43
    
I doubt that the distribution of reproductive success is exactly the same for men and women. you are assuming that women and men have the same partners all through their lives, which is not so true at least in the US. In a society where a large percentage of children are born out of wedlock men and women do not need to have the same reproduction distribution. Also in polygamous societies or where there is gender based infanticide it doesn't hold. No judging here, just saying. Overall the point holds, but not sure you needed this base assumption. –  shigeta Oct 28 '12 at 3:01
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@shigeta, for every child a man has, somewhere a woman has had one also and vice versa. Wedlock has nothing to do with it, nor does infanticide or anything else. This holds true for all (exclusively) sexually reproducing species. By definition. –  terdon Oct 28 '12 at 3:34
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"That does not mean that all couples will have 2.5, or even that most couples will have 2.5." In fact, it's safe to say that no couples have exactly 2.5 kids. –  Bruce Alderman Feb 1 '13 at 17:27

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.

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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.

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I just want to emphasise that "races" is more of a social construct than a scientifically strictly defined category (see Razib Khan's comment here: blogs.discovermagazine.com/crux/?p=1350#.UR45L1qcYVo) . So while I agree with a term "alleles associated with some populations" the other terms you use ("race has a genetic component", "less white") are more murky (unless, in the latter example, you're talking about alleles associated with skin color - but then again, they do not necessarily correlate with ancestry). –  yotiao Feb 15 '13 at 13:37
    
I mean that there are differences in allele frequencies among the census bureau race/ethnicity categories. These are probably self-identified groups, but there are differences in allele frequencies between the group of people who self-identify as hispanic vs those who self-identify as white/caucasian. Alleles common in the former group are becoming more frequent relative to alleles common in the second group. Those would presumably include some alleles for skin pigmentation, among others (like variations in lactose tolerance, renal sodium channels, sickle-cell anemia, etc). –  RecursivelyIronic Feb 15 '13 at 20:48
    
To rephrase that paragraph in more PC language: some self-identified ethnic groups have higher birthrates than others. To the extent that allele frequencies in high-birthrate ethnic groups are different from the mean frequency for the population as a whole, the US population is "evolving" to increase the frequencies of those alleles. I added "race has a genetic component, but..." to specifically avoid an interpretation that genetics were the cause of increased birthrates. –  RecursivelyIronic Feb 15 '13 at 21:05

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.

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Evolution will ("in theory") continue as long as:

  1. there is variance in lifetime reproductive success (LRS) (a.k.a. fitness) - we know there is even just from your statement of the average being 2.5 children per male, because no one is having 2.5 children, that average is made up (even with low variance) of people who have 2 or 3 children - this is variance. in truth the variance is much greater. LRS is one of the best ways to measure fitness, in an evolutionary sense, in any species.
  2. the variance in LRS within the breeding population has some genetic underpinning. If modern cultural selection was based on a non genetic trait then evolution would cease. For example, if fitness was purely based on wealth, and this was in no way related to their genes, then selection could not affect the genetic composition of the population. (But wealth is partly genetic because intelligence & athletic ability are affected genetically and smart people & sports stars are often rich)

This is a combined version of Darwin's three principles of evolution (variation, heredity, selection). He stated that within a breeding population there must be variation in a trait, that trait must be heritable (heredity) to some degree (see Falconer & Mackay IQG chapters 9 & 10 for how to estimate heritability), and the fitness of individuals must be related to the trait creating selection (i.e. some values are fitter than others). Thus, there will be evolution if there is heritable variation in traits, within a population, that have fitness consequences.

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