82
T.J. Givnish
4 Visual Selection, Sexual Selection, Predators,
and Species Diversity
Limited dispersal may also have played a key role in the diversification of the
African rift-lake cichlids. This group is considered one of the most dramatic cases
of adaptive radiation, with species in different lakes evolving a wide range of different feeding strategies and associated dentition, body shape, and coloration (Fryer
and Iles 1972; Greenwood 1984; Nishida 1991; Kocher et al. 1993; Meyer et al.
1994; Stiassny and Meyer 1999). Yet, while more than 1,000 species of cichlids
have arisen in the seven major lakes, surely there are no more than 20 or 30 adaptive types per lake. The large remaining component of diversity must surely reflect
the effects of limited dispersal, at least in part (Givnish 1997). Dispersal between
the three largest lakes (Victoria, Tanganyika, Malawi) is minimal and molecular
data indicate that parallel, largely independent radiations have occurred in each
(Meyer et al. 1990, 1991, 1996; Kocher et al. 1993). In cichlids, extensive
intralacustrine speciation is favored by (1) mouth brooding and limited dispersal of
young; (2) use of rocks for shelter from predatory species, resulting in philopatry of
adults in several groups; (3) the insular nature of suitable rock outcrops around the
periphery of each lake; and (4) periodic drought and the resulting dissection of
lakes into separate basins (see Mayr 1970; Fryer and lies 1972; Greenwood 1974,
1978; Meyer et al. 1990, 1996; Johnson et al. 1996; Verheyen et al. 1996; Reinthal
and Meyer 1997). Such limited dispersal fosters convergent radiations in individual
lakes, and extensive speciation within lakes, associated with low vagility associated
with mouth breeding and philopatry associated with isolated areas of rock outcrops.
Recent research by Albertson et al. (1999) supports this perspective. Their molecular phylogeny, based on DNA fingerprinting, resolves relationships among
closely related cichlids from Lake Malawi. Adaptive divergence in feeding morphology appears to have occurred early in the history of the Malawi lineage; subsequent diversification has arisen with little change in trophic morphology, suggesting that processes other than adaptive radiation (e.g., limited dispersal) may have
played a greater role in recent speciation.
Sexual selection on visual cues used for mate recognition also appears to playa
key role in cichlid diversification, with many species displaying some of the most
vivid coloration patterns of any freshwater fishes. Seehausen et al. (1997) have
recently shown a strong correlation between water clarity and cichlid species diversity within and between the African rift lakes, and have experimentally demonstrated that water turbidity increases mismating by female cichlids. Seehausen et
al. (1997, 1999) argue that, under better viewing conditions, sexual selection based
on visual cues can generate a greater diversity of pre-mating barriers between species, leading to greater species diversity; greater turbidity led to more mismating
and an erosion of cichlid diversity. Indeed, male nuptial coloration (blue vs. red
dorsum and/or ventrum) varies rapidly through evolutionary time, especially in
species with promiscuous mating systems (Seehausen et al. 1999); mating barriers
between species often disappear under dim light or incomplete spectra (Seehausen
T.J. Givnish
4 Visual Selection, Sexual Selection, Predators,
and Species Diversity
Limited dispersal may also have played a key role in the diversification of the
African rift-lake cichlids. This group is considered one of the most dramatic cases
of adaptive radiation, with species in different lakes evolving a wide range of different feeding strategies and associated dentition, body shape, and coloration (Fryer
and Iles 1972; Greenwood 1984; Nishida 1991; Kocher et al. 1993; Meyer et al.
1994; Stiassny and Meyer 1999). Yet, while more than 1,000 species of cichlids
have arisen in the seven major lakes, surely there are no more than 20 or 30 adaptive types per lake. The large remaining component of diversity must surely reflect
the effects of limited dispersal, at least in part (Givnish 1997). Dispersal between
the three largest lakes (Victoria, Tanganyika, Malawi) is minimal and molecular
data indicate that parallel, largely independent radiations have occurred in each
(Meyer et al. 1990, 1991, 1996; Kocher et al. 1993). In cichlids, extensive
intralacustrine speciation is favored by (1) mouth brooding and limited dispersal of
young; (2) use of rocks for shelter from predatory species, resulting in philopatry of
adults in several groups; (3) the insular nature of suitable rock outcrops around the
periphery of each lake; and (4) periodic drought and the resulting dissection of
lakes into separate basins (see Mayr 1970; Fryer and lies 1972; Greenwood 1974,
1978; Meyer et al. 1990, 1996; Johnson et al. 1996; Verheyen et al. 1996; Reinthal
and Meyer 1997). Such limited dispersal fosters convergent radiations in individual
lakes, and extensive speciation within lakes, associated with low vagility associated
with mouth breeding and philopatry associated with isolated areas of rock outcrops.
Recent research by Albertson et al. (1999) supports this perspective. Their molecular phylogeny, based on DNA fingerprinting, resolves relationships among
closely related cichlids from Lake Malawi. Adaptive divergence in feeding morphology appears to have occurred early in the history of the Malawi lineage; subsequent diversification has arisen with little change in trophic morphology, suggesting that processes other than adaptive radiation (e.g., limited dispersal) may have
played a greater role in recent speciation.
Sexual selection on visual cues used for mate recognition also appears to playa
key role in cichlid diversification, with many species displaying some of the most
vivid coloration patterns of any freshwater fishes. Seehausen et al. (1997) have
recently shown a strong correlation between water clarity and cichlid species diversity within and between the African rift lakes, and have experimentally demonstrated that water turbidity increases mismating by female cichlids. Seehausen et
al. (1997, 1999) argue that, under better viewing conditions, sexual selection based
on visual cues can generate a greater diversity of pre-mating barriers between species, leading to greater species diversity; greater turbidity led to more mismating
and an erosion of cichlid diversity. Indeed, male nuptial coloration (blue vs. red
dorsum and/or ventrum) varies rapidly through evolutionary time, especially in
species with promiscuous mating systems (Seehausen et al. 1999); mating barriers
between species often disappear under dim light or incomplete spectra (Seehausen