Hybrids

Abstract

In Chapters 4–14, I have given examples of embryos and larvae in a great variety of taxa, all consistent with the hypothesis that they were transferred from other taxa. In the present chapter, I discuss hybridization as the most probable means of transfer. I became convinced that larval forms had been transferred between taxa some two years before I seriously considered hybridization as the mechanism for transfer. I originally thought that all transfers were of forms that were larvae both before and after transfer, but the concept of transfer itself evolved. The first step in this evolution was the proposal that urochordate tadpole larvae can be traced back to an appendicularian: an example of a larval form with an adult origin (Williamson, 1992), and the second step was the proposal that all embryos and larvae have adult origins (Williamson, 2001, 2002). I put forward hybridization as the probable cause of transfer in Larvae and Evolution (1992), when I had taken only the first step, but the latest version of larval transfer considerably strengthens this probability. If all embryos and larvae can be traced back to adult forms, the genomes of these adults are the units of transfer, and such units occur in sperm and unfertilized eggs.