Thomas J. M. Schopf

GENETIC VARIABILITY IN THE DEEP SEA: RELATION TO ENVIRONMENTAL VARIABILITY

A current concept of population biology holds that much genetic variability is maintained in populations as an adaptation tc environmental heterogeneity in time and space (Battaglia, 1963; Bretsky and Lorenz, 1971; Burns and Johnson, 1971; Powell, 1971). Environmental heterogeneity is thought to promote genetic variability by mechanisms of diversifying selection (reviewed in Dobzhansky, 1970). High levels of heterozygosity have also been proposed to increase the developmental homeostasis of individuals confronted by environmental variability, and to confer evolutionary flexibility on populations subjected to long-term changes (Lerner, 1954; Lewontin, 1958). In predictable stable environments high adaptive values might be achieved by homozygous gene ensembles and genetic variability would decline. In recent years the techniques of electrophoresis genetics have yielded estimates usually in the range of 20 to 60% polymorphic loci for outbreeding terrestrial and aquatic animals (tables in OBrien and Maclntyre, 1969; Kojima et al., 1970; Selander et al., 1970; Gottlieb, 1971). These studies have dealt with species occupying relatively heterogeneous environments and have not tested the hypothesis that genetic variability is a function of environmental variability. We have tested this hypothesis in relation to the deep sea by obtaining estimates of genetic variability in eight species of benthic invertebrates from 1,000-2,000 m in the Atlantic and Pacific Oceans.

PROTEIN POLYMORPHISM OF THE HYBRIDIZING SEASTARS ASTERIAS FORBESI AND ASTERIAS VULGARIS AND IMPLICATIONS FOR THEIR EVOLUTION

The seastars Asterias forbesi and A. vulgaris share 67% of their genes in common (based on 27 loci). These species are normally easily characterized by 7 prominent phenotypic differences but naturally occurring hybrids are found in localities with typical adults of the two species.A. forebesi and A. vulgaris are thought to have evolved during the mid to late Pleistocene as a result of a restriction in the range of a more widely distributed Miocene or early Pleistocene form due to lowering of sea level and the coincident emergence of a disrupting land barrier (Cape Cod-Georges Bank). At least one local population of the ancestral species evolved into the present cold water form (A. vulgaris) during selection in an arctic-fed Gulf of Maine. Coincidently, at least one other local population of the ancestral species evolved with selection in warmer, southern waters into the present shallow water, temperate form (A. forbesi).If A. forbesi and A. vulgaris have been derived from a late Tertiary wide-ranging spec...

GENE FREQUENCIES IN A MARINE ECTOPROCT: A CLINE IN NATURAL POPULATIONS RELATED TO SEA TEMPERATURE

The concept that gene flow is continuous and widespread within a species has recently been questioned (Ehrlich and Raven, 1969), but no data could be cited for marine organisms. Indeed, the spatial and temporal distribution of gene frequencies is unknown for all but a minute fraction of the three million existing animal species. Hitherto, field information on genic differentiation in animals that lack phenotypic polymorphism has been very difficult to obtain. The development of high-resolution zone electrophoresis coupled with staining techniques for specific proteins has now made it routinely possible to identify marker genes and to determine gene and genotype distributions (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966). Factual information on this question of gene flow among populations and the degree of genic differentiation is especially critical for marine ectoprocts because the adults of several species with short-lived larvae (hours) are unable to be distinguished phenotypically over distances of hundreds to thousands of kilometers. Results presented below support the notion that selection adjusts genotype frequencies over very short distances for these sessile marine animals with short-lived larvae. The change in frequency of alleles in populations can be observed over 10-20 kms distance even though populations are nearly continuous over the intervening area. We analyzed the distribution of allele frequencies along a coastal and island transect of 35 kms along southern Cape Cod. From 29 to 47 colonies of the widespread encrusting ectoproct Schizoporella unicornis (see Appendix) were collected from 1/? to 3 m depth from pilings or floating docks at each of five localities, Cape Cod, Massachusetts (Fig. 1). Local populations were estimated to contain several hundred colonies. Irregular aggregations of colonies of S. unicornis have been recorded nearly every kilometer of the way from one end o,f our transect to the other (Sumner et al., 1913) and our impression from examining dredged material is that they are even more closely spaced. Elsewhere along the Atlantic coast of the United States, S. unicornis is also an abundant local species.

POPULATION GENETICS OF MARINE SPECIES OF THE PHYLUM ECTOPROCTA

Proteins of marine species of the Phylum Ectoprocta can be easily separated by zone electrophoresis on polyacrylamide gels. Many bands revealed by electrophoresis represent the product of individual genetic loci from which gene and genotype frequencies were calculated. Results are reported for 19 loci which are responsible for the formation of esterases, malate dehydrogenase and leucine aminopeptidase in Schizoporella unicornis and Bugula stolonifera.Observed genotype frequencies correspond closely with those predicted by Hardy-Weinberg equilibria indicating that these marine ectoprocts are dominantly outbreeding and that panmictic local populations cover an area of square meters. In addition, 25.0-54.5% of the diagnosed loci are polymorphic, depending on the species. Forty-two % of all loci identified are polymorphic.The most important implication of these findings is that the population structure and amount of genetic variability in ectoprocts (and by implication in other marine organisms) is fundamen...

A Critical Assessment of Punctuated Equilibria. I. Duration of Taxa

The purpose of this paper is to present a reasonable paleontological basis for interpreting species durations over geologic time. The context is to evaluate the currently nmuch discussed model of punctuated equilibria. I will take the position of a devils advocate and explore the biases that are inherent in the model as customarily used.

GENETIC VARIATION IN THE MARINE ECTOPROCT SCHIZOPORELLA ERRATA

Schizoporella errata is sessile and has larvae that live only hours. On a purely local scale, the spatial distribution of gene frequencies and genotypes appears random in most comparisons. In addition, genotypes and gene frequencies remained relatively stable after the passage of a year. Over a distance of 1000 km from the southern edge of the Acadian Faunal Province through the Virginian Faunal Province and into the Carolinian Faunal Province, from 80 to 89 per cent of the sampled genome is identical. That is, genetic polymorphism stands at 1 of the 9 well-established loci (11 per cent) and an estimated 2 of 10 total loci in pooled material from 9 localities. The proportion of alleles at the single, clearly polymorphic locus (Lap-3) varies directly as a function of environmental temperature measured at the warmest time of the year.

Survey of genetic differentiation in a coastal zone invertebrate: the ectoproct Schizoporella errata.

Gene and genotype frequencies are reported for two polymorphic loci which make the enzymes leucine amino peptidase and glutamate oxalate transaminase for a ubiquitous coastal zone species, the marine ectoproct Schizoporella errata (S. unicornis of most literature). Allele frequencies were determined from 14 localities in the Cape Cod region for 1972 and repeated in 1973. Gene frequencies for both loci change in a regular manner that is correlated with local water temperature during the warmest season. Allele frequencies have remained fairly uniform in local populations for as long as 5 successive years.

A Natural Experiment to Test the Hypothesis That Loss of Genetic Variability was Responsible for Mass Extinctions of the Fossil Record

Animals living under constant environmental conditions have been hypothesized to lose genetic variability. If so, this would account for a lack of adaptation to changing environments and hence be responsible for the significant mass extinctions of the fossil record. However, eight modern deep-sea species (1,000-2,000 m) in as constant a major environment as exists on earth, have as much genetic variability as do shallow marine and terrestrial species (20%-50% of the loci polymorphic). Accordingly, species do not appear to lose genetic variability as the environment becomes constant, and this explanation of the cause for mass extinction fails.