Foraminifera and parallel evolution - How ofrwhy?

Abstract

Directional changes are distinguished from yes-or-no changes. The latter lack the quality of direction; in the evolution of larger foraminifera they manifest themselves at first at a late ontogenetic stage as individual monstrosities, but lead, by the deuterogenetic mode, to new genera or families. Further evolution within a lineage is characterized by the directional gradualism of neptonic reduction. Various hypotheses why this complex evolution could have been of selective advantage are reviewed; complicating factors are discussed; there is still a large unexplained residue. Gradualism is considered to be rare and to be associated with faunas of low diversity, consisting of related but genetically isolated subunits which react to extra-group factors as one population, but which may evolve through intra-group competition; hence the frequent coexistence of related lineages. Still unexplained is the following of the same evolutionary pathway by forms of no near relationship. This may be due to severe structural limitation of possibilities; one pathway is followed by forms derived from planispiral ancestors, another by forms derived from trochoid ancestors. Three appendices give details on the phylomorphogenesis of Cycloclypeus, notes on lndonesian larger foraminifera, and notes on the stratigraphy of South Sumatra.