Download Lecture 6

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EEB 210
Spring 2008
Lecture 6: Sexual selection; learning and evolution
Sexual selection: This phenomenon was first suggested by Darwin, especially in his
book, The Descent of Man. Sexual selection occurs when (phenotypic) traits are selected
because they are attractive to members of the opposite sex. Often such traits may
actually reduce an individual’s potential for survival, while at the same time increasing its
potential to obtain opportunities for reproduction. For example, the showy tail of the
male peacock is attractive to the female, but also makes the male more visible to
predators---to say nothing of the energy required for producing and carrying all those
extra long feathers. Sexual selection can be viewed as a special case of natural selection,
where individuals of the opposite sex are seen as a very important part of an organism’s
environment, exerting selection pressure that contributes to the evolution of various
traits. So it may be that the disadvantages of the male peacock’s big tail are outweighed
by the advantage it provides him as he competes with other males to obtain mates.
Female peacocks must surely gain in at least one way by mating with males with the
showiest tails: To the extent that showiness is heritable, females that mate with showy
males will produce sons with showy tails who will be more likely to succeed in
competition for mates as compared to males that are less showy. (As with natural
selection, for sexual selection to have evolutionary significance the trait(s) that is selected
(preferred by the opposite sex) must have some genetic basis (i.e., must be heritable).
Sexual selection can be responsible for patterns of evolution of:
a. morphological traits, such as bright colors in many male birds and fishes,
large antlers in deer and elk (The extinct Irish elk appears to be an especially
spectacular example.)
b. behavioral traits, such as elaborate mating displays
There continues to be debate over the question of how sexual selection begins;
that is, why are particular traits ‘attractive’ to members of the opposite sex? One idea is
that a trait like, say, the male peacock’s showy tail may be linked to another trait that has
significant survival value, such as a robust immune system. This link could be via one or
more genes with pleiotropic effects. (Pleiotropism refers to the ability of a single allele to
produce more than one overt effect. For example, the gene that codes for the production
of the peptide, oxytocin has multiple effects because oxytocin is a hormone that
stimulates ejection of milk from the mammaries during nursing and also stimulates
uterine contractions during labor. In addition, oxytocin is also produced in certain brain
regions where it acts as a neurotransmitter; its functions as a neurotransmitter are not as
well understood as are its endocrine functions, but there is evidence that it may be
involved in certain social behaviors.) Thus, it might be that a particular gene that
contributes to immune function is also responsible for the production of bright eyespots
in the tail. In this event, females that choose to mate with the males that have showy tails
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would be (unwittingly) getting mates with genes for good immune systems and would
pass these good genes on to both their male and female offspring. In addition, females
that mate with ‘showy’ males would also tend to produce male offspring that would
develop showy tails. These male offspring would have an advantage in the competition
for mates as compared to males with less elaborate plumage. For these reasons, a
selective advantage would be conferred on the females that make a choice of mates with
showy tails. Once this pattern becomes established with a genetic basis, then males with
even more showy tails would likely have an advantage in the competition for mates, and
this would now be the case even if the additional ‘showiness’ of the tail was not linked to
a further improvement in immune function. That is, the showy tail itself has now become
a force for selection; females choose males with the showiest tails, and by mating with
such males they will produce sons with showy tails which will tend to win competitions
for mating opportunities---and on it goes. This escalating process would be expected to
go on until the disadvantages (to the male) of having too big a tail balance the advantages
that accrue in the competition for female mates.
Zahavi proposed an alternative hypothesis to explain sexual selection---the socalled handicap hypothesis. He suggested that some sexually selected traits evolved
because they serve as an ‘advertisement’ or ‘proof’ that their bearer has been able to
survive despite carrying a handicap; thus, in this view the male peacock is demonstrating
that he must have superior traits in that he has survived despite his unwieldy and showy
tail. Zahavi proposes that females that mate with such males are ‘choosing’ males with
superior characteristics as demonstrated by their ability to survive despite a handicap.
Think about what features are generally considered attractive in humans who are
looking for mates. Might some of these features be subject to the process of sexual
selection as originally postulated by Darwin? Keep in mind that though sexual selection
is most often discussed with respect to masculine traits that are important in male-male
competition for mates, the phenomenon logically could apply to female traits that become
attractive to males. The reason for the emphasis on male traits is that, for most species,
male-male competition for mates is more intense than female-female competition. This
is because (1) males produce more gametes and thus a male can inseminate multiple
females in a short period of time and (2) females generally invest more energy in the
offspring, whether simply by making large eggs (as compared to much smaller sperm) or
by providing care for newborns as well. Since the sex ratio is usually close to 1:1, if some
males mate with multiple females, this must mean that some males fail to father any
offspring; hence the competition between males can be very intense.
A strongly monogamous system may tend to increase competition among females
for choosing the best mates, as compared to the situation in a polygamous species---this
would tend to balance the forces of sexual selection with selection acting more equally on
both males and females to choose best partners and therefore acting to produce sexually
selected traits in both sexes.
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Sex differences: In any given sexually reproducing species, there are a number of
morphological and behavioral differences between the sexes. Some of these differences
may be at least partly the result of sexual selection, but many are not. Most sex
differences presumably evolve in relation to the different reproductive strategies of the
two sexes. In many organisms, there are sex differences in body size, aggressive
behaviors, and sex behaviors.
In mammals, there appears to be an interesting correlation between the degree of
sex dimorphism in body size and the reproductive pattern. Specifically, species that are
polygamous tend to show marked sex dimorphisms in size, with males typically larger
than females. This type of dimorphism is especially pronounced in species where the
males establish territories and attract harems; examples would be sea lions and elk. The
large size of the males in these species is likely related to the intense male-male
competition for territories and for females, with larger males presumably having an
advantage in this competition. In contrast, monogamous species tend to show less sex
difference in body size, and this may be related to a reduced degree of male-male
competition. Humans are sexually dimorphic with respect to body size, but much less so
than gorillas and other primate species that are polygamous. It has been suggested that
this may indicate that early humans were either monogamous or at least largely
monogamous.
Is there a relation between sexual selection and sexual differentiation? There is
no direct relation between these two processes, one of which refers to an evolutionary
process and the other a developmental process. Not all sexually differentiated traits are
sexually selected. (For example, many internal structures are sexually differentiated but
are not subject to sexual selection. Male mammals develop glands like the prostate that
secrete substances into the semen and females do not have such glands.) However, a
great many sexually selected traits are sexually differentiated. Sexually selected traits
generally either appear in only one sex (i.e., specific patches of bright color in males of
many bird species that are attractive to females of the same species) or are markedly
more pronounced in one sex (i.e., larger body size in males of some species where
females prefer to mate with the largest males). Thus, a general understanding of
mechanisms of sexual differentiation is bound to be useful in understanding some of the
developmental mechanisms that underlie cases of sexual selection.
Learning and evolution:
Baldwin effect: One evolutionary concept that deserves special mention in this course is
the so-called Baldwin effect. James Baldwin was a social psychologist who worked in
the late 19th/early 20th centuries. He suggested that some biological traits that initially
appear via learning could ultimately influence genetic evolution in such fashion that the
trait eventually becomes innate (largely genetically determined) in a population. For
example, woodpecker finches of the Galapagos use twigs or cactus spines to probe for
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grubs. This behavior is largely innate in the current population of woodpecker finches. It
involves a number of components, including finding appropriate twigs or spines, holding
them in the beak in the right orientation, and using them as a probe. If this behavior first
arose as a learned behavior, it is conceivable that natural selection could have improved
on the behavior by selecting for birds that had genetically controlled behavior
components that made them better tool users. This concept suggests how a particular
complex behavior might evolve and eventually be “incorporated into the genome” with
some aid from learning. Thus, we might start with a population of birds in which no
individuals have an ‘innate’ proclivity to perform the behavior, but in which some
individuals have the ability to learn the behavior through trail-and-error. These
individuals, because they possess certain genetically-based abilities that facilitate the
learning of the useful behavior, have improved chances of being winners in the natural
selection process. This would be expected to lead to an increase in the frequency of the
appropriate genes in future populations of the species. Note that this particular
combination of genes might not have had any particular selective advantage in the
original population were it not for the ability of the ancestral birds to learn. (Thus,
simply picking up a cactus spine doesn’t help, nor does holding the spine in the beak, etc.
The bird must learn---presumably by trial-and-error---to use the spine as a tool.) The
concept of the Baldwin effect need not apply only to learned behaviors. It might apply to
virtually any feature of organismal development that exhibits plasticity---that is, the
ability to be modified either by experience or by environmental conditions.
The Baldwin effect has not received a great deal of attention from biologists as a
potential factor in evolution. However, it would be expected to be most significant in
organisms that have the best developed learning abilities; thus, it may be something that
should receive more attention as a potential factor in human evolution.