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Evolution (PCB 4674).
Chapter 10. Studying adaptation: Evolutionary
analysis of form and function
Main topics of lecture:
I: Introduction:
1.- All hypotheses must be tested: The giraffe's neck reconsidered
2.- Experiments and observational studies
3.- The comparative method
II: Phenotypic plasticity:
III: Trade-offs and constraints:
4.- Factors that limit adaptive evolution
5.- Female flower size in a Begonia: A trade off
6.- Flower color change in a Fuchsia: A constraint
7.- Shifts in a herbivorous beetle: Constrained by lack of
genetic variation
I: Introduction:
1.- All hypotheses must be tested: The giraffe's neck reconsidered
1.1.- How can a researcher rigorously test the hypothesis that a particular trait is adaptive?
This is an important question in evolutionary biology, mostly because we showed in the
previous lectures that other evolutionary mechanisms different from selection are also
important in evolution. In this lecture will give an introduction on how researchers can
detect natural selection in all of its forms.
1.2.- As we mentioned in previous lectures, a trait, or integrated suite of traits, that
increases fitness of its possessor is called and adaptation and it is said to be adaptive.
Roughly speaking, in order to demonstrate that a trait is an adaptation, we need first
to determine what the trade is for and then show that individuals possessing the trait
contribute more to future generations lacking it.
1.3.- Obvious explanations for adaptation are no so obvious!!! The traditional
adaptive hypothesis for the origin of the giraffe's neck (to forage trees above the
reach of their competitors) provides an example that no explanation for the
adaptive value of a trait should be accepted simply because it is plausible and
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charming.
1.4.- Traditionally the hypothesis used to explain the adaptive value is known as
the foraging-competition hypothesis, if this hypothesis is correct, then during
the dry season, when food is scarce, giraffes should spend most of their time
foraging above the reach of their competitors. In fact, THIS IS NOT THE CASE,
giraffes spend much of their dry-season foraging time browsing on low bushes
and not tall trees (!!!!).
1.5.- Even when giraffes forage above the reach of their competitors, they seldom
forage at a level anywhere near their maximum height (Fig. 8.2).
Figure 8.2.: These graphs show the proportion of time males
(left) and females (right) spend feeding at different heights
1.6.- Simmons and Scheepers offered and alternative scenario for the evolution
of the giraffe's neck. They suggested that the giraffe's neck evolved as a
weapon, used by the males in combat over opportunities to mate (Fig. 8.3)
Check this site:
http://www.ctap3.org/_lperry/africa/giraffes.htm
you will see how giraffes use their necks and heads as clubs for fighting!!!
Figure 8.3.: Males giraffes use their heads and necks as
clubs when they fight opportunities to mate
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1.6.- Consistent with this hypothesis, male giraffes have necks that are
30 to 40 centimeters longer and 1.7 heavier than the necks of females of
the same age.
1.7.- Also consistent with this hypothesis are data on giraffe behavior.
Researchers divided the individuals of a giraffe population in three classes:
i.- Class C: Young males.
ii.- Class B: Adult males with thin necks, small horns.
iii.- Class A: Adult males with thick necks, massive horns.
1.8.- Studies of interactions between these males in which one bull displaced
another from a social group showed that Class A bulls were dominant over
classes B and C, and Bs were dominant over Cs.
1.9.- Courting and mating interactions between males and females showed that
females preferred to mate with A and B males than with C males. Based on
these data suggested that among the ancestors of today's giraffes, the long-necked
males had higher reproductive success NOT because they got more to eat but
because they intimidated their rivals and attracted more mates. Why, then, do
females have long necks too? Perhaps is simply because males pass genes
for long necks to their daughters as well as to their sons.
1.10.- This example demonstrates that we cannot uncritically accept a hypothesis
about the adaptive significance of a trait simply because it is plausible. We
must submit all the hypothesis to rigorous tests.
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1.11. Evolutionary biologists use three methods to test hypothesis about the
adaptive significance of traits:
i.- Experiments.
ii.- Observational studies.
iii.- Comparative method.
2.- Experiments and observational studies
Experiments.2.1.- Experiments allow us to isolate and test the effect that a single, well-defined
factor has on the phenomenon in question. A good example of an experiment
designed to test an adaptive trait concerns the existence of distinctive dark bands
on the wings of the tephritid fly Zonosemata vittigera.
2.2.- When this fly is disturbed it holds its wings perpendicular to its body and
waves them up and down. This display seems to mimic the territorial threat
display of jumping spiders. Two hypothesis are been suggested to explain this
behavior of the tephritid fly:
i.- Because the jumping spiders are fast and have a nasty bite, a fly
mimicking a jumping spider might be avoided by a wide variety
of predators.
ii.- Because jumping spiders are Zonosemata's main predators,
it has been suggested that the fly uses its wings to make markings
and wing-waving display to intimidate the jumping spiders themselves.
2.3.- Greene and his co-workers set out an experiment to test which of the two
hypothesis account for the adaptive value of this trait. The three hypothesis to test
were:
i.- The flies do not mimic jumping spiders. It is known that in other
species of flies use their markings and displays during courtship.
ii.- The flies mimic jumping spiders. But the flies behave like spiders to
deter other non-spider predators.
iii.- The flies mimic the territorial threat of jumping spiders, and this
mimicry functions specifically to deter predation by the jumping
spiders themselves.
2.4.- These researchers found that they could cut the wings of a Zonosemata
fly and glue them back on. They also found that they could cut the wings of
a Zonosemata fly and replace them with the wings of a housefly, which
are clear and unmarked. The surgically altered Zonosemata continued
to wave their wings in the normal way.
2.5.- By performing various treatments, Green an colleagues created five
experimental groups (A to E):
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A.- Zonosemata untreated.
B.- Zonosemata with own wings cut and reglued.
C.- Zonosemata with housefly wings.
D.- Housefly with Zonosemata wings.
E.- Housefly untreated.
2.6.- The researchers starved 20 jumping spiders for two days. Then they
presented one of each of the five experimental fly types to each spider,
in random order. Results showed clearly (Figure 8.5 of textbook) that
jumping spiders tended to retreat from flies of experimental groups
A and B, these are flies with marked wings. In contrast they attacked
flies that lacked either wing markings, wing waving, of both
(experimental groups C, D and E).
2.7.- When researchers tested these treatment to other predators, all the
test flies were captured and eaten.
2.8.- These results re consistent with the third hypothesis (see heading 2.3
of this lecture): tephritid flies mimic their own jumping-spider predators
to avoid being eaten by them.
2.9.- In terms of experimental design, the Greene et al. study illustrates four
important points:
2.9.1.- Defining and testing effective control groups is critical. Groups A and
B served as control. These control groups demonstrate that: Wing surgery
(wings have been cut and glued) itself had not effect on the behavior of the flies
or spiders.
2.9.2.- All the treatments must be handled exactly alike. The same test arena
was used, the same interval, the same definitions of predator response were also used.
2.9.3.- Randomization is an important technique for equalizing other,
miscellaneous effects among control and experimental groups. For example
the different kinds of flies were presented to the spiders and other predators
in random order.
2.9.4.- Repeating the test on many individuals is essential. They help to:
i.- Reduce the amount of distortion caused by unusual
individuals or circumstances. Four of the flies were killed before
they had a chance to display.
ii.- Understand how precise the estimates are by measuring the
amount of variation.
Observational studies.2.10.- Some hypothesis about adaptation are difficult or impossible to
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test with experiments. Experiments may also be inappropriate when a
hypothesis makes predictions about how organisms will behave in
nature. When experiments are impractical or inappropriate, careful
observations can sometimes yield sufficient information to evaluate
a hypothesis. A good example of observational studies was provided in
headings 1.1 - 1.9 when we discussed the adaptive value of long necks
in giraffes.
3.- The comparative method
3.1.- Comparisons among species can be used to study the evolution
of form and function. The example that we will show comes from
a groups of bats called the Megachiroptera (e.g. fruit bats and flying
foxes).
3.2.- Males in some of these bat species have larger testis for their
body size than others. It has been suggested that this is an adaptation
for sperm competition. Sperm competition occurs when a female mates
with two or more males during a single estrus cycle, and the sperm
from the males are in a race to the egg. Males can increase his
reproductive success in the face of sperm competition by producing
large ejaculates. Larger testis will produce larger ejaculates.
3.3.- To test this hypothesis Hosken (1998) noticed that although
fruit bats live in groups, the size of a typical group varies
dramatically among species, from two or three individuals
to thousands.
3.4.- Hosken reasoned that females living in larger groups will
have more opportunities for multiple matings. Therefore he
expected that males living in larger groups will experience greater
sperm competition. Hosken predicted that whenever a bat species
evolves larger roosting group sizes, its males will also evolve
larger testis for their body size.
3.5.- Hosken gathered data for a variety of species and made a
scatter plot showing relative testis size as function of roost group
size. As he expected he found that species that live in larger
groups have larger testis for their body size (Fig. 8.11).
Figure 8.11.: Relative testis size as a function
of roost group size for 17 species of fruit
bats and flying foxes
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3.6.- However the data of this graph are not as robust as they
seem. A phylogenetic approach to these data will show that the
evidence from this scatter diagram is not so strong.
3.7.- Let's suppose that we plot a graph for only six hypothetical
species (Fig. 8.12.a). This graph also shows a positive correlation
between the two traits (Group size and testis size).
Figure 8.12.a: Relative testis size as a function
of roost group size for 6 hypothetical species
of bats
3.8.- Let's now suppose that we know the evolutionary relationships
(phylogeny) of the six species shown in the scatter diagram
(Fig. 8.12.b).
Figure 8.12.b: A phylogeny for the six species
depicted in Fig. 6.12.a
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3.9.- This phylogenetic tree shows that species A, B, and C are
all closely related to each other, as are species D, E, and F.
It may be that species A, B, and C all inherited their
small group size and their small testis from
their common ancestor (green arrow) (Likewise
for species D, E, and F). The possibility that our six species.
JUST TWO
inherited their traits from
ancestors deflates
the strength of our evidence considerably. Why?? Because
we want to be sure that when we plot the data in our graph
they are independent of each other. If they are independent,
then each data makes a separate statement for or against
our claim.
3.10.- To be sure that our scatter plot accurately reflects the
nature of the evidence, we should therefore replace the
points for species A, B, and C with a single point,
representing their common ancestor (the value for the
common ancestor is obtained as the weighted average
of the descendants' phenotypes, see Box. 8.2. of textbook
on how to calculate this value). We should do the
same with the points for species D, E, and F.
However a scatter plot with only two data points
(Fig. 8.12.c) is weak evidence on which to base that there
is a strong correlation between testis size and populations size.
Figure 8.12.a: Relative testis size as a function
of roost group size for the two major ancestor
species identified in the phylogenetic tree of
Fig. 6.12.b.
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3.11.- Felsenstein (1985) developed a method to evaluate
cross-species correlations among traits. This method
is known as the "Method of Phylogenetically
Independent Contrasts". This method looks
at patterns of divergence as sister species evolved
INDEPENDENTLY AWAY from their common ancestors.
Felsenstein's method will be shown using a graphical
interpretation. Let's suppose we have a phylogeny for
five species (Fig. 8.13.a). In this phylogeny species A and
B are sister species that diverged from common ancestor G.
Likewise, G and C are sister species that diverged from
common ancestor H.
Figure 8.13.a: Hypothetical phylogenetic tree
for six species of bats
3.12.- The question that we want to address is:
When species diverged from a common ancestor,
does the species that evolves larger group sizes
also evolve larger testei?
3.13.- We can answer this question by first plotting all of
the pairs of sister species on a scatter plot, with lines
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connecting their data points (Fig. 8.13.b).
Figure 8.13.b: Relative testis size as a function
of roost group size for the 6 hypothetical species
of bats shown in Fig. 8.13.a
3.14.- We then grab each pair of points by the
point closest to the vertical axis, and drag that
point to the origin (Fig. 8.13.c).
Figure 8.13.c: Group size contrasts as a function
relative testis size contrasts for the 6 hypothetical
species of bats shown in Fig. 8.13.a
3.15.- Finally we can erase the points and the connecting
lines. We are left with a scatter plot with four data points.
Each data point represents THE DIVERGENCE (OR
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CONTRAST) that arose between a pair of sister species
evolved away from their common ancestor (Fig. 8.13.d).
Figure 8.13.d: Group size contrasts as a function
relative testis size contrasts for the 6 hypothetical
species of bats shown in Fig. 8.13.a
3.16.- Hosken used the Method of Phylogenetically
Independent Contrasts to tests the hypothesis that correlation
between species population size and testis size really reflects
adaptation. Figure 8.14a shows a phylogeny of the 17
bat species whose data were plotted in Figure 8.14.b.
Figure 8.14.a: A phylogeny for the 17 species
of bats shown in Figure 8.11
Figure 8.14.b: Independent contrasts for relative
testis size versus group size
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3.17- Figure 8.14b shows a plot of the contrasts in relative
testis size versus the contrasts in group size. There is a significant
positive correlation among the contrasts. In other words, the data
show that when a bat species evolved larger roosting group sizes
than its sister species, it also tended to evolve larger testes for its
body size.
3.18.- Hosken concluded that the evidence from flying foxes and
fruit bats is consistent with the hypothesis that large testes are
an adaptation to sperm competition.
II: Phenotypic plasticity:
II.1- When phenotypes are plastic, individuals with identical genotypes
may have different phenotypes if they live in different environments
Phenotypic plasticity is itself a trait that can evolve. Here we
will provide an example that illustrates phenotypic plasticity in
the water flea (Daphnia).
II.2.- Daphnia individuals reproduce asexually most of the time.
This makes them ideal for studies of phenotypic plasticity. Researchers
can grow genetically identical individuals in different environments
and compare their phenotypes.
II.3.- We can distinguish two kind of phenotypes in this species:
i.- Positively phototactic: The swim toward light.
ii.- Negatively phototactic: The swim away from light.
II.4.- De Meester (1996) conducted an experiment with different clones
of this species to study phenotypic plasticity. He placed 10 genetically
identical individuals in a graduate cylinder, illuminated them from
above, and then recorded where in the cylinder they swan.
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II.5.- De Meester assigned to each clone an index of phototactic
behavior. The index can range from +1 to -1 as follows:
+1 = All the Daphnia in the test swan to the top of the
column, toward the light.
-1 = All the Daphnia in the test swan to the bottom of the
column, away from the light.
II.6.- He measured phototactic behavior of 10 water flea genotypes
in three lakes of Belgium:
i.- Lake Blankaart (This lake has a sizeable population of fish)
ii.- Lake Driehoekvijver (This lake has few fish).
iii.- Lake Citadelpark (This lake has no fish).
The results of this experiment are indicated with blue dots in Fig. 8.17.
They show that each lake harbors considerable genetic variation for
this phototactic behavior.
Figure 8.17.: Variation in phototactic behavior in
Daphnia magna.
II.7.- De Meester also measured the phototactic behavior of the same
30 water flea genotypes in water that had been previously occupied by fish.
The results of this experiment are indicated with red squares in Fig. 8.17.
These results show clearly that phototactic behavior is phenotypically
plastic. Lake Blankaart, in particular, most water flea genotypes score
considerably lower on the phototactic index when tested in the presence
of chemicals released by fish. It is believed that water flea have evolved
to avoid well-lit areas when fish is present.
II.8.- The most important message from this experiment is that it
proves that phenotypic plasticity is a trait that can evolve. Each of the
water flea populations contains genetic variation for phenotypic
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plasticity. Therefore, some genotypes in each population will alter their
behavior more than others in the presence versus the absence of fish.
Genetic variation for phenotypic plasticity is called
genotype-by-environment interaction.
II.9.- The average genotype in Lake Blankaart shows considerable more
phenotypic plasticity than the average genotype in either of the other lakes.
Blankaart is the only one of the lakes with a sizeable population of fish.
Fish are visual predators, and they eat Daphnia. A reasonable
interpretation is that fish select in favor of Daphnia that avoid well-lit
areas when fish are present.
III: Trade-offs and constraints:
4.- Factors that limit adaptive evolution
4.1.- It is impossible for any population of organisms to evolve optimal
solutions to all selective challenges at once. the giraffe's long
neck, for example, may be useful for males attempting to
obtain a mate, but appears to be inconvenient for drinking.
4.2.- There are additional examples of research into factors that
limit adaptive evolution. These factors include:
i.- Trade-offs.
ii.- Functional constraints.
iii.- Lack of genetic variation.
5.- Female flower size in a Begonia: A trade off
5.1.- The tropical plant Begonia involucratra is monoecious (that is
there are separate male and female flowers on the same plant). Flowers
of this species are pollinated by bees. They travel among male flowers
gathering pollen and sometimes also transfer pollen from male flowers
to female flowers.
5.2- The male flowers offer the bees, a reward, in the form of the pollen
itself. The female flowers offer no rewards. The female flowers resemble
the male flowers in color, shape, and size (Fig. 8.22.a). This resemblance
is presumably adaptive. Given that bees avoid female flowers in favor of
male flowers, the rate at which female flowers are visited should depend
on the degree to which they resemble male flowers.
Figure 8.2.a: Begonia involucrata. Male (left) and
female (right) flowers
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5.3- The ability of female flowers to attract pollinators will influence
fitness through female function because seed set is limited by pollen
availability. Schemske and Agren proposed two possible hypothesis about
the mode of selection imposed by bees on female flower size:
i.- Hypothesis 1: The more closely a female flower resemble a "typical
average" male flowers, the more often it will be visited by bees. Selection
of female flowers is stabilizing with best phenotypes for females identical
to the mean phenotype of males (Fig. 8.23.a).
Figure 8.23.a. Stabilizing selection hypothesis for
female flowers in Begonia involucrata
ii.- Hypothesis 2: The more closely female flowers resemble the most
rewarding male flowers, the more often they are visited by bees. If larger
male flowers offer bigger rewards, then selection on female flowers always
favored over smaller flowers (Fig. 8.23.b).
Figure 8.23.b. Directional selection hypothesis for
female flowers in Begonia involucrata
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5.4- Schemske and Agren made artificial flowers of three different sizes to
test whether pollinators prefer to visit larger flowers. The results were
clear: The larger the flower the more bees approaches and visits attracted.
Therefore selection by bees on female flowers is strongly directional.
5.5.- Therefore we would expect for female flowers to be bigger than
male flowers. However this is not the case and female flowers are not
bigger than male flowers. Schemske and Agren gave two
possible explanations for this paradox:
i.- Begonia involucratra simply lacks genetic variation for female
flowers that are substantially larger than male flowers.
ii.- There is a trade off between flower size and number of flowers
per inflorescence.
5.6.- Schemske and Agren favored the second hypothesis as they expanded
their focus from individual flowers to inflorescences. They measured the
size and number of the female flowers on 74 inflorescences. They
discovered a trade off: The larger the female flowers on an inflorescence,
the fewer flowers there are (Fig. 8.23 c).
Figure 8.23.c. Number of female flowers per inflorescence
as a function of flower size
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5.7.- If an individual plant has a finite supply of energy and nutrients to
invest in flowers, it can slice this pie into a few large pieces or many small
pieces, but not into many large pieces.
5.8.- Inflorescences with more flowers may be favored by selection for
two reasons:
i.- Bees may be more attracted to inflorescences with more flowers.
ii.- More female flowers means greater potential for seed production.
5.9.- Schemske and Agren hypothesize that female flower size in Begonia involucratra
has been determined, at least in part, by two opposing forces: directional selection
for large flowers, and the trade-off between flower size and number.
6.- Flower color change in a Fuchsia: A constraint
6.1.- Fuchsia excorticata is a bird-pollinated tree endemic to New Zealand.
Its flowers hang downward like bells (Fig. 8.24.a). The ovary is at the top
of the bell. The body of the bell consists of the hypanthium or floral
tube, and the sepals. The style resembles an elongated clapper. It is
surrounded by shorter stamens and a set of reduced petals.
Figure 8.24. Fuchsia excorticata flower
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6.2.- The hypanthium and sepals are the most conspicuously showy parts
of the flower. They remain green for about 5.5 days after the flower opens,
then begin to turn red. The transition from green to red lasts about 1.5 days,
at the end of which the hypanthium and sepals are fully red. The red
flowers remain on the tree for about 5 days. The red flowers then
separate from the ovary at the abscission zone and drop from the tree.
6.3.- Pollination occurs during the green phase and into the intermediate
phase, but it is complete by the time the flowers are fully red. The
flowers produce nectar on days 1 through 7. Most flowers have exported
more than 90% of their pollen by the end of that time.
6.4.- Bellbirds and other avian pollinators strongly prefer green flowers,
and virtually ignore nectarless red flowers. Why do the flowers of this tree
change color? A general answer, is that color change serves as a cue to
pollinators alerting them that the flowers are no longer offering a reward.
6.5.- This answer is only partially satisfying. Why does F. excorticata not just
drop its flowers immediately after pollination is complete? Dropping
the flowers will have to effects:
i.- It will give an unambiguous signal to pollinators that a reward is no
longer being offered.
ii.- It would be metabolically cheaper than maintaining the red flowers
for several days.
Therefore retention of the flowers beyond time of pollination seems
maladaptive.
6.6.- Delph & Lively (1989) considered two hypothesis for this feature.
6.6.1- Red flowers may still attract pollinators to the tree displaying them if
not to the red flowers themselves. Once drawn to the tree, pollinators could then
forage on the green flowers still present. However these researchers compared
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the amount of pollen deposited on green flowers in red-free trees and branches
versus red-retaining trees and branches. They found not significant differences.
Therefore this pollinator-attraction hypothesis does not explain the retention
of the red flowers in Fuchsia excorticata.
6.6.2.- Physiological constraints prevents Fuchsia excorticata from
dropping its flowers any sooner than it does. This physiological constraint is
the growth of pollen tubes from the stigma to the ovary. The growth of pollen
tubes takes time, especially in a plant like Fuchsia excorticata, which has
long styles. It takes approximately tree days for the pollen tube to reach the ovary.
6.7.- Fuchsia excorticata cannot start the process of dropping a flower until
about three days after the flower has finished receiving pollen. Dropping a
flower involves forming an abscission zone between the ovary and the flower.
In Fuchsia excorticata takes at least 1.5 days.
6.8.- Delph & Lively suggest that flower change in Fuchsia excorticata is an
adaptation that evolved to compensate for the physiological constraints that
necessitate flower retention. Given that the plants had to retain its flowers
because this physiological constraint, selection favored individuals offering
cues that allow their pollinators to distinguish the receptive (green color)
versus unreceptive flowers (red color) on their branches.
7.- Shifts in a herbivorous beetle: Constrained by lack of
genetic variation
7.1.- Natural selection is the process that produces adaptations, genetic
variation is also the raw material from which adaptations are molded.
Populations of organisms may be prevented from evolving particular
adaptations simply because they lack the necessary genetic variation to do so.
7.2.- Futuyama et al. have sought to determine whether lack of genetic
variation has constrained adaptation in species of Ophraella, a genus of
small beetles that feeds as larvae and adults on the leaves of one or a few
closely related species of composites (plants in the sunflower family, the
Asteraceae).
7.3.- Each species of host plants makes a unique mixture of toxic chemicals
that serve as defenses against herbivores. Each beetle species is adapted to
the chemicals of its particular host plant.
7.4.- A phylogeny for 12 species of Ophraella shows that the evolutionary
history of the beetle genus has included several shifts from one host plant to
another. These shifts included changes from plant of one tribe of the
Asteraceae to a plant in another tribe. Other shifts involved movement to
a new host in the same genus.
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7.5.- Futuyama et al. have attempted to elucidate why some hosts shifts have
actually happened while others have remained hypothetical. They proposed
two hypothesis:
i.- Hypothesis 1: Most of the shifts are genetically possible. That is,
every beetle species harbors sufficient genetic variation in its feeding and
detoxifying mechanisms to allow at least some individuals to feed and
survive on every potential host species. If a few individuals can feed
and survive, they can be founders for a new population of beetles that
will evolve to become well-adapted to the new host. Because all host shifts
are genetically possible, the pattern of actual host shifts has been determined
by ecological factors and chance. Ecological factors might include the
abundance of various host species within the geographic ranges of the beetle
species, and the competitors and predators associated with each host species
i.- Hypothesis 2: Most host shifts are genetically impossible. That is,
most beetle species lack sufficient genetic variation in their feeding and
detoxifying mechanisms to allow any individual to feed and survive on any
but a few of the potential host species. The pattern of actual host
shifts has been largely determined by what was genetically possible.
7.6.- Futuyama et al. used a quantitative genetic approach to determine
how much genetic variation the beetles harbor for feeding and surviving
on other potential hosts. The researchers examined various combinations
of four of the beetle species (O. notulata, O. communa, O. conferta, and
O. artemisiae) shown in the phylogenetic tree (Fig. 8.25) with six of the
host plants (Iva frutescens, Chrysopsis villosa, Eupatorium perfoliatum,
Artemisia vulgaris, Solidago bicolor, Solidago altissima).
7.7.- Their tests revealed that there is little genetic variation in
most of the beetle species for feeding and surviving on most potential
host species. In 18 of 39 tests of whether larvae or adults of a beetle
species would recognize and feed on a potential host plant, the
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EVOLUTION/LECTURE1
researchers found no evidence of genetic variation for feeding. In
14 of 16 tests of whether larvae could survive on a potential host
plant, the researchers found no evidence of genetic variation for
survival.
7.8.- Futuyama et al. performed an additional test of hypothesis 2 by
looking for patterns in their data on genetic variation for larval and adult
feeding. If hypothesis 2 is correct, then a beetle species is more likely
to show genetic variation for feeding on a potential new host if the
new host is a close relative of the beetle's present host. Futuyama et al.'s
data confirm this prediction
7.9.- Likewise if hypothesis 2 is correct, then a beetle species is more
likely to show genetic variation for feeding on a potential new host
if the new host is the actual host of one of the beetles's close relatives.
Futuyama et al.'s data confirm this prediction.
7.10.- These results suggest that hypothesis 2 is at least partially correct.
The history of host shifts in the beetle genus has been constrained
by the availability of genetic variation for evolutionary change. It is
necessary to emphasize that little genetic variation does not mean
NO GENETIC variation. The little genetic variation found in this
group had been, however, enough to produce the host shifts shown
in Fig. 8.25, although most of the host shifts have taken place within
particular sunflower tribes.
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