Ingvarsson PK, Olsson K, Ericson L
Extinction-recolonization dynamics in the mycophagous beetle Phalacrus substriatus
Evolution: 1997 51:187-195
The population structure of the mycophagous beetle Phalacrus substriatus is characterized by many small, local populations interconnected by migration over a small spatial scale (10 X 75 m(2)). Each local P. substriatus population has a relatively short expected persistence time, but persistence of the species occurs due to a balance between frequent local extinctions and recolonizations. This nonequilibrium population structure can have profound effects on how the genetic variation is structured between and within populations. Theoretical models have stated that the genetic differentiation among local populations will be enhanced relative to an island model at equilibrium if the number of colonizers is less than approximately twice the number of migrants among local populations. To study these effects, a set of 50 local P. substriatus populations were surveyed over a four-year period to record any naturally occurring extinctions and recolonizations. The per population colonization and extinction rate were 0.237 and 0.275, respectively. Mark-recapture techniques were used to estimate a number of demographic parameters: local population size (N = 11.1), migration rate ((m) over cap = 0.366), number of colonizers (k = 4.0), and the probability of common origin of colonizers (phi = 0.5). The theoretically predicted level of differentiation among local populations (measured as Wright's F-ST) was 0.070. Genetic data obtained from an electrophoretic survey of seven polymorphic loci gave an estimated degree of differentiation of 0.077. There was thus a good agreement between the empirical results and the theoretical predictions. Young populations ((young) = 0.090) had significantly higher levels of differentiation than old, more established populations ((old) = 0.059). The extinction-recolonization dynamics resulted in an overall increase in the genetic differentiation among local populations by c. 40%. The global effective population size was also reduced by c. 55%. The results give clear evidence to how nonequilibrium processes shape the genetic structure of populations.
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