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Adaptive radiation is defined as “the evolutionary
divergence of members of a single phylogenetic line into a variety of different
adaptive forms” (Futuyma, 1998). During adaptive
radiation descendants from a common ancestor diversify through adaptation, in
accordance to an ecological niche. Ecological opportunity is said to precede adaptive
radiation, although this is not a necessity and some groups fail to radiate
even in its presence. Ecological opportunity is a facet as it is believed that through a variety of
different resources being available a species has greater opportunity to
diversify, with each being adapted to utilising a different area of the
resource spectrum (Losos, 2010). Central to this
theme is the ability of a species to adapt. Simpson proposed the first framework
of fundamentals needed for adaptive radiation to occur according to four
different scenarios; the presentation of new resources, extinction of species
previously using resources, a species colonizing an area where these resources were
not previously being used and evolution of a specific trait that allows utilization
of resources in ways not previously possible (Simpson,
1953).

 

Adaptive
radiation in respect to Anolis

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A species having undergone adaptive radiation is the
Anolis lizard (Genus: Anolis). The
lizards are situated across multiple Caribbean islands known as the Greater
Antilles with 154 of the 300 species being located on these islands, all from
two colonizing species (Losos et al., 2001). There
are multiple different species within the communities of the islands, each displaying
morphological and behavioural disparities to one another. It is highlighted that
the habitat influenced evolution of these ecomorphs and the dispersal of the
Anoles is what caused these evolutionary jumps (Eastman
et al., 2013). The Anolis lizard for these reasons is an ideal species for
investigating Simpsons theory of adaptive zones resulting in adaptive
radiation, as two of the four fundamental scenarios for adaptive radiation have
occurred in the Anole community.

 

Causes
of the Anolis radiation

The main aspect which drove the species radiation and
therefore evolutionary divergence seen in Anolis lizards is believed to be
interspecific competition for resources. This interspecific competition is
reportedly great in Anoles because of their strong interactions (Losos and Scheider, 2009). This causes species to
inhabit various areas of the island resulting in adaptation and the occupation
of a niche. This factor has most likely influenced the evolutionary divergence
seen between different Anolis ecomorphs. The various Anole species can be
separated both sympatrically as a result of varying courtship behaviours, habitat use and morphology. As
many as 11 species of anoles live sympatrically within the Caribbean islands (Losos and Scheider, 2009). Allopatric separation is also present as species
are reproductively isolated by inhabiting different islands.

 

 

 It has been
hypothesised that Anolis lizards’
reproductive isolation can stem from differences in dewlap design, this is rare
or occurs by chance in allopatric speciation, but in sympatric speciation it’s invariable
as it acts as a species recognition signal and is key to species divergence.
This behaviour would therefore support the species recognition hypothesis (Glor et al., 2006). The role of colour in species recognition has been tested
in lab conditions with species pairs and has shown support for the hypothesis (Rand and Williams, 1970). However, a study conducted on a large Anolis population revealed
no significance between dewlap colour and species recognition (P values ranged from 0.87 to 0.99) (Harmon et al., 2007).      

 

Radiation
in reference to the Antillean islands

The most apparent
diversity of this species is found across all four Greater Antillean islands
(Cuba, Hispaniola, Jamaica and Puerto Rico) *. The various Anolis lizard that
inhabit the island are deemed ecomorphs as they are uniquely evolved and
adapted to their environmental niche. There are six ecomorphs across the four
islands: trunk, trunk-crown, trunk-ground, grass-bush, twig and crown-giant.
Apart from the trunk and grass-bush ecomorphs all Anolis lizards are found on
every island. However, on each island the ecomorphs are not represented by one
species but many with Cuba showing the most with 14
trunk-ground species and 15 grass-bush species (Table 1.). It has been proven
through phylogenetic analysis that other than a few, most habitat specialist
Anoles have evolved independently on the various islands (Losos and Scheider, 2009). Furthermore, analysis has shown
Anolis lizards that inhabit different microhabitats have morphological
differences (Fig 1.). Through using Multivariate analysis of variance (MANOVA) morphology
has been highlighted as the most important source of variation in relation to
the Anolis radiations (Jonathan et al., 2007). With
body size being one of the many traits which has shown evidence for ecological differentiation,
it is shown to correlate with the species habitat and the species functional performance
(Eastman et al., 2013). Other morphological and physiological
differences according to Anolis species habitat are larger hind legs being present in Anolis lizards that live in
broad tree trunks such as the crown-giant Anole, and Short legs in species that
inhabit the canopies to balance on thin irregular twigs such as the trunk-crown
Anolis (Losos and Scheider, 2009).

 

 

Although the Anole ecomorphs have evolved multiple
times on different islands, there are some specialised species such as Anolis vermiculatus a semi-aquatic Anole
found only in Cuba (Glor et al., 2006). Species
radiation however has not only presented in the six ecomorphs but also within
highly specialised Anolis species, such as four large Cuban species from the
genus Chamaeleolis that are specially
adapted for narrow perches and feed on molluscs (Leal
and Losos, 2000). 

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