Species as the basic units in evolution and biodiversity: Recognition of species in the Recent and geological past as exemplified by larger foraminifera

• Non-molecular species delimitation within a time horizon.
• Split-off and split-up speciation.
• Application to the fossil record.

Methods for species recognition and delimitation based solely on morphological characters are presented. Species can be described as pools of contemporarily interconnected genotypes possessing their own history leading to evolutionary lines. Interconnection in genotypes is expressed by homogeneous phenotypes, but not in a one to one relation. The proof of interconnection by phenotypic homogeneity must be based on the four criteria ‘shape homogeneity’, ‘ontogenetic cohesion’, ‘homogeneous ecological niches’ and ‘evolutionary continuity’. While in all eukaryotes, three of the four – homogeneities in shape and ecological niches, ontogenetic cohesion – can be checked both in living individuals and fossil forms, the detection of birth (speciation), lifetime and death (ending) of a species, factors that determine an evolutionary line, is only possible in organisms with a fossil record. Speciation can be grouped in split-off and split-up processes. Split-off processes where a daughter species derives from a mother species are easier to recognize than split-up processes where several species originate more or less contemporaneously within a geological time interval. This makes it difficult to delimitate species when they are in a reticulate speciation process in which hybridization between subspecies is a common feature. In contrast to evolutionary continuity, homogeneities in shape and ecological niches as well as ontogenetic cohesion are more difficult to recognize in fossil species due to low specimen numbers, incomplete preservation caused by taphonomic processes and the fragmentary representation of fossil environments in the sedimentary strata, hindering the acquisition of gradients. Nevertheless, the four criteria enable the recognition of species without molecular–genetic investigations. Only the combination of these criteria makes identification of species in the fossil record possible.

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