Evolutionary change in flowers and inflorescences: evidence from naturally occurring terata.

Records of naturally occurring, heritable floral abnormalities considerably enhance our understanding of floral evolution. Peloric mutants, frequent in natural populations of orchids and mints, have radially symmetric flowers but occur in species characterized by bilaterally symmetric flowers. Three distributions of peloric flowers across an inflorescence are: (1) complete (all flowers peloric, as in the cycloidea mutant of Antirrhinum), (2) scattered (with both peloric and zygomorphic flowers, as in the epigenetic cycloidea mutant of Linaria), and (3) terminal (only the terminal flower peloric, as in the centroradialis mutant of Antirrhinum). Genetic relationships between lateral and terminal peloria, and between peloric and pseudopeloric flowers, remain ambiguous. Complete peloria probably caused occasional evolutionary reversals from zygomorphy to actinomorphy, whereas the 'terminal-flower effect' is a less likely cause of floral evolution.