Behavioral Genetics and Sociobiology
Chromosomes, Genes, and Environment
Within the nuclei of living human cells are 23 pairs of rod-like structures
called chromosomes. The chromosomes contain the units of inheritance, genes,
which, in interaction with the environment, guide the individual's physical
development, including the development of the brain and nervous system. Since the
brain and nervous system mediate mental and behavioral processes, it is reasonable to
include genetics in one's study of psychology.
An individual gene may be thought of as coding for the production of one or
another or no protein. Genes are found within the nucleus of every cell in your body.
They are composed of DNA. This DNA serves as the template for the production of
RNA. The RNA serves as the template for producing proteins from amino acids.
Collectively, the combination of the proteins produced by all the genes is a living
organism, like you.
Of course there must be available in the environment certain basic ingredients
for normal development of the organism. Development is best thought of as being due
to the joint operation of genes and environment:
• Different environments lead to different developments, so two genetically
identical zygotes (fertilized eggs) in two different environments would be
expected to develop differently.
• Likewise, two zygotes with different genes would be expected to develop
differently even if they were in identical environments.
• Furthermore, the environment that would lead to the "best" development for an
individual of one genetic constitution may not be the same as the environment
that would lead to the best development for a genetically different individual.
This is known as Gene x Environment interaction.
Mitosis and Meiosis
Cells in your body can reproduce themselves through a process known as
mitosis. Barring errors, mitosis results in exact duplication of the DNA in the parent
cell. Germ cells produce eggs or sperm through a process known as meiosis. While it
is not important, for this class, for you to know all of the details of meiosis, you should
know the following:
• There is an exchange of genetic material between paired chromosomes
• Each gamete (sperm or egg) receives only one of each of the paired
• Which one of the two chromosomes in each pair goes to an individual gamete is
random, not related to which one of the two chromosomes in any of the other 22
pairs of goes to that gamete. This is known as random assortment.
• This crossing-over and random assortment produces a remarkable “shuffling” of
the genes that go into the gametes, such that it is nearly impossible that any two
gametes produced by the same individual are identical.
In sexual reproduction the zygote formed by the union of one sperm and one
egg has approximately half of its DNA from its father and half from its mother. The
shuffling of the genes that took place during meiosis combined with the contribution of
genes from two different parents produces great genetic diversity among the
offspring. This genetic diversity is likely to be of great benefit in helping a species
adapt to changing environments. In that circumstance, it is likely that a few of the
new genetic combinations will be especially good at living in a new environment, even if
most of the new genetic combinations are not.
Now, if the environment was not changing rapidly, then the best strategy would
be to reproduce asexually, and many organisms do exactly that. After all, if you have
put together a good combination of genes, why mix them all up again. Think of a good
combination of genes as being like a good hand in poker. An organism has a lot more
that 6 genes, but, to keep it simple, imagine that we are playing 6 card poker. One
parent has a very good hand, a straight flush, 2, 3, 4, 5, 6, 7 of hearts. The other
parent has an equally good hand, 2, 3, 4, 5, 6, 7 of spades. If they sexually reproduce,
each gives half of its genes (cards) to the offspring. Who is going to have the better set
of genes (cards), the parents or the offspring?
Some organisms have adopted the marvelous strategy of reproducing asexually
when the environment is stable, but shifting to a sexual mode of reproduction during
times of great environmental change, so they can find a new combination of genes
better suited to the changed environment, then back to asexual reproduction once
things have settled down again. One example of such a creature is the hydra. Point
your browser to http://biology.about.com/library/weekly/aa090700a.htm for more
information on this. It has been suggested that long ago all species reproduced by this
strategy, but then most of them lost that ability during a period of prolonged
environmental change and/or during a time when the number of individuals left in the
breeding population was very small.
Defining Sex Biologically
When biologists determine which morph of a species is male and which female,
it is the size of their gametes that is important. The sex that produces the larger
gametes (eggs) is female, that producing the smaller gametes (sperm) male. In
humans having two alike sex chromosomes (XX) is associated with the female sex and
two different sex chromosomes (XY) with the male sex. This is not universally so. In
birds the males have two alike sex chromosomes, the females two different sex
The male human typically produces more male sex hormones (androgens,
including testosterone) and less female sex hormones than does the female human.
There are also well known differences between the genitalia of males (penis and
scrotum) and females (clitoris and vagina). Gender identity (with which of the two
social categories, "female" or "male," does one identify) usually but not always agrees
with biological definitions of sex.
If you were to look at a single locus (location of a gene) on each member of pair
of your chromosomes, you might find that the gene on the one member is identical to
the gene on the other chromosome. You would then be said to be homozygous at
that locus. However, the genes that can be found at a given locus can come in
different varieties, alleles. If you have two different alleles at a given locus, then you
are heterozygous at that locus.
The phenotype of an organism is its observable characteristics, while the
genotype of an organism is the particular set of genes it has inherited. Most
observable characteristics result from the interplay of genes at several loci (and thus
are polygenic), but for some characteristics, such as the wrinkling of Gregor Mendel’s
pea seeds, the characteristic results from the genes at a single locus.
Rather than dealing with the wrinkling of pea seeds, I shall discuss a simple
model of the inheritance of a blood type in humans. One allele for blood type is the A
allele. Individuals who have one or two A genes produce a particular protein (we shall
just call it protein A) in their blood. Because it takes only one A gene for the A
characteristic to be expressed in the blood, the A gene is considered dominant.
Another allele is the o gene. The o gene may be thought of as coding for the
production of no extra protein in the blood. Because an individual has to have two o
genes to have the phenotype of Type O blood, the gene o is recessive.
There is a third common allele for blood type, the B gene. The B gene is
dominant -- that is, it takes only one B gene for the B protein to be produced in the
blood. When there are two or more dominant genes available at a locus, then we have
codominance. An individual with two different dominant genes will express the
characteristics of both dominant genes.
Genotype oo Ao AA Bo BB AB
Blood Type O A A B B AB
Genetic Context and Inheritance
I have grossly oversimplified the concept of dominance in genetic inheritance.
Whether a gene is dominant, codominant, or recessive is a function of the other genes
which are involved in the chain of reaction that leads to a product that modifies a
phenotype. Even the simplest of traits are generally regulated by a long chain of
processes that interact with one another in constructing the final product. Each step in
this chain is under the control of a gene. If one of the steps is rate-limiting, then the
gene at that step may appear to be the only gene that influences the phenotypical trait.
For example, think about water flowing through a series of funnels, from each to the
next and finally into the phenotypic basin. The wider a funnel at its base, the faster the
water can flow through it. The one funnel with the most narrow base will provide the
rate-limiting step. Water cannot pass through the system any faster than it can through
the slowest funnel. Now think of each funnel’s size as being determined by a different
gene. A gene that affects the size of the rate-limiting funnel will affect the phenotype,
but genes that affect the other funnels will not (unless they reduce the size of one of the
other funnels to less than that of the rate-limiting funnels). In this way, it appears that
a single gene is affecting the phenotype, but, in fact, the phenotypic product
results from the interaction of many genes.
Consider the inheritance of purple color in snapdragon flowers. The gene that
controls the rate-limiting step has two alleles, one dominant and one recessive. The
recessive gene codes for the production of only a small amount of the biological
product that is involved in making the flowers purple. The dominant gene codes for a
large amount of that product. A plant that has two of the recessive genes will have very
little of the purple-producing product. A plant that has one recessive gene and one
dominant gene will have a medium amount of the purple-producing product. A plant
that has two dominant genes will have much of the purple-producing product. Now,
imagine that there is a certain amount (a threshold) of the purple-producing product
that must be made for there to be any effect on the phenotype. A plant with less than
the threshold value will be white, and a plant with more than the threshold value will be
purple. A homozygous recessive snapdragon will not reach the threshold, so it will
have no purple flowers, just white flowers, but the heterozygous plant with a single
dominant gene will produce enough of the purple-producing product to exceed the
threshold and cause the plant to have purple flowers. Having two dominant genes is no
more effective in getting across the threshold than is having just one dominant gene, so
the homozygous dominant plant looks just like the heterozygous plant, with purple
A more complete discussion of the effects of genetic context on inheritance can
be found in the excellent article The Importance of Context in Genetics, 2003,
American Scientist, 91, 416-423.
Simple Inheritance of Fearfulness in Dogs
John Paul Scott and John Fuller demonstrated simple inheritance of a sort of
fearfulness in dogs. They started with basenjis, who are fearful of strange people, and
cocker spaniels, who do not show such fear. When they crossed basenjis with cockers,
all of the F1 offspring expressed such fear. This is consistent with the hypothesis that
such fearfulness is controlled by a genes at a single locus with the allele coding
Father F f
F FF Ff
f fF ff
fearfulness being dominant. Let us use the letter F to stand for the dominant, fearful
gene and f to stand for the recessive, non-fearful gene. The genotype of the F1
offspring is then Ff.
Now, what should happen if we cross the F1 offspring with one another? As you
can see in the diagram to the right, we would expect to get 75% fearful pups, 25% not
fearful, which is exactly what Scott and Fuller observed.
Scott and Fuller also backcrossed the F1 generation with the purebred cockers
and purbred basenjis. As you can see from the below tables, the expectation is that the
F1 - cocker matings would produce 50% fearful pups and the F1 - basenji matings 100%
fearful pups. The expected percentages were obtained.
F1 f f F1 F F
F Ff Ff F FF FF
f ff ff f fF fF
I should add that Scott and Fuller also found other traits in these dogs that were
inherited simply and independently of the fearfulness trait.
Nature and Nurture, Genetics and Environment
“Nature” refers to the effects of genes, “nurture” refers to the effects of
Some scientists produce estimates of the heritability of various characteristics
-- that is, in an entire population of individuals, what percentage of the differences
between individuals on that characteristic is due to genetic differences between
individuals rather than to differences in the environments to which the individuals have
been exposed. These estimates can be easily misinterpreted.
Consider the characteristic of aggressiveness in mice. Suppose that I am
working with inbred mice, mice that are nearly genetically identical to one another.
Since the mice do not differ from one another genetically, 100% of the observed
differences in their aggressiveness must be due to environmental factors, 0% to
genetics. Now suppose that I work with outbred mice, but I rear them in highly
controlled laboratory conditions so that each mouse has the same environment as all
other mice. In this circumstance 100% of the observed differences in aggressiveness
must be due to genetics, 0% to environment. When someone gives you a heritability
estimate for some characteristic, do keep in mind the fact that such an estimate is
specific to a population with a particular amount of environmental variance and a
particular amount of genetic variance. If the population is changed such that individuals
differ from one another less in terms of their individual environments or more in terms
of their genotypes, then the heritability would increase.
Psychologists have often made the mistake of believing that if a trait can be
modified through environmental change then it must have been initially established
environmentally also. For example, if a behavioral disorder can be treated with
environmental change (such as behavior modification techniques, changing response
contingencies), the psychologist may conclude that the disorder was caused by a bad
environment (the mother is often blamed). Such inferences are not justified, since
genetically caused events can be environmentally modified. For example, PKU
(phenylketonuria) is a disease caused by a person having a pair of recessive genes --
not having the normal dominant gene, these persons do not produce an enzyme that is
necessary properly to metabolize certain foodstuffs. One consequence of their
abnormal metabolism of such foodstuffs is the production of poisons that damage their
brains and lead to mental retardation. Despite the fact that PKU is inherited, a simple
environmental treatment (avoiding foods that contain phenylalanine) solves the
Genetic constitution may cause an animal to select or create a particular type of
environment. That environment may then cause the animal to develop in a way
different than it would if it were in a different environment. That is,
genes → habitat selection → behavioral development. In such a case, what is
causing the differences in behavior for animals who have experienced different
environments?. At the proximal level the environment is causing the differences, but
at the ultimate level the differences are genetically caused, since genes caused the
animals to choose or create their environments.
Wecker (1964, Scientific American) studied two subspecies of prairie deer mice.
Prairie deer mice are cute little critters. They have a baby face with big bulging eyes,
and a short bicolored tail, cinnamon brown on top, cream on bottom. I worked with
them when I lived in Ohio. A similar species can be found here in North Carolina. They
are native American mice, as opposed to the imported European house mouse. I
should warn you that deer mice have been identified as one of the species that carries
the deadly Hanta virus.
Photo of Peromyscus
One breed of these mice usually lives in the forest, the other in the plains. If
you give these mice a choice between living in a field or living in a wooded area the
plains breed chooses the field and the forest breed chooses the woods. This habitat
preference is not caused by early experience, it is genetically caused. If you raise the
plains breed in the woods they still select the field when given a choice. So, what if you
try to explain other naturally occurring behavioral differences between these two
subspecies of prairie deer mice? The psychologist would be tempted to say that they
act differently because they are raised in different environments, and, at a proximate
level, this may be true. Being raised in the plains might well lead to different behavioral
development than being raised in the woods -- but if it is genetics that is causing the
one breed to choose to live in the plains and the other breed to choose to live in the
woods, isn't genetics a higher-level cause of the behavioral differences between the
A similar confounding (entanglement) of genetics and environment may
occur in human development. Certain parenting styles tend to be associated with the
development of "competent" children, other styles with less desirable development
(recall our discussion of Diana Baumrind’s research, from Chapter 2 of Gray.
Psychologists have often used such associations to confirm their belief that different
parenting styles (providing the children with different social environments) cause
different development. That is, their explanation of the observed correlation is:
parental style → child’s competence.
An alternative explanation is tenable. Children seem to be born with different
temperaments and behavioral dispositions. Some love contact, others avoid it, some
fuss, others are quiet, etc. Although the fetal environment may cause some such
differences, genetics likely account for most of them. Babies who act differently
evoke different responses from their parents, so the association between
characteristics of the child and parental behavior may be as much a case of the child's
behavior affecting the parent as vice versa. Maybe competent children are those who
were born with the genetic constitution that disposes them towards developing into
competent persons and also disposing them into acting in ways that cause their parents
to adopt that parenting style associated with the development of competence. This
could account for the observed association between parenting style and development
of competence even in the absence of any effect of parenting style on the development
of competence. That is,
child’s genes → child’s competence → parental style.
Another possible exists. A baby's genetic constitution may cause it to act in
ways that evoke from its parents the type of parental care that causes the child to
develop a certain way. That is, the child's genes cause it to select/create the
environment (make the parents act a certain way) that determines the child's
child’s genes → child’s behavior → parental style → child’s development of
So, what is causing differences in children's competence, parental care
(environment) or genetics?
Inheritance of Polygenic Characteristics
The strongest evidence for inheritance of behavioral dispositions comes from
nonhumans, upon whom we can do experiments such as selective breeding. Consider
a study done long ago by Tryon (1940, Yearbook of the National Society for Studies in
Education, 39, 111-119). The maze-solving "intelligence" of rats was measured.
The "smartest" males were mated with the smartest females and from their offspring
only the smartest mated with the smartest et cetera across six generations. The most
"stupid" males were paired with the most stupid females across six generations too.
The result was the creation of two different strains of rats, one very good at solving
mazes and one very poor at doing so. This is quite convincing evidence that
maze-intelligence in rats is heritable, genetically influenced. Many such studies have
been done with nonhumans, clearly establishing the heritability of various psychological
attributes. The more sophisticated studies control for the effects of parental care. Why
do intelligent parents produce intelligent offspring? Is it because they give their
offspring their intelligence-enhancing genes or because they raise their offspring
differently than do less intelligent parents or both? One technique for separating the
effects of parental care from the effects of genetics is cross-fostering. Using Tryon's
study as an example, we take half of the smart parents' offspring and foster them onto
dull parents and half of the dull parents' offspring and foster them onto smart parents.
This gives us 2 BP (biological parents) x 2 FP (foster parents) = 4 combinations to
evaluate: smart BP & FP, smart BP dull FP, dull BP smart FP, and dull BP & FP. Often
both genetic (BP) and environmental (FP) factors prove to be important.
So what about humans? The research with humans is bound to be more
complicated and difficult to interpret since we cannot do selective breeding experiments
with humans. One approach is to measure the psychological similarity between
monozygotic (identical) twins, dizygotic (fraternal) twins, siblings, and other pairs with
varied degrees of genetic relatedness. When psychological similarity is correlated
with genetic relatedness, then the trait is considered to be influenced by genetics.
For example, monozygotic twins are psychologically more alike one another than are
dizygotic twins. Such evidence is even stronger when the data include "natural
cross-fostering experiments," that is, persons who have been adopted. Again, the
usual result is that both genetic and environmental factors prove important.
Among the psychological attributes now considered to be influenced by genetics are:
extroversion (Loehlin et al., 1982, J. Person. Soc. Psychol., 42, 1089-1099),
neuroticism (Scarr et al., 1981, J. Person. Soc. Psychol., 41, 885-898), dominance
and aggressiveness (Goldsmith, 1983, Child Development, 54, 331-355), and
intelligence (Bouchard & McGue, 1981, Science, 212, 1055-1059). We shall consider
the evidence for the heritability of intelligence later in the semester, when we cover
If you still have trouble accepting the notion that genes influence psychological
characteristics, ask yourself this question: What would have happened if you were
mistakenly exchanged with another baby at the hospital where you were born?
Would your parent's foster-baby have grown up to be exactly the same person
you are now? If you answer "no," then you accept the influence of genes. Would you
(raised by other parents) have grown up to be exactly the same person you are
now? Your "no" to this question indicates you also accept the influence of
Behavior Genetics, Free Will, and Egalitarianism
Most people have little difficulty accepting the fact that the genes they received
from their parents influence their physical characteristics such as skin/hair/eye color,
blood type, type of ear lobe, etc. Some have difficulty, however, with the idea that
one's mental and behavioral dispositions are also influenced by genetics.
Perhaps this difficulty stems in part from our clinging to the delusion of "free will,"
and we did not choose our genes. If you accept the idea that mental and behavioral
events are the product of an interaction between the brain/nervous system and the
environment, then you should be able to deduce that mental and behavioral events are
genetically influenced, since the brain and nervous system are physical structures, and
genes clearly affect organisms' physical characteristics.
Few people would argue that the differences between human beings and
other animals stem only from environmental effects. A dog raised in the same
environment as a human remains quite different from a human. Clearly genes are
important in determining differences between different species. Actually the genetic
differences between humans and other apes are relatively small. The genetic
differences between you and me are yet smaller. Humans share a common genetic
constitution, with only relatively minor differences among us. These minor differences
are, however, associated with physical and psychological differences among us. Part
of our reluctance to accept the notion that genetics in part determines the differences
among us may result from our egalitarianism. Egalitarianism is the belief that all
persons should have equal political and social rights, a belief to which I very strongly
subscribe. Egalitarianism does not, however, require that all humans be identical
on all characteristics. You and I can differ on various attributes but still be granted
equal rights, can't we? Another part of our fear of genetic explanations of human
differences may stem from past and present abuses of genetic explanations by
racists. To the best of my knowledge there is no evidence that the genes that
determine skin-color have any direct effect on psychological attributes. Such
genes may, of course, interact with the environment in a racist society, leading to
psychological differences. For example, suppose that people with blue skin are the
subject of unfair discrimination, not being given the same political, economic, and social
rights that others are given. Such unequal treatment would likely lead to psychological
differences between blue people and others, thus producing an association between
blueness and psychological characteristics.
By the way, there actually are blue humans. They are blue (almost purple)
because they lack a hormone that converts a blue type of hemoglobin
(methemoglobin) into normal red hemoglobin. Curiously, injection of a blue dye,
methylene blue, will turn these people pink by converting the blue hemoglobin into
normal hemoglobin. The condition is not associated with ill health. This congenital
abnormality appears to be inherited via a recessive gene or genes. As would be
expected for a rare condition due to recessive genes, its occurrence is associated with
inbreeding. Cathy Trost’s article in the November issue of Science 85 (p. 35 - 39) is
an interesting and very readable story about Benjamin Stacy, who descended from a
Martin Fugate, a blue Frenchman who settled on the banks of Troublesome Creek in
Eastern Kentucky in 1820. Martin Fugate married a woman who carried the recessive
gene. Four of their seven children were blue. The Fugates inter-married for many
years in this isolated, mountainous part of Eastern Kentucky, and the number of blue
people grew and grew. Once coal mining and the railroads brought the outside world to
this isolated community, the Fugates started marrying outside of their clan, and the
frequency of blue people dropped. Ben Stacy is one of the last know blue Fugates.
Although very blue at birth, now he is a relatively normal color, except that his lips
and finger-nails turn purple when he is cold or angry. For reasons unknown to me,
some persons with this condition retain their baby blueness as they grow up while
others nearly loose it.
The Blue People of Troublesome Creek – The Science82 article with
background music. You will also find there a link to a message posted by Ben Stacy in
March of 1999. He reported that he was currently a student at Eastern Kentucky
The Blue People of Troublesome Creek – PowerPoint show that
includes a scanned image of an artist’s rendition of Martin Fugate and his family.
One of the most remarkably new genetic technologies developed late in the 20th
century involves the ability to move genes from one species to another. In 1985
Monsato transfered a human gene into petunia plants (Science 85, April, p. 76). The
transferred gene made these plants produce a human hormone. Such research clearly
has the prospect of providing a way to produce human hormones for medical
In 1986 researchers at the University of California at San Diego transferred
firefly genes into tobacco plants and monkey cells. To see a picture of one of the
resulting “glow in the dark” tobacco plants, just point your web browser to
Glow in the Dark Tobacco .
In 1993 researchers at the Albert Einstein College of Medicine inserted firefly
genes into tissue samples taken from patients with tuberculosis (Time, May 17, 1993,
p. 25). Within 2 hours the TB bacteria start glowing. These tissue samples can then be
treated with different antibiotics to see which antibiotic can successfully be used to treat
the patient. An effective antibiotic stops the glowing of the bacteria. This greatly
reduced the time needed to determine which antibiotic is the one to use.
Glow in the Dark Pigs
Genetic tests are available to determine whether or not one carries the genes
associated with several diseases, and I expect the number of disease-associated
genes which can be detected will increase greatly in the near future. One of the
potential problems associated with such testing is that the results of such a test can be
used against the person tested. For example, an employer could require such tests as
a condition of employment and then refuse to hire a people whose genes indicated they
were likely to develop one or another disease. If such information were made available
to others, such people might find themselves unemployable and uninsurable. One
example of the sort of potential abuse was discussed in Time in 1994 (January 167,
page 53). An HMO in California learned that a genetic test had revealed that the baby
being carried in the womb of one it its clients had the gene associated with cystic
fibrosis. The HMO advised the client that they would pay for an abortion, but that if
she allowed the baby to be born the HMO would not pay for any treatments for the
baby. That is pretty scary.
Assessing the Chromosomes in Sperm
Sometimes a man will want to have the chromosomes in his sperm evaluated to
see if his sperm have chromosomal damage (from exposure to environmental
pollutants, for example). Apparently the sperm’s chromosomes cannot be evaluated
until after they have joined with an egg, but then it is difficult to determine which
chromosomes are from the sperm and which from the egg. Researchers at the
University of Hawaii hit upon a remarkable way to solve this problem. They take
hamster eggs, strip them of their protective outer coating, and then fertilize them with
human sperm (Science 81, p. 22- 25). In such a hamster-human zygote it is easy to
tell the human sperm’s chromosomes from the hamster egg’s chromosomes.
Physiological Psychology and Proximate Causation of Behavior
When attempting to explain a behavior one usually looks somewhat myopically
backwards in time to find potential causal events that occurred shortly before the
behavior of interest. For example, suppose one asks why it is that, unlike other
primates, human females do not have a distinct period of sexual heat? The
physiological psychologist would look for what hormonal and neural events precede
sexual heat in nonhuman primates and would then tentatively label these as the causes
of sexual heat. She might then experimentally manipulate these putative causal events
to see if she can control the occurrence of sexual heat in nonhuman primates -- if she
can, then she is more certain that she has found the cause. She might then look for the
same hormonal and neural events in human females, and if she did not find them, then
she would say that the absence of such events in women is the cause of their not
having a distinct period of sexual heat. Because the cause is so close in time to the
behavior of interest we call causal investigations like this one investigations of
Natural Selection and Ultimate Causation of Behavior
Earlier we discussed the English naturalist Charles Darwin, who argued that the
bodily and behavioral characteristics of plants and animals change across generations
through a process of natural selection: Characteristics which promote the survival
and reproductive success of an organism become more frequent across generations.
We now know that it is the frequency of genes which is altered by natural
selection. Genes which are associated with characteristics that make the individual
better at reproducing will be propagated better than will be genes associated with lower
levels of reproductive success. Such genes are said to be high in reproductive
Some scientists (sociobiologists and evolutionary psychologists) attempt to
look far back in time to find how natural selection might have caused certain behaviors
to become very common in a species. Such a search is an attempt to find ultimate
causation. Later this semester I shall discuss a few potential ultimate causes of the
absence of sexual heat in women, but for now let me tackle a much easier problem:
Why is it that most people are interested in taking care of their children? Not to
assure "survival of the species" (a long debunked but still common myth), but because
genes that make one appropriately care for their children are much more reproductively
fit (good at reproducing themselves) than are genes that make one an uncaring parent.
Imagine that we start a new species in which a single gene pair determines
whether or not a particular type of parental care is likely to be show by the reproductive
individual. There are two forms of the gene, P for parental (a new gene that arose from
mutation) and N for nonparental. The P gene disposes one to provide the parental
care, the N gene disposes not to. Suppose we start out with 1% P genes and 99% N
genes in the entire population. Individuals with P genes will take better care of their
children than will those with N genes. Accordingly, children with P genes (from their
parents) will be more likely to survive and subsequently reproduce themselves. Having
the P gene rather than the N genes confers a reproductive advantage, and each
generation the frequency of the P genes in the population increases, while the
frequency of the N genes declines. Eventually all, or nearly all, of the members of the
population will have P genes and will exhibit the parental care.
Of course, taking care of the children is not the only important determinant of
successful reproduction. One must be smart enough and/or strong enough to survive
to sexual maturity and to attract a mate, so there will be selective pressures favoring
attributes that are associated with survival and attracting mates.
Not all species invest as heavily in parental care as do humans. Zoologists
sometimes speak of two different reproductive strategies here, r-selection and
K-selection. R-selection refers to the strategy of putting all reproductive effort into
making lots of offspring and little into caring for them. If an animal makes 1,000 babies
at a time, it doesn't matter much if 95% of them die, it still has successfully reproduced.
This strategy seems to work well in small, short-lived species like mosquitoes where
premature death (such as a mosquito larva being eaten by a hungry fish) is pretty much
a chance factor that is unaffected by how well the parents raised the offspring.
K-selection refers to the strategy of making only a few offspring, but then heavily
investing in them, caring for them well to assure that they survive.
Adaptation by Natural Selection
Organisms' environments sometimes change in ways that change the relative
reproductive fitness of different genes, and this may provide the species a way to
adapt, across generations, to the environmental change. For example, there is a
species of moth in England which, before the industrial revolution, was almost
always light colored. This moth tended to rest against light colored surfaces, so being
light colored helped hide it from predators. There was a little genetic variability with
respect to color -- dark colored moths were sometimes seen, but not often, as they
were too easily seen by predators. During the industrial revolution soot from burning
coal coated almost everything in England. This resulted in a change in the relative
reproductive fitness of light versus dark moths in England, shifting the advantage to the
dark moths, who did not stand out when resting on sooty surfaces. Accordingly, the
dark moths were better at surviving to reproduce, and soon almost all moths of this
species were dark. Now that the English have cleaned up their air, the light-colored
morph of these moths is once again most common.
The Case of Industrial Melanism – includes photos of the moths .
Gray (page 68) gives another example of adaptation by natural selection:
Finches in the Galapagos have bigger bodies and stronger beaks during years of
drought, when the large size and strength is necessary to eat all of the little food
available, even seeds whose shell is hard to crack, but during years of plenty the small
finches gain the advantage because they can grow to mature and reproduce more
quickly that the big guys.
Coadapted Gene Complexes, Positive Assortment,
and Reproductive Isolation
Suppose that our moths live in an environment where both dark and light
backgrounds are available. The dark moth that chose to rest on a light background or
the light moth that chose to rest on a dark background would be more likely to be seen
by the hungry predators and eaten than would the moth that rested only on
backgrounds the same color as its body. A gene that disposed a moth to rest on dark
rather than light backgrounds would be a good idea for dark moths but not for light
moths. Likewise a gene that disposed a moth to rest on light rather than dark
backgrounds would be a good idea for light but not for dark moths. In such a situation it
would seem wise to somehow link the body-color gene with the rest-color gene, that is,
these body-color and rest-color genes may co-evolve such that moths with dark
body-color genes also have dark rest-color genes and those with light body-color genes
also have light rest-color genes. To protect such favorable combinations of co-evolved
genes our moths should show positive assortment in their mating -- that is, they
should choose as mates individuals similar to themselves. If dark-body dark-rest moths
mated with light-body light-rest moths, some of the offspring would end up with genes
causing them to rest on backgrounds that don't match their body-colors. In extreme
cases, the reproductive isolation of dark from light moths might even lead to the
creation of two different species of moths.
Characteristics With No Adaptive Value
It is tempting to try to ascribe some adaptive value (value for surviving and
reproducing) to every observable characteristic of every organism. Gray mentions
racial differences in the shape of noses. Are these differences the result of natural
selection, with some shapes better for races that evolved in hot climates and other
shapes for those that evolved in cooler climates? Perhaps, perhaps not. The racial
differences in nose shape could be due to genetic drift: When the size of a breeding
population gets very small, it becomes more likely that genetic variation in any
characteristic, such a shape of nose, could be eliminated. Perhaps the groups of
humans that moved out of Africa to evolve into different races were so small in number
that chance factors caused them to loose the genes for all but the now characteristic
shapes of their racial group’s noses.
Another possibility is that an observed characteristic is vestigial -- that is, it was
adaptive in the species’ evolutionary history but is no longer. For example, we have an
appendix that apparently was useful in our distant ancestors, but is of no apparent use
now. Since it is not very detrimental, we are stuck with it. If it were very detrimental to
have an appendix, over time natural selection would remove it. Gray (page 80) uses
the clinging response of prematurely born infants as an example of a vestigial
behavior -- they cling in a way that would be adaptive in our distant ancestors, who
were a lot more hairy than are we now.
Some characteristics may have desirable effects in one stage of life but
undesirable effects later in life. For example, some diseases may be caused by genes
that give younger humans a reproductive advantage but which cause illness later in life.
For example, some genes that contribute to the development of high blood pressure
later in life might, during early adulthood, make the individual more capable of
competing for mates.
It is also possible that an observed characteristic is not adaptive in its own right
but is just a side-effect of another characteristic that is adaptive. Gray uses the
example of the amazingly large clitoris (about the same size as the male’s penis)
found in spotted hyenas. During copulation the male’s penis must be inserted through
the female’s tubular clitoris and during birth the babies must pass through it. Ouch!
One explanation of the evolution of this enormous clitoris is that it is a side effect of
natural selection for large size and aggressiveness in female spotted hyenas -- the
females are larger than the males and dominate them. Proximally, the large size and
aggressiveness of the female hyena is produced by her having high levels of
androgens (hormones, including testosterone, typically found in high levels in male but
not female mammals). Spotted hyena mothers’ androgens affect the prenatal
development of her daughters, causing them to grow enormous clitorises.
Sociobiology of Sex Differences in Reproductive Behavior
The basic premise of sociobiology is that our (and other species') common
behavioral patterns are the result of many generations of natural selection, selecting
behavioral patterns that enhance reproductive success. In short, genes that are
especially successful at reproducing themselves, including genes that influence social
behaviors, will eventually be the most common genes in a population. Can
sociobiology explain differences between females’ and males’ sexual strategies?
In many species, the females tend to be cautiously selective in their choice of a mate.
They look for evidence of health and vitality and for a commitment in time and
resources to help in raising their young.
Males seem to be less selective in choosing with whom to have sex and
are typically more assertive in seeking sex. Why?
1. Cheap Sperm, Expensive Eggs. One potential answer has to do with the
fact that females have more invested in each egg than do males in each
sperm. Eggs are much larger and take more energy and materials to
produce. The female produces far fewer eggs in her lifetime than does a
male produce sperm. Female mammals’ investment also includes the costs
of bearing the fertilized eggs in the womb and then nursing the offspring
after birth. Given the female's great investment in each egg, she might be
expected to be more choosy in picking a mate than is the male (whose sperm
are a relatively cheap expense to risk).
2. Testing the Male. In some species, the female may seem to resist the
male's sexual advances at first, testing his resolve. If the male does not
persist, he may not be the healthy, vigorous sort of male that is most
likely to have "good genes" for her offspring. Accordingly, genes that
dispose a male to be sexually persistent should replicate themselves better
than do genes that dispose a male to give-up quickly. The female may
enhance her own reproductive fitness by accepting only persistent males,
males who will give her sons the genes that make them persistent maters
also, and such sons should further the female's reproductive success by
giving her lots of grandchildren.
Parental Care. Why is it that in most species it is the female rather than
the male that is the primary provider of parental care? Here are some potential
1. Parental Certainty: It is wasteful (in terms of propagating your own genes)
to spend a lot of time investing in offspring that are not yours (unless they are
nonetheless very closely related to you, for example, your nieces and nephews). In
mammals, the female is always more certain of her maternity than is the male of his
paternity, for obvious reasons!
2. Greater Initial Investment: As discussed earlier, females have much more
invested in each egg than do males in each sperm. Some opine that this should make
the females more interested in caring for the offspring (protecting their investment) than
are males. Others argue that past investment ("sunk cost") is irrelevant in making
correct decisions regarding present or future investments. They speak of the
"Concorde fallacy," referring to the continued French and British investment in a flying
machine at a time that it appeared it would never make money (much money had
already been poured into the project). Having foolishly spent millions on something is
in itself no good reason to continue to do so. Thus, even though his initial investment is
much less than that of the female, the male should invest heavily in offspring that are
his own, unless he can more effectively channel his reproductive energies elsewhere.
Assuming that the mother and the father have equal numbers of offspring (which would
not be the case in a "harem" mating system, where each successful male would have
many more offspring than each mated female), the father should then give each
offspring as much care as does the mother.
3. Facultative (optional) Paternal Care: Males may be quite willing to stay
home and invest heavily in their offspring, unless and until the opportunity arises to
engage in a yet more effective way to propagate their genes, such as seeking
copulations with other females. Males may be especially likely to seek extra mates
when times are good, when the environment is rich, since during those times the
mother (who cannot so easily enhance her reproductive fitness by seeking extra
copulations -- remember, sperm are cheap, eggs are not) can probably take care of the
offspring without as much help. Also, the male is more likely to "stray" when his
mate is not receptive (for example, when she is incubating eggs), since he has no risk
of being cuckolded in his absence. This alternative strategy (increase the number of
children you produce by seeking extra copulations) is not available to the female -- the
maximum number of offspring she could produce is very much less than that a male
could. This should also mean that she should be very interested in mating only with
very good males, males who have good genes and who have control of resources
essential to help her reproduce. Of course, the female may be able to raise her
reproductive fitness somewhat with some careful "extra-pair copulation" -- if she
can obtain genes (and maybe also parental resources) from another "better" male, and
if she does not get caught (in which case her mate may abandon her or he may kill the
Sociobiology of Mating Systems
Robert Trivers, with whom I presented a symposium on paternal care at the
annual meeting of the American Society of Zoologists back in 1983, has proposed that
there will be a correlation between the mating system typical of a species and the
parental investment of females versus males. “Parental investment” involves the
allocation of total lifetime resources devoted to rearing current offspring, Investing
heavily in the rearing of current offspring can reduce the amount of investment possible
for producing future offspring.
Polygyny. In this breeding system each breeding male mates with several
females, and most males do not breed. This system is associated with species in
which the maternal investment is much greater than paternal investment. This
describes the situation for mammals, as we discussed earlier (“Cheap Sperm,
Expensive Eggs), and most mammals are, in fact, polygynous. In this circumstance, a
male’s reproductive success is largely a function of how many females he can
breed with. This leads to natural selection of characteristics that enable males better
to compete for access to females. Such characteristics typically include great size,
strength, and aggressiveness. Females cannot greatly enhance their reproductive
success by mating with many males, but they may well be able to enhance their
reproductive success by choosing to mate with the male who wins the competition
among males, since that male should provide her sons with the genes that would
dispose them to be successful breeders too. In some species, when a male takes over
a breeding group from another male, he may kill all babies in the group and may even
induce abortion in pregnant females. In that way the new stud can start passing on his
own genes more quickly than if the females were allowed to continue raising their
Polyandry. In this breeding system, each breeding female mates with more
than one male, and some females don’t get to mate. This system is associated with
paternal investment being greater than maternal investment, and has been
observed in some species of birds and fish. One interesting example is the American
jacana (pronounced zhá-sa-ná). These birds live in wetlands, where their elongated
toes enable them to walk on aquatic vegetation. Each male defends a small breeding
territory. Each female defends a large territory, which includes the territories of as
many as four males. Only the males provide parental care. Females exclude other
females from their territories. If a female is successful in taking over another female’s
mate, she may kill the eggs he is incubating or the chicks he is rearing and replace
them with her own.
Jacanas and Polyandry – includes a nice PBS video clip.
Monogamy. The basic idea here is that each breeding animal is mated with one
member of the opposite sex. These pair bonds may be for lifetime or may only be for
one breeding season. Monogamy is rarely absolute -- that is, monogamous animals
do sometimes mate with individuals other than their primary partner -- so called “extra-
pair copulation.” It has been determined that as many as one third of the offspring of
monogamous birds were actually fathered by a male other than the one attending the
nest. This mating system seems to have evolved in species where conditions make it
very difficult for a single parent to raise offspring without assistance from the other
parent. Most birds are monogamous. Unlike with mammals, the male and the female
are equally able to feed the young, but one parent needs to guard the nest while the
other is out gathering food. There are some monogamous mammals, most notably
foxes and coyotes. These carnivores need to feed their pups not only milk, but also
meat, and the male can assist with the provision of meat.
Polygyandry. Here there is more than one female and more than one male in
the breeding group. This mating system is hypothesized to have evolved in species for
which the benefits of living in a tightly knit group outweigh the benefits of
individualism. Two species closely related to humans are classified as
polygyandrous: Chimpanzees and bonobos. Bonobos and humans are, by the way,
the only primates in which females mate at all times of their reproductive cycle, not only
during a period of sexual heat near ovulation.
Sociobiology of Altruism
Altruistic behavior is that which increases the reproductive potential of another
individual at the cost of reducing the reproductive potential of the altruistic individual.
Sociobiologists argue that natural selection could not favor any truly altruistic
behavior, since genes are “selfish” -- that is, the genes that will be favored by natural
selection are those that are better at reproducing themselves than are competing
genes. Gray discusses two of the potential explanations which have been offered to
explain apparently altruistic behavior.
Kin Selection. According to this theory, a gene that disposes an individual to
behave in apparently altruistic ways can actually have a selective advantage if such
altruistic behavior is directed only at others who are likely to have the same gene,
that is, only towards close kin. A large body of research indicates that many animals
are indeed much more likely to show altruism towards kin than towards nonkin.
Reciprocal Altruism. The basic idea here is that apparent altruism may
actually be a case of “I’ll help you if you’ll help me.” That is, it is long-term
cooperation, in which both parties gain. For reciprocal altruism to evolve, individuals
must be able to remember who has returned the favor and who has not, and then
decline to be helpful in the future to those who have not returned the favor.
Ethology and Species-Typical Behaviors
A species-typical behavior is one which is the same in all members of a
species. Such behaviors are thought to be automatically produced by specific
environmental stimuli. The stimuli are referred to as sign stimuli, and the behavior as
a fixed action pattern. Gray gives an example involving Tinbergen’s study of
reproductive behavior in the stickleback, an European fish. During the breeding
season, the male stickleback builds a nest and defends it against any male intruders.
When a female stickleback enters his territory, he displays a “zig-zag dance” which is
designed to convince the female to lay her eggs in his nest. During the breeding
season the males’ belly swells up and turns red. Tinbergen found that the swollen red
belly serves as the sign stimulus which releases the attack on intruding conspecific
males. Models of objects with swollen red bellies released the fixed action pattern,
even when the model did not otherwise even look like a stickleback. Models in which
the swollen red belly was even more prominent than it is in a real stickleback were
especially effective at releasing the attack behavior.
Ethologists have a great interest in studying the species-typical behaviors of
animals and comparing different species of animals with the goal of determining
why different species have evolved their particular species-typical behaviors.
Gray mentions the use of homologies and analogies in such comparisons. A
homology between two species is a similarity that stems from their having a close
common ancestor. For example, the flipper of a whale and the arm of a human
have very similar structures, despite their having quite different functions. A analogy is
a similarity that stems not from common ancestry, but rather from having independently
evolved similar ways of dealing with similar problems. For example, consider the
wings in birds, insects, and bats, species that are not closely related.
Lesson on Homologies and Analogies
• Gray discusses hive building in bees and smiling in humans as examples of
attempts to reconstruct the evolution of a species-typical behavior. I shall
use a different example, vampirism in moths. No, I am not kidding, there
are actually vampire moths. Hans Banziger offered his cut finger to a species
of moth found in Southeastern Asia, Calpe eustrigata, and discovered that
the moth not only consumed the blood, but actually pierced the flesh with its
proboscis. How could this species have evolved vampirism?
• Most moths have a delicate proboscis used to suck nectar from flowers. If
most members of a group of closely related animals share some trait, then a
common ancestor probably had that trait too, so we conclude that the first
moths were nectar-feeders.
• Many moths supplement their diet by feeding on the sugars of damaged
fruits, and by obtaining salt and other minerals from drinking mud puddles,
urine, or other animal secretions. It is assumed that one of these sorts of
moths was ancestral to vampire moths.
• Only a small number of moths use a stronger, raspier proboscis to scrape
fruits and cause them to leak. These moths have stronger hairs at tip of their
proboscis. This represents another step towards vampirism.
• In even fewer species of moths, the hairs at the tip of the proboscis are
firmer, allowing the moth to use the proboscis to cut directly into thick-skinned
fruits, such as oranges.
• Vampire moths (there are four species of them) must have evolved from a
fruit piercing ancestor. We assume that they derive extra nutrients from
Asian Vampire Moth – see the Asian vampire moth
Butterfly Proboscis – includes mention of the Asian Vampire Moth
Carolina Sphinx Moth – check out the proboscis on this beast.
Cautions Regarding the Sociobiology of Human Behavior
Gray warns us of two fallacies to avoid when attempting to use sociobiological
principles to explain human behavior:
The Naturalistic Fallacy. This is the belief that what is natural is good. For
example, if natural selection has resulted in human beings being war-like creatures,
then that is a good thing. This does not really make any sense, because natural
selection is not a moral force, it is a biological process. Just because natural selection
has resulted in our having certain dispositions does not mean that those dispositions
are morally correct.
The Deterministic Fallacy. This is the belief that we cannot do anything to
change behavior which is, at least in part, determined by our genes. In fact, we
may not be able to do anything about the behavioral dispositions we inherit, but we
certainly can restrain some of them and encourage others of them by changing the
environment in which we live. In fact, humans have put into place a number of
institutions which function to encourage moral behavior from the human animal. The
sort of institutions I am thinking of are religious, cultural, social, and governmental
institutions. If only such institutions were to act in the best interest of the people. I fear,
however, that such institutions are subject to the same sort of selfish evolutionary
processes that make individuals most motivated by self-interest. Those institutions that
have attributes that promote their institutional survival and reproduction will dominate,
even it that involves sacrifice of the individuals within such institutions -- until the
revolution, that is.
Revised March, 2006. Illustrations now have alternative text.
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