0198566123.pdf

FROM VALLEY ISOLATES TO ISLAND-HOPPING RADIATIONS 231

Box 9.2 The explosive radiation of cichlid fishes in the African Great Lakes

When considering freshwater organisms, lakes

can be viewed as islands, and the African Great

Lakes (Tanganyika, Malawi, Victoria, etc.),

clustered in the Rift Valley, can be considered as

an archipelago. They possess one of the most

impressive examples of radiation in the form of

about 1000 species of cichlid fishes (Cichlidae),

with some 200 in Lake Tanganyika, 300 in Lake

Victoria, and over 400 in Lake Malawi. Although

Lake Tanganyika’s cichlids very likely derive from

multiple riverine ancestors, those in Lake Victoria

apparently derive from just two separate ancestral

species and those in Lake Malawi from a single

founder population (Danley and Kocher 2001).

Lake Tanganyika developed as a shallow

depression linked to the Congo River system

about 20 Ma, and achieved its isolation about

9–12 Ma. Molecular clock estimates indicate that

the Tanganyika lineages diverged from their

ancestors at least 5 Ma. Lake Malawi is at most

1–2 million years old, and the cichlid radiation is

estimated to date back only approximately

0.7 Ma. Finally, Lake Victoria is considered to date

to between 0.2–0.7 Ma, but may have been

completely dry as recently as 12 400 years ago,

suggesting a truly explosive rate of radiation

subsequently (Johnson et al. 1996).

Many models have been proposed in

explanation of these spectacular cases of

radiation. One of the more straightforward is the

three-stage radiation model(Streelman and

Danley 2003):

1 Major habitat diversification.In the first

adaptive radiation stage, an initial divergence of

the lineages takes place, resulting in the appear-

ance of different clades related to the major

available habitats: the rock-dwellers, the sandy-

substrate dwellers, and the pelagic species.

2 Trophic diversification.This second adaptive

stage is well studied in the rock-dwellers of Lake

Malawi, and involves the rapid directional

selection of the feeding apparatus, attaining

resource partitioning between species. This

process has yielded 10–12 recognized endemic

genera, featuring morphological differences in

features such as jaw shape and tooth structure.

3 Sexual selection.The most intriguing step of

the model is the third phase, a non-adaptive radi-

ation process, involving sexual selection through

mate choice by females. This mechanism relies on

colour differences in males and is thought to

occur only in transparent waters, where the

colours are evident. As cormorants are thought to

be able to predate the colourful males more easily

in transparent waters, it is assumed that bright

colouration must come at a cost to fitness and

thus can be considered essentially non-adaptive.

distribution of a formerly more widely distributed
species, which may or may not have originated on
the island to which it is currently restricted. For
instance,Saxicola dacotiae(the Canary island chat) is
today restricted to Fuerteventura, having been
extirpated from the nearby islet of Alegranza a cen-
tury ago. Although not endemic, another similar
case is Pyrrhocorax pyrrhocorax(red-billed chough),
now restricted within the Canaries to La Palma but
known from fossil evidence to have occurred on
Tenerife, La Gomera, and El Hierro (Martín and
Lorenzo 2001). Such losses may be the product of
natural causes as envisioned within the taxon cycle
but in this case are attributable to humans.

In contrast, multiple-island endemics(MIEs) are

attributable to three different scenarios.

●First, they may have originated on one island,

most typically the oldest island, and have subse-

quently colonized a younger island or islands,

where as yet there has been insufficient time (or too

much interisland gene flow) for speciation to have

occurred.

●Second, they may be palaeoendemics that have

evolved little since colonizing an archipelago, but

have been lost from their continental source areas.

●Third, in a small number of cases they may be

examples of parallel microevolution, in which sim-

ilar environmental pressures on separate islands