Bio5445 Lecture 21.pdf - Biology Courses Server
Bio5445 Lecture 21.pdf - Biology Courses Server
Bio5445 Lecture 21.pdf - Biology Courses Server
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Insect-Plant Interactions<br />
• Phytophagous insects account for approximately 40% of all described<br />
insects. In 1964 Paul Ehrlich and Peter Raven published a paper that argued<br />
that the incredible proliferation of phytophagous insects and higher plants is<br />
the result of a coevolutionary process between them. According to this<br />
scenario the evolution of terrestrial plants presented a new adaptive zone for<br />
insects to exploit. As insects evolved means to exploit plants as food, plants<br />
evolved countermeasures which led to greater diversification of plants and<br />
further diversification of insects. Increased diversification of plants also led to<br />
increased structural diversity in habitats, and increased diversification of<br />
phytophagous insects led to increased diversification at higher trophic levels.<br />
Thus much of present day diversity on earth may be the result of evolutionary<br />
interactions between insects and plants.<br />
• In an earlier lecture we examined the physiological adaptations of insects for<br />
feeding on plants protected by toxic secondary compounds. In today's lecture<br />
we will explore some of the more long-term aspects of coevolution between<br />
plants and insects. First we will ask whether in fact phytophagy was an<br />
evolutionary innovation that led to increased diversification of insects. And<br />
then we will examine the evidence for coevolution between insects and<br />
plants.
Coevolution and Adaptive Radiation<br />
Coevolution is the evolution of characteristics of two or more species<br />
in response to changes in each other. Coevolution occurs when two or<br />
more species produce reciprocal changes in one another. It has two<br />
components:<br />
1. Coadaptation is the degree of mutual modification between lineages. It can<br />
be expressed as gene for gene changes in two lineages or in a more diffuse<br />
way, involving many genes. Coadaptation represents the microevolutionary<br />
aspects of coevolution.<br />
2. Cospeciation is the degree of mutual phylogenetic association between two<br />
lineages. It is said to occur when the phylogenies of two lineages are<br />
concordant. Cospeciation represents the macroevolutionary aspect of<br />
coevolution.<br />
Adaptive radiation is the evolution of a variety of forms from a single<br />
ancestral stock, often after colonizing an island group or entering a new<br />
adaptive zone. This may include speciation, but not necessarily.
Adaptive Radiation of Phytophagous<br />
Insects<br />
A major tenet of the Ehrlich & Raven hypothesis is that plants initially<br />
represented a new, unexploited adaptive zone for insects. Insect that<br />
successfully colonized this adaptive zone then underwent an adaptive<br />
radiation, leading to enhanced diversification. Can this tenet be tested<br />
To test the adaptive-zone hypothesis we must asked whether adaptive<br />
shifts are repeatedly associated with accelerated diversification across<br />
many independent groups. Is the phytophagous habit associated with<br />
accelerated diversification in insects<br />
How do we compare diversification rates among lineages Sister-group<br />
analysis is one approach.
Sister Group Analysis of Adaptation<br />
• By definition, sister groups are the same age.<br />
• Any differences in diversity between sister groups reflect<br />
different rates of diversification.<br />
• An adaptive shift occurs when a lineage moves from an<br />
ancestral adaptive zone to a new one. The hypothesis of<br />
adaptive radiation is supported if the sister group that has<br />
undergone the adaptive shift is consistently more diverse than<br />
the sister group that remains in the ancestral adaptive zone.<br />
• The statistical power of sister-group analysis is increased<br />
when a particular adaptive shift occurs in many independent<br />
groups.
Test of the phytophagous insect<br />
diversification hypothesis<br />
• Higher-plant feeding is found in 9<br />
orders of insects. It has probably<br />
arisen at least 50 times in just the<br />
extant forms with known habits.<br />
• Present phylogenetic information<br />
allows the identification of 13 pairs<br />
of sister groups, one of which<br />
feeds on higher plants and the<br />
other of which does not.<br />
• In 11 of these 13 sister-group<br />
pairs, the phytophagous lineage is<br />
more diverse than its presumed<br />
non-phytophagous sister group.<br />
Thus the phytophagous feeding<br />
habit is associated with increased<br />
diversification. This provides<br />
tentative support of the Ehrlich &<br />
Raven hypothesis.
Diversification of plants in response to<br />
feeding by phyotophagous insects<br />
• As phytophagous insects<br />
diversified on plants, plants<br />
should respond by escalating<br />
their defenses against insects.<br />
• Resin and latex canals found in<br />
many plants presumably serve<br />
as a defense against plantfeeding<br />
insects.<br />
• Are plants with resin and latex<br />
canales more species rich<br />
compared to their sister groups<br />
• In 13 out of 16 groups the<br />
answer is yes.
Scenarios for the evolution of insectplant<br />
associations<br />
• Concordant cladogenesis<br />
(association by descent).<br />
• Discordant cladogenesis<br />
(insect colonization of preexisting<br />
plants; resource<br />
tracking).<br />
• Concordant cladogenesis due<br />
to homoplasy or convergent<br />
evolution of secondary plant<br />
compounds.<br />
• Partial concordant.
Example of Concordant Cladogenesis<br />
• In 14 phylogenetic<br />
analyses, only 1 showed<br />
extensive concordance, 3<br />
showed partial concordance<br />
and 10 showed<br />
no concordance.<br />
• Phyllobrotica on<br />
Scutellaria in the<br />
Lamiaceae (mint family).<br />
Strong evidence of<br />
cospeciation.
Example of Discordant Cladogenesis<br />
• Ophraella on Asteraceae<br />
(sunflower family). Little evidence<br />
of cospeciation.<br />
• Differences in the degree of<br />
phylogenetic concordance in<br />
these groups may reflect the<br />
relative strength of constraints<br />
operating in the two systems.<br />
Phyllobrotica depends on its host<br />
plant throughout all life stages<br />
(adults use host-plant compounds<br />
for defense against predators),<br />
whereas Ophraella does not.<br />
• Although strict, prolonged,<br />
pairwise cospeciation b/w insects<br />
and plants is rare, they have<br />
experienced a long history of<br />
coadaptation.