17 research outputs found
Generalized Chaplygin Gas, Accelerated Expansion and Dark Energy-Matter Unification
We consider the scenario emerging from the dynamics of a generalized
-brane in a spacetime. The equation of state describing this
system is given in terms of the energy density, , and pressure, , by
the relationship , where is a positive constant and
. We discuss the conditions under which homogeneity arises
and show that this equation of state describes the evolution of a universe
evolving from a phase dominated by non-relativistic matter to a phase dominated
by a cosmological constant via an intermediate period where the effective
equation of state is given by .Comment: 5 pages, 4 figures, revte
A seed predator drives the evolution of a seed dispersal mutualism
Although antagonists are hypothesized to impede the evolution of mutualisms, they may simultaneously exert selection favouring the evolution of alternative mutualistic interactions. We found that increases in limber pine (Pinus flexilis) seed defences arising from selection exerted by a pre-dispersal seed predator (red squirrel Tamiasciurus hudsonicus) reduced the efficacy of limber pine's primary seed disperser (Clark's nutcracker Nucifraga columbiana) while enhancing seed dispersal by ground-foraging scatter-hoarding rodents (Peromyscus). Thus, there is a shift from relying on primary seed dispersal by birds in areas without red squirrels, to an increasing reliance on secondary seed dispersal by scatter-hoarding rodents in areas with red squirrels. Seed predators can therefore drive the evolution of seed defences, which in turn favour alternative seed dispersal mutualisms that lead to major changes in the mode of seed dispersal. Given that adaptive evolution in response to antagonists frequently impedes one kind of mutualistic interaction, the evolution of alternative mutualistic interactions may be a common by-product