A major challenge in evolutionary biology is understanding why some evolutionary lineages and some ecosystems are more diverse than others. This unevenness in species diversity can be attributed to three major classes of factors: time, chance, and ecology.
Time. Evolutionary lineages can be diverse simply because they have been around for a long time. For example, there are 20 species of hutias, and 285 species of squirrels, but hutias have only been around for 15 million years and squirrels have been around for more like 50 million years. Likewise, some habitats are older than others, and that may explain some of the differences in species richness across environments.
Chance. A certain amount of unevenness in species diversity is expected to result from dumb luck. That is, much of the observed disparity in species diversity across evolutionary lineages can be explained by models of random diversification.
Ecology. This is where things get really interesting. Much of the unevenness of species diversity can’t be explained by time or chance. What that means is that there are specific properties of organisms, populations and ecosystems that change the rates of speciation and extinction (or alter the limits on diversity). These properties are poorly understood. And so one of our big goals is to understand the ecological drivers of taxonomic diversification in plant-feeding insects — the most species-rich metazoan trophic guild on earth.