Coevolution: Patterns and Processes 1. Introduction Definition ? • mutually induced evolutionary change between two or more species or populations • reciprocal evolutionary change in interacting species; the partial coordination of non-mixing gene pools over evolutionary time • Key questions – Do competitors evolve in response to each other? – Do predators (or parasites) evolve in response to prey (or hosts)? – Does coevolution promote biodiversity (ie. lead to speciation and radiation) ? 2. Historical context • First indication in Darwin’s (1859) ‘entangled bank’ – ‘mutual relations’, ‘mutual adaptations’, ‘co-adaptations’ – First indication of ‘arms race’ ideas of coevolution 3. Coevolution as reciprocal evolutionary change (insects) • Ehrlich and Raven (1964) (butterflies on plants) • Described process, and linked it to process of speciation • The ‘original’ process (Ehrlich and Raven) – – – – – Plants produce novel secondary compounds via mutation and recomb These compounds reduce rates of herbivory, so are favoured by NS Plants containing these compounds radiate, free from herbivores Novel mutation in insect population reduces toxicity to compound Insects enter new ‘adaptive zone’ and radiate • The evidence – 1) Observation (but take care!) – 2) Empirical studies (Berenbaum and the parsnip web worms) – 3) Phylogenetic studies (cospeciation) Incongruence versus cospeciation Host Parasite incongruence (not cospeciation, e.g. host switching) congruence (cospeciation) Phoebetria palpebrata Diomedea chrysostoma Diomedea bulleri 79 Paraclisis diomedeae 78 Paraclisis diomedeae 77 Paraclisis diomedeae 81 Paraclisis hyalina Diomedea epomophora 82 Paraclisis hyalina Diomedea exulans 80 Paraclisis giganticola Diomedea irrorata 83 Paraclisis miriceps Diomedea nigripes Diomedea immutabilis 76 Paraclisis confidens 0.1 Gophers and Lice O. hispidus G. chapini O. underwoodi G. setzeri O. cavator G. panamensis O. cherriei G. cherriei O. heterodus Z. trichopus P. bulleri G. costaricensis G. trichopi G. nadleri G. expansus C. castanops C. merriami G. bursarius (b) G. geomydis G. oklahomensis G. bursarius (a) G. ewingi G. breviceps G. texanus G. personatus T. bottae T. talpoides G. actuosi G. perotensis G. thomomyus T. minor T. barbarae • Counter-evidence – Functions of secondary plant compounds ? – Coevolution does not necessarily equal cospeciation 4. Requirements for coevolution • Identify mutual traits – Variation among individuals – Heritable – Variation in trait is linked to variation in fitness • *Verify traits are mutually induced* • Lab studies demonstrating natural selection on traits 5. Case studies - Garter snakes and newts Skin toxin (tetrodotoxin,TTX) in newts vs. resistance to toxin in snakes (Brodie and Brodie and others…). Natural history (general): • TTX produced by newts (and some others!) - powerful skin toxin that interferes with nerve function • some populations of garter snakes contain individuals that can detoxify TTX • ’resistance’ measured on a race track • both reciprocal traits are heritable • allopatric snakes are highly susceptible to TTX • sympatric snakes show close phenotypic matching Natural history (specific): • tube-shaped proteins on snake nerve cells control rate of sodium ion in-flow • TTX enters through a hole on the surface of the protein, severely interfering with rate of flow • sodium channel genes are highly conserved, but not in snakes • some snakes don’t allow TTX to bind to the protein (but probably at a cost) • molecular arms race involving toxification/detoxification genes In regions of sympatry, concentration of toxin is matched to level of resistance (ie. phenotypic matching). Snake pops without Newts are highly susceptible to TTX. Coevolutionary hotspots, surrounded by regions where reciprocal selection is less intense. • Conclusion – example of predator/prey coevolution – strong trait-for-trait coevolution – strong evidence for arms race model of coevolution 6. Types of Coevolution • Gene-for-gene coevolution – Each host gene affecting resistance is matched by a parasite gene that affects attack (or virulence) – e.g. Flor (1950’s) flax/rust fungi system - genetic crosses involving 2 strains of flax and 2 strains of rust - host resistance and susceptibility were Mendelian - parasite virulence was Mendelian - genetic systems were perfectly complimentary – Link to Ehrlich and Raven (1964) – Commonness of gene for gene coevolution? • Escape and radiation coevolution – includes periods when intimate interactions do not occur – e.g. Berenbaum’s parsnip webworm – e.g. catipillars on milkweed • Guild or diffuse coevolution – – – – Coevol occurs among groups of species, not pairs pollinators and flowering plants fruit-eating birds and fleshy-fruited plants evolution of immune systems, evolution of parasite deception • e.g. immunoglobulin diversity (i.e. humoral immunity) 7. The geographic mosaic of coevolution (J. Thompson) • outcomes of interactions vary among populations – coevolutionary hotspots (where there is reciprocal selection) – coevolutionary coldspots (no reciprocal selection) Lithophragma and Greya moths, Thompson, 2002 Conclusions • • • • Outcome varied spatially AND yearly Reciprocal selection varies over time and space Relevance to conservation biology Parallels to snake/newt system 8. Summary • Coevolution is an umbrella term, aimed to describe various outcomes of evolving interactions • Diversity of outcomes within given interactions • High spatial and temporal variation = fluctuating selection pressures • “look for coevolutionary vortices within an evolutionary stream”