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Exam #2: Dec 22. 2-3.30 pm. Exam HallChapters: 7, 10, 11, 12, 13, 14, 16(Adib Kfoury proctoring)

Homework assignment: due today

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Chapter 16: Population Genetics and Evolution

Robert E. RicklefsThe Economy of Nature, Fifth Edition

+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 (frequency-dependent 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.

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+ 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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Chapter 17: Predation and Herbivory

Robert E. RicklefsThe Economy of Nature, Fifth Edition

+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 combine traits of parasites and

predators

Herbivores eat whole plants or parts of plants: may act as predators (eating whole plants)

or as parasites (eating parts of plants): grazers eat grasses and herbaceous vegetation browsers eat woody vegetation

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+ 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

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+ 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

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+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

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+ 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 fly24

+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 26

+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

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+Harmless mantid (b) and moth (c) evolved to resemble a wasp (a)

+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

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+ Costa Rican butterflies and moths

+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

+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 are well-adapted to capturing prey.

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 of life.

Parasites and hosts remain in a delicate 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.

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