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Exam #2:
Dec 22. 2-3.30 pm. Exam Hall
Chapters: 7, 10, 11, 12, 13, 14, 16
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(Adib Kfoury proctoring)
Homework assignment: due today
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Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Chapter 16: Population
Genetics and Evolution
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Maintenance of Genetic Variation 1
A
paradox:
 natural
selection cannot produce evolutionary
change without genetic variation
 however, both stabilizing and directional
selection tend to reduce genetic variation:

how does evolution continue under such circumstances?

does availability of genetic variation ever limit the rate of
evolutionary change?
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Maintenance of Genetic Variation 2

Mutation and migration supply populations with new
genetic variation.

Spatial and temporal variation tend to maintain
variation by favoring different alleles at different times
and places.

When heterozygotes have a higher fitness than
homozygotes, the relative fitness of each allele depends
on its frequency in the population (frequencydependent selection):


alleles are selected for when at low frequency and against
when at high frequency
heterozygote superiority is also called heterosis
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How much genetic variation?
 About
1/3 of genes that encode enzymes
involved in cellular metabolism show
variation in most species:
 about
10% of these are heterozygous in any
given individual
 however, most genetic variation is apparently
neutral or has negative effects when expressed
 thus most variation has no fitness consequences
or is subject to stabilizing selection
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Genetic Variation is Important
 Under
changing environmental conditions,
the reserve of genetic variation may take on
positive survival value.
 There
seems to be enough genetic
variation in most populations so that
evolutionary change is a constant
presence.
+ Evolutionary Changes in Natural
Populations
 Evolutionary
changes have been widely
documented, particularly in species that have
evolved rapidly in the face of environmental
changes caused by humans:
cyanide resistance in scale insects (Chapter 9)
 pesticide and herbicide resistance among agricultural
pests and disease vectors
 increasing resistance of bacteria to antibiotics

 In
each case, genetic variation in the gene pool
allowed these populations to respond to
changed conditions.
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Useful Conclusions from
Population Genetics Studies
 Every
population harbors some genetic
variation that influences fitness.
 Changes
in selective factors in the
environment are usually met by
evolutionary responses.
 Rapid
environmental changes caused by
humans will often exceed the capacity of a
population to respond by evolution; the
consequence is extinction.
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Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Chapter 17: Predation and
Herbivory
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Consumer-Resource Interactions
 All
life forms are both consumers and
victims of consumers.
 Consumer-resource
interactions organize
biological communities into consumer
chains (food chains):
 consumers
benefit at the expense of their
resources
 populations are controlled from below by
resources and from above by consumers
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Some Definitions
 Predators
catch individuals and consume them,
removing them from the prey population.
 Parasites
consume parts of a living prey organism,
or host:


parasites may be external or internal
a parasite may negatively affect the host but does not
directly remove it from the population
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More Definitions
 Parasitoids
consume the living tissues of
their hosts, eventually killing them:
 parasitoids
predators
combine traits of parasites and
 Herbivores
plants:
eat whole plants or parts of
 may
act as predators (eating whole plants) or as
parasites (eating parts of plants):


grazers eat grasses and herbaceous vegetation
browsers eat woody vegetation
+ Parisitoid wasps
develop inside
the larvae or
pupae of other
insects
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Detritivores occupy a special
niche.
 Detritivores
consume dead organic material, the
wastes of other species:


have no direct affect on populations that produce these
resources:
 do not affect the abundance of their food supplies
 do not influence the evolution of their resources
are important in the recycling of nutrients within
ecosystems
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Predators have adaptations for
exploiting their prey.
 Predators


vary in size relative to their prey:
predators may be much larger than their prey (whales are far
larger than krill and small fish)
prey are rarely much larger than their predators:
 beyond a certain prey size, a predator cannot successfully
subdue and consume the prey
 cooperative hunters are an exception, taking prey substantially
larger than themselves
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Form and Function Match Diet
 Form

and function of predators are closely tied to diet:
vertebrate teeth are adapted to dietary items:
 horses have upper and lower incisors used for cutting fibrous
stems of grasses, flat-surfaced molars for grinding
 deer lack upper incisors, simply grasping and tearing
vegetation, but also grinding it
 carnivores have well-developed canines and knifelike
premolars to secure and cut prey
+ A predator’s form and function are closely tied to its
diet. (a) upper incisors are used to cut plant
material; (b) flat-surfaced molars for grinding plant
material; (c) knifelike premolars secure prey and
tear flesh
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More Predator Adaptations
 The


variety of predator adaptations is remarkable:
consider grasping and tearing functions:
 forelegs for many vertebrates
 feet and hooked bills in birds
 distensible jaws in snakes
digestive systems also reflect diet:
 plant eaters feature elongated digestive tracts with
fermentation chambers to digest long, fibrous molecules
comprising plant structural elements
+ Distensible jaws: shift the articulation of
the jaw with the skull from the quadrate
bone to the supratemporal
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Digestive tracts of consumers are adapted to their
diets. Digestive organs of herbivores > carnivores
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Prey have adaptations for escaping
their predators.
 Prey


escape mechanisms are remarkably diverse:
in animals:
 swift escape
 capability of early predator detection
 hiding or seeking refuge
 sensitive mechanisms for detecting predators
in plants:
 thorns and defensive chemicals that dissuade herbivores
+ A bombardier beetle sprays a noxious liquid at the
temperature of boiling water toward a predator
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Crypsis and Warning Coloration
 Through
crypsis, animals blend with their
backgrounds; such animals:



are typically palatable or edible
match color, texture of bark, twigs, or leaves
are not concealed, but mistaken for inedible objects by
would-be predators
 Behaviors
of cryptic organisms must correspond to
their appearances.
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Cryptic appearances (a) mantid; (b) stick
insect; (c) lantern fly
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Warning Coloration
 Unpalatable
animals may acquire noxious
chemicals from food or manufacture these
chemicals themselves:

such animals often warn potential predators with warning
coloration or aposematism:
 predators learn to avoid such animals after unpleasant
experiences
 certain aposematic colorations occur so widely that
predators may have evolved innate aversions
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Unpalatable organisms
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Why aren’t all prey unpalatable?
 Chemical
defenses are expensive, requiring large
investments of energy and nutrients.
 Some
noxious animals rely on host plants for their
noxious defensive chemicals:


not all food plants contain such chemicals
animals utilizing such chemicals must have their own
means to avoid toxic effects
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Batesian Mimicry
 Certain
palatable species mimic unpalatable
species (models), benefiting from learning
experiences of predators with the models.
 This
relationship has been named Batesian
mimicry in honor of discoverer Henry Bates.
 Experimental
studies have demonstrated
benefits to the mimic:
predators quickly learn to recognize color patterns of
unpalatable prey
 mimics are avoided by such predators

+Harmless mantid (b) and moth (c) evolved to
resemble a wasp (a)
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Müllerian Mimicry
 Müllerian
mimicry occurs among
unpalatable species that come to resemble
one another:
 many
species may be involved
 each species is both model and mimic
 process is efficient because learning by predator
with any model benefits all other members of the
mimicry complex
 certain aposematic colors/patterns may be
widespread within a particular region
+ Costa Rican butterflies and moths
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Parasites have adaptations to
ensure their dispersal.
 Parasites
are usually much smaller than
their hosts and may live either externally or
internally:
 internal

parasites exist in a benign environment:
both food and stable conditions are provided by host
 parasites


must deal with a number of challenges:
host organisms have mechanisms to detect and destroy
parasites
parasites must disperse through hostile environments,
often via complicated life cycles with multiple hosts, as
seen in Plasmodium, the parasite that causes malaria
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Parasite-Host Systems: A Balancing
Act
 The
parasite-host interaction represents a
balance between parasite virulence and
host defenses:
 immune
system of host can recognize and
disable parasites
 but parasites may multiply rapidly before an
immune response can be deployed
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Parasites may defeat a host’s
immune response.
 Circumventing
the host’s immune system is a
common parasite strategy:



some parasites suppress the host’s immune system (AIDS
virus)
other parasites coat themselves with proteins that mimic
the host’s own proteins (Schistosoma)
some parasites continually coat their surfaces with novel
proteins (trypanosomes)
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Cross-Resistance
 Some
parasites elicit an immune response
from the host, then coat themselves with
host proteins before the immune response
is fully mobilized:
 initial
immune response by host may benefit the
host later when challenged by related parasites
in a phenomenon known as cross-resistance
 Once
an immune response has been
elicited, antibodies can persist for a long
time, preventing reinfection.
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Plants have antiherbivore
defenses.
 Plant-herbivore “warfare” is
waged
primarily through biochemical means.
 Full
spectrum of plant defenses includes:
 low
nutritional content of plant tissues
 toxic compounds synthesized by the plants
 structural defenses:



spines and hairs
tough seed coats
sticky gums and resins
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Spines protect the stems and leaves
(a) cholla cactus and (b) prickly pear cactus
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Digestibility
 Animals
typically select plant food
according to its nutrient content:
 especially
important to young animals, which
have high demands for protein
 Some
plants deploy compounds that limit
the digestibility of their tissues:
 tannins
produced by oaks and other plants
interfere with the digestion of proteins
 some animals can overcome the effect of tannins
through production of digestive dispersal agents
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Secondary Compounds
 Secondary
compounds are produced by
plants for purposes (typically defensive)
other than metabolism.
 Such
compounds can be divided into three
major classes:
 nitrogen
compounds (lignin, alkaloids,
nonprotein amino acids, cyanogenic glycosides)
 terpenoids (essential oils, latex, plant resins)
 phenolics (simple phenols)
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Induced and Constitutive Defenses

Constitutive chemical defenses are maintained at high levels in
the plant at all times.

Induced chemical defenses increase dramatically following an
attack:

suggests that some chemicals are too expensive to maintain under light
grazing pressure

plant responses to herbivory can reduce subsequent herbivory
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Herbivores control some plant
populations.

Examples of control of introduced plant pests by herbivores
provides evidence that herbivory can limit plant populations:

prickly pear cactus in Australia


controlled by introduction of a moth, Cactoblastis
Klamath weed in California

controlled by introduction of a beetle, Chrysolina
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Effects of Grazers and Browsers on
Vegetation

Herbivores consume 30-60% of aboveground vegetation in
grasslands:


demonstrated by use of exclosures limiting access to vegetation
by herbivores
Occasional outbreaks of tent caterpillars, gypsy moths, and
other insects can result in complete defoliation of forest
trees.
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Summary 1
 Among
consumers, ecologists recognize
predators, parasitoids, and parasites.
 Predator-prey
relative sizes may vary
dramatically.
 Predators
 Prey
are well-adapted to capturing prey.
avoid predation by avoiding detection
and by means of chemical, structural, and
behavioral defenses.
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Summary 2
 Batesian
mimicry involves an unpalatable
model and palatable host.
 Müllerian
mimicry complexes involve two or
more unpalatable species that resemble one
another.
 Parasites
have unique adaptations for their way
 Parasites
and hosts remain in a delicate
of life.
balance.
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Summary 3

Plants use structural and chemical defenses to deter
herbivores.

Some herbivores can detoxify secondary plant compounds,
enabling them to consume otherwise toxic species.

Herbivores may control populations of plants at levels far
below their sizes in the absence of specialized consumers.