Comparing apples and oranges (and pinecones): understanding patterns of reproductive disparity across seed plants

Seed plant reproductive structures are highly diverse, but analysing morphological disparity across the group is challenging because taxa are difficult to directly compare. Here, we develop an approach that accommodates divergent structures and unclear homologies, treating basic structural units as analogous and analysing characters within these units. We scored 284 acrogymnosperm and angiosperm cones and flower-fruits for 47 discrete characters and calculated morphological distances based on character mismatches among organ sets, treating character absence as a state. We then performed two analyses: one focused on the diversity of structural units and another focused on character diversity, where information from all structural units was collapsed into a single space. In both analyses, we found low reproductive disparity within most acrogymnosperm lineages but high disparity across them. Major angiosperm clades, by contrast, consistently show high disparity and occupy broad, overlapping regions of morphospace. Seed plant groups therefore appear to evolve disparity in fundamentally different ways: acrogymnosperms exhibit high levels among major lineages while angiosperms show high disparity both within and among major lineages. The approach we use to identify these patterns is also generalizable to other systems with serial homology and/or highly divergent morphologies.