Lauren Carpenter
Dr. Ely
Term Paper
Bio 770
Oceanic Dispersal Plays a Key Role in the Divergence of Different Species
Oceanic dispersal has been an important contributor to biogeographic
patterns in various taxa. Oceanic dispersal occurs when an organism is transferred
from one landmass to another by crossing the sea on what is essentially a floating
island (Tolley et al. 2014). An example of a species that demonstrated this
phenomenon is the chameleon. The age of the species suggests they appeared postGondwanan break-up. Gondwanan was a part of Pangaea that broke off and
included Africa, Madagascar, South America, Australia, and Antarctica (Tolley et al.
2014). With this in mind, scientists observed that chameleons were located in
Africa and Madagascar.
Tolley et al. (2014) conducted a study to see from where chameleons
originated and how oceanic dispersal played a role in their migration. The authors
composed a dated phylogeny that used 6-13 markers for roughly 174 taxa that
represented more than 90 percent of all the chameleon species. In the study, Tolley
et al. (2014) used three different ancestral-state reconstruction mechanisms, which
included the Bayesian, DEC, and the Mesquite. The Bayesian analysis was used to
form the chameleon’s phylogenetic tree. The DEC was used to determine the time
constraints that were put on the dispersal of chameleons. The Mesquite was used to
reconstruct the ancestral habitats of the chameleons. The outcome of these analyses
was the conclusion that the Chameleon family most likely originated in Africa and
had two different oceanic dispersals to Madagascar.
Tolley et al. (2014) concluded that these two oceanic dispersals probably
occurred between the Paleocene and the Oligocene because the oceanic currents
present at that time were believed to have favored eastward dispersal. The
Paleocene is a geological series that occurred 66-56 million years ago and was
during the Paleogene Period in the modern Cenozoic Era (Zimmerman KA 2013).
The Oligocene is a geological series that occurred 33.9-23 million years ago and was
also during the Paleogene Period in the modern Cenozoic Era (Zimmerman KA
2013). When Tolley et al. (2014) examined the groups of organisms that are
considered to have evolved at the same time, also called clades, they observed that
the diversification of chameleons at the genus level took place in the Eocene (time
period between the Paleocene and Oligocene), while the species-level of
diversification occurred in the Oligocene. The point of this discovery was to show
that the chameleons diversified into new species after they were separated by
oceanic dispersal (Tolley et al. 2014).
Figure 1, taken from Tolley et al. (2014) study, is a chronogram of
chameleons. A chronogram is a phylogenic tree that shows evolutionary
relationships of different species based on their physical and genetic differences.
Every time a branch diverges it symbolizes their most recent common ancestor,
which is called a node. The chronogram created in this study used black dots to
represent a deep node while the triangles symbolize a constrained node. A deep
node shows the emergence of a new type of chameleon while a constrained node is a
node within a specific type of chameleon (Tolley et al. 2014). Each color represents
where the species most likely are found (Africa, green; Madagascar, blue). Also, a
time chart is located at the bottom of the chronogram, which shows when the
species most likely evolved. These data illustrated that the chameleons migrated
during the Paleocene and Oligocene when the oceanic currents were headed
eastward toward Madagascar (Tolley et al. 2014).
Figure 1: Cladogram that shows the ancestral linage of Chameleons
Today, this phenomenon of oceanic dispersal is still a major topic in scientific
studies. Samonds et al. (2014) researched the spatial and temporal patterns of
Madagascar’s vertebrate species as a result of oceanic dispersal and ancestor type.
In this study, they proved that there was a tipping point in the Cenozoic Era that
provided an increase in oceanic dispersal. The data collected by Samonds et al.
(2014) provided additional evidence that during the Cenozoic Era, the only way of
transportation to Madagascar was via transoceanic dispersal. Table 1 shows that
their expected rates of dispersal were lower than the observed values for the
Cenozoic time period. In other words, more species migrated during this time
period than expected.
Table 1: Observed and Expected Rates of Chameleons During the Different
Epochs of the Cenozoic and Mesozoic Eras.
Epoch
Observed
n
Expected
n
Arrival rate
(arrivals/Myr
)
χ2
n
P
Original Database
MidMiocene to
present
Oligocene
to midMiocene
Paleocene
to Eocene
Cretaceous
Late
Jurassic
Crottini et.
MidMiocene to
present
Oligocene
to midMiocene
Paleocene
to Eocene
Cretaceous
Late
Jurassic
35
7.1
2.33
14
8.9
0.74
14
14.9
0.44
9
4
37.8
7.3
0.11
0.26
136.
4
76
<0.001
124.
3
76
<0.001
al. database (16)
33
7.1
2.20
14
8.9
0.74
18
14.9
0.57
10
1
37.8
7.3
0.13
0.06
These results are consistent with previous research that has been conducted
to understand oceanic dispersal using frogs. Vences et al. (2004) observed how
oceanic dispersal of amphibians influenced phylogenetic relationships and genetic
variation. In this experiment, the authors sequenced the mitochondrial 16S rRNA
gene of the Ptychandena mascareniensis, a type of frog, from different populations
over several continents and made a phylogenic tree. When the data was examined
thoroughly, Vences et al. (2004) concluded that there were 5 deep clades in the
phylogenic tree that had pairwise divergences of >5%. Vences et al. (2004) believe
that one of the clades was restricted to Madagascar. The authors realized the only
logical way this dispersal could have happened was if the colonization happened by
overseas rafting. This was a new way of dispersal compared to what scientists had
previously thought. Figure 2 is a phylogenic tree that shows the divergence of the P.
mascareniensis and where each of these species could be found. This is the same
type of data found in the results section that was used in Tolley et al. (2014).
Figure 2: Cladogram that shows the ancestral linage of Ptychandena
mascareniensis
In another study, Emadzade and Horandi (2010) used phylogenetic analyses
of nuclear and chloroplast DNA to reconstruct the biogeographical ancestry of
Ranunculeae, which is a plant. The cladogram illustrated deep nodes, and the
Ranunculeae originated in Northern Hemisphere during the Eocene epoch.
Emadzade and Horandi (2010) also discovered that there was weak evidence
supporting a vicariance, separation of a population by a physical barrier, event that
took place between North America and Eurasia during that time period. In Figure 3,
it is noticeable that the Eurasian clade differentiated between the Oligocene and
Miocene epochs, which resulted in three independent migrations into the Southern
Hemisphere. Furthermore, Figure 3 shows that the North American clade
differentiated in the Miocene period. It was also concluded that during this time
frame the “oceanic barriers already existed between continents and thus dispersal is
most likely the explanation for the current distribution of the tribe” (Emadzade and
Horandi 2010).
Figure 3: Cladogram that traces the ancestral linage of the species
Ranunculaea
This study that Emadzade and Horandi (2010) conducted provides evidence
that another species that was diversified during the same era as chameleons,
Ptychandena mascareniensis, as well as different vertebrate fauna. Oceanic dispersal
of a species was most common and most prominent during this specific time period
known as the Cenozoic Era. Without this phenomenon, we may not have the various
kinds of species we have today.
In future studies, the analyses that were used can aid other scientists in
tracing a species ancestral linage to its origin. This could be beneficial when trying
to determine when a species diverged or when a specific species became extinct. A
cladogram can be helpful in seeing how different species are related or if they came
from the same biogeographical areas. This would aid in scientists’ understanding of
the similarities and differences between certain species and how they are able to
live in the areas they inhabit.
Literature Citied
Emadzade K and Horandi E (2010) Northern Hemisphere origin, transoceanic
dispersal, and diversivication of Ranunculeae DC. (Ranunculaceae) in the Cenzoic.
Journal of Biogeography 38:517-530 doi:10.1111/j.1365-2699.2010.02404.x
Samonds KE, Godfrey LR, ali Jr, Goodman SM, Vences M, Sutherland MR, Irwin MT,
Krause DW (2014) Spatial and temporal arrival patterns of Madagascar’s vertebrate
fauna explained by distance, ocean currents, and ancestor type. Proceedings of the
National Academy of Sciences 109:14 doi:10.1073/pnas.1113993109
Tolley KA, Townsend TM, Vences M (2013) Large-Scale phylogeny of chameleons
suggests African origins and Eocene diversification. Proceedings of the Royal Society
Biological Sciences 280:20130184. http://dx.doi.org/10.1098.rspb.2013.0184
Understanding Evolution. 2014. University of California Museum of Paleontology.
Available from: http://evolution.berkeley.edu/evolibrary/article/phylogenetics_01
Vences M, Kosuch J, Rodel MO, Lotters S, Channing A, Glaw F, Bohme W (2004)
Phylogeny of Ptychadena mascareniensis suggests transoceanic dispersal in a wide
spread African-Malagasy frog lineage. Journal of Biogeography 31:593-601
doi:10.1046/j.1365-2699.2003.01031.x
Zimmerman KA 2013. Cenozoic Era: Facts About Climate, Animals, and Plants.
Available from: http://www.livescience.com/40352-cenozoic-era.html