Download Bio5445 Lecture 21 - Biology Courses Server

Insect-Plant Interactions
•  Phytophagous insects account for approximately 40% of all described
insects. In 1964 Paul Ehrlich and Peter Raven published a paper that argued
that the incredible proliferation of phytophagous insects and higher plants is
the result of a coevolutionary process between them. According to this
scenario the evolution of terrestrial plants presented a new adaptive zone for
insects to exploit. As insects evolved means to exploit plants as food, plants
evolved countermeasures which led to greater diversification of plants and
further diversification of insects. Increased diversification of plants also led to
increased structural diversity in habitats, and increased diversification of
phytophagous insects led to increased diversification at higher trophic levels.
Thus much of present day diversity on earth may be the result of evolutionary
interactions between insects and plants.
•  In an earlier lecture we examined the physiological adaptations of insects for
feeding on plants protected by toxic secondary compounds. In today's lecture
we will explore some of the more long-term aspects of coevolution between
plants and insects. First we will ask whether in fact phytophagy was an
evolutionary innovation that led to increased diversification of insects. And
then we will examine the evidence for coevolution between insects and
plants.
Coevolution and Adaptive Radiation
Coevolution is the evolution of characteristics of two or more species
in response to changes in each other. Coevolution occurs when two or
more species produce reciprocal changes in one another. It has two
components:
1. Coadaptation is the degree of mutual modification between lineages. It can
be expressed as gene for gene changes in two lineages or in a more diffuse
way, involving many genes. Coadaptation represents the microevolutionary
aspects of coevolution.
2. Cospeciation is the degree of mutual phylogenetic association between two
lineages. It is said to occur when the phylogenies of two lineages are
concordant. Cospeciation represents the macroevolutionary aspect of
coevolution.
Adaptive radiation is the evolution of a variety of forms from a single
ancestral stock, often after colonizing an island group or entering a new
adaptive zone. This may include speciation, but not necessarily.
Adaptive Radiation of Phytophagous
Insects
A major tenet of the Ehrlich & Raven hypothesis is that plants initially
represented a new, unexploited adaptive zone for insects. Insect that
successfully colonized this adaptive zone then underwent an adaptive
radiation, leading to enhanced diversification. Can this tenet be tested?
To test the adaptive-zone hypothesis we must asked whether adaptive
shifts are repeatedly associated with accelerated diversification across
many independent groups. Is the phytophagous habit associated with
accelerated diversification in insects?
How do we compare diversification rates among lineages? Sister-group
analysis is one approach.
Sister Group Analysis of Adaptation
•  By definition, sister groups are the same age.
•  Any differences in diversity between sister groups reflect
different rates of diversification.
•  An adaptive shift occurs when a lineage moves from an
ancestral adaptive zone to a new one. The hypothesis of
adaptive radiation is supported if the sister group that has
undergone the adaptive shift is consistently more diverse than
the sister group that remains in the ancestral adaptive zone.
•  The statistical power of sister-group analysis is increased
when a particular adaptive shift occurs in many independent
groups.
Test of the phytophagous insect
diversification hypothesis
•  Higher-plant feeding is found in 9
orders of insects. It has probably
arisen at least 50 times in just the
extant forms with known habits.
•  Present phylogenetic information
allows the identification of 13 pairs
of sister groups, one of which
feeds on higher plants and the
other of which does not.
•  In 11 of these 13 sister-group
pairs, the phytophagous lineage is
more diverse than its presumed
non-phytophagous sister group.
Thus the phytophagous feeding
habit is associated with increased
diversification. This provides
tentative support of the Ehrlich &
Raven hypothesis.
Diversification of plants in response to
feeding by phyotophagous insects
•  As phytophagous insects
diversified on plants, plants
should respond by escalating
their defenses against insects.
•  Resin and latex canals found in
many plants presumably serve
as a defense against plantfeeding insects.
•  Are plants with resin and latex
canales more species rich
compared to their sister groups?
•  In 13 out of 16 groups the
answer is yes.
Scenarios for the evolution of insectplant associations
•  Concordant cladogenesis
(association by descent).
•  Discordant cladogenesis
(insect colonization of preexisting plants; resource
tracking).
•  Concordant cladogenesis due
to homoplasy or convergent
evolution of secondary plant
compounds.
•  Partial concordant.
Example of Concordant Cladogenesis
•  In 14 phylogenetic
analyses, only 1 showed
extensive concordance, 3
showed partial concordance and 10 showed
no concordance.
•  Phyllobrotica on
Scutellaria in the
Lamiaceae (mint family).
Strong evidence of
cospeciation.
Example of Discordant Cladogenesis
•  Ophraella on Asteraceae
(sunflower family). Little evidence
of cospeciation.
•  Differences in the degree of
phylogenetic concordance in
these groups may reflect the
relative strength of constraints
operating in the two systems.
Phyllobrotica depends on its host
plant throughout all life stages
(adults use host-plant compounds
for defense against predators),
whereas Ophraella does not.
•  Although strict, prolonged,
pairwise cospeciation b/w insects
and plants is rare, they have
experienced a long history of
coadaptation.