Biology Reference
In-Depth Information
times, will more frequently encounter conditions where genetic isola-
tion can occur. One can model this by using ''pitted speciation land-
scapes'' (compare the ''rugged fitness landscapes'' used in population
genetics): such a landscape has many ''pits'' which are regions where
isolation is achieved. Selection will normally try to keep species in these
pits (i.e., there is stasis), and the pits are of course not permanent but
evolve depending on various changing factors, such as the presence of
other species. Overall, populations that change faster, driven by accel-
erated mutation rates, faster selection, and shorter generation times, will
move around the speciation landscape faster and ''fall'' more frequently
into the isolation pits. However, experimental evidence for direct
temperature effects on speed of election are not available. Such evidence
is urgently needed.
The combined effects of the above should be an increase in speciation
rates at higher temperatures. Rohde (
1992
) gives some references in
support of this assumption. Jablonski (
1993
) presented paleontological
evidence that post-Paleozoic marine orders have appeared more
frequently in tropical waters. Cardillo (
1999
) found, for birds and
butterflies, that relative rates of diversification per unit time increase
towards the tropics, (but see Bromham and Cardillo
2003
, who found
no effect of latitude on evolutionary rates in 45 pairs of phylogenetically
independent bird species using two mitochondrial genes and DNA-DNA
hybridization distances). Most importantly, Allen et al.(
2002
)have
shown that species diversity can bepredictedfromthebiochemical
kinetics of metabolism. Their model predicts quantitatively how species
richness increases with environmental temperature. They conclude that
''evolutionary rates are ultimately constrained by generation times of
individuals and mutation rates. Both of these rates are correlated with
metabolic rates and show the same Boltzmann relation to temperature.
The results therefore support the hypothesis that elevated temperatures
increase the standing stock of species by accelerating the biochemical
Vivien's (
2002
) conclusion of her chapter on ''Energetics and Fish
Diversity on Coral Reefs'' is very similar to the hypothesis that effective
evolutionary time is largely responsible for the latitudinal gradients in
species diversity. According to her, synergy between constant high
temperature and long-term stability may speed up the rate of molecular
evolution and increase genome variability allowing the emergence of
new species by natural selection. Most recently, Wright et al.(
2003
)
provided support for the hypothesis that speciation rates are higher in
