Agriculture Reference
In-Depth Information
3.2.2 Recent evolution
Evolution of legumes is still occurring, both by dispersal and vicariance and
some examples are given in this section. In the species-rich neotropical genus
Inga
(Mimosoideae), speciation appears to have been concentrated in the last 10 million
years, with a significant number of species being as recent as 2 Ma. Factors causing this
increase in diversity include rapid changes of climate in tropical areas, closing of the
Isthmus of Panama, and the uplifting of the Andes (Richardson et al., 2001). It has re-
cently been suggested that this uplifting took place at a particularly rapid rate (1.5 km in
amillion years) about 8Ma (Garzione et al., 2008) although the interpretation of isotope
data leading to this conclusion has still to be tested (Kerr, 2008). Uplifting of the Andes
allowed the most rapid evolution of species yet known for plants, in
Lupinus
.Eighty-
one species of lupin have evolved in the high Andes (see Chapter 2) in less than the last
2 million years. This equals the record for eukaryotic diversification, held by cichlid
fish in East African lakes (Hughes & Eastwood, 2006). The high plateaux of the Andes
were likened to islands in their isolation, allowing rapid evolution. A similar situation
obtains in parts of Northern Eurasia, where genera such as
Hedysarum
are undergoing
speciation, and in the islands of the Russian archipelago, above 80
◦
north, where
Astra-
galus, Oxytropis
and
Hedysarum
have distinct species (Yakovlev et al., 1996). InAustralia,
members of the endemic tribes Mirbelieae and Bossiaeae are diversifying into areas of
increasing aridity (Crisp et al., 2004). Crisp&Cook (2007), in a comprehensive examina-
tion of the Southwestern and SoutheasternAustralian flora, concluded that a vicariance
event associated with the formation of the Nullabor Plain led to speciation of genera in
these two legume tribes about 2 to 4 Ma. In a detailed study of the neotropical dalber-
gioid genus
Platymiscium
, Saslis-Lagoudakis et al. (2008) concluded, inter alia, that this
genus originated in seasonally dry tropical forests and then invaded theAmazon region
twice within the last 5.6 million years. This is consistent with the suggestion of Schrire
et al. (2005) that legumes evolved in arid areas and later colonised the wet tropics.
It should be emphasised that as more data become available ideas may change. For
example, it is commonly thought that the species-rich Cape Floristic region of South
Africa (see Section 2.5.1) resulted from the development of a Mediterranean climate
about 5 Ma. However, recent work on two legumes tribes Crotalarieae (pro parte) and
Podalyrieae has cast some doubt on this, suggesting that they may be much older, 44
to 46 Ma (Edwards & Hawkins, 2007).
3.3 How was the information for nodulation acquired?
There are two major routes by which acquisition of the information for nodulation may
have occurred. One involves recruitment of genes that evolved for other purposes, and
the other involves genes that have been duplicated, with some of the duplications
being modified for use in symbiosis (Sprent, 2008).
3.3.1 Ancient genes that have been recruited for symbiotic purposes
With the exception of lichens, the first symbioses to have evolved between green
plants and fungi were arbuscular mycorhizas (AM). Fossil evidence for these dates
