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    Human Evolution - Petering Out?
    By Michael White | April 29th 2009 12:03 PM | 17 comments | Print | E-mail | Track Comments
    About Michael

    Welcome to Adaptive Complexity, where I write about genomics, systems biology, evolution, and the connection between science and literature,

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    Have we really stopped evolving? In Cosmos magazine, Steve Jones argues that human evolution is coming to an end:
    The question I have is: will human evolution really continue? I think the evidence shows that human evolution has largely come to a halt.
    He lists three components necessary for human evolution:
    First of all there's variation, which comes from mutation. Second, natural selection, which comes from inherited differences between individuals and their ability to reproduce.
    Finally, evolution is greatly promoted by isolation.
    Jones is right about the first two elements: you need variation and natural selection for evolution to happen. But Jones is wrong about the third element - it's not necessary for evolution.

    We'll tackle all three elements, but let's start with the third. Isolation is important for speciation - the generation of two (or more) new species from one, but it is absolutely not necessary for evolution in general. One species can change dramatically over time, without splitting into isolated populations, as it adapts to changes in its environment, like new diseases, changes in climate, or even culturally-driven changes that affect survival. The human species itself has changed dramatically in the last 300,000 years, without splitting into new species: archaic Homo sapiens that lived 300,000 years ago were physiologically different from humans today, or even those that lived 80,000 years ago.

    Species can undergo evolutionary change without isolation. Humans aren't as geographically isolated from each other as they once were, but that doesn't mean that human evolution has stalled.

    Next element: variation. Jones argues that the pipeline of genetic variation is drying up: rates of new germ-line mutations (mutations that happen in sperm or egg cells, and are thus passed on to the next generation - the only kind of mutation that counts when it comes to evolution) are going down, because of changes in human reproductive patterns:
    We see little evidence to say that human evolution is going to speed up because there are more mutations coming into the process. Nowadays, people start their families late, but stop early; changing demographics show that there are fewer older fathers today than in the past. So, if anything, the rate of mutation is going to slow down.

    That claim is incredibly difficult to substantiate, and it's not obvious that it should be true: very recent (20th century) changes in child-bearing patterns may or may not be significant enough to have a serious impact on the rate of new germ-line mutations (and thus the generation of new genetic variation). There is no convincing evidence yet that such recent changes have dramatically altered the rate of appearance of new mutations on a global scale. 
    Jones can speculate that these demographic changes have reduced germline mutation rates, but it's just as easy to think of other factors that could increase germline mutations - like laptops, whose heat output has probably increased the rate of germline mutations, at least among men in developed countries. In any case, whether the mutation rate has changed or not, germ-line mutation hasn't stopped - we're all born with new mutations.
    Mutation rates aside, there is already a tremendous amount of genetic variation among humans for natural selection act on - and this variation isn't going to go away as the human population becomes more and more like a giant melting-pot: a large population with individuals that mate randomly with each other preserves its genetic variation. (You may remember the Hardy-Weinberg principle from your high school or college biology class).

    Compared to say, rats or cockroaches, the human population is not that large and doesn't engage in totally random mating, but the point is this: we're not draining the pool of genetic variation by interracial marriage. Jones' is simply wrong when he claims this:
    But now the world's populations are mixing at an extraordinary rate. In Britain, one marriage in 50 or so is between members of different ethnic groups. We are mixing into a global mass; the future is brown.
    And this:
    What we find is, even more today than in 1950, that our genes are mixing and averaging out. More and more people are subject to this averaging process. And for humans, this averaging means an end to evolution.

    We may "average out" our skin color by all this mixing, but, as a species, we're not averaging out our genetic variation. There is no shortage of raw material left for selection to act on, and it's not going away any time soon.

    On to the final Jones' final element of evolution: what about natural selection?

    Certainly one of the most dramatic (and tragic) sources of natural selection, childhood disease, has been significantly reduced, especially in the developed world. And with fertility treatments, more and more people are able to have children.

    But that doesn't mean the difference in the numbers of children people have (i.e., differences in reproductive fitness) is negligible. Jones claims:

    Between 1880 and 2000, the huge variation in the average number of children born per couple between countries diminished. So, once again, there's no evidence that evolution has anything left to work with.
    The flaw here is that natural selection doesn't need "huge variation in the average number of children born per couple" - very small differences add up to huge changes in the long run. (And I'm not sure why Jones' thinks that the differences have to be between countries or populations - evolutionary change can take place within a population.) Look around your neighborhood, and you'll see differences that are large enough for selection to act on. In the US, we don't all have 2.5 children: there are many, many adults who never have children, and some who have more than most of us could handle.

    It doesn't take much - phenomena like the slight reproductive advantage of more closely related couples in Iceland is enough. (This study found that couples who are third or fourth cousins are more fertile than couples that aren't so closely related.)

    So is human evolution grinding to a halt? There is not reason to think it is.

    Comments

    Gerhard Adam
    This viewpoint is interesting only because it tends to show more about the author's biases than anything resembling science.

    In the first case, evolution cannot stop, since by it's nature it is simply change.  We can certainly ponder the direction that an evolutionary path can take, but to suggest that it can simply stop is absurd on the face of it. 

    In addition, humans have different selection pressures on them now than they did hundreds of thousands of years ago (or even a few thousand years ago).  In fact, I would argue that human evolution is actually being dominated by a sort of "second phase" where the cooperative/social elements of human society are the primary selection forces at work.  In the same way that cells evolved to "cooperate" with one another and established a division of labor to form bodies, so do societies evolve.  We can see this in some of the social insects where the division of labor extends to individuals and the animal itself is no longer truly representative of the species, but rather the colony or group is.

    Similarly, humans are evolving in a similar fashion where our social roles often dominate our biological roles.  Increasingly it is the social net which ensures survivability and even caretaking of offspring in the event the biological parent fails.  While this may seem a bit to esoteric for some, it is easy to see that those individuals that embrace this type of change will tend to be selected for (since they will individually have the potential to achieve higher reproductive success), so that over time, it isn't difficult to imagine how human beings can evolve into a super-organism.

    All the elements are in place, where humans have increasingly lost their knowledge of how to survive and replaced it with how to cooperate.  Humans simply lack the knowledge to replicate the lives they have as social animals (in other words the technology in use today is simply unattainable by any individuals) and therefore there is a strong pressure to conform and participate in the group. 

    Humans tend to embrace mythologies about themselves (such as the "rugged individual"), but in truth, human society has more in common with the ant hill than the grizzly bear.

    On another point, .... I thought the concept of "averaging" out genetic traits was already put to rest by Mendel?
    Mundus vult decipi
    adaptivecomplexity
     I thought the concept of "averaging" out genetic traits was already put to rest by Mendel?
    That's where Hardy and Weinberg come in - they made the implications of Mendel's discoveries explicit and testable.
    Mike

    It seems to me that inherent in Jones’s argument is the common misconception that evolution is an “onward-and-upward” process of improvement – a pyramid with Homo sapiens at the apex. This is related to a similar misconception – that all evolution is the product of natural selection. Relaxed selection does not equal an end to evolution. In Sewall Wright’s fitness landscapes, in fact, relaxed selection could act as a major evolutionary force by freeing a population to explore a greater region of evolutionary space that it could if tightly constrained to a local optimum – or “fitness peak”.

    adaptivecomplexity
    Relaxed selection does not equal an end to evolution. In Sewall Wright’s fitness landscapes, in fact, relaxed selection could act as a major evolutionary force by freeing a population to explore a greater region of evolutionary space that it could if tightly constrained to a local optimum – or “fitness peak”.
    Good point. The misconception that evolution is all about selection is unfortunately pervasive.
    Evolutionary biologist Michael Lynch has been arguing vigorously (and IMHO persuasively) that many features of our genomes are shaped by non-selective processes. More people need to read his stuff and at least consider non-adaptive, alternate hypothesis before they jump to the conclusion that nothing interesting happens in the absence of selection.
    Mike
    Gerhard Adam

    I might go so far as to say that ALL the features of our genomes are shaped by non-selective processes, since by definition, they are fundamentally chemistry.  The role of selection is to determine which of these features are preserved or lost.  If they are preserved, then they must enhance the survival to reproduction (or minimally do no harm).  If they are lost, then clearly they must be detrimental (or too expensive to maintain).

    I think the issue with selection is that it tends to imply (at least to the general viewpoint) that evolution has a directed objective and hence survival is contingent on somehow being "better" than the competition.  This terminology invariably leads to the perception that there is a purpose at work instead of considering that any change can only be assessed (in terms of reproductive success)  after it occurs and therefore must exist outside the scope of any selection pressures.

    Even negative traits are preserved so long as they have some probability of being in the reproductive pool, although they would never reproduce at a sufficiently high level to become a generally pervasive part of the species. 

    Mundus vult decipi
    adaptivecomplexity
    I think the issue with selection is that it tends to imply (at least to the general viewpoint) that evolution has a directed objective and hence survival is contingent on somehow being "better" than the competition.  
    In evolution, "better" or more fit is, as you suggest, only a relative term, with meaning only in a given environmental context, and relative to other members of the species. I agree that the language can be misleading - it's easy to fall into the trap of thinking that the language is implying some sort of ultimate goal.  The key is to focus not on the possible dual meaning of various terms, but the logical structure of the argument for natural selection - the argument doesn't make any assumptions about teleology.
    Under natural selection, there are some clear, predictable consequences that differ from what we expect under a neutral model, but even neutral processes can produce complex structures.
    Mike
    Gerhard Adam

    This comment from the abstract seems a bit strange though:

    "...genetic drift, mutation and recombination, raising questions about whether natural selection is necessary..."

    My point is that genetic drift, mutations, and recombination are all the agents of change.  In other words, they are the means by which change is introduced at the molecular level.

    Natural selection can only occur after the fact.  Once a change has occurred, then it may be selected for (by improving reproductive capabilities), but until then there is no such thing as natural selection.  Whatever changes occur (good or bad) will be represented in subsequent generations based solely on how well they hold up to the reproduction requirements.  One would think that favorable traits would tend to be more attractive to future mates, and consequently lead to better reproductive success, but this would certainly not be guaranteed. 

    Mundus vult decipi
    Gerhard Adam
    BTW .... my extracted quote is clearly not complete, but I realized that the way I presented it may make it sound like the article is suggesting that natural selection isn't necessary or doesn't occur.  My apologies if that appears to be a misrepresentation.
    Mundus vult decipi
    adaptivecomplexity
    It's true that natural selection doesn't have anything to work with without heritable variation - variation which is generated through mutation and recombination.

    Drift is like selection in the sense that it occurs 'after the fact' of mutation - and Lynch's point is that, given that you alway have mutation and recombination, genetic drift can build build complex regulatory structures in the absence of selection. In other words, the null hypothesis of no selection does not imply a featureless, simple genome - complexity by itself is not a sign of natural selection.
    Mike
    I really don't think we will ever stop evolving because evolution never stops. I'm also starting to understand more about human behavior and evolution from author M.A. Curtis of "Dominance & Delusion." When viewed as an animal, and as part of the animal kingdom, some surprising conclusions are reached when the question is asked, “Why do we do the things we do?”

    Fossil Huntress
    Have we really stopped evolving? In Cosmos magazine, Steve Jones argues that human evolution is coming to an end:

    I have to admit something to you Michael. When I first read your opening paragraph I thought the source was Cosmo trying for a science angle... not Cosmos. Is fashion evolving? Read here to get all the hard science. Equally charming from either angle, angel.
    Hank
    If fashion can save the planet, I don't see why it can't do science too.   

    Funny email I got from the Pew Institute after I wrote that; one of the women that worked there asked the person who showed my article to her, "So is he going to buy one?"
    adaptivecomplexity
    Put a Scientific Blogging logo on it, and we'll all buy one.
    Mike
    adaptivecomplexity
    I thought the same thing at first when I first saw the link.  Humans may stop evolving, but science never will.
    Mike
    iramjohn
    Yeah, that's an odd article.  Sure, humans are probably subject to less selection than they were in the past, and yes, we're mixing more than we used to.  But I don't see how that translates into an "end" of evolution.

    Having fewer children die certainly reduces the role of selection and modern medicine allows certain deleterious alleles to survive.  But the stresses of modern life, especially modern urban life, are also selective factors.  Sure, it doesn't kill you as a child, but I strongly suspect that people with schizophrenia or severe depression or autism are going to leave fewer offspring than the average person.  We're probably selecting against the genes that pre-dispose people to early onset type II diabetes.  And, of course, HIV is having a huge effect on the population genetics of southern Africa.  But simply choosing to have fewer children, and delaying the age of first reproduction is probably going to have some selective effect of its own.

    I'm unconvinced by the assertion that there are fewer children born to older fathers these days.  My grandfather had his 9th (and last) child when he was 38.  Becoming a father for the first time at 38 isn't all that unusual these days.  Similarly, delaying the age of first reproduction also increases the proportion of children born to older fathers.  Jones may have data to back up his assertion, but without sources I'm dubious. 

    Finally, I find the assertion that mixing somehow overcomes evolution to be odd.  Mixing increases genetic diversity, which gives selection more to work with.  As for the whole world becoming "brown", skin colour appears to have evolved very rapidly; there's no reason to assume that it won't continue to be subject to selection.  (People still die of skin cancer, and not everyone is going to get enough Vitamin D in their diet).  Not to mention that bemoaning mixing sounds like a throwback to outdated ideas of racial purity and eugenics.  I'm a little offended by the subtext that there's something wrong with people like me...
    Since when is evolution strictly a DNA thing? Life that is evolving and adapting, has the best advantage if it can adapt the fastest. It evolves to evolve faster. Technology is just the next step in the evolutionary ladder.

    adaptivecomplexity
    Since when is evolution strictly a DNA thing?
    Biological evolution is about heritable change, and thus DNA is key. But cultural and technological evolution is interesting too.
    Mike