Character Displacement

    Character displacement is the evolutionary divergence of ecological requirements of two lineages.  When the physical barrier responsible for allopatric lineage splitting breaks down and the two daughter lineages' ranges overlap, coexistence is possible in the region of overlap only if individuals of the two lineages are not too ecologically similar (i.e. only if they do not compete too intensely for the same limiting resource).  If they are too similar, one lineage will become extinct (Gause's Law).  During allopatric divergence, the two lineages may have also diverged in ecological requirements.  If so, then competitive exclusion will not occur and both species may coexist.  If, however, ecological divergence has not occurred, then one of two outcomes are possible: competitive exclusion or ecological character displacement. The figures below describe the process of character displacement.

In this figure, the curve K(z) represents the relative abundance of resources of different types.  For example, the x-axis may represent seed size for a seed-eating bird, and the curve K(z) indicates the relative abundances of seeds of different size.  The individual resource use curves below K(z) represent the relative proportions of resources of different types (e.g. seeds of different size) that are used by particular individuals.  Each individual uses a range of seed sizes, but concentrates on seeds of a particular size within that range.  Mean seed size eaten is designated by the inverted triangles at the top of the graph for individuals 1-4.
 
 
 
 
Evolution of a single population in response to resource abundance.  In this figure the resource abundance curve, K(z) is again portrayed.  Each curve below it represents the resource utilization curves of a population--a frequency distribution of means of the individual resource use curves portrayed in the previous figure.  The left curve represents the initial state of a population.  Because individuals with means to the right side of the curve have more resources available (higher K(z)), their fitness is higher than individuals with means on the left side of the curve.  Consequently, natural selection causes in increase in mean resource used and the population's resource use curve evolves to the right.  The equilibrium corresponds to the second curve, in which the most commonly used resource is the most common resource (i.e. the peaks of the resource availability and resource use curves correspond.)
 
 

Character displacement.  The two fainter, central resource use curves represent two lineages that are ecologically similar when they come into contact.  The presence of each species depresses the resource availability curve (K(z)) for each lineage in the central region (the altered resource availability curve for the left lineage is shown in red), effectively shifting its peak to the right or left.  This causes natural selection to favor individuals in each species with resource preferences corresponding to the new peaks, and causes the populations to diverge in resource use, as indicated by the arrows and darker resource use curves.
 

(Figures from D. J. Futuyma.  1998.  Evolutionary Biology.  Sinauer, Sunderland, MA.)
 

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