Download Evolution Lecture 28

Evolution Lecture 28
Chapter 18
Topics for today:
Coevolution
• Enemy-victim
• Predator-prey
• Fossil record
• Extant populations
• Host-parasite
• Mutualism
• Competition
Evolution in Enemy-Victim relationships
Evidence for predator-prey coevolution
1. Predator-prey from fossil record
o Mesozoic fish evolved ability to crush and rip shells
o Diversity of shell form in molluscs increased
o Thicker shell margins
1. Protected aperature
o Fig. 18.9
2. Predator-prey from extant populations
o Potent neurotoxin TTX in skin
o One newt has enough toxin to kill 25,000 mice
o Population on Vancouver Island has almost no TTX
o Garter snakes that are sympatric with toxic newts feed on them
Predictions regarding resistance?
o Garter snake
o Rough-skinned newt
o Garter snakes in sympatry with toxic newts have higher resistance
to TTX
Fig. 18.10
Evidence for predator-prey coevolution
Infectious disease and their hosts
o Level of virulence depends on both host and parasite
o Common idea is virulence should decline
Why?
o Evolution of resistance
o Loss of virulence
Ro = bN
v+d+r
Ro = fitness of parasite (number of new hosts infected)
b = probability of infection
N = number of hosts available
v = mortality rate of hosts due to disease
d = mortality rate of hosts due to other causes
r = rate of recovery of hosts
Example: Coevolution in contemporary time
o European rabbit introduced in to Australia in1859
o By 1880’s became major pest
o 1950 myxoma virus introduced
o Mosquito vectors spread across continent
o Rabbit population drastically declined
Test for evolution of resistance?
o Rabbits tested against original virus strain for susceptibility after each
disease epidemic
 r
Fig. 18.12
Test for loss of virulence?
o Virus sampled from field over time and tested on susceptible rabbits
o Original virus killed rabbit in 6-10 days
o Less-virulent virus kills rabbit in 21-28 days
o Milder strain more effective in infecting rabbits ( v)
o Spreads more rapidly than virulent strains ( Ro)
Fig. 18.12
Is the hypothesis of the evolution of benign parasite naïve?
o Gut parasite
o Spores released in feces
o Tested pairs  Local
 Nonlocal
o Parasites locally adapated
o Contradicts prevailing hypothesis
Fig. 18.13
Evolution of mutualisms
o Benefit both species
o Can result in extreme adaptations
Movie clip: Nice example of:
1. Coevolution leading to extreme adaptation
2. Diffuse coevolution that involves multiple participants
3. Coevolution in competitive interactions
Hover fly
How do mutualisms evolve?
Example of yucca and yucca moth
o Female pollinates with specialized mouth parts
o Lays a few eggs in each flower
o Leaves scent marking
o Larvae survive only in pollinated flowers
o Hatch, consume developing seed, pupate
Many incremental changes over time
o Parasitize flower for larval resources
o Pollinate incidentally
o Those that pollinate have higher larval survival
o Evolve structures to aid in intentional pollination
o Plant protects itself from over utilization
o Moths evolve cheating
How can this be tested?
o Map traits onto a phylogeny
o Includes more ancestral moths
Evolution of competitive interactions
o Competition for resources drives evolution of divergence in resource use
o Major cause of origin and divergence of species
Fig. 18.18
Example of resource partitioning through spatial segregation
o Bumble bees use that use flowers of same corolla length are spatially
segregated by elevation
Fig. 18.17
Example of character displacement
o Divergence due to competition
o Sympatric populations of two species differ more than allopatric populations
Fig. 9.27
Example of ecological release from character displacement
o Bill size correlated with feeding behavior and resource use
o Competition with other woodpecker species on continent
o No competition on island
Fig. 18.19
Is character displacement predictable?
Not always
o 56 species of lizard in co-occur in the Great Victoria Desert of Australia
o 20 species in Kalahari semidesert of southern Africa
o 6-17 species in similar North America deserts
Sometimes
o Species on different islands have evolved similar ways to avoid competition
Fig. 18.20