COEVOLUTION AMONG PLANTS AND HERBIVOROUS INSECTS
I. Introduction
A. Definitions
Janzen 1980: “ An evolutionary change in a trait of an array of populations or
species, followed by reciprocal change in the array of species exerting the
initial selective pressure”
B. Scale:
1. Microevolution: Alteration of gene frequencies
2. Macroevolution: Speciation
C. Types, and definitions
1. Gene for gene coevolution: Hosts and parasites have complementary genes for
resistance and virulence
Mode 1958
2. Pair-wise coevolution: Coadaptation of two species. The adaptive responses
between two species lead to evolutionary changes in each other. Not
necessarily a gene-for-gene basis of heredity. Also called “Specific
coevolution”
Examples: Ehlrich & Raven, Berenbaum (Biochemical arms-race),
Thompson (Highly specific pollination systems)
3. Diffuse coevolution: Reciprocal adaptation of a suite of species imposed by
another suite of species. Emphasis is on a complex of selective pressures.
Janzen, Thompson, Futuyma, Strong
4. Species tracking: Changes in host induced largely by forces separate from the
herbivore; herbivore must adapt. “Sequential evolution”: T. Jermy
D. Selective Forces
1. Herbivory
2. Pollination
3. Reward: Fruits & seed dispersers
4. Refuge: Ants & Acacias
5. Associations: Insects & microorganisms exploiting plants;
E. Current Thought
II. Types of Evidence
A. Functional Analysis: Evaluates the effects of particular traits of plants & insects
B. Comparative Approach: Relates distribution of convergently evolved characters over
taxa
C. Population Biology and Ecological Genetics: Quantifies the distributions of allele
frequencies
D. Phylogenetic approaches
1. Cladogenesis
2. Fossil record
3. Determine change in polarity of character
E. Difficulties in evaluating evidence, and the biases they cause
1. Certain types of evidence more available than others
2. Certain types of coevolution more difficult to evaluate than others
3. Genetic structure
4. Preadaptations
5. Exaptive traits:
6. Laboratory artifacts
7. Uneven availability of appropriate methodologies
III. Processes (Berenbaum 1983)
A. Production by plants of novel secondary substances (or other traits) by mutation or
recombination
B. New traits alter the suitability of plants for insects
C. Plants undergo evolutionary radiation in a new adaptive zone
D. Insects evolve resistance to new trait
E. Adapted Insects enter new adaptive zone, undergo evolutionary radiation
F. Parallel evolution of insects adapted to particular plant traits
IV. Key Questions regarding evolution between insects & plant secondary chemicals: Futuyma
&
Keese 1992
A. Do phytochemicals act as barriers to herbivores?
B. Is defense against herbivores the function of phytochemicals?
C. Are plant defenses broadly or narrowly targeted?
D. Do novel resistance characters confer better defense?
E. Does escape from herbivory promote plant diversification?
F. Do herbivores adapt to chemical defense?
G. Does adaptation to chemically novel plants promote insect diversification?
H. What accounts for the host range of herbivore species?
I. Are host shifts mediated by chemical similarities of plants?
J. Have phytophages and their hosts undergone congruent cladogenesis?
K. Has there been pairwise coevolution (or escalation)?