  In a geographically distributed population, assortative clustering plays an
important role in evolution by modifying local environments. To examine its
effects in a linear habitat, we consider a one-dimensional grid of cells, where
each cell is either empty or occupied by an organism whose replication strategy
is genetically inherited to offspring. The strategy determines whether to have
offspring in surrounding cells, as a function of the neighborhood
configuration. If more than one offspring compete for a cell, then they can be
all exterminated due to the cost of conflict depending on environmental
conditions. We find that the system is more densely populated in an unfavorable
environment than in a favorable one because only the latter has to pay the cost
of conflict. This observation agrees reasonably well with a mean-field analysis
which takes assortative clustering of strategies into consideration. Our
finding suggests a possibility of intrinsic nonlinearity between environmental
conditions and population density when an evolutionary process is involved.
