Mutualisms
Parasites:
Parasites can significantly reduce host fitness, creating pressure on
hosts to evolve responses. For example, behavioural responses
(chimpanzees infected with nematodes eat a plant that contains
chemicals that kill or paralyse the nematodes) and immune
responses (encapsulation in insects)
Thus, host-parasite interactions are likely to affect the evolution of
both species leading to coevolution.
Does this coevolutionary process occur by creation of new loci
(‘Arms Race’, results in positive selection – a conflict between 2 or
more parties with a goal of having superior forces and weapons,
resulting in a rapid escalation of technology)? Or by changes in the
allele present at resistant loci (trench warfare, balancing selection)?
Cost of resistance and virulence may prevent arms race
Frequency-dependent selection maintains genetic variation and
perhaps explains sexual recombination. An evolutionary process
where the fitness of a phenotype depends on its frequency relative
to other phenotypes in a given population.
Is there any way to stop the coevolutionary process? Tolerance
traits: reduce the impact of parasites on host fitness. Do NOT exert
selection on parasite.
Beneficial interactions:
Mutualism – mutually beneficial interaction between individuals of 2
species (+/+)
Commensalism – individuals of one species benefit, while individuals
of the other species do not benefit and are not harmed (+/0)
Sometimes difficult to determine which one it is
Is there such a thing as ‘neutral’ interaction? Commensalisms or
mild parasitism? Classification of these species interactions requires
careful experimental studies: cleaner fishes – initially thought as
commensalism, cleaner fish clearly benefit by obtaining resources,
but why does the host fish allow the interaction? Host fish without
cleaners have more parasites, suggesting benefit for the host fish –
therefore, a mutualism.
Are there any real commensalisms? Anemone’s tentacles produce
sufficient poison to kill/ paralyse small fish, yet clownfish are able to
live among the tentacles without harm. Fish obtains protection, but
for the anemone there is no gain/ cost. Probably a commensalism
(+/0)
Do not confuse mutualism with Symbiosis – a relationship in which
the 2 species live in close physiological contact with each other,
such as corals and algae. For example, the wood eating cockroach
Parasites:
Parasites can significantly reduce host fitness, creating pressure on
hosts to evolve responses. For example, behavioural responses
(chimpanzees infected with nematodes eat a plant that contains
chemicals that kill or paralyse the nematodes) and immune
responses (encapsulation in insects)
Thus, host-parasite interactions are likely to affect the evolution of
both species leading to coevolution.
Does this coevolutionary process occur by creation of new loci
(‘Arms Race’, results in positive selection – a conflict between 2 or
more parties with a goal of having superior forces and weapons,
resulting in a rapid escalation of technology)? Or by changes in the
allele present at resistant loci (trench warfare, balancing selection)?
Cost of resistance and virulence may prevent arms race
Frequency-dependent selection maintains genetic variation and
perhaps explains sexual recombination. An evolutionary process
where the fitness of a phenotype depends on its frequency relative
to other phenotypes in a given population.
Is there any way to stop the coevolutionary process? Tolerance
traits: reduce the impact of parasites on host fitness. Do NOT exert
selection on parasite.
Beneficial interactions:
Mutualism – mutually beneficial interaction between individuals of 2
species (+/+)
Commensalism – individuals of one species benefit, while individuals
of the other species do not benefit and are not harmed (+/0)
Sometimes difficult to determine which one it is
Is there such a thing as ‘neutral’ interaction? Commensalisms or
mild parasitism? Classification of these species interactions requires
careful experimental studies: cleaner fishes – initially thought as
commensalism, cleaner fish clearly benefit by obtaining resources,
but why does the host fish allow the interaction? Host fish without
cleaners have more parasites, suggesting benefit for the host fish –
therefore, a mutualism.
Are there any real commensalisms? Anemone’s tentacles produce
sufficient poison to kill/ paralyse small fish, yet clownfish are able to
live among the tentacles without harm. Fish obtains protection, but
for the anemone there is no gain/ cost. Probably a commensalism
(+/0)
Do not confuse mutualism with Symbiosis – a relationship in which
the 2 species live in close physiological contact with each other,
such as corals and algae. For example, the wood eating cockroach
