Although the significance ( p-value) of the phylogenetic correlation
between stream gradient and PLD length depended on
the topology used, trade-offs associated with environmental
variables like stream gradients could be shaping the evolution
of darter PLD. Longer darter PLD might increase
population connectivity via dispersal in large, slow rivers.
Alternatively, a reduced time in the water column might be
particularly advantageous in high gradient streams with
rapid flows, reduced pelagic zones and little suitable downstream
habitat. Despite the potential advantages, reduced
dispersal associated with lower PLD probably leads to isolated
populations with small ranges that are less stable and
more vulnerable to environmental changes [22].