End of Chapter Questions
CHAPTER 9
1. a. The figure shows that variation in antisocial behavior is correlated with
differences in genotype. It is possible that the correlation is due to some other
factor (for example, certain genotypes may be more common in certain races,
which may be exposed to certain environments). However, leaving that possibility
aside for the moment, we can tentatively conclude that MAOA genotype
influences antisocial behavior.
b. Yes, because different levels of childhood maltreatment (an environmental
condition) are associated with different levels of antisocial behavior. If there were
no effect of environment, both lines would have a slope of 0.
c. Men with different genotypes respond in the same direction-in both groups of
men, increased maltreatment is associated with heightened antisocial behavior.
However, the strength of this effect is different in the two groups. Men with the
low MAOA activity genotype appear much more strongly influenced by
environment than are men with the high MAOA activity genotype.
d. Yes. This means simply that some of the variation in antisocial behavior is
attributable to genotype.
e. The answer to this question is left to the reader.
2. a. Yes. A QTL, or quantitative trait locus, is any locus at which genetic variation
in alleles is statistically associated with variation in a given quantitative trait.
QTLs may be identified by various mapping techniques (generally, investigating
large sections of the genome for markers that are statistically linked to the trait) or
by investigation of certain loci that are already suspected to affect the trait. In this
case, the serotonin transporter gene was suspected to affect neuroticism, since
serotonin is a neurotransmitter known to affect mood.
b. No. Like most quantitative traits, neuroticism is almost certainly influenced by
many genes, only some of which have been discovered, and only some of which
vary in genotype. (Variation in serotonin transporter genotype explains only about
5% of variation in neuroticism. Many other QTLs associated with neuroticism
have since been found.)
c. Several other explanations are possible. The serotonin transporter gene might
be physically linked to another locus that affects neuroticism. The frequencies of s
and l alleles might vary in different human populations, which in turn might have
various differences in other genes or may be exposed to different environments.
3. The breeders wish to identify QTLs associated with racetrack performance. Many
different research programs are possible. In general, we would analyze genetic
samples from as many racehorses as possible (winners and losers), for as many
genetic markers as possible. We would then look for statistical associations
between presence of each genetic marker and racetrack performance. Performance
could be measured in several ways: winning of stakes races, fastest recorded
times of certain distances (regardless of whether they won), siring offspring that
raced well, etc. Alternatively, we might also investigate candidate loci that are
4.
5.
6.
7.
8.
already suspected to contribute to performance, such as loci known to contribute
to endurance, muscle contraction speed, aerobic capacity, and bone density.
High heritability within a population does not mean that variation between
populations is due to genetic differences. If the populations differ markedly in an
environmental factor, the variation between populations can be due entirely to
environment. In this case, your roommate is comparing height across populations
- medieval populations vs. modern populations - that differed markedly in dietary
environment.
a. Yes, the trait may be strongly affected by environment. But no matter how
important the environment may be, if there is little variation in environment, the
heritability value will typically be very high.
b. Heritability values will often change if the environment changes. Typically, if a
formerly invariant environment begins to vary, heritability values will fall.
a. Heritability (h2) is approximately 0.045, the slope of the least-squares linear
regression line of a scatterplot of midparent speed vs. midoffspring speed.
b. No. A heritability of 0.045 is quite low; only 4.5% of the variation in running
speed is explainable by genetic variation. One generation of selection is unlikely
to cause a large change in running speed.
c. The breeder should concentrate on environmental factors, which apparently
explain 95% of the variation in running speed. Some obvious factors include
behavioral training, physical conditioning, racing experience, and diet. Other
possible factors include fetal and neonatal effects such as diet of the mother and
litter size.
a. The selection differential is 4.82 m/s, the difference between mean speed of
breeders (13.3 m/s) and mean speed of nonbreeders (8.5 m/s). The selection
gradient is 0.32, the slope of the best-fit line of midparent speed vs. relative
fitness, where relative fitness is 3 for breeders and 0 for nonbreeders.
b. The predicted response to selection is 0.22 m/s (selection differential of 4.8 m/s
multiplied by heritability of 0.045). Predicted average running speed of the next
generation is 10.30 m/s, the sum of the average speed of the parental generation
(10.09 m/s) plus the response to selection (0.22 m/s). (Numbers have been
rounded.) However, the actual running speed of the offspring of the breeding
families is only 8.95 m/s, lower than predicted. This is because of the high scatter
in the data-that is, in the scatterplot of midparent vs. midoffspring values, data
points do not fall exactly on the best-fit line. In particular, in this particular data
set, average running speed of all puppies is lower than average running speed of
all adults. This unexpected result is almost certainly due to low sample size,
which increases probability of genetic drift (e.g. puppies may just happened to
have inherited low-speed alleles rather than high-speed alleles from heterozygous
loci of their parents), and of variation in environmental effects (e.g., puppies
might not have been trained optimally). In a much larger data set of thousands of
dog families, we would expect to see closer correspondence of predicted and
actual response to selection.
The basic strategy in any assessment of heritability is simply to measure the trait
in as many parents as possible and as many offspring as possible, and plot
midparent vs. midoffspring values. In this case, we could measure gall size of
larva, then collect the emerging larvae, raise them to adulthood in captivity, let
them breed, let the females lay their eggs in a new set of plants, and finally
measure the size of the offspring's galls. If paternity turned out to be difficult to
assess or control, it is possible to estimate heritability using just the mother's data
(as in Figure 9.20). As in any heritability analysis, close attention must be paid to
shared environmental effects and other possible confounds. A likely confound
here is the mother fly's choice of plant, since the plant's species, genotype and
environment may all affect gall size.
9. There are several possibilities. Alpine skypilots may still be evolving larger size;
bumblebees may prefer a certain size of flower, but not larger sizes; or there
might be opposing forces of selection in the tundra that limit large flower size.
The first possibility could be tested by monitoring skypilot flower size over
several generations; the second by testing bumblebee preference for flowers of
many sizes, possibly including artificially enlarged flowers (e.g. attaching extra
petals to existing flowers); and the third by measuring fitness in an experimental
tundra population that is hand-pollinated instead of bumblebee-pollinated, to see
if larger flowers have any disadvantages. Many variations of each of these
experiments are possible.
10. a. Stabilizing selection occurs when individuals with average values of a trait
have highest fitness. This tends to trim the tails off of the population distribution,
reducing variation but not changing the population mean. Directional selection
occurs when a value to one side of the population average (higher or lower, but
not both) has highest fitness. This trims one tail off the population distribution and
expands the other tail, shifting the population mean. Variation tends to reduce
(because one tail is trimmed off) but not very much (because the other tail tends
to lengthen, depending on available genetic variation). Finally, disruptive
selection results when high and low values have higher fitness than the average
value. This tends to split the population into two morphs (forms) and eliminate the
middle peak of the bell curve, increasing variation but not changing the mean.
b. Gall size is under stabilizing selection, which in this case is due to opposing
directional selection from birds and parasitoid wasps. If parasitoid wasps
vanished, gall-making flies would be under directional selection by birds only,
average gall size should decrease.
CHAPTER 10
1. An experimental study is one in which the researchers are able to directly
manipulate the variable of interest, typically changing it in one group of
individuals and leaving it unchanged in a control group. Note that the
experimenters are able to control which individuals are assigned to each group.
Examples include Clayton et al.'s pigeon beak experiment, Weeks' oxpeckerexclusion experiment, and Greene et al.'s experiment on wing-waving in tephritid
flies. Experimental studies are extremely powerful because they can control for
other confounding variables, but not all questions can be studied this way,
particularly those that involve large-scale evolutionary changes.
An observational study is one in which researchers simply observe the patterns
that occur in nature, such as Huey et al.'s study of rock selection in garter snakes.
(Sometimes, observational studies may compare two groups of animals that occur
in nature. However, the researchers do not assign individuals to the different
groups; rather, the individuals have "assigned themselves" to the different groups,
which can introduce considerable confounds.)
A comparative study is one that compares different taxa of organisms, often
studying the distribution of a trait on a phylogeny, and seeking to understand why
some clades evolved the trait and others did not. Examples include Hosken's study
of testis size in bats, and Futuyma et al.'s study of host shifts in leaf beetles. A
comparative approach is very useful when many taxa have evolved a similar trait.
Frequently, the overall research plan will include a combination of several of
these different approaches.
2. Futuyma et al.'s two hypotheses were, (1) all host shifts are possible, and
ecological factors and random chance are the only factors that determine which
host shifts actually occur; and (2) not all host shifts are possible, because the
beetles lack genetic variation to eat certain species of host plants. Futuyma et al.
tested the hypotheses by using quantitative genetic trait analysis. They examined
the genetic variation associated with four of the beetle species' ability to survive
(or not) on six of the possible hosts. They found that, in fact, not all host shifts are
possible - that is, these four beetle species often lacked the necessary genetic
variation to detoxify the leaf toxins in different plant species.
Futuyma et al.'s results show that some traits (in this case, the trait of only eating
the leaves of one host plant species) may not be adaptive, or not entirely adaptive;
they may simply reflect an evolutionary constraint due to lack of genetic
variation.
3. Felsenstein's method is useful for case A, but not for the other two studies.
Felsenstein's method was designed to test questions in comparative studies only,
particularly those that involve comparisons of quantitative traits, (e.g., testis size
and group size in bats). Felsenstein's method is a method for controlling for
phylogenetic relatedness, which otherwise can add a major confounding factor to
comparative studies.
4. A trade-off is a situation in which an increase the fitness of one trait will
inevitably lead to a decrease in fitness of another trait. This can occur due to
developmental constraints, or simply because organisms have a limited pool of
energy and cannot develop all traits to a maximum degree simultaneously.
Examples include testis size vs. brain size in bats, and flower size vs. number in
begonias. The occurrence of trade-offs demonstrates that not all traits are
perfectly adaptive.
5. An evolutionary constraint is an obstacle that prevents a taxon from evolving a
certain trait, often due to developmental pathways, or some other competing
process of physiology or ecology. Examples include pigs' failure to evolve wings
(due to a developmental program that does not allow multiple pairs of forelimbs),
the retention of flowers on Kotukutuku trees for several days after fertilization
(possibly due to the physiological constraint of slow growth of pollen tubes), and
the lack of host shifts in body feather lice of birds (possibly due to an ecological
constraint of limited dispersal opportunities). The occurrence of evolutionary
constraints demonstrates that not all traits are perfectly adaptive.
6. Apert syndrome appears to be a case of competing selection at different levels.
The Apert syndrome mutation may increase fitness of spermatogonia relative to
other spermatogonia. However, it also decreases fitness of the man whose
spermatogonia have the mutation, since some of his offspring may inherit the
Apert syndrome mutation. The occurrence of competing selective forces at
different levels (e.g., mitochondria, cells, individuals) can result in the occurrence
of traits that are maladaptive from the individual's point of view.
7. Many answers are possible. Traits may not be adaptive, or not fully adaptive, due
to: trade-offs with other traits; evolutionary constraints (developmental,
physiological, ecological, etc.); insufficient time for natural selection to have
operated; insufficient genetic variation; and opposing selection at other levels
(e.g., selection at the level of the cell).
8. a. This group of flies controlled for the effects of the operation alone. Without this
group, any difference between intact flies and altered flies might have been due to
other unanticipated effects of the operation, not necessarily the change in wing
markings. (For example, the operation might have affected subtle details of the
flies' wing-waving behavior, or their general health and hence their ability to
dodge the spiders.)
b. Only by cutting wings were they able to test the effects of wing-markings and
wing-waving independently.
c. If flies had been presented to the spiders in the same order every time, the
results from the last flies might have been affected by the spider's experience with
previous flies. For example, the spiders might have learned about those flies'
particular type of wing markings and wing-waving behavior, and might react to
subsequent flies differently.
9. One nonadaptive hypothesis is that perhaps snakes are unable to find medium
rocks. According to Huey et al.'s data, thin, medium, and thick rocks are equally
abundant, but medium rocks make up 61% of the rocks chosen by snakes. Some
snakes may have to make the best of a bad deal by settling for a less favorable
rock, rather than having to try to oust another individual from a preferred rock.
The hypothesis could be tested in a few different ways. Another prediction from
this hypothesis is that, given ready access to medium rocks, all snakes will choose
medium rocks. We could artificially add medium rocks until there were enough
for all snakes, then count the number of snakes that still selected thick rocks.
Another prediction is that, given insufficient medium rocks, snakes compete over
access to them. This could be tested both in the field and in controlled contest
trials in the lab.
There are many other possible hypotheses as well. One adaptive hypothesis is that
thick rocks provide better protection from predators than do medium rocks, so
some snakes are optimizing predator avoidance rather than thermoregulation. This
could be tested by monitoring survivorship among snakes who habitually used
medium vs. thick rocks. It could also be tested by observing predator behavior.
You might think of further hypotheses of your own.
10. A major difficulty geckos face in trying to use behavioral thermoregulation at
night is the lack of direct sunlight available to raise body temperature. Based on
the lack of direct sunlight, we could predict that the optimal sprinting temperature
in geckos would be lower than for a typical diurnal lizard. One possible reason
why it's not is that geckos may live in warm environments that don't experience
much cooling at night. If air temperature remains high all night, sunlight would
not be necessary to keep body temperature warm.
11. A logical prediction is that Daphnia in Lake Citadelpark would evolve greater
phenotypic plasticity in phototactic behavior if fish were introduced into the lake.
To test this prediction, we could allow a number of generations to pass, then
repeat DeMeester's experiment. We could then breed a number of clones from a
number of individuals, producing many samples of many different genotypes.
Each set of clones could then be tested for phototactic behavior, both in water that
had never had fish in it, and also in water that came from the Lake. If the
prediction is correct, then members of each genotype will show strong negative
phototaxis in normal lake water and much weaker negative phototaxis in water
that had never had fish in it.
12. A major potential advantage of large body size is a decrease in vulnerability to
predation. For birds and mammals, another advantage is lower mass-specific
metabolic rate (i.e., large mammals and birds can maintain a high body
temperature with less energy expenditure [per unit body mass] than can small
mammals and birds); this can represent a significant energy savings. For
ectotherms, larger body size provides greater thermal inertia, making it easier to
maintain a body temperature above ambient. Costs of small body size are
increased vulnerability to predators and, in birds and mammals highly elevated
mass-specific metabolic rates (shrews, the smallest mammals, must eat almost
continuously to stay alive). These advantages and disadvantages can be tested by
comparative studies of closely related species who differ in body size.
13. The hypothesis that muntjacs have been the selective force causing the evolution
of leaf toxins on Island A leads to several predictions. First, given a choice
between the leaves of Island A and Island B shrubs, the muntjacs should prefer
the Island B leaves. If this prediction is met, it indicates that the toxins do have
the effect of deterring herbivory.
A second prediction is that differences in temperature and rainfall do not cause
the difference in leaf toxins. This could be tested by growing Island A and Island
B shrubs under controlled conditions mimicking those of both islands. If, under
both sets of conditions, Island A shrubs continue to produce toxins and Island B
shrubs do not, we can eliminate the hypothesis that abiotic conditions themselves
do not lead to toxin production.
A third prediction is that herbivorous insects are not the selective force favoring
toxin production. This could be tested by performing tests of the preference of
insects to either Island A or Island B shrubs in a controlled setting. If insects
exhibit no preference, we can eliminate them as a selective force.
A final prediction is that, if muntjacs were removed as a selective force, Island A
shrubs would evolve lower levels of toxicity. This prediction assumes, crucially,
that toxin production is costly - if that assumption is incorrect, the prediction
doesn't hold. If it does, and if we have sufficient time for long-term experiments,
then we could fence off a group of Island A shrubs to prevent muntjacs from
feeding on them and measure the levels of leaf toxins in individuals and in the
population over time. Because these shrubs are still exposed to the same abiotic
conditions and the same insect herbivores, a decrease in leaf toxicity over time
would be strong evidence that the muntjacs were the selective force favoring
toxicity in the first place.
14. Human skin color shows each of these phenomena. Skin color is heritable (it's a
quantitative trait); in general, dark-skinned parents have dark-skinned children
while light-skinned parents have light-skinned children. It is also phenoypically
plastic, especially in some individuals who have light skin during the winter but
who acquire deep tans when exposed to sunlight. Furthermore, skin color exhibits
genotype-by-environment interaction; genetic variation for phenotypic plasticity
exists. In other words, some individuals' skin color respond to environment (these
are the individuals who are pale in winter, but tan in summer), while others are
not (for example, many people of Celtic heritage remain very pale and do not tan
at all despite exposure to sunlight; and many people of African heritage remain
very dark even when they are not exposed to sunlight at all). Variation in
phenotypic plasticity can happen among children in the same family, suggesting
that the variation is due to genetic, rather than environmental, variation. That is,
among a group of light-skinned siblings, some will tan well and others will not;
some will freckle more than others; etc.
Phenotypic plasticity for skin color could evolve if light and dark skin were
adaptive at different times of the year. In temperate zones, for example, light skin
might be adaptive when limited amounts of sunlight limit vitamin D synthesis;
low levels of melanin at those times could optimize vitamin D synthesis. In
contrast, during seasons of high levels of sunlight, melanin might provide
important selection against damage by ultraviolet radiation. If this was the case
and genetic variation in phenotypic plasticity existed, it could evolve.
15. a. Moose antlers: These may be selected for use in competition with other males
for access to mates; they may be selected against because of the energy
investment required to build, maintain, and carry them around all year, and
because of the enormous loss in calcium that occurs when antlers are discarded
every year. (Antlers represent a large fraction of the moose's entire skeleton, and
large deer like moose can suffer osteoporosis if they cannot obtain enough
calcium in the diet while they are growing antlers.)
b. Douglas fir trees: Height is almost definitely selected for because the advantage
it confers in competition for sunlight; but it may be selected against because of the
energy investment required to build and maintain the "extra" tissues required to
grow that tall, particularly the impressive investment in woody support tissue and
water- and food-transporting tissue.
c. Termite gut symbionts: Cellulose-digesting organisms are favored because of
the nutrients they provide the termites; but high population levels may be selected
against because they limit the amount of food a termite can store, or because they
use much of the energy of the cellulose for their own metabolism rather than for
the termite.
d. Maple trees: Loss of leaves in fall can be favored because otherwise, leaves
may be destroyed (i.e., by heavy snowfall) or frozen. However, the complete loss
of leaves requires an enormous investment in new leaf growth every spring. (Note
that conifers have a different strategy - they retain their leaves, or needles. But
each needle is very skinny and small, presumably to reduce risk of damage during
snowfall, and does not offer much photosynthetic leaf area during summertime.)
e. Male moths: Large antennae may be favored because they make individuals
more sensitive to female pheromone, allowing them to find mates more
efficiently. They might be selected against because of the energy demands they
represent, or because they are fragile and easily damaged.
f. Barnacles: Potential advantages to being motile include the ability to find and
exploit favorable habitats; the ability to seek out mates; and a decrease in
competition with other individuals of the same species. Potential disadvantages
are that favorable habitats and mates might not be found, and the barnacle is very
vulnerable to any environmental changes in the location that it settled in. For
example, if the particular rock (or ship) that it is on is overturned, the barnacle is
out of luck.
16. By using artificial flowers, Schemsky and Âgren were able to precisely control
for flower size, pollen reward, and any other cues bees might have been using to
target blossoms. The major disadvantage is that we can never know whether some
feature of the artificial flowers (subtle scent, texture, color, etc.) skewed the bees'
preferences.
17. It is selectively advantageous for the P1 virus to carry the addiction module
because it prevents selection at the level of bacterial populations from acting
against the transmission of the plasmid from parent to daughter cell.
This explanation assumes that, without the addiction module, bacteria without the
plasmids would have higher rates of reproduction. Under these circumstances,
selection at the level of the bacteria would favor mechanisms that allowed
individuals to divide without passing the plasmid on to their daughter cells. If
selection acted this way, though, the addiction module would be favorable for the
virus because any bacterial cell that succeeded in reproducing without passing on
the plasmid would leave no descendants. In effect, the presence of the module
selects for bacteria that don't succeed in eliminating the plasmid when they
reproduce.
18. One way to start brainstorming questions is simply to observe some organism in
its environment. Virtually every observation can be turned into a question by
putting the words "how" and "why" in front of it.
CHAPTER 11
1. The fundamental cause of sexual dimorphism is an asymmetry in the amount of
parental investment in a given mating and the care of any resulting offspring. The
sex that invests less time and energy in this process tends to be limited merely by
access to mates, and hence is under strong selective pressure to attract as many
mates as possible. The result is evolution of showy traits that attract the opposite
sex, and competitive traits for competition with the same sex. This sex is often
(but by no means always) the male, since ejaculates tend to be relatively cheap
and males of many species do not provide parental care.
The sex that invests more time and energy in this process is usually not limited by
access to mates, and hence is not under strong selective pressure to find as many
mates as possible. Rather, this "parental" sex is under pressure to select just a few
good mates-sometimes just one. This sex is often, but by no means always, the
female, since eggs are relatively large and females often provide parental care.
2. Intersexual selection refers to sexual selection for increased attractiveness to
members of the opposite sex. This form of sexual selection tends to lead to
"display" or "advertisement" traits, such as showy or colorful body parts, or
exaggerated mating displays. Examples include long tails in male red-collared
widowbirds and calling in male frogs. Intrasexual selection refers to sexual
selection for increased ability to compete directly with members of the same sex
for access to the opposite sex. Intrasexual selection tends to lead to weaponry,
armor, fighting ability, and threat displays. Examples include large body size in
iguanas, infanticide in lions, antlers in deer, and so on.
Note that some traits may serve both functions-for example, bird song often
functions both as a display that attracts females and also a threat that deters rival
males.
3. In marine iguanas, many males are larger than the "optimum" body size (the size
that can be maintained long-term), and experimental data confirms that survival
rates are lower for the largest iguanas than for medium-sized iguanas. Similarly,
in long-tailed widowbirds, males with intact long tail feathers lost weight at a
greater rate than males with experimentally shortened feathers. These results
indicate that sexual selection is working contrary to natural selection, and that
"attractive" males may ultimately pay a price for attractiveness, in the form of
lowered survival, shorter life spans, or reduced health.
4. Females may gain "good genes" for their offspring by choosing traits that indicate
that the male is healthy and fit. An example is Welch's experiment on gray tree
frogs, which demonstrated that offspring of long-calling males outperform those
of short-calling males in several measures of health and rapid growth. Second,
females may select males who provide them with a valuable resource, such as a
food gift. An example is male hangingflies, who provide a food gift to the female.
Third, females may have pre-existing sensory biases that can be exploited by
males. An example is male water mites, which employ a mating display that
appears to take advantage of females' tendency to turn toward vibrations-a trait
that originally evolved for hunting, not for mating. Fourth, once a trait is preferred
by a majority of females, the trait may become self-perpetuating because females
that prefer that trait will tend to have "sexy sons". An example is spotted
cucumber beetles, in which most females prefer fast-stroking males-a trait that
appears to provide no advantage other than the fact that sons of those males will
themselves be fast-strokers.
5. The pollen-producing parts are under more intense selective pressure for
showiness because showy flowers attract more pollinators. Pollen-producing
plants (or parts of plants) produce large amounts of tiny pollen, and are limited in
reproductive success primarily by the number of mates they have access to-which
means, the number of pollinators that they can attract. On the other hand, seedproducing plants (or parts of plants) produce just a few large fruits. Even a few
pollinator visits is enough for them to receive enough pollen to fertilize all the
seeds that they can bear. They do still need flowers, because they obviously do
need to attract a few pollinators, but they do not need especially gaudy flowers
because they do not need to attract enormous numbers of pollinators. Therefore,
pollen-producers are under intense sexual selection for large and showy flowers,
while seed-producers are not.
6. It is a reasonable inference for any species that a morphological difference
between males and females is probably a result of sexual selection. It is
particularly reasonable for this species, since data show that male reproductive
success is strongly affected by access to mates, and males must, therefore, be
under selective pressure for traits that increase their likelihood of obtaining mates.
However, either male-male competition or female choice could explain the crests.
To test the hypothesis that female choice has selected for this trait, a sample of
females could be given visual access to males with different crest sizes. If female
choice is operating, females should consistently prefer the male with the largest
crest. Male-male competition could be responsible for the trait if, for example,
having a larger crest allowed those males to dominate access to preferred breeding
spots. This could be tested with a combination of laboratory dominance trials and
careful observations in the field.
7. As the ratio of males to females increases, females will be competing to a lesser
degree for access to males. Most females should succeed in finding a mate, and
the variance in female reproductive success should decrease (that is, the
difference between the "winners" and "losers" will be smaller). The frequency
distributions of the number of mates and the number of offspring become more
even. The relationship between number of mates and number of offspring should
be correspondingly shallower.
A change in the ratio of females to males should have little to no effect on male
reproductive success as they are already the limiting sex in this species. All males
should garner at least some mates and all should have some reproductive success.
Therefore, the graphs for male reproductive success should change minimally, if
at all.
8. Extreme variance in male reproductive success implies that a few males within a
breeding population are doing most of the mating; put another way, most females
are mating with only a few males. A graph of the hypothesis for the relationship
between number of mates and reproductive success for males and females would
resemble that for rough-skinned newts: the slope of the line will be statistically
indistinguishable from 0 for females and steeply positive for males.
Male elephant seals are likely four times larger than females because of sexual
selection for winning male-male combat competition events. Male elephant seals
fight one another for control over harems of females on nesting beaches; larger
males are likely to have a significant advantage in these encounters. Size may be
limited by energetic constraints imposed by the ability of males to find enough
food to fuel their bulk. In this species, in which the male lies on top of the female
during mating, extremely large males may even crush or smother females during
mating attempts.
9. In general, the sex with the greatest reproductive investment should experience
strong selection against making such mistakes, as any mistake in identifying an
appropriate partner can cause a significant fitness cost. In contrast, the sex with
minimal reproductive investment can, in effect, afford a few mistakes, and may
benefit from a wider range of partners. A male frog that will mate with a large
variety of females-females of various sizes, colors, behaviors, etc.-may
occasionally make a "mistake" and stray completely outside its species. But the
cost is minor (some sperm), and may be offset by the benefit of being willing to
mate with a large variety of females that are indeed the correct species. Females,
in contrast, may benefit greatly from not only identifying the appropriate species,
sex, and level of physiological maturity of potential mates, but also their overall
quality. And for females, the cost of making so flawed a choice that an expensive
mating attempt is wasted is extremely high.
10. The sage grouse is most likely a male, displaying to females to attract mates. (We
can guess this simply because males are most often the showier sex; but this is not
universally true. See Exploring the Literature, below, for a counterexample.) The
fact that this species has evolved elaborate plumage and a behavioral display
suggests that access to mates limits reproductive success in males. This is most
likely to be the case when males provide little or no parental care. A number of
different social systems are possible under these conditions; the most likely is
polygyny (one male mates with multiple females while females mate with a
single, best male), but promiscuity (both males and females mate with more than
one individual) is also a good possibility.
11. This sounds like a classic case of female choice based on resources provided by
the male. If so, selection would favor mechanisms that allowed females to reliably
choose the males that provided the most sodium for her eggs. Those mechanisms
should include, minimally, variation in male phenotype (morphological or
behavioral) correlated with the amount of sodium he sequesters and neurological
mechanisms in the female that allow her to assess phenotypic variation in the
males and select the ones with the most sodium. These mechanisms, in turn,
should be associated with a general courtship pattern in which males "advertise"
to females and females select among rival males.
To test these ideas, a first step would be to measure many aspects of male
phenotype and explore them for correlations with the amount of sodium males can
provide. If specific phenotypic features are correlated with sodium, then
controlled mate choice tests should reveal that females prefer the phenotypes
generally associated with high sodium levels. If the phenotypic trait in question is
behavioral and flexible (i.e., males behave differently when they've been able to
sequester large amounts of sodium than when they haven't), then we could do
more sophisticated tests in which the amount of sodium available to a male is
controlled and male behavior and female choice are assessed.
12. Taken at face value, the data in the graph illustrate a positive correlation between
parasite prevalence and the importance of physical attractiveness, suggesting that,
in populations where parasite infections are most likely, physical appearance is a
mechanism of mate choice. This pattern is consistent with the hypothesis that
selection favors mate choice mechanisms that enhance fitness.
Under this scenario, males and females are selecting attractive mates because they
are the least likely to be carrying parasite infections. This could be because the
parasites themselves cause disfiguration, or because the features that are
associated with attractiveness cannot be maintained if a portion of the body's
energy is being used to fight an ongoing infection. By selecting an attractive mate,
an individual reaps the benefit of obtaining a strong genotype (one that confers
the ability to ward off parasitic infections) for his or her offspring.
A possible cultural explanation for this pattern is that, while attractiveness is still
associated with lack of parasites (for the reasons explained above), the mechanism
protecting individuals from parasites is simply the wealth and education needed to
avoid infection in the first place. Under this scenario, resistance to parasites isn't
heritable. Rather, the most attractive individuals are the wealthiest and healthiest,
and are preferred because of their material resources.
13. Males were choosy and females competitive under conditions of low-food
availability (the control). Males called less (spent less energy trying to attract
females) and rejected females a greater percentage of the time, while females
tended to seek multiple matings and exhibited more competition with other
females. When food is limiting, the male spermatophore represents a significant
investment of energy on the part of the male. In this case, female reproductive
success is limited by access to mates, whereas male reproductive success is
limited by the ability to make spermatophores.
When extra food is provided, male spermatophores represent much less of an
investment by the males, returning the system to the more usual condition in
which male reproductive success will be limited by access to mates and female
success by the ability to lay eggs. Under these conditions, males become
competitive and females choosy: males spend more time calling (advertising) and
reject fewer females; females seek fewer mates, reject more males, and compete
less with one another.
14. The cost of male symbiosis for the male is that he can never change his choice of
mate. If his female has reduced fertility or poor health, or cannot produce a large
number of eggs, he is simply stuck with her (literally). In fact, if she dies, he will
die, too. In addition, he may have to share the female with other symbiotic males,
so he is not even guaranteed paternity of the eggs. The cost for the female of is
that she bears a physiological and energetic cost of supplying energy and nutrients
to the males, and the attached males also cause hydrodynamic drag while she is
swimming. However, there are also benefits, primarily in mate-finding. It is very
difficult to find mates in the vast and very sparsely populated environment of the
deep ocean floor. Permanent attachment spares both sexes from having to find a
mate or having to struggle to not lose each other.
Overall, the males' symbiotic habit is likely the result of both natural and sexual
selection. In this environment, sexual selection would favor traits that allowed a
male to stay with one female for life, since mates are so difficult to find. Sexual
selection, therefore, may have initially selected for the initial stages of the
evolution of parasitism (physical attachment to the female). Subsequently, natural
selection may have favored those aspects of male anatomy and physiology that
allowed males to usurp the energy and nutrient resources of the females.
However, males that usurped too much of the female's resources would bear a
reproductive cost in terms of a reduction in the number of eggs the females could
produce-so natural selection should also have favored a reduction in body size to
limit the amount of energy required from the female.