{"id":26945,"date":"2009-08-21T11:55:59","date_gmt":"2009-08-21T11:55:59","guid":{"rendered":"http:\/\/scienceblogs.com\/gregladen\/2009\/08\/21\/falsehoods-has-evolution-stopp\/"},"modified":"2009-08-21T11:55:59","modified_gmt":"2009-08-21T11:55:59","slug":"falsehoods-has-evolution-stopp","status":"publish","type":"post","link":"https:\/\/gregladen.com\/blog\/2009\/08\/21\/falsehoods-has-evolution-stopp\/","title":{"rendered":"Falsehoods: Has evolution stopped for humans?"},"content":{"rendered":"

In my initial \n

  • s.php”>list of Falsehoods<\/a>, I included these two closely related items:<\/li>\n
  • Evolution has stopped for humans; and<\/li>\n
  • Serious scientists often entertain the question: “Has evolution stopped for humans?”<\/li>\n<\/ol>\n

    I’ll make the distinction between them clear eventually. For now, let’s focus on the first: Has evolution stopped for humans or not?
    \n
    \nThe reason we bring up these falsehoods to begin with is because     examining them exposes assumptions or requires the investigation of knowledge that is worth talking about.<\/a> In this case, the deeper question is this: What is evolution exactly?<\/em> So, this is a true litmus test kind of question. With a clear understanding evolution it should be sufficient to get to the point where this question can not be answered! It can’t be answered because it really shouldn’t even be asked. It’s like asking “OK, so the earth is a globe. Fine. But how did it get from being flat to being a globe?” That question has no answer.<\/p>\n

    Neither does “Has evolution stopped for humans?” The only species for which evolution has stopped are the extinct ones.<\/p>\n

    First a definition of evolution. There are many such definitions, and much ink has been split, pixels flashed on the screen, and breadth spewed into the atmosphere on this. It is not clear why. Biological evolution is change in allele frequency over time. This is the correct definition. Let me underscore this by giving you an alternative definition that is wrong, one that I saw on the Internet. In this particular place on the Internet, someone argued that while “change in allele frequency over time” was a nice definition, it failed to truly grasp the real meaning of evolution because this did not include, or address, “Descent with modification.” That particular inter-tubule concluded that “descent with modification” is a better definition than “change in allele frequency over time.” But really, that is wrong.<\/p>\n

    I assume that this was meant to refer to phenotypic change that we can normally detect or that matters with respect to the organism’s interaction with the environment. Larger teeth, thinner fur, change in body size, better bodily fluids, and so on. So, with “descent with modification” if there is no physical change over time, there is no evolution over that period of time.<\/p>\n

    But in fact, most evolution is all about things not<\/strong> changing over time. You saw that “not” in the previous sentence, right? (Good, just checking.)<\/p>\n

    In every generation, mutations are introduced that have no phenotype correlate. You know that an allele is a version of a gene. All genes have one allele or more. If there are two different DNA sequences that are in the same locus and code for the same product (a protein) then there are two alleles of that gene. If there are three different such DNA sequences then there are three alleles, etc. In practice, there are more real alleles than there are known alleles, and this is important. (Some people will disagree with me on this, but they would be wrong IMO.) If one DNA sequence codes for two proteins that have different functions, then it is clear that there are two alleles because we can see the phenotype difference. One fruit fly has smooth eyes and one fruit fly has wrinkled eyes. (I’m mixing up fruit flies and peas on purpose … more about that another time. Suffice it to say that I won’t be using any real examples in this essay.) Anyway, everybody agrees that this is a gene with two alleles.<\/p>\n

    But what if a gene has two versions, with different DNA sequences, but both DNA sequences code for the same exact protein? Some people would say that there is only one allele, and for all practical purposes, this is OK and will not mess you up, depending on what you are doing with this information. But say you are using allelic variation to measure mutation rates. Then you know you have to make an adjustment for “silent” mutations such as this one. Or, you could simply sequence the DNA and recognize that difference means not the same, so “different DNA sequences” means that, well, the DNA sequences are different even if there are fewer proteins being coded for than there are differences.<\/p>\n

    A quick review of something you probably know, but just in case: A mutation is a change in DNA whereby one of the base pairs is changed at a particular point in the DNA sequence. There are other kinds of mutations which we shall ignore at this time (like insertions and deletions and stuff). A silent mutation is a mutation that does not have a phenotypic effect. For now we’ll call a visible trait, such as blue eye color or tendency to be a Republican, a trait, and not worry about the small technicality that if you were really really tiny and could look at a protein and see altered base pair that is not expressed at the bigger scale (like eye color or voting habits).<\/p>\n

    A mutation can be silent because it changes a base pair in a codon that does not in turn change the amino acid that codon specifies. Proteins are made up of a sequence of amino acids, and “coding for a protein” means having a series of codons (three base pairs, a kind of genetic “word”) that specifies the series of amino acids that the little fairies that live inside your cells will put together to make the proteins. (I’m leaving out some details and substituting said details with the word “little fairies” … I figure it will help my search engine score on Google.)<\/p>\n

    You probably already know that many different codons (there are 64 in all) code for a smaller number of amino acids (about 20). Like skillet and frying pan both mean the same thing, so do many sets of codons mean the same thing … the same amino acid.<\/p>\n

    A mutation can also be silent because it does change which amino acid is being placed in that position while making the protein, but in a way that does not matter. For example, if the phenotype relies on a certain part of the protein working a certain way, then the other part .. where the hypothetical mutation happened … can vary within a certain range and the protein still acts the same.<\/p>\n

    Like screwdrivers. If you change the bit end of a screwdrivers you may have screwed yourself out of a screwdriver that works. But if you slightly change the color or size of the handle, who cares? So a mutation can be silent because it is a “who cares” mutation. This can be taken to several levels. You can get a protein that has a real functional difference, but it is one protein of seven that essentially serve the same purpose because of some kind of crazy redundancy thing and no one notices. For instance, maybe this is a protein that is essential in the synthesis of a really important molecule, but in an organism that consumes copious quantities of this same molecule found in nature in usable form. So the inability to synthesize the molecule will not be a visible trait until it is noticed by someone, like when certain crazy primates started to sail around in the ocean for months at a time and started getting scurvy because they did not synthesize Vitamin C because an ancestral primate got this then-silent mutation. The mutation never got noticed because all the other monkeys and apes were sitting around eating copious quantities of Vitamin C (fruit, vegetables, fresh blood, etc.). But these boat-monkeys were eating dried pork rind or something else without Vitamin C, so they got quite ill. No one noticed for tens of millions of years in that case! So a lot of what some people might call “neutral evolution” is going on all the time in the background with nobody caring and nobody noticing.<\/p>\n

    That is change in allele frequency over time. It is not decent with modification in the usual sense of the term.<\/p>\n

    Now, suppose something gets noticed? If an allele arises that causes a change in phenotype that matters, then it is possible that selection will act on it. Actually, I’ve misspoken. When I said “that matters” I meant an allele that confers a difference subject to selection. It will be selected against or for depending on the interface between the functional change and the environment. Usually against. (All change is bad, right?) But selection comes and goes and not everything is given the same ranking in the selective milieu. In other words, a single allele change with nothing else happening in a population or even a lineage is very unlikely. Usually in a population at any given time there are a bunch of novel alleles. Let’s say that there is a one in a thousand chance that a single mutation will have a meaningful effect. (This is quite possibly a gross underestimate). Depending on who you believe, this could mean that in a population of 100,000 individuals, there are about 200 different meaningful alleles introduced every generation. Many populations of larger animals are probably much smaller than this. Many populations of plants probably inter pollinate among millions of individuals. So this thought experiment is pretty limiting. But just figure that it is quite possible that when you think of a “species” there may be many novelties introduced in any one generation.<\/p>\n

    Most of these meaningful changes probably have weak effects. Being a winner or loser in the mating game, for instance, may not mean you are the mother of all future individuals or an evolutionary dead end. It probably just means that you will have a little more or a little less of a contribution to future generations. Then the genes get all mixed up each generation because of a crazy little thing called love, and so on. So novel mutations that are under selection may be around for many generations. If hundreds or thousands of novelties are introduced every generation and most get to stay around for several generations, that means that there are anywhere from scads to zillions (to be exact) of mutations in a given “species” (population) at any one point of time. So now you get the picture of a constant ongoing interconnected complex mess of a situation. Dawkins’ river out of Eden.<\/p>\n

    And all this time, it may be that many different selective forces are working on the phenotype. How broad and long is the tail of an alligator or a crocodile? What genes cause that particular feature? Over time, it turns out that in most systems, the genes that underlie particular major features in related forms (like all crodillians) stay the same. (But not always … the subject of another discussion, another time.) But it may be that the particular combination of alleles that produce that phenotype vary. Here’s why: There is no gene that affects only one trait, and there is no trait that is affected by only one gene (and I’m only slightly exaggerating). Therefore, the size and shape of the tail of a crocodile may be affected by, say, a dozen genes, each with a range of alleles, that in combination come up with a particular tail size and shape, but whereby that same size and shape could be obtained with several different combinations.<\/p>\n

    A metaphor is in order. Imagine starting, say, an Internet hosting company, with servers and such. You hire 12 people to run the company. They provide sales, technical maintenance, support, make purchasing decisions for new equipment, brew coffee and change light bulbs, etc. They have something like 50 or 60 jobs divided among the 12 of them to get the whole bigger job of running this company done.<\/p>\n

    Now clone the company so that there are 100 identical companies, and let them run without communicating with each other for five years. At the end of ten years assume you still have the same 100 companies that are doing the same thing. Now interview each person and see what their job is exactly. Are the 60 specific jobs divided among them being done by exactly the same nominal positions in the company as when you first started it up and cloned them? Many key jobs probably are. You need a Unix Geek to do the Unix stuff, and your sales rep is probably not that person. But many jobs will have migrated back and forth among different categories of individuals. Who makes the coffee, who has input into web design, etc. etc. could all shift around quite a bit. You may end up with some very different models for internal functioning among these 100 companies that are all doing the same thing on the outside. In other words, you may end up with 100 different combinations of alleles all making the same exact phenotype.<\/p>\n

    This is a LOT of change in allele frequency over time. It is ZERO descent with modification.<\/p>\n

    OK, that is how evolution works. Now let’s get back to the original question: “Has evolution stopped for humans.” I feel like I don’t even want to answer that question at this point, don’t you agree? Are humans organisms that use genes to encode protein sequences and pass them on over time? Do humans have phenotypes coded for by complex interactions of many many genes? And so on?<\/p>\n

    How can evolution stop? What would that mean? It means nothing other than extinction and at the moment we are not extinct. So we are evolving. Evolution has not stopped for humans.<\/p>\n

    So, if we measure phenotypic change in humans over time and see some change, then that must mean we are evolving. But, if we measure phenotypic change in humans over time and don’t see any, then that does not mean we are not evolving. We are still evolving even if there is no phenotypic change.<\/p>\n

    There are many other aspects to this question, which I’ll explore later. For now, I’ll just briefly touch on the second iteration mentioned above: The fallacy that serious scientists ever consider that evolution has stopped\/not stopped for humans. By now you probably understand what this is about.<\/p>\n

    Now and then you will see a headline in the science section of the NY Times or somewhere that says “Scientists show that evolution has not stopped for humans after all!” or words to that effect. There are two things going on here. One is the press playing off the fallacy that evolution has stopped for humans. The other is that some scientists who is probably not really a biologist, or certainly not a population geneticist, woke up one day and realized that evolution has not stopped for humans. No serious scientists who actually studies human evolution would ever have this thought.<\/p>\n

    Above I talk only about neutral evolution. This does not mean that I don’t think meaningful, adaptive change is not happening now with humans. I’m just not talking about that in this post. Another time.<\/p>\n

    There is also an interesting discussion to be had as to why humans tend to gravitate towards presuming the evolution has stopped for our species. The reasons are often not just born of ignorance, but can be rather nefarious. As you might expect given that we evolved from apes …<\/p>\n

    More Falsehoods !!!<\/em><\/h3>\n

    This post is one of a series on the topic of falsehoods. The following is a list of falsehoods posts in order:<\/p>\n