With recently derived algorithms, it is possible to calculate the relative phylogenetic distinctiveness of taxa with respect to patterns of phylogenetic branching. We extended this approach by exploring the relative extent to which taxa represent phenotypic biodiversity. We devised a method, based on the use of independent contrasts, that measures the amount of phenotypic change that occurs when lineages diverge. We use this method to quantify the extent to which a taxon represents the phenotypic diversification that has occurred in the past. We applied our method to an analysis of variation in clutch size across 133 avian families. All families did not contribute equally to representing clutch size diversification. The top 10 avian families in terms of representing clutch size diversification were the mesites (Mesitornithidae), cranes (Gruidae), bustards (Otidae), new world quail (Odontophoridae), seriemas (Cariamidae), finfoots ( Heliornithidae), swallows (Hirundinidae), megapodes (Megapodiae), and guans (Cracidae). The 217 species in these 10 families (2.3% of all bird species, 7.5% of families) represented 19.3% of diversification in clutch size. Seventeen percent of overall clutch size diversification was represented by taxa threatened with extinction. The 10 families that represent the greatest proportion of overall clutch size diversification threatened by extinction were the mesites (Mesitornithidae), kagu (Rhynochetidae), cranes (Gruidae), kiwis (Apterygidae), new world quail (Odontophoridae), megapodes (Megapodiae), cassowaries (Casuariidae), finfoots (Heliornithidae), guans (Cracidae), and logrunners (Orthonychidae). The 42 threatened species (0.5% of all bird species, 3.8% of all threatened bird species) in these 10 families encompassed 53% of the clutch size diversification whose representation was threatened with extinction. Our results suggest that this type of analysis could potentially help prioritize species-based conservation efforts by identifying those taxa that contribute most toward representing the evolutionary processes that lead to current phenotypic biodiversity.