A key observation from natural communities is that different species vary widely in their abundance and distribution. Understanding what factors are most important in explaining this variation is a fundamental goal of ecology. Here I take a comparative phylogenetic approach to address this problem. Using two clades of diverse tropical understory plants, I use information garnered from species' evolutionary relationships to test hypotheses about why some species are common while other species are rare. In a study of geographic range size variation of Neotropical Piper (Piperaceae) species, I used published DNA sequences to infer species' divergence times and herbarium collection records to infer their range sizes. I found that younger species have significantly smaller range sizes than older species. I examined a similar question using Mesoamerican Psychotria subgenus Psychotria (Rubiaceae) species. To infer the evolutionary relationships of species, I sequenced DNA from two loci of > 60 species in this clade. I concurrently inferred the phylogenetic relationships and absolute divergence times of species using a Bayesian relaxed-molecular clock method. I calculated two metrics of geographic range size using herbarium collection records, and predicted species' potential ranges using species distribution modeling. I found that Mesoamerican Psychotria subgenus Psychotria species have diversified primarily over the past 17 million years (Mya), and species largely fall into two clades that diverged approximately 15 Mya. In one clade, younger species have colonized a significantly smaller proportion of their potential range extent than older species. Finally, using two genera in the clade Psychotrieae (Rubiaceae), I examined the impact of phylogenetic relatedness on the co-occurrence and variation in abundance among these species in Costa Rica, Central America. Using data collected on 240 transects nested in seven assemblages across Costa Rica and a phylogenetic hypothesis of species relationships based on DNA sequences, I found that Psychotrieae assemblages are significantly phylogenetically overdispersed, indicating that co-occurring species are less related than expected by chance. Within one heavily sampled assemblage, I found an inverse relationship between species' phylogenetic relatedness and their variation in abundance. The opposite trend was found across assemblages, where phylogenetic relatedness and variation in abundance were positively correlated.
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