Coevolution, With Particular Reference To Herbivor Essay

This essay has a total of 2252 words and 13 pages.

Coevolution, With Particular Reference To Herbivory











COEVOLUTION
with particular reference to herbivory









BIOL 0106
ASSESED COURSEWORK
RORY AULD
JANUARY 2000




COEVOLUTION WITH PARTICULAR REFERENCE TO HERBIVORY

Of all the extant organisms in the world, it is believed that terrestrial plants and their
natural ‘enemies' constitute more than forty percent. Moreover, plants exhibit a
remarkable diversity of supposedly defensive characteristics including trichomes, spines,
silica, secondary chemical compounds, temporal avoidance of enemies, and structures along
with chemicals that attract predators of their natural enemies. In addition, the
exploitation of the plants and their defences is facilitated by a vast number of
behavioural, morphological and physiological adaptations by herbivores

Accounting for this diversity has been a major area of research for nearly a century. The
seminal article, attributing this diversity to coevolution, was published in 1964 by
Ehrlich and Raven. They suggested plants and herbivorous insects evolved reciprocally by
the following events: Plants, through occasional mutations and recombinations, produced a
series of chemical compounds not directly related to their basic metabolic pathways. Some
of these compounds, by chance, serve to reduce or destroy the palatability of the plant in
which they are produced. Such a plant, protected from the attack of phytophagous animals,
would in a sense have entered a new adaptive zone. Evolutionary radiation of plants might
follow.

If a new recombinant or mutant appeared in a population of insects that enabled
individuals to feed on some previously protected plant, selection could carry the line
into a new adaptive zone. Here it would be free to diversify in the absence of competing
herbivores. Ehrlich and Raven (1964) emphasised the importance of the reciprocal selective
responses between ecologically linked organisms.

Since 1964, studies have questioned Ehrlich and Ravens postulates. Due to the nature of
evolutionary study, ideas are only as strong as the background in the literature; that is,
acceptance by the scientific community depends upon its knowledge. In time people learn
more and previously weak theories become more feasible. Alternatively, and more so in
science, accepted work in time becomes disregarded (example; until the 1950's geologists
believed in static continents, now all believe in plate techtonics and continental drift).
The significance of this is that any theory published is only speculation of what is
happening in these interactions. The knowledge is blind in that historical findings
leading to these assumptions are not concrete. What happened in the past might be a
different picture to what we have envisaged so far.

Thompson (1999) has proposed that there are crucial components to coevolution. These need
to be recognised before we can fully understand coevolution. Firstly, phylogenetic studies
are providing five kinds of data important in interpreting the historical context of
coevolving interactions. 1) Shared traits. Phylogenetic studies are allowing us to
evaluate which traits of interacting species were already present in the hosts ancestors.
This allows us to determine whether traits are coevolved or merely a trait exhibited as a
consequence of the organisms genotype. For example, Yucca plants provide a source of food
for host specific Yucca moths, with which they are believed to have coevolved. Examining
the phylogenetic trees of these moths elucidated this. Most moths in this family
(Prodoxidae) exhibited host specificity (Davis et al 1992). Before this technology, people
would have assumed the specificity of the Yucca moth to be a product of the coevolution.

This brings up a useful comment by Vermeij (1994). Almost all inferences about coevolution
are derived from the existence of trends in the expression of traits that function during
interactions between species. Evolutionary trends have often been found by analysing
ancestor-dependant relationships within monophyletic groups, or clades. Although many
trends are best sought this way, others cannot in principle be detected within single
clades and instead arise when ecologically and functionally comparable clades replace each
other through time.

2) Unique traits. The Yucca moths as described above have tentacles on their mouthparts
used to hold pollen for later active transfer to floral stigmas. Using phylogenetic
studies, it has been found that the ancestors to these moths did not have tentacles,
suggesting a coevolutionary adaptation.

3) Relative malleability of traits. Regardless of selection intensities, some traits may
be more malleable than others. Recognising this enables us to discriminate between
organisms that appear to be evolutionarily constrained (low malleability) and those that
appear to be evolutionarily dynamic (high malleability). Those that have low malleability
of a particular trait are not necessarily ‘fixed' and using phylogenetic studies can
determine how ‘fixed' they are.

4) Multiple origins of interactions. Selection for convergence of traits, especially in
mutualisms, can confound interpretation of the coevolutionary process in the absence of a
good phylogenetic template.

5) Relationships between traits and patterns of diversification. Our understanding of how
evolution of new traits have shaped diversification in interacting taxa has been enhanced
by phylogenetic studies. There are currently four hypotheses on the interrelationship
between species interactions and diversifications of taxa.

 Parallel cladogenesis (phylogenetic tracking) suggests host specific parasites
speciate with their hosts simultaneously but do not cause speciation. They may or may not
be coevolving through coadaptation but the host speciation is independent of any
coadaptation. That is, coevolution is not a cause of reciprocal speciation.

 Sequential evolution. This theory suggests parasites track their hosts
speciation, but do so in a much more general way. The theory assumes they are not
coevolving. Hosts undergo periodical diversification due to other factors, then the
parasites colonise the new host.

 Escape and radiate coevolution. This is the formal version of Ehrlich and Ravens
(1964) hypothesis as explained earlier. This theory does not predict strict parallel
cladogenesis.

 Diversifying coevolution. This group of hypotheses suggest; 1) populations of a
species evolve to specialise (often on different species) as a result of reciprocal local
adaptation. 2) Hybrids among the specialists populations are at a selective disadvantage,
thereby favouring reproductive isolating mechanisms.

Coevolution demands some degree of reciprocal specialisation among interacting species if
the interaction is to affect the fitnesses of individuals and favour evolutionary
reciprocal change. Evolutionary arguments are often based upon this fact.

Opposition to coevolution note that coevolution occurs when only a few species interact.
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