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Lecture 17

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University of Toronto Scarborough
Biological Sciences
Jason Weir

Lecture 17: Coevolution Coevolution: The joint evolution of two or more interacting species, each of which evolves in response to selection imposed by the other. • Coevolution is an important process of both micro- and macroevolutionary change Three types of coevolution: i. Specific coevolution: two species evolve in response to each other. ii. Guild coevolution: multiple species coevolve, and their effects are not independent. iii. Escape-and-radiate coevolution: A species escapes predation or parasitism after evolving a major defense, then diversifies. Latter a different predator or parasite adapts to the host clade and diversifies . Ex latex in plants that is poisonous to insects escapes predation and radiates like a motherfucker. Sometime in the future the insects evolve to counteract the latex 2) Phylogenetic detection of Coevolution Figure 19.2 The phylogeny of endosymbiotic bacteria included under the name Buchnera aphidicola is perfectly congruent with that of their aphid hosts. Coevolution their phylogenies correspond to one another and look alike. The bacteria has almost the same phylogentic tree as its aphid host. That’s because the bacteria live in the host and diverse endosymbiotically with their hosts. Especially in endosymbiotic species. Figure 19.3 (A) A phylogeny of specialized feather lice is mostly congruent with that of their hosts. These feather lice don’t correspond to the phylogeny of their host . that’s because the lice jump from one host to the other. Host species mizing so they are not correlated to one another. 3) Processes of Coevolution We will consider processes of coevolution in 4 types ofrelationships between species A. Coevolution of enemies and victims B. Mutualism C. Competition D. Mimicry 3.1) Coevolution of enemies and victims: Predator-prey coevolution • Newt and garter snakes and TTX o Rough-skinned newt (Taricha granulosa) has the potent neurotoxin tetrodotoxin (TTX) in its skin. o Most populations have high levels of the toxin (typically, a single newt has enough toxin to kill 25,000 mice). o Some populations, like those from Vancouver Island, lack the toxin. o Resistance to the toxin in the garter snake Thamnophis sirtalis was measured in garter snake populations where the newt is absent, where it is present but lacks TTX, and where it is present but has high levels of TTX. o Garter snake populations showed high levels of resistance to the toxin only where occurred sympatrically with high TTX newt populations. o • A fledgling common cuckoo being fed by its foster parent, a much smaller reed warbler. (B) Mimetic egg polymorphism in the European cuckoo o Each common cuckoo population contains several different genotypes that prefer differ hosts. o Each genotype lay eggs closely resembling those of their preferred hosts o Other individuals lay non-mimetic eggs o Host species vary in their ability to recognize mimetic eggs. • Davies and Brooke 1998 place artificial cuckoo eggs in a variety of host species nests and followed the fate of the eggs. • Species not parasitized by cuckoos: rarely rejected eggs • Commonly parasitized species: high levels of egg rejection. • Rarely parasitized species: equally high levels of egg rejection.: THIS IS WEIRD? • These results suggest that rarely parasitized species probably once served as common hosts (sometime in the past), and their ability to reject cuckoo eggs has selected against the cuckoo genotypes that parasitize these species. 3.1) Coevolution of enemies and victims: Coevolution in plants and herbivores • The fruit of the Japanese camellia has a thick, woody fruit wall (pericarp) that encloses the seeds. • North we have smaller fruit: shorter rostrums and south we have big fruit and the weevils have longer rostrums. • The larva of the camellia weevil feed exclusively on this species. • Adult weevils bore a hole through the pericarp into the seed chamber using their rostrum (mandible) and deposit their eggs. • There is a latitudinal gradient in rostrum length and pericarp thickness with southern populations having exaggerated traits. • Studies: Joju and Sota (2006) determined weevils success in boring through to the seed chamber as a function of their rostrum length relative to the thickness of a fruits pericarp (C) (D) shows what combinations of rostrum length and pericarp thickness would result in a 50% success rate for the weevil. Northern populations are ahead in this conflict – their rostra are long enough to ensure a success rate well over 50% - whereas in the south, the plant population is ahead with pericarps thick enough to reduce the weevil’s success. These species may be involved in an “arms race”, although the reason for the thinner pericarp in the north is not clear. 3.1) Coevolution of enemies and victims: Infectious disease and evolution of parasite virulence QUESTION: What would be the optimal level of virulence for a parasite? High virulence : too much will kill the host Slow Virulence: 3.1) Coevolution of enemies and victims: Infectious disease and evolution of parasite virulence a. How Virulent should a parasite be? • Rabbits introduced to Australia: a. Multiplied like a motherfucker b. So to counteract this increase they introduced myxoma virus to kill off the rabbits. • myxoma virus introduced to reduce rabbit numbers • Result: Myxoma: south America virus so this Australian rabbits did not have adaptation against the virus. First year : Virelnt grade of 1 : bby 1964 : virulent grade declined to 3 or 4. Same virus was introduced in France and Britain and similar decline in virulence over time. What happened was that this high virulence was not optimal for the virus . 3.2) Mutualisms • Mutualisms: interactions between species that benefit individuals of both species • In The Origin of Species, Darwin challenged his readers to find one example of a species having been modified solely for the benefit of another species. So far, no one has met Darwin’s challenge. • Mutualisms exemplify not altruism, but reciprocal exploitation, in which each species obtains something from the other. • Some mutualisms, for example, may have originally started out as parasitic or other types of exploitative relationships which evolved to become benign and eventually mutualistic. • Yuccas, pollinated by yucca moths are one such example.  Most of the basal species of yucca moth do not pollinate and are thus parasites.  A few species in the basal genera (like Greya) incidentally pollinate flowers in which they lay eggs. Such species may have lead to a transition from a parasitic to a mutualistic relationship.  Active pollination, and thus a mutualistic relationship, evolved in the ancestor of Tegeticula and Parategeticula.  Several Tegeticula have evolved to become cheaters and no longer actively pollinate. This brings up the issue of conflict within mutualisms.: “Cheaters” 2. Within mutualisms there is always the potential for conflict. A genotype that “cheats” (by exploiting its partner without paying the cost of providing a benefit) is likely to have a selective advantage. 3. “Cheaters” don’t always have a selective advantage. In some cases, mechanisms are in place to punish “cheaters”.  For example, in the case of the yucca species, the yucca moth is the sole pollinator.  Typically a moth lays only a few eggs in each flower, so that only a few seeds of each fruit are consumed by the larva. This leaves some seeds which can then germinate.  Fruit in which many eggs have been laid are more likely to be aborted by the plant, than fruit with only a few eggs. Larva in aborted fruit all die.  Thus there is a real selective incentive for a female moth not to “cheat” by laying to many eggs in each flower.  Hawkmoth doesn’t like having pollen on his face. Don’t think of mutualism as organisms helping each other out. Its like they have both positive effects  Orchid had long spur and hawkmoth have long tongues  Both species are mutually exploiting each other and have evolved th
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