NOTE: These are lecture notes for Biology 391, Organic Evolution, at The University of Tennesee at Martin. Anyone outside of UT Martin wishing to use these notes or to contact me for additional information should first read the information obtained by clicking here.
Goals: evaluate studies in which evolutionary theory is applied to human behavior. Review methods of studying heritability; discuss problems in identifying heritability of human behavioral traits. Review kin selection and sexual selection and discuss studies in which these concepts have been applied to human behavior.
Related Textbook Material: Freeman and Herron (2001) Chapters 7 and 16
Lab Manual Questions over this material are in Lab Manual Chapter XXII
The Lecture:
An area of particular interest in the evolution of humans is the evolution of human behavior. Clearly we have some of the most complex social behavioral characteristics of any animal. One difficulty in studying human behavior from an evolutionary perspective is that much human behavior is learned. Behavior that is learned, not genetic, is not subject to evolution. Presumably the ability to learn is something that evolved, but once it evolved, then the specific things we learn to do are not evolved traits.
It has been extremely difficult to determine how much of human behavior is learned and how much is genetic. Recent studies coming from the Human Genome Project have identified some alleles that seem to be responsible for some behavioral characteristics -- alleles associated with having a nervous temperament for example. Such alleles may affect how people behave, but their affects can also apparently be modified by learning.
Human behavioral characteristics may also be polygenic (quantitative). Click here to review quantitative traits and heritability. Various people have tried to estimate the genetic variability of human behavioral traits by estimating their heritability. This turns out to be difficult. Remember that we typically try to estimate heritability through mid-parent offspring regression, but that a possible bias in mid- parent offspring regression is that offspring may be similar to their parents for environmental reasons rather than genetic reasons, making the heritability estimates too high. This is particularly likely for human behavior since children learn from their parents and are expected to behaviorally like their parents because of this learning.
Some studies attempting to estimate heritability have looked at identical twins who were separated at birth and adopted by different families. The idea behind this is that these individuals are genetically identical, but raised in different environments. If they develop behavioral traits that are more similar to each other than to their adopted parents or adopted siblings, it suggests a genetic basis to these behavioral traits; if they develop behavioral traits that are more similar to their adopted parents and adopted siblings, it suggests these traits are learned, or environmental. Such studies have suggested that some traits, such as IQ, have a heritability of about 0.5; that would mean that about half of the variation among individuals in IQ is genetic and half environmental. However, these studies also have potential problems. First, they do not really rule out environmental similarities because even though the twins were separated at birth, they shared the same environment before birth, in their mother's uterus. We know that the prenatal environment can affect behavior of offspring from some of the effects of drug and alcohol use on offspring. For example, one affect of excessive alcohol use in pregnant mothers is shorter attention span in their offspring; a short attention span will affect various other behavioral characteristics and is likely to be similar for identical twins who come from mothers who used too much alcohol during pregnancy. But note that this is an environmental, not a genetic, effect. Another reason for similarities in behavior between identical twins, even if they are raised separately, is that they may be treated similarly in some ways because of their identical appearance. Social perception and treatment of them because they are the same ethnic group and same sex may be similar.
The real point to the preceding paragraphs is that it is difficult to determine how much genetic variation there is in human behavior. Studies of this area also tend to make people respond emotionally -- get mad, for example. They may be hard to evaluate without including our own bias; I'm sure my bias is evident in this lecture. For example, I spent a lot of time talking about the problems with studies of the genetic basis and noting that it appears that genetic tendencies toward certain behavioral characteristics can be modified by learning. Someone else might have focused more on the evidence that these traits are genetic and spent less time on the problems.
Without knowing the heritability of human behavior, it's hard to know what aspects are subject to evolution. There have been studies, however, that examine human behavioral characteristics to see whether they fit what we would predict if human behavior has evolved through natural selection and is adaptive. Most of these studies have focused on social behavior. Sociobiology is the study of the evolution of social behavior in general (throughout animals); human sociobiology, the study of the evolution of human social behavior, is an active area of research.
Some studies of human sociobiology evaluate whether human behavior fits predictions of some of the areas of behavioral evolution we have studied in this course, including sexual selection and mate choice, and kin selection and altruism.
Here's a study of mate choice that tests an honest advertising hypothesis (Click here if you need to review sexual selection theory.) Women were given a choice of different male odors; the odors were obtained by having men wear the same t-shirt for 2 days, then women were given the t-shirts and asked questions on how attractive or unattractive the odors were and whether they did or did not remind them of previous or current boyfriends/husbands. Men and women were also tested for alleles related to immune system function (from the MHC, or major histocompatability complex, loci.) Women's answers to the questions indicated that they had a preference for male odor from men whose MHC differed from their own. Since immuns sytem diversity may increase health, the idea is that if women reproduced with men with different MHC, their offspring might be healthier.
Here's a study of human behavior that tests predictions of kin selection. Human cultures vary in sexual promiscuity so that there are some cultures where men can be very sure that the children of their marriage are really their own offspring and other cultures where men are not very sure who the father of "their" children -- the children of their wife, in their marriage -- really are. These cultures differ from each other in how men pass their wealth to future generations. In cultures where men have high certainty of paternity, they leave their money and possessions to their children (the children of their marriage.) In cultures where men have low certainty of paternity, they leave their money and possessions to the offspring of their sisters. Think about it and you will see that they are most certain to be genetically related to the offspring of their sisters, so leaving their wealth this way fits what we would expect through kin selection. Click here to review altruism and kin selection
I'd like to end with a few comments on another way evolutionary biology and studies of the natural world have been applied to understanding human life. Some people have attempted to use patterns that have evolved in the natural world to determine how humans should behave -- what is "natural" is used to say somehow what ought to happen among humans or what is moral. In my opinion, and I think in the opinion of most other evolutionary biologists (certainly it is expressed in several of the textbooks on evolution) this is totally inappropriate. I'd like to give some examples of how it has been used and why I think it's wrong. People have argued that because there is more parental care by females in other species, that the "natural" and right thing for humans is for women to stay home with children and men to work. People have argued that because altruism is typically directed toward relatives, that hatred among human races is "natural" since one is likely to be less related toward members of a different rate, and they've argued that because it is natural it is somehow excusable. People have argued that because others species do not show homosexuality, that it is unnatural and therefore wrong in humans. Now, a first problem with these arguments is that there are exceptions in the "natural" world of animals other than humans to these trends -- species where males do the parental care (such as seahorses, for example), species where there is altruism between non-relatives (what is called "reciprocal altruism" -- individuals help others AND receive help so they are both altruists and recipients; this occurs, for example, in vampire bats), and species where there is a lot of homosexual behavior (for example, in one of the closest relatives to the humans, the bonobos, there are long-term bonds between females that involve sexual behavior, and short term sexual contacts between males.) So it is very hard to say that something is or is not "natural." Different people could choose different examples and say "this is natural" to justify what they thought was appropriate for humans. A more important problem, though, is that there's no reason to say that what occurs in the natural world should be what is moral for humans. As humans we have our own cultures and religions that guide us morally. We started out this term reading a paper by Gould that argued that science and religion were two different "magisteria" -- different ways of knowing things -- and that the areas where we should be guided by these magisteria are different. Throughout this course we've been using the magisterium of science to study evolution based on observation, data, and the scientific method, and we have not applied religion to understanding these questions. Similarly, I would argue that we should not apply what we learn from the results of evolutionary biology to our moral behavior -- science is the wrong magisterium for studying morality; there are other ways to determine what is right, or moral, for humans to do. Well, I won't test you over my opinions on this, but I couldn't tell you that until the end of the paragraph because after all they are my opinions and I wanted you to read them.