Dissertation - HQ

General discussion 165
such as the tail of the Peacock. At the level of the species, the energy
allocated to producing tails and parading would be more efficiently
used in producing more offspring. However, Peacocks which displayed
larger tails had more individual reproductive success than the others,
so the frequency of the character increased. In general, if a particular
behaviour/trait is advantageous for individuals, it will be selected for,
no matter how deleterious it is for the species. There is evidence that
such characters may even lead to the species’ own extinction (also
called evolutionary suicide) 288,289 . A less academic example would be
us, human beings, who are probably the only species conscious of its
existence as such and who still exploit our environment individually, in a
way that may well lead to our extinction in the near evolutionary future.
Regarding dispersal during the larval phase, this means that, unless
there are advantages at the level of the individual which compensate
the risk of loosing progeny in an hazardous dispersive stage, dispersal
will not persist solely “because it is good for the species”.
The evolutionary causes of dispersal must be sought at the level of
the individual. In the, mostly terrestrial, literature 32 some processes are
classically recognised to favour dispersal: local environment variability
at the scale of the life of the parent, which makes it better, for the parent
and for the offspring, to spread the risk by dispersing at each reproductive
event; high percentage of local inbreeding; and strong parasitism or
competition pressure in the habitat of the parents. Few have been investigated
to explain natal dispersal in marine systems and we only risk
conjectures here. The Introduction (section I.5.1, page 20) highlighted
that even coral reefs, which were considered very stable, are in fact
subject to catastrophes. The remaining question is whether these perturbations
are frequent enough to make systematic dispersal advantageous.
According to Bonhomme & Planes 285 , they are not. Inbreeding depression
is little considered in fishes except in an aquaculture context 290 .
Finally, given the small space available for demersal species (compared
to pelagic ones for example) and the densities observed in favourable
environments such as rocky shores or coral reefs, parasitism and competition
are likely to be strong, albeit everywhere. On the other hand,
for each individual, the parental habitat is of demonstrably sufficient
quality for reproduction. This should favour self-recruitment; as should
the potential for local adaptation 291 . As already highlighted, the fact that
two species with very different early life histories (demersal eggs and
short PLD vs. pelagic eggs and long PLD) achieve the same 60% rate
of self-recruitment 44 suggests that there are some forces which favour
this behaviour. Finally, from a mechanistic point of view, large scale
orientation toward a known point of origin can be explained (through
imprinting 103,255 and/or solar or magnetic compass calibration 146 ) but
orientation toward a hypothetical other habitat cannot 292 .
In a nutshell, marine organisms may present a larval stage for reasons
other than dispersal, still not fully understood. Most larvae probably
enhance retention actively (otherwise they would take the risk to roam
Individual-level selective
forces are required
Classic causes of
dispersal seem absent
Arguments for
self-recruitment exist