Character Displacement
Brown & Wilson’s Definition:
Two closely related species have overlapping
ranges. In the parts of the ranges where one species
occurs alone, the populations of that species are
similar to the other species and may even be very
difficult to distinguish from it. In the area of
overlap, where the two species occur together, the
populations are more divergent and easily
distinguished, i.e., they "displace" one another in
one or more characters. The characters involved
can be morphological, ecological, behavioral, or
physiological; they are assumed to be genetically
based.
Lack’s Work
Beak size similar where
allopatric, but G. fulginosa
has smaller beak than G.
fortis when sympatric
The Pattern
What else could cause this?
• Competitive exclusion (character sorting)
• Environmental gradients (sympatry happens
to correspond with largest differences)
• Random
• Phenotypic plasticity
Character displacement = just so story!
How can we improve observations?
1) Know what to look for
2) Null models
3) Do experiments instead!
Six Criteria for Character Displacement
(know what to look for)
1) Differences between sympatric taxa are
greater than expected by chance
How?:
Null models for comparison
Six Criteria for Character Displacement
(know what to look for)
2) Differences in character states are related to
differences in resource use
How?:
Correlate resource use with character. Duh.
Six Criteria for Character Displacement
(know what to look for)
3) Resources are limiting, and interspecific
competition for these resources is a function of
character similarity.
How?:
Demonstrate competition is occurring (e.g.
fewer prey consumed in sympatry vs. allopatry);
Experiments help here.
Six Criteria for Character Displacement
(know what to look for)
4) Resource distributions are the same in
sympatry and allopatry such that differences in
character states do not simply reflect differences
in resource availability
How?:
Show that environment is sufficiently similar in
both places.
Six Criteria for Character Displacement
(know what to look for)
5) Differences must have evolved in situ
How?:
Trait differences in sympatry should exceed
whole range in allopatry; use population
genetics/phylogenetics to model past events
Six Criteria for Character Displacement
(know what to look for)
6) Differences must be genetically based
How?:
Reciprocal transplants, experimental rearing,
genetics.
Criteria Met?
Meta-analysis of 61 studies; from Schluter 2000
Is character displacement biased?
Meta-analysis of 61 studies; from Schluter 2000
Types of Character Displacement
• Ecological Character Displacement:
Competition over resources causes divergence
• Reproductive Character Displacement:
“Competition” over correct gamete causes
divergence
Case study: Grant & Grant 2006
Green = Invader
Red = Invaded
Case study: Grant & Grant 2006
Severe drought
Severe drought
G. magniostris arrives
Case study: Grant & Grant 2006
Case study : Grant & Grant 2006
Case study: Grant & Grant 2006
Which criteria are met?
1) Change greater than chance?
2) Character states related to resource use?
3) Demonstrated resources are limiting?
4) Resource distributions same in sympatry and
allopatry?
5) Differences evolved in situ?
6) Characters heritable?
Case study: Schluter 1994
Uses experiments!
Case study: Schluter 1994
White =
Intermediates only
Black =
Intermediates plus
limnetic specialist
Benthic
Limnetic
Paper choices for discussion
• Option 1: Losos 1990 – Uses phylogenetics
and various models to determine why Anolis
species pairs on Antilles islands are
large/small, but single species are
intermediate: character displacement or
character sorting?
Paper choices for discussion
• Option 2: Muchhala and Potts 2007 – Looks at
reproductive character displacement in batpollinated flowers; looks at nature of
pollination mechanisms experimentally and
observationally.
Paper choices for discussion
• Option 3: Pfennig and Pfennig 2005 – Examines
character displacement in spade foot toads to
test if displacement may lead to a ‘winner’ and
‘loser’; is character displacement the best of a
bad situation?
Paper choices for discussion
• Potential 4th option – If you liked the case
studies, we could read Grant & Grant 2006,
plus Schluter 2000, which reviews his labs
work, including the experiment presented
today. Both are pretty short.