Thesis (Ph.D.) - Indiana University, Biology, 2012
In this dissertation I explore broad-scale regularities in the distribution of North American ferns and lycophytes. I test models for the evolution of these regularities. To this end, I construct a comprehensive database of all North American fern and lycophyte species. In the final empirical chapter I extend my analyses to New World mammals and global amphibians.
First, I describe patterns of North American fern and lycophyte species and family richness and describe the relationships of these patterns with climate. Next, I explore the geography of fern polyploid species and present biogeographical evidence that most polyploid speciation events are evolutionary dead-ends. These results contrast with notions that polyploid speciation leads to evolutionary novelty and adaptive geographical expansion. Next, I test the "Out of The Tropics" model (OTT) for the evolution of latitudinal gradients in diversity. The OTT model proposes that higher taxa tend to originate in the tropics, and subsequently expand into temperate regions while maintaining a tropical presence. The OTT model predicts that older and diverse higher taxa will have expanded from their region of tropical origin, while younger and less diverse higher taxa will still be located in the tropics. I find partial support for the OTT model. Endemic fern families are found only in the tropics suggesting fern families more frequently originate in the tropics than the extra-tropics. Fern families with many species extend further into temperate regions than do less diverse families. However, family age is not correlated with the degree of extension from the tropics.
In the final empirical chapter, I describe a global biogeographical rule observed in ferns, mammals and amphibians. The new rule states that diversity of a taxon at the taxon's richness peak predicts the amount of geographical expansion of that taxon from its center, and the number of species completely outside the taxon's center. I call these patterns diversity-expansion relationships, and I explore these patterns using multiple measures of diversity and geographical breadth (expansion). I propose alternative explanations for the existence of diversity-expansion patterns and discuss their macroevolutionary implications.