Exam #2: Dec 22. 2-3.30 pm. Exam HallChapters: 7, 10, 11, 12, 13, 14, 16(Adib Kfoury proctoring)Homework assignment: due today
Chapter 16: Population Genetics and Evolution Robert E. Ricklefs The 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?
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
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
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.
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.
Chapter 17: Predation and Herbivory Robert E. Ricklefs The 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
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
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
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
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
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
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
Digestive tracts of consumers are adapted to their diets. Digestive organs of herbivores > carnivores
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
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.
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
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
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
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
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
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
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)
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.
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
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
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)
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
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
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.
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.
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.
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.