On homology of arthropod compound eyes.

Eyes serve as models to understand the evolution of complex traits, with broad implications for the origins of evolutionary novelty. Discussions of eye evolution are relevant at many taxonomic levels, especially within arthropods where compound eye distribution is perplexing. Either compound eyes were lost numerous times or very similar eyes evolved separately in multiple lineages. Arthropod compound eye homology is possible, especially between crustaceans and hexapods, which have very similar eye facets and may be sister taxa. However, judging homology only on similarity requires subjective decisions. Regardless of whether compound eyes were present in a common ancestor of arthropods or crustaceans + hexapods, recent phylogenetic evidence suggests that the compound eyes, today present in myodocopid ostracods (Crustacea), may have been absent in ostracod ancestors. This pattern is inconsistent with phylogenetic homology. Multiple losses of ostracod eyes are an alternative hypothesis that is statistically improbable and without clear cause. One possible evolutionary process to explain the lack of phylogenetic homology of ostracod compound eyes is that eyes may evolve by switchback evolution, where genes for lost structures remain dormant and are re-expressed much later in evolution.