Propagule Pool Research Articles

Rate of evolution slowed by a dormant propagule pool

Theoretical models of plant seed banks and common sense suggest that stores of dormant propagules must slow down the rate of genetic change1,2 because they sequester a substantial fraction of the gene pool from the influence of microevolutionary processes in each generation. The magnitude of the effect must represent a balance between the extent of selection and the fraction of the dormant pool that returns to the active population in each generation (and the other microevolutionary processes of migration, drift and mutation). For most populations, in which to observe the action of selection is difficult in itself, detecting the added influence of a dormant propagule pool will be more difficult still. Here we report the effect of hatching of diapausing eggs from lake sediments on the rate of phenotypic change in two populations of the freshwater copepod, Diaptomus sanguineus. When the input of new eggs to the sediments was eliminated by a brief but intense two-year appearance of predators, the role of dormant stages in slowing evolution was clearly seen.

EXTINCTION AND RECOLONIZATION: THEIR EFFECTS ON THE GENETIC DIFFERENTIATION OF LOCAL POPULATIONS.

In this paper, we use a model by Slatkin (1977) to investigate the genetic effects of extinction and recolonization for a species whose population structure consists of an array of local demes with some migration among them. In particular, we consider the conditions under which extinction and recolonization might enhance or diminish gene flow and increase or decrease the rate of genetic differentiation relative to the static case with no extinctions. We explicitly take into account the age-structure that is established within the array of populations by the extinction and colonization process. We also consider two different models of the colonization process, the so-called "migrant pool" and "propagule pool" models. Our theoretical studies indicate that the genetic effects of extinction and colonization depend upon the relative magnitudes of K, the number of individuals founding new colonies, and 2Nm, twice the number of migrants moving into extant populations. We find that these genetic effects are surprisingly insensitive to the extinction rate. We conclude that, in order to assess the genetic effects of the population dynamics, we must first answer an important empirical question that is essentially ecological: is colonization a behavior distinct from migration?

Comparison of Three Techniques for the Study of Aquatic Hyphomycete Communities

The aquatic hyphomycete unit-community on a single leaf, the aquatic hyphomycete community on 60 randomly collected leaves, and the aquatic hyphomycete propagule pool in the Sangamon River, Illinois, were sampled simultaneously and the resulting data compared. No single species was clearly dominant according to all three techniques and although a large number of the same species occurred in data from all three techniques, species rankings were dissimilar. Generally, the propagule pool did not accurately reflect the structure of the aquatic hyphomycete community or the extent of colonization on randomly sampled leaves. The aquatic hyphomycete community on leaves (diversity = 13) was clearly dominated, according to relative importance values, by Filosporella annelidica and Tetracladium marchalianum. Other than a lower average number of species per leaf for oak, no major differences in aquatic hyphomycete community structure occurred between oak and maple leaves.

Comparison of Three Techniques for the Study of Aquatic Hyphomycete Communities

The aquatic hyphomycete unit-community on a single leaf, the aquatic hyphomycete community on 60 randomly collected leaves, and the aquatic hyphomycete propagule pool in the Sangamon River, Illinois, were sampled simultaneously and the resulting data compared. No single species was clearly dominant according to all three techniques and although a large number of the same species occurred in data from all three techniques, species rankings were dissimilar. Generally, the propagule pool did not accurately reflect the structure of the aquatic hyphomycete community or the extent of colonization on randomly sampled leaves. The aquatic hyphomycete community on leaves (diversity = 13) was clearly dominated, according to relative importance values, by Filosporella annelidica and Tetracladium marchalianum. Other than a lower average number of species per leaf for oak, no major differences in aquatic hyphomycete community structure occurred between oak and maple leaves.

Group selection on a quantitative character

We derive a model of group and individual selection on a quantitative character that is similar to the single-locus "metapopulation" models of group selection. Two alternative methods for the colonization of new or vacant habitats are examined and their effects are contrasted. In one model, all populations contribute migrants to a common pool, the "migrant pool," from which colonists are drawn at random to fill vacant sites. In the migrant pool there is complete mixing of individuals from different populations. This model of colonization is the one used in all previous models of group selection. In the other model, the "propagule pool" model, each propagule is made up of individuals derived from a single population and there is no mixing of colonists from different populations during propagule formation. The analysis shows that much more between-population genetic variance can be maintained with the propagule pool model than with the migrant pool model. Consequently, group selection can be much more effective in natural populations than is commonly supposed.

Gene flow and genetic drift in a species subject to frequent local extinctions

Two models of the effect of extinction and recolonization on the genetic differentiation of local populations are analyzed. One model is Wright's “island model” in which there is gene flow from a source of fixed gene frequency. The other is an island model with a continuous production of new alleles and gene flow among all the populations. Individual and group selection are not considered. It is shown that the extent of population differentiation and the direction of the effect of the colonization and extinction process depend on the manner in which the propagules that establish new colonies are formed. Two extreme cases are considered. In the “propagule pool” model all the individuals in a single propagule are derived from one population while in the “migrant pool” model, the individuals in a propagule are derived from a random sample of the entire collection of populations.