Linear model
The lm identified a significant latitudinal gradient in BGF
frequency distribution along the Brazilian coast. A significant negative
correlation between BGF frequency and latitude was observed. The number
of BGF within a given region, represented here as the frequency of
putative BGF concordances across multiple taxa and markers per site,
tend to decrease as latitude increases. This result concurs with one of
the most ubiquitous large-scale biotic pattern on Earth, thelatitudinal gradient in species diversity (LGD). Several
hypotheses have been proposed and a plethora of papers, text books and
reviews have been published, to explain the causes of LGD
(e.g. Fine, 2015; Pianka, 1966; Stevens, 1989). To make matters more
complicated, our latitude BGF gradient can be both, a result of
particular drivers used to explain LGD (e.g. climate stability,
available energy, temperature, body size, metabolic rate) and as a
driver itself (confounded with other hypotheses such as the ‘higher
spatial heterogeneity’ hypothesis, Pianka, 1966). As a driver, all other
factors being equal, our “BGF concentration hypothesis” predict that a
greater number of BGF in a particular location would inevitably drive
higher species diversity across evolutionary time.
Jablonski, Huang, Roy, & Valentine (2017) over-simplistically
classified LGD hypotheses into two non-mutually exclusive categories:in situ hypotheses (also known as the ‘producer hypothesis’) and
spatial dynamics hypotheses (also known as the ‘receiver hypothesis’)
(se also Stebbins, 1974). The former category suggests that causes for
higher species diversity in lower latitudes are driven by local
environmental factors driving higher speciation. The latter category
suggests that higher species diversity in the tropics is driven by the
arrival and concentration of species arising elsewhere (e.g. temperate
regions). Our results support the theory that speciation rates are
higher at lower latitudes not only due to factors related to species’
intrinsic biology such as smaller body size, faster metabolic rates and
shorter life cycles (Fine, 2015) but also due to the occurrence of
higher frequency of BGF in lower latitudes – an in situhypothesis. According to our data, higher BGF frequency in lower
latitudes tend to produce more genetically structured populations, which
in due time give rise to different species locally. Additionally, our
results also help identify one of the processes driving Rapoport’s rule,
that species latitudinal ranges are usually smaller at lower latitudes
than at higher latitudes. Smaller latitudinal ranges between species,
particularly between sister-species and within species complexes, at
lower latitudes can be the result of higher frequency in BGF occurring
within smaller spatial scales.
We call the reader’s attention to the fact that worldwide patterns of
LGD are a result of several millions of years of evolution and most of
the phylogeographic data used in this study was shaped in far smaller
time scales (≤ 2 million years). The extent at which higher frequency of
BGF at lower latitudes is either a recent, localized, phenomenon (i.e.
Brazil-centric) or a universal evolutionary process helping drive LGD in
the planet remains to be determined. However, marine and terrestrial
fossil records (Mittelbach et al., 2007) and, more recently, large scale
phylogenetic studies (Jablonski et al., 2017; Verbruggen et al., 2009)
suggest that higher speciation rates in lower latitudes has occurred
across geological time, worldwide.