Ch17 Evolution of Species Interactions.docx
Ch17 Evolution of Species Interactions.docx

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University of Toronto St. George
Spencer Barrett

BIO120H © Li| Page 12011 chapter 17: EVOLUTION OF SPECIES INTERACTIONS  biological factors which have shaped adaptations differ from physical factors:  BF stimulate mutual evolutionary responses in the traits of interacting pop.  ex. predators shape prey’s adaptations for escape which shape their own adaptations for pursuit and capture  but adaptations of animals have no reciprocating effect on the envmt (BF)  they foster diversity of adaptations rather than promoting similarity  org. specialize in response to BF  in response to similar stresses, species converge (evolve similar adaptations)  coevolution: the reciprocal evolution in 2 or more interacting species of adaptations selected by their interaction  adaptations in one pop. promote the evolution of adaptations in the other  ex. when a herbivore evolves a way to detoxify a noxious chemical that has evolved in a plant  hyena’s bone-crushing teeth is NOT coevolution  their prey didn’t evolve to resist being eaten by hyenas  when the evolutionary relationship b/w species is antagonistic, the species can be involved in an evolutionary stuggle to increase their own fitness  may result in an evolutionary stalemate where both antagonists continually evolve in response to each other, but the net outcome of their interaction is a steady state  when one doesn’t respond fast enough, it may become extinct  species in mutually beneficial relationship evolve adaptations that promote their interaction ADAPTATIONS IN RESPONSE TO PREDATION DEMONSTRATE SELECTION BY BIOLOGICAL AGENTS  colouration is a trait that can evolve in prey under selection from predators  changes in colouration feed back on predator adaptations: find increasingly well- camouflaged prey or to avoid prey w/ colouration that signal noxious qualities Crypsis versus Warning Colouration  crypsis: an appearance that allows in org. to blend into the background to avoid detection by others  ex. resembling sticks, leaves, bird droppings  strategy for palatable (edible) animals  warning colouration aposematismm): conspicuous patterns or colours adopted by unpalatable prey org. to advertise their noxiousness or dangerousness to potential predators  produce noxious chemicals or accumulate them from food plants  predators learn to avoid these colourations (org. tastes bitter)  some predators evolved innate aversions to black and red/yellow  not all potential prey species are noxious or unpalatable because chemical defences use NRG and org. who get toxicity from plants need mechanisms to avoid the toxic effects BIO120H © Li| Page 22011 Mimicry  Henry Bates  Batesian mimicry: a predator deterrence strategy in which a palatable species (mimic) resembles an unpalatable species (model)  Fritz Müller  Müllerian mimicry: a predator deterrence strategy in which several unpalatable species adopt a single pattern of warning colouration  predators learn to avoid these more efficiently b/c a bad experience w/ one species confers protection on all other members of the mimicry complex ANTAGONISTS EVOLVE IN RESPONSE TO EACH OTHER  Charles Mode coined the term coevolution - modeling  concerned w/ the relationship b/w crops and their fungal pathogens (esp. rusts)  developed a model of continual evolution of a pathogen and its host in response to evolutionary changes in each other  assumed pathogen virulence and host resistance was each controlled by a dominant gene (V, R) – both costly to org.  fitness dependant on genotype of the other  thus, frequencies of virulence and resistance genes should tend to oscillate over time  cycle: r (host)  V (pathogen)  R (host)  v (pathogen)  r (host) …  when host is susceptible (rr), selection favours virulent pathogens (VV, Vv) o virulent pathogens, selection favours host resistance (RR, Rr)  when host is resistant, selection favours avirulent pathogens (vv) o avirulent pathogens, selection favours susceptible hosts (rr)  Paul Ehrlich and PPeter Raven– observed patterns  noticed that closely related groups of butterflies tended to feed on closely related species of host plants  suggested evolutionary history linked butterflies to their host plants  evolution of butterflies tolerate the particular defences of their host plants  plants evolved ex. passionflower evolved to minimize herbivory by butterfly larvae  control: wasps allowed to parasitize a fly pop. that was kept at a constant #, emerging flies from pupa removed to maintain “naïve” hosts  experimental: fly pop. was kept at the same constant #, let David Pimentel et emerging flies remain in cage David Pimentel  wasp’s reproductive rate dropped from 135 to 49 progeny, al. longevity decreased from 7 to 4 days, pop. decreased (evolution of parasitoid-host  thus, flies evolved resistance to wasps (after 3 years – evolution occurred) relationships)  experiment 2 - # of flies allowed to vary housefly pupa vs.  control: flies and wasps had no previous contact wasp  wasps were efficient parasitoids – dramatic oscillations ecologists in the field occurred  experimental: flies and wasps from experimental cage from before  wasp pop. remained low, flies attained high constant pop. level BIO120H © Li| Page 32011 COEVOLUTION IN PLANT-PATHOGEN SYSTEM REVEALS GENOTYPE- GENOTYPE INTERACTIONS  responding evolution = genetic variation for traits that influence interactions  plant geneticists develop strains of crops resistant to pathogens  when new strain appears, they select new resistant strains  but new strains of pathogen continue to appear; by migration/mutation  results in continual evolutionary change in the sys. (Mode’s model)  genotype-genotype interactions: variation in the expression and fitness of genotypes genotype-genotype interactions in one species depending on the genotypes of another species w/ which it interacts  evolution of most species is driven in part by their interactions w/ their consumers, resources, competitors, and mutualists CONSUMER AND RESOURCE POPULATIONS CAN ACHIEVE AN EVOLUTIONARY STEADY STATE  when does evolution stop?  when a species interacts w/ many others simultaneously, no single virulence or resistance factor is likely to convey a unique advantage over all the others  the ability of virulent pathogens to switch to a more abundant host species (which gives a reduced host pop. a chance to recover) might lead to an equilibrium state of maintained genetic diversity  in strict coevolution, time delays occur b/c each pop. responds to only one other pop. resulting in cyclic changes in gene frequencies  but when multiple consumer an resource pop. affect one another simultaneously, time delays are less imp.  most eco. sys. evolve toward a steady state in which evolution continues  rate of adaptation evolution in a resource pop. is proportionate to rate at which it is exploited  thus, any adv. a consumer evolves over its resource pop. is only temporary  the strength of selection for new adaptations in consumers for exploiting resources decreases as resources are heavily exploited  when resource is not heavily exploited, adaptations of consumers that enable them to use that resource are selectively favoured, and their exploitation of that resource increases  as exploitation increases, resources reduced, and further increases in consumer efficiency have less selective value  high rates of consumption favour consumers that shifted their diets toward other, more abundant resource pop.  evolution favour less efficient use of a resource as a consequence of adaptations to exploit another, more abundant resource pop.  evolutionary steady state – when selection on the resource pop. for adaptations to reduce consumption balances selection on consumer pop. to increase consumption  both continually evolve to maintain balance COMPETITIVE ABILITY RESPONDS TO SELECTION  competitors exert pressure on each other  competitors may be selected to diverge as a result of the resources consumed  evolution of o
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