Jan 26 JDN 2460702
It frustrates me that in American society, evolutionary biology is considered a controversial topic. When I use knowledge from quantum physics or from organic chemistry, all I need to do is cite a credible source; I don’t need to preface it with a defense of the entire scientific field. Yet in the United States today, even basic statements of facts observed in evolutionary biology are met with incredulity. The consensus in the scientific community about evolution is greater than the consensus about quantum physics, and comparable to the consensus about organic chemistry. 95% of scientists agree that evolution happens, that Darwinian natural selection is the primary cause, and that human beings share a common ancestor with every other life form on Earth. Polls of scientists have consistently made this clear, and the wild success of Project Steve continues to vividly demonstrate it.
But I would rather defend evolution than have to tiptoe around it, or worse have my conclusions ignored because I use it. So, here goes.
You may think you understand evolution, but especially if you doubt that evolution is true, odds are good that you really don’t. Even most people who have taken college courses in evolutionary biology have difficulty understanding evolution.
Evolution is a very rich and complicated science, and I don’t have room to do it justice here. I merely hope that I can give you enough background to make sense of the core concepts, and convince you that evolution is real and important.
Foundations of genetics
So let us start at the beginning. DNA—deoxyribonucleic acid—is a macromolecular (very big and complicated) organic (carbon-based) acid (chemical that can give up hydrogen ions in solution) that is produced by all living cells. More properly, it is a class of macromolecular organic acids, because differences between DNA strands are actually chemical differences in the molecule. The structure of DNA consists of two long chains of constituent molecules called nucleotides; for chemical reasons nucleotides usually bond in pairs, adenine (A) with thymine (T), guanine (G) with cytosine (C). Pairs of nucleotides are called base pairs. We call it a “double-helix” because the two chains are normally wrapped around each other in a helix shape.
Because of this base-pair correspondence, the two strands of a DNA molecule are complementary; if one half is GATTACA, the other half will be CTAATGT. This process is reversible. Either strand can be reproduced from the other; this is how DNA replicates. A DNA strand GATTACA/CTAATGT can split into its GATTACA half and its CTAATGT half, and then the original GATTACA half will acquire new nucleotides and make a new CTAATGT for itself; similarly the original CTAATGT half will make a new GATTACA. At the end of this process, two precise copies of the original GATTACA/CTAATGT strand will result. This process can be repeated as necessary.
DNA molecules can vary in size from a few base-pairs (like the sequence GATTACA), to the 16,000 base-pairs of Carsonella bacteria, up to the 3 billion base-pairs of humans and beyond. While complexity of DNA and complexity of organism are surely related (it’s impossible to make a really complicated organism with very simple DNA), more base pairs does not necessarily imply a more complex organism. The single-celled amoeboid Polychaos dubium has 670 billion base-pairs. Amoeboids are relatively complex, all things considered; but they’re hardly 200 times more complex than we are!
The copying of DNA is exceedingly precise, but like anything in real life, not perfect. Cells have many physical and chemical mechanisms to correct bad copying, but sometimes—about 1 in 1 million base-pairs copied—something goes wrong. Sometimes, one nucleotide gets switched for another; perhaps what should have been a T becomes an A, or what should have been an A becomes a G. Other times, a whole sequence of DNA gets duplicated and inserted in a new place; still other times entire pieces of DNA are lost, never to be copied again. In some cases a sequence is flipped around backwards. All of these things (a single-nucleotide substitution, an insertion, a deletion, and an inversion, respectively) are forms of mutation. Mutation is always happening, but it can be increased by the presence of radiation, toxins, and other stresses. Usually cells with mutant DNA are killed by the immune system; if not, mutant body cells can cause cancer or other health problems. Usually it’s only mutations in gametes—the sperm and egg cells that carry DNA to the next generation—that actually have a long-term effect on future generations. Most mutations do not have any significant effect, and most of those that do have bad effects. It is only the rare minority of mutations that actually produces something useful to an organism’s survival.
What does DNA do? It makes proteins. Technically, proteins make other proteins (enzymes called transcriptases and polymerases and so on), but which protein is produced by such a process is dependent upon the order of base pairs in a DNA strand. DNA has been likened to a “code” or a “message”, but this is a little misleading. It’s definitely a sequence that contains information, but the “code” is less like a cryptographer’s cipher and more like a computer’s machine code; it interacts directly with the hardware to produce an output. And it’s important to understand that when DNA is “read” and “decoded”, it’s all happening purely by chemical reactions, and there is no conscious being doing the reading. While metaphorically we might say that DNA is a “code” or a “language”, we must not take these metaphors too literally; DNA is not a language in the same sense as English, nor is it a code in the same sense as the Enigma cipher.
Genotype and phenotype
DNA is also not a “blueprint”, as it is sometimes described. There is a one-to-one correspondence between a house and its blueprint: given a house, it would be easy to draw a blueprint much like the original blueprint; given a blueprint, one can construct basically the same house. DNA is not like this. There is no one-to-one correspondence between DNA and a living organism’s structure. Given the traits of an organism, it is impossible to reconstruct its DNA—and purely from the DNA, it is impossible to reconstruct the organism. A better analogy is to a recipe, which offers a general guide as to what to make and how to make it, but depending on the cook and the ingredients, may give quite different results. The ingredients in this case are nutrients, and the “cook” is the whole of our experience and interaction with the environment. No experience or environment can act upon us unless we have the right genes and nutrients to make it effective. No matter how long you let it sit, bread with no yeast will never rise—and no matter how hard you try to teach him, your dog will never be able to speak in fluent sentences.
Furthermore, genes rarely do only one thing in an organism; much as drugs have side effects, so do genes, a phenomenon called pleiotropy. Some genes are more pleiotropic than others, but really, all genes are pleiotropic. In any complex organism, genes will have complex effects. The genes of an organism are its genotype; the actual traits that it has are its phenotype. We have these two different words precisely because they are different things; genotype influences phenotype, but many other things influence phenotype besides genotype. The answer to the question “Nature or Nurture?” is always—always—“Both”. There are much more useful questions to ask, like “How much of the variation of this trait within this population is attributable to genetic differences?”, “How do environmental conditions trigger this phenotype in the presence of this genotype?”, and “Under what ecological circumstances would this genotype evolve?”
This is why it’s a bit misleading to talk about the “the gene for homosexuality” or “the gene for religiosity”; taken literally this would be like saying “the ingredient for chocolate cake” or “the beam for the Empire State Building”. At best we can distinguish certain genes that might, in the context of many other genes and environmental contributions, make a difference between particular states—much as removing the cocoa from chocolate cake makes some other kind of cake, it could be that removing a particular gene from someone strongly homosexual might make them nearer to heterosexual. It’s not that genes can be mapped one-to-one to traits of an organism; but rather that in many cases a genetic difference corresponds to a difference in traits that is ecologically significant. This is what geneticists mean when they say “the gene for X”; it’s a very useful concept in evolutionary theory, but I don’t think it’s one most laypeople understand. As usual, Richard Dawkins explains this matter brilliantly:
Probably the first point to make is that whenever a geneticist speaks of a gene `for’ such and such a characteristic, say brown eyes, he never means that this gene affects nothing else, nor that it is the only gene contributing to the brown pigmentation. Most genes have many distantly ramified and apparently unconnected effects. A vast number of genes are necessary for the development of eyes and their pigment. When a geneticist talks about a single gene effect, he is always talking about a difference between individuals. A gene `for brown eyes’ is not a gene that, alone and unaided, manufactures brown pigment. It is a gene that, when compared with its alleles (alternatives at the same chromosomal locus), in a normal environment, is responsible for the difference in eye colour between individuals possessing the gene and individuals not possessing the gene. The statement `G1 is a gene for phenotypic characteristic P1‘ is always a shorthand. It always implies the existence, or potential existence, of at least one alternative gene G2, and at least one alternative characteristic P2. It also implies a normal developmental environment, including the presence of the other genes which are common in the gene pool as a whole, and therefore likely to be in the same body. If all individuals had two copies of the gene `for’ brown eyes and if no other eye colour ever occurred, the `gene for brown eyes’ would strictly be a meaningless concept. It can only be defined by reference to at least one potential alternative. Of course any gene exists physically in the sense of being a length of DNA; but it is only properly called a gene `for X’ if there is at least one alternative gene at the same chromosomal locus, which leads to not X.
It follows that there is no clear limit to the complexity of the `X’ which we may substitute in the phrase `a gene for X’. Reading, for example, is a learned skill of immense and subtle complexity. A gene for reading would, to naive common sense, be an absurd notion. Yet, if we follow genetic terminological convention to its logical conclusion, all that would be necessary in order to establish the existence of a gene for reading is the existence of a gene for not reading. If a gene G2 could be found which infallibly caused in its possessors the particular brain lesion necessary to induce specific dyslexia, it would follow that G1, the gene which all the rest of us have in double dose at that chromosomal locus, would by definition have to be called a gene for reading.
It’s important to keep this in mind when interpreting any new ideas or evidence from biology. Just as cocoa by itself is not chocolate cake because one also needs all the other ingredients that make it cake in the first place, “the gay gene” cannot exist in isolation because in order to be gay one needs all the other biological and neurological structures that make one a human being in the first place. Moreover, just as cocoa changes the consistency of a cake so that other ingredients may need to be changed to compensate, so a hypothetical“gay gene” might have other biological or neurological effects that would be inseparable from its contribution to sexual orientation.
It’s also important to point out that hereditary is not the same thing as genetic. By comparing pedigrees, it is relatively straightforward to determine the heritability of a trait within a population—but this is not the same as determining whether the trait is genetic. A great many traits are systematically inherited from parents that have nothing to do with DNA—like language, culture, and wealth. (These too can evolve, but it’s a different kind of evolution.) In the United States, IQ is about 80% heritable; but so is height, and yet nutrition has large, well-documented effects on height (The simplest case: malnourished people never grow very tall). If, as is almost certainly the case, there are many environmental influences such as culture and education that can affect IQ scores, then the heritability of IQ tells us very little.
In fact, some traits are genetic but not hereditary! Certain rare genetic diseases can appear by what is called de novo mutation; the genes that cause them can randomly appear in an individual without having been present in their parents. Neurofibromatosis occurs in as many people with no family history as it does in people with family history; and yet, neurofibromatosis is definitely a genetic disorder, for it can be traced to particular sections of defective DNA.
Honestly, most of the debate about nature versus nurture in human behavior is really quite pointless. Even if you ignore the general facts that phenotype is always an interaction between genes and environment, and feedback occurs between genes and environment over evolutionary time, human beings are the species for which the “Nature or nurture?” question reaches its most meaningless. It is human nature to be nurtured; it is written within our genes that we should be flexible, intelligent beings capable of learning and training far beyond our congenital capacities. An ant’s genes are not written that way; ants play out essentially the same program in every place and time, because that program is hard-wired within them. Humans have an enormous variety of behaviors—far outstripping the variety in any other species—despite having genetic variation of only about 0.1%; clearly most of the differences between humans are environmental. Yet, it is precisely the genes that code for being Homo sapiens that make this possible; if we’d had the genes of an ant or an earthworm, we wouldn’t have this enormous behavioral plasticity. So each person is who they are largely because of their environment—but that itself would not be true without the genes we all share.
Human Economics 