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The Tropical Conservatism Hypothesis-by Wiens and Donoghue (2004)

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The Tropical Conservatism Hypothesis-by Wiens and Donoghue (2004)

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1. Less time for species richness to accumulate in temperate regions because groups with high tropical species richness originated in tropics have dispersed recently or not at all.
2. Tropical groups are limited by their inability to adapt the freezing temperature during winter.
3.Large number of extant groups originated in the tropics because the tropics were more extensive until recently (~30-40 mya). It is related to the idea that species richness is correlated to area.

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-distributed widely  (all major continental regions except sub-Saharan Africa and Antarctica)
- few species in temperate North America
- many in tropical Middle America ( Mexico to Panama) and tropical South  America
- few in temperate South America

Why Hylid Frogs?
~ they have been the recent subject of intensive phylogenetic study and systematic revision
~ updated range maps are available for all species
~ hylid fauna at the interface between temperate and tropical regions in the Northern Hemisphere has been especially well studied

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1. Phylogenetics
2. Ancestral area reconstruction
3. Divergence date estimation
4. Analysis of diversification rates
5. Ecological niche modeling

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~ ancestral area reconstruction will show that hylids originated in tropical South America
( relationship between how long a clade has been present in a given region and how many species are in that region, regardless of whether regions are tropical or temperate)
~no general relationship between where clades occur and their rate of diversification
~ diverse lineages of tropical hylids will independently converge on similar northern range limits adjacent to temperate regions and that ecological niche modeling will show that the distribution of cold winter temperatures predicts the poleward range limits of these taxa (tolerance to climatic factors)
~ hylids originated before expansion of the temperate regions 30–40 mya, and that their major clades arose before this period as well

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- . Hylid frogs are strongly supported as monophyletic, as are the three subfamilies (Hylinae, Pelodryadinae, and Phyllomedusinae)
- Pelodryadinae and Phyllomedusinae are strongly supported as sister taxa
- Cophomantini is the sister group to all other Hylinae
- The remaining species fall into four strongly supported clades: the Dendropsophus clade, the Scinax clade, the Lophiohylini or Phrynohyas clade, and the Middle American clade of Wiens et al. (2005) or Hylini of Faivovich et al. (2005).

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Time and species richness
-Findings support species-rich clades of hylids originated in tropics and spread to temperate region more recently
- strong relationship between the time when a region was colonized and how many species are present there today, regardless of whether the region is tropical or temperate
- no significant relationship between the diversification rate of individual clades and the latitudinal midpoint of those clades

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The findings support the tropical conservatism hypothesis with 2 caveats: 1) temperature seasonality (rather than extreme winter temperature) and 2) the study offers only weak test  of the role of the Cenozoic expansion of temperate regions in generating the latitudinal gradient. The tropical conservatism hypothesis should apply to most groups of organisms with high tropical species richness.