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.