James W. Valentine

Genetic variability in a temperate intertidal phoronid, Phoronopsis viridis.

The Phoronida are a coelomate phylum consisting of only two genera and about 12–15 described species. Phoronids probably represent the common ancestral stock of all lophophorates, and may be the most primitive living deuterostomes. Using the techniques of starch gel electrophoresis, we have studied genetic variation at 39 loci in 120 individuals of Phoronopsis viridis collected in Bodega Harbor, Bodega Bay, California. Allelic variation was found at 27 (69.2%) loci. If a locus is considered polymorphic when the frequency of the most common allele is no greater than 0.99, the proportion of polymorphic loci in the total sample is 48.7%. The average number of alleles per locus is 2.23. The expected frequency of heterozygous loci per individual on the assumption of Hardy-Weinberg equilibrium is 9.4%. There is evidence of inbreeding; the mean value of F, Wrights fixation index, is 0.21±0.02. Genetic variability in P. viridis is intermediate among marine invertebrates. The tropical clam, Tridacna maxima, has on the average 20.2% heterozygous loci per individual. At the other extreme, a brachiopod from Antarctica, Liothyrella notorcadensis, has an average of 3.9% heterozygous loci per individual. Among marine invertebrates, there seems to be a gradient of decreasing genetic variability from low to high latitudes, which may reflect their different adaptive strategies.

Phanerozoic Taxonomic Diversity: A Test of Alternate Models

1. K. R. L. Hall, Curr. Anthropol. 4, 479 (1963). 2. Specific examples of tool use by birds: A. Alcock, Ibid 112, 542 (1970); A. H. Chisholm, ibid. 96, 380 (1954); D. Lack, Sci. Amer. 188, 66 (April 1953); H. B. Lovell, Wilson Bull. 70, 280 (1957); G. C. Milliken and R. I. Bowman, Living Bird 6, 23 (1967); J. van Lawick-Goodall Nature 212, 1468 (1966); and H. van Lawick, Nat. Geogr. Mag. 133, 631 (1968). General reviews of the tool-using literature: Hall (1); J. Alcock, Evolution 26, 464 (1972); J. van LawickGoodall, Advan. Study Behav. 3, 195 (1970); W. H. Thorpe, Learning and Instinct in Animals (Harvard Univ. Press, Cambridge, 1963). 3. A. C. Bent, Life Histories of North American Jays, Crows and Titmice (Bulletin 191, Smithsonian Institution, United States National Museum, Washington, D.C., 1946); J. W. Hardy, Univ. Kans. Sci. Bull. 42, 13 (1961). 4. M. W. Hunter and A. C. Kamil, Psychonom. Sci. 22, 271 (1971); A. C. Kamil, M. Lougee, R. I. Shulman, J. Comp. Physiol. Psychol. 82, 394 (1973). 5. We thank Saul Balagura and Theodore Sargent for their critical comments on an earlier version of the report. Supported by NSF grant GB-30501 to A.C.K.

Climatic Regulation of Species Diversification and Extinction

Models for marine species diversity are proposed in which world oceans with mild, uniform climates are characterized by low species diversity while oceans with cooler polar waters and higher temperature gradients are characterized by high provinciality, high species diversity, and perhaps higher rates of phyletic evolution. Climates that fluctuate between cooling and warming trends will act to create species diversity and enrich the marine biota, whereas times of warming should be times of greatest extinction rates.

Genetic variation in Frieleia halli, a deep-sea brachiopod☆

Electrophoretic study of a population of the deep-sea brachiopod Frieleia halli shows That it is genetically highly variable. Using a conservative criterion of polymorphism, this brachiopod population is polymorphic at about 50% of its loci, while an average individual is heterozygous at about 17% of its loci. Despite this high genetic variability, F. halli is not particularly variable morphologically. These findings agree with previous indications from electrophoretic studies of genetic variability in organisms from stable environments and suggest that the body of ecological-genetic theory that predicts a positive correlation of genetic variability with environmental instability is incorrect.

Radiometric Ages of Pleistocene Terraces from San Nicolas Island, California

The ages of coral skeletons from three localities on San Nicolas Island, California, have been estimated by uranium series methods. Two localities are on a low terrace and their radio-metric ages of 120,000 ± 20,000 and ≥ 120,000 years B.P. suggest that this terrace is roughly contemporaneous with the Palos Verdes sand and correlative mainland terraces. One locality from a high terrace that contains an extinct faunal element yielded a significantly older age (>200,000 years B.P.).

RADIOMETRIC AGES OF PLEISTOCENE FOSSILS FROM CAYUCOS, CALIFORNIA

Age estimates based on uranium and thorium isotopes in invertebrate skeletal material from marine terrace deposits at Cayucos, California, range from 130,000 to 140,000 years B.P. This age range suggests that the Cayucan (central California) invertebrate shelf province was contemporaneous with the Verdean (southern California) shelf province. Both provinces were probably chiefly Sangamonian but may have persisted into early Wisconsin time.

Mass Extinctions and Genetic Polymorphism in the “Killer Clam,” Tridacna

Mass extinctions of marine invertebrates have been attributed to genetic depauperation in specialized lineages. Tridacna maxima is a plausible modern analog of the lineages that were commonly associated with mass extinctions; it is restricted to a relatively stable biogeographic province, lives in shallow water, is highly specialized, and is associated with reef communities. Our studies show, however, that it is highly polymorphic and heterozygotic, and thus fails to support the depauperate gene-pool hypothesis of mass extinction.