Michelle Bess
Final Summary
This research article is about the increase in species richness from the poles to the
tropics, referred to as the latitudinal diversity gradient. The question has been why do nearly all
groups of life have more species in the tropics? There have been hundreds of hypotheses on
latitudinal diversity gradient but there are still four unexplained components. The four major
components are the time since the clade colonized the region, speciation rates, extinction
rates, and dispersal events.
These scientists that have done the experiments and tests have been doing this so that
there can be an answer to why there are more species in the tropics than other places in the
world. They had two main hypotheses. Under the first one dispersal is higher out of than into
the tropics, thus acting against the latitudinal gradient. The second one is known as the tropical
niche conservation. Which lineages originate in the tropics and have difficulties to disperse and
adapt into temperate regions which makes them stay in the tropics.
There are many results of the experiments about the increase in species richness from
the poles to the tropics. According the current range distribution data from the PanTHERIA
database, mammalian species richness peaks near the equator. They categorized each species
reported in the mammalian phylogeny as living in the tropical biome, the temperate biome, or
both. They analyzed the resulting worldwide data using recent bigeographic birth-death models
of diversification. The results of these tests help prove their theories to be correct or incorrect.
Their goal is to prove or disprove their hypotheses.
The main analyses were preformed on 100 species-level time calibrated phylogenetic
trees randomly sampled from the Bayesian pseudo-posterior distribution of trees. They
obtained minimum and maximum latitudinal data from the PanTHERIA database. When they
tested for association between latitude and diversification rates, they used Geographic State
Specification and Extinction model. For them to be able to test for an association between
latitude and diversification rates they used the Geographic State Speciation and Extinction
model. This birth-death model is a geographic extension of character-dependent diversification
The results show the process underlying the latitudinal diversity gradient is
poorly understood. The results in this discussion show that both speciation and extinction rates
vary with latitude. The results show that speciation rates in mammals are higher in the tropics.
This was supported by analyses of the global phylogeny, and in dispersal-constrained analyses
for five of the eight orders. For primates and lagomorpha, no latitudinal difference in speciation
was detected, and for carnivora a higher speciation rate was found in temperate regions. They
found lower extinction rates in tropical regions for the global phylogeny and in dispersalconstrained analyses for the majority of the studied orders.