0198566123.pdf

ADAPTIVE RADIATION 227

Box 9.1 Canarian spiders, beetles, and snails

The Canarian invertebrates provide an
outstanding example of radiation on rather less
isolated islands than Hawaii and the Galápagos.
There may be as many as 6000 native Canarian
invertebrate species, of which half are considered
to be endemic (Oromí and Báez 2001). In
general, the Canarian invertebrate fauna is
characterized by the absence of certain groups
(e.g. of scorpions, light-worms, Cicadae,
Solifugae, and dung-beetles) but this has been
compensated for through active speciation within
colonist lineages, producing a high species–genus
ratio. Additionally, 99 genera are considered
exclusive to the archipelago, 58 of them being
present in more than one island (multi-island
genera) and 41 restricted to one island, of which
25 are found on the island of Tenerife (Oromí
and Báez 2001). Not all lineages have radiated
extensively; for example, as many as 57 of the
Canarian endemic genera are monotypic,
indicating a lengthy period of insular isolation
and change without radiation.
Radiation is particularly evident in land snails,
spiders and beetles, with at least 24 different
genera producing 15 or more endemic species (see
Table). The largest genera of beetles (Laparocerus),
diplopods (Dolichoiulus), and spiders (Dysdera) are
represented in all the major habitats, from the
coast up to 3000 m, in arid and wet zones, in
forested and open areas (even in lava tubes), all
over the archipelago, a pattern strongly supportive
of the label adaptive radiation. Several of the
genera listed in the table are also present on other
Macaronesian islands, although the degree to
which they have radiated is variable between
archipelagos (Oromí and Báez 2001). For instance,
among diplopods, there are 2 species of
Dolichoiuluson Madeira but 46 on the Canaries; in
contrast, the figures for Cylindroiulusare 25
species on Madeira and 2 on the Canaries. This
may reflect the timing of the colonization of each
archipelago by different genera, but this
explanation requires testing via molecular
phylogenies.

As on other geologically young volcanic islands

(e.g. Hawaii), there are fine examples of lava

tubes on the Canaries. These features provide very

similar habitats that are also very different from

the environment above ground. Moreover, they

are typically quite isolated within each island.

Different tubes often feature their own sets of

albino and blind Dysderaspiders and Loboptera

cockroaches. Given the difficulty of such forms

dispersing between lava tubes, they are most

parsimoniously explained as examples of parallel

evolution from above-ground relatives.

Number of species of endemic invertebrates of the Canary

Islands, within Canarian endemic (bold) and non-endemic genera

represented on the islands by at least 15 endemic species

(source: Oromí and Báez 2001)

Group Name Number of species

Snails Hemicycla 76

Napaeus 45

Obelus 21

Plutonia 18

Spiders Dysdera 43

Oecobius 35

Spermophorides 22

Pholcus 16

Diplopods Dolichoiulus 46

Hemiptera Cyphopterum 24

Asianidia 17

Issus 15

Beetles Laparocerus 68

Attalus 51

Cardiophorus 31

Tarphius 30

Acalles 27

Calathus 24

Hegeter 22

Nesotes 20

Oxypoda 16

Pachydema 16

Trechus 15

Isopods Porcellio 18