Geology Reference
In-Depth Information
15.4.2 Neogene Events: Evidence from
Phylogeographic Studies of Fishes
and Mammals
occupy definitive niches (stenotypes), and are thus forced to
track their local environments. Over evolutionary
timescales, these stenotypes either successfully track their
specific physical habitats, or become extinct. Therefore
stenotopic fish may record the timings of river events, such
as disruption or connections of river channels, when water
transfer between the river systems ends or begins. The
events dates by molecular proxies, record of the start of the
current phase of the river drainage structuring (assuming
equal ability of dispersal up and down rivers, that is no
barriers) as connections may be seasonal (i.e. yearly flood)
or event based (i.e. 1 in 10 year floods). The use of primate
evidence goes some way in solving problems with the timing
of prolonged river channel emplacements as many primate
taxa distribution patterns are often contained by major rivers
that serve as barriers to movement, both on a genus and
species levels (Anthony et al. 2007 ; Harcourt and Wood
2012 ).
Yet not all rivers are equally effective barriers, with wider
and faster flowing rivers being formidable barriers compared
to narrower, smaller tributaries and thus the lower reaches of
a river are often more effective barriers compared to their
upper reaches; for example, the lower sections of the CR
seem to be more effective barriers than its smaller tributaries
and headwaters (Harcourt and Wood 2012 ). As African
rivers form the border of distributions for more subspecies
rather than genera, Harcourt and Wood ( 2012 ) suggest that
primate taxonomic level may be a useful, albeit rough, proxy
to estimate times of population isolation by river emplace-
ment. Therefore primate divergence estimations (calculated
from the MRCA) may serve as proxies of rivers achieving a
sufficient size, and continuity of flow, to become a barrier to
dispersal. The CR acts a major, regional barrier to primate
dispersal, with fewer primate species and subspecies strad-
dling the CR compared to other rivers (Harcourt and Wood
2012 ). Owing to the relationship between rivers as a barrier
and primate size (Harcourt and Wood 2012 ), it can be argued
that timings of divergence of large primates (i.e. bonobos,
chimpanzees and gorillas) relate to the development of large
rivers (i.e. Congo) with medium and smaller primates
serving as proxies for emplacement of medium and smaller
rivers. However the inherent heterogeneity of a river in both
space and time must be borne in mind when considering
rivers as barriers to primate dispersal. This is highlighted by
a genetic study of chimpanzees either side of the Malagarasi
River, an assumed barrier river (Piel et al. 2013 ). Although
the Malagarsi River is over 100 m wide for extensive
stretches, both field observations and genetic analysis of
chimpanzees populations either of the river, close to a ford,
indicated that the chimpanzees made use of this natural
bridge to cross the river (Piel et al. 2013 ). Thus the
Malagarasi River is likely only a seasonal barrier to primate
movement in areas where natural rock bridges may have
While there is some geologic and geomorphic data regarding
the timing of river re-arrangements, this data is often discon-
tinuous or lacking in terms of resolution. This lack of data of
river developments can be circumvented by using biological
data as proxies to identify changes to drainage topology.
These proxies use phylogenetic reconstructions of related
species that have become geographically isolated by
changes in the landscape in which they live. Over evolution-
ary time, the isolated populations accumulate changes in
their DNA, with the genetic difference between the
populations allowing for an estimation of time since separa-
tion based on a molecular clock (Avise 2000 ; DeSalle and
Rosenfeld 2013 ). It is possible to determine the average rate
at which a species has accumulated genetic differences
(mutation rate) over time. By measuring the number of
genetic differences between two related groups and applying
the average mutation rate, a time estimate of when the two
populations were once freely breeding can be calculated
(Avise 2000 ). This process gives the approximate age of
genetic divergence of the two groups from their last shared
common population, or most recent common ancestor
(MRCA) (Avise 2000 ). The use of Bayesian methods allows
for age estimates to be assigned a measure of statistical error,
usually expressed at the 95 % level, which constrains the
probability of the true age value (DeSalle and Rosenfeld
2013 ).
The challenges involved in using phylogenetic data (such
as calibration methods and sampling density of species) can
be mitigated by using phylogenetic data from several spe-
cies. The congruence of age estimates across different spe-
cies provides insights into the spatio-temporal changes in the
fluvial geomorphology of central Africa during the late
Cenozoic (i.e. Cotterill and de Wit 2011 ; Goodier et al.
2011 ; Schwarzer et al. 2011 ; Hart et al. 2012 ; Kawamoto
et al. 2013 ). For example, it appears that the CR is the only
river that contains the extent of the large primates, namely
chimpanzees ( Pan troglodytes ) and gorillas ( Gorilla
gorilla ), preventing these primates from occurring south of
the CR, although both having successfully dispersed across
smaller rivers (see Fig. 15.8a ). Fish are the most accessible
of these biological indicators and being restricted to water
systems their biogeography and evolution are tightly linked
to the evolution of fluvial systems (Cotterill and de Wit
2011 ). This link of biogeography and landform can be seen
in the broad congruence that differenct fish assemblages
show with regional drainage basins. Although fish
assemblages may differ locally, owing to the interplay of
numerous abiotic and biotic factors (i.e. water pH, tempera-
ture, biological competitors and predators) some fish species
Search WWH ::




Custom Search