Host-switching
After the first host speciation, parasite individuals in a host species
can switch to another host. For each parasite individual, we randomly
selected a host species, including the one in use. If the selected host
species is not the original host (donor), we follow a probability
function for the host-switching event. This probability of
host-switching events (Phs ) decreases over time,
representing the loss of opportunity and/or compatibility of parasites
associated with the evolutionary history of hosts. The probability of a
parasite individual successfully migrate (host-switching) from one host
to another host species, in a given generation n , is defined as:
Phs( \(n\))=exp \([-r*(n-n_{s})]\), (1)
where r is a positive parameter that controls the decay of the
host-switching probability, and ns is the
generation that the common host ancestor had speciated (then,
n-ns is how long the two host species had
diverged). If r = 0, these probabilities are equal to 1
regardless of the host divergence time, meaning that there is no
restriction to host-switching. As a consequence, parasite gene flow is
continuous and speciation does not occur. At the other extreme, for
sufficiently large r values
(Phs ~0), host-switching is
absent, and cospeciation between hosts and parasites is expected. For
intermediary r value, some parasite individuals can eventually
switch hosts. This will increase the host repertoire of the parasitic
species, and also enable speciation by isolation (by host use), similar
to the speciation by founder’s effect (Mayr 1999; Gavrilets & Hastings
1996). In order to better interpret the effect of parameter r on
the trajectories and compare the results between the communities, we do
not present our results in terms of r, but how much it changes
the overall host-switching events. To obtain this overall metric, we
calculated the expected percentage of parasite individuals that switch
hosts over the entire simulation and we call it ashost-switching intensity .