Clines and Other Genotypic/Phenotypic/Evolutionary Responses to Geography or Environment Among Neogene Planktonic Foraminifera

Roberta K. Smith

Morphologic variation along geographic or ecologic clines is not widely recognized among planktonic foraminifera. A search among living taxa clearly reveals Neogloboquadrina as a cline. Its existence probably demonstrates a (genotypic/phenotypic) response to varying temperature-related conditions. This clinal response apparently has continued, with parallel evolving species, through the late Neogene.

A good fossil record shows other probable genotypic/phenotypic/evolutionary responses to varying environmental conditions. These include morphological convergence--as strikingly with menardiform Globorotalia--derived from two distinct lineages. Divergence is demonstrated by Neogloboquadrina^rarrPulleniatina and Globigerinoides^rarrSphaeroidinellopsis/Spheroidinella. Parallel evolution is illustrated among Globigerinoides lineages.

Possibly Pulleniatina and Sphaeroidinella also represent clines (and/or morphological responses to environmental conditions). They live in tropical latitudes with their closest ancestral-lineage relatives. Neogloboquadrina dutertrei and Globigerinoides succulifer. Pulleniatina-Sphaeroidinella depth ranges are not well understood, however, and they may live under different conditions than do their ancestral-lineage "cousins." Juveniles can be very difficult to distinguish from the "ancestral" forms and intergrades appear.

Perhaps partly because the biostratigrapher or taxonomist desires to recognize "species," clines may have gone largely unnoticed. Samples of some taxonomic groups can be morphologically variable and include apparent intergrades among taxa, for example, (1) Globigerina falconensis-G. bulloides-G. calida-Globigerinella aequilateralis, and (2) Globoratalia inflata-G. "puncticulata" (and/or G. crassaformis), and even G. scitula-G. hirsuta. What is the genetic meaning of clines and of apparent convergences--and even the species concept? Morphologic variation may reflect gene flow across taxonomic lines because of the available diversity of breeding systems and other potential mechanisms for gene exchange (such as symbionts).

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.