30
RICHARD S. MILLER
non-intersecting fundamental niches (Fig. 3 A). Only the first of these
alternatives requires the action of interspecies competition.
D. CHARACTER DISPLACEMENT
Brown and Wilson (1956) proposed the term “character displacement” to describe a situation in which, when two species become
sympatric, the differences between them are accentuated in the zone of
overlap and are weakened or lost entirely in the parts of the range
outside this zone. Any of a variety of morphological, ethological, ecological or physiological characters may diverge in this manner and they
are assumed to have a genetic basis. While the most obvious function
of character displacement may be to reinforce existing isolating mechanisms, it is usually thought to reflect a competitive interaction which
has led also to niche differentiation (Brown and Wilson, 1956; Kohn
and Orians, 1962; Mayr, 1963). Once a secondary contact between two
cognate populations has been established, the species may interact in
two ways to augment their initial divergence: (1) if inbreeding occurs,
hybrid sterility or non-viability will lead to the reinforcement of reproductive barriers, and natural selection will favor a reduction in “gamete
wastage” and further ethological or genetic divergence, and ( 2 ) ecological
displacement and a reduction in competition will also be favored by
natural selection if the ecological characteristics have a genetic basis
(Brown and Wilson, 1956). According to this theory interspecies competition leads initially to ecological divergence in the physical space of
the intersection between the two fundamental niches. Ecological divergence is accompanied by character divergence, reduced competition,
and reinforcement of ethological and ecological isolation. This theory
describes a positive feedback mechanism which will continue to increase
diversity, as long as it has selective value and hybridization does not
intervene at an early stage disrupting the process.
The adaptive differences between two sympatric species of nuthatches
(Sitta) have been cited as a particularly clear example of character
displacement (Brown and Wilson, 1956; Mayr, 1963). Xitta neumayer
and X. tephronota largely replace one another in eastern and western
Eurasia but overlap broadly in Iran. Vaurie (1951) showed that the
two species are nearly identical in areas outside the zone of overlap,
and in fact can only be distinguished by an experienced taxonomist;
but where the two species occur in more or less equal numbers in the
zone of sympatry in Iran, S. neumayer shows a marked reduction in
bill length and overall bill size, and in the width, size and distinctness
of the facial stripe. Conversely, S. tephronota shows a positive augmentation of all these characters in specimens from the zone of sympatry.
RICHARD S. MILLER
non-intersecting fundamental niches (Fig. 3 A). Only the first of these
alternatives requires the action of interspecies competition.
D. CHARACTER DISPLACEMENT
Brown and Wilson (1956) proposed the term “character displacement” to describe a situation in which, when two species become
sympatric, the differences between them are accentuated in the zone of
overlap and are weakened or lost entirely in the parts of the range
outside this zone. Any of a variety of morphological, ethological, ecological or physiological characters may diverge in this manner and they
are assumed to have a genetic basis. While the most obvious function
of character displacement may be to reinforce existing isolating mechanisms, it is usually thought to reflect a competitive interaction which
has led also to niche differentiation (Brown and Wilson, 1956; Kohn
and Orians, 1962; Mayr, 1963). Once a secondary contact between two
cognate populations has been established, the species may interact in
two ways to augment their initial divergence: (1) if inbreeding occurs,
hybrid sterility or non-viability will lead to the reinforcement of reproductive barriers, and natural selection will favor a reduction in “gamete
wastage” and further ethological or genetic divergence, and ( 2 ) ecological
displacement and a reduction in competition will also be favored by
natural selection if the ecological characteristics have a genetic basis
(Brown and Wilson, 1956). According to this theory interspecies competition leads initially to ecological divergence in the physical space of
the intersection between the two fundamental niches. Ecological divergence is accompanied by character divergence, reduced competition,
and reinforcement of ethological and ecological isolation. This theory
describes a positive feedback mechanism which will continue to increase
diversity, as long as it has selective value and hybridization does not
intervene at an early stage disrupting the process.
The adaptive differences between two sympatric species of nuthatches
(Sitta) have been cited as a particularly clear example of character
displacement (Brown and Wilson, 1956; Mayr, 1963). Xitta neumayer
and X. tephronota largely replace one another in eastern and western
Eurasia but overlap broadly in Iran. Vaurie (1951) showed that the
two species are nearly identical in areas outside the zone of overlap,
and in fact can only be distinguished by an experienced taxonomist;
but where the two species occur in more or less equal numbers in the
zone of sympatry in Iran, S. neumayer shows a marked reduction in
bill length and overall bill size, and in the width, size and distinctness
of the facial stripe. Conversely, S. tephronota shows a positive augmentation of all these characters in specimens from the zone of sympatry.