Robert K. Selander

SEXUAL DIMORPHISM AND DIFFERENTIAL NICHE UTILIZATION IN BIRDS

Adaptive radiation has been defined as the evolutionary divergence of members of a phyletic line into different niches or adaptive zones (Mayr, 1963:633). Although it has been customary to think of adaptive radiation solely in terms of species or races, a growing body of evidence indicates that some degree of radiation occurs also within populations, as individuals come to occupy different subniches or adaptive subzones, subdividing and, perhaps, expanding the total niche or zone utilized by the population. Probably all species show some degree of ecological variation, either polymorphic or continuous. But this phenomenon is being studied in only a few groups of organisms, notably in Drosophila, in which chromosomal polymorphism has been interpreted as a. means of adaptation of populations to heterogeneous environments (Dobzhansky, * 1961, 1963, 1965). Theoretical bases for research on ecological variation in animal populations have been provided by Ludwig (1950), Levene (1953)) da Cunha and Dobzhansky (1954), Dempster (1955), Li (1955), Carson (1959), and Levins (1962, 1963). In birds, as in other vertebrates, the sexes usually differ in size if not also in proportions of body parts, including those used in feeding (Amadon, 1959) ; and, especially where the degree of sexual dimorphism, which is a form of polymorphism (Ford, 1961: 12), is marked, it seems probable that the morphological divergence has ecological significance in adapting the sexes to different subniches. However, there is only an occasional reference in the literature to sexual dimorphism in relation to niche utilization (e.g., Pitelka, 1950; Rand, 19.52), and, in general, the whole problem of ecological variation in populations has been neglected by vertebrate ecologists. The primary purpose of this report is to present evidence of an adaptive function of sexual dimorphism in size in woodpeckers by relating degrees of morphological dimorphism and sexual divergence in foraging behavior in two melanerpine species, the strongly dimorphic Hispaniolan Woodpecker (Centurus striatus) of Haiti and the Dominican Republic and the moderately dimorphic Golden-fronted Woodpecker (Ce&zmus awifrons) of continental North and Central America. In addition, the paper surveys other evidence that sexual dimorphism in birds is related to differential niche utilization. Finally, some evolutionary aspects of sexual dimorphism and ecological variation are considered.

Elephant Seals: Genetic Variation and Near Extinction

Blood samples from northern elephant seals (Mirounga angustirostris), representing five breeding colonies in California and Mexico, were surveyed electrophoretically for protein variation reflecting underlying genetic differences. No polymorphisms were found among 21 proteins encoded by 24 loci. This uniform homozygosity may be a consequence of fixation of alleles brought about by the decimation of this species by sealers in the last century.

EVOLUTIONARY GENETICS OF CAVE-DWELLING FISHES OF THE GENUS ASTYANAX

Attempts to understand evolutionary processes in cave organisms have been largely limited to speculation on the causes of loss of photoreceptor organs and pigmentation, increase in size and complexity of tactile sensory structures, and certain modifications in physiology (see review in Barr, 1968). Little is known of the population genetics of cave organisms or of the genetic changes accompanying the transformation of epigean (surface-dwelling) forms to troglobites (obligate cavernicoles). Yet genetic information is essential to the confident development of theories of troglobite evolution. For this reason, we have compared the genic character and degree of variability in troglobitic and epigean populations of the characid fish Astyanax mexicanus in Mexico. Students of cave biology have generally accepted the thesis that the ancestors of many troglobites entered caves as troglophiles (facultative cavernicoles) before the end of the Pleistocene, and became isolated with the local extinction of surface populations as a result of climatic changes associated with glaciation (Barr, 1968). If this is true, many troglobites have been living

Biochemical genetics of hybridisation in European house mice.

SummaryTechniques for demonstrating allozymic variation in seven enzymes (four esterases, isocitrate dehydrogenase, malic enzyme, and malate dehydrogenase) were employed to study genie variation in a narrow zone of hybridisation between allopatric semispecies of the house mouse (Mus musculus musculus and M. m. domesticus) on the Jutland Peninsula of Denmark. Material consisted of 2696 mice collected at 152 farms representing 44 sample areas on the peninsula and adjacent islands. The history of movements of early farming cultures with which mice were associated as commensals suggests that musculus and domesticus have been in contact and hybridising in northern Europe since 3000 B.C. The zone in Jutland and another in Germany lie in regions transitional between Atlantic and continental climates, the two parental forms meeting where they are equally well adapted to ecological conditions.The zone in Jutland has not shifted since Ursin defined its position in 1952 on the basis of morphological characters. An analysis of genotypic proportions in populations in the zone of hybridisation failed to demonstrate assortative mating, thus supporting laboratory evidence of free interbreeding between the semispecies. Genie heterozygosity levels are “ normal “ on the large islands of Falster, Fyn, and Als but reduced on the small islands of Airø and Hjaelm, presumably through the founder effect or genetic drift. The zone is strongly asymmetrical north to south, with extensive introgression of domesticus alleles into musculus, but little introgression in the other direction. A marked increase in width of the zone in western Jutland is associated with a more extensive gradient of environmental factors, particularly precipitation. In the narrow eastern part of the zone, 90 per cent of the transition in genetic character (as measured by a hybrid index) occurs over a distance of 20 km. The extent of introgression varies markedly among loci. Linkage between the loci studied is not a major factor affecting patterns of introgression. The extreme steepness of the gradient of transition in genetic character and the occurrence of major changes in frequencies at all loci along the same line are cited as evidence that the selective values of alleles are determined in part by the internal genetic environment. The “new” genetic environment created by introgression of domesticus alleles into musculus populations apparently favours the occurrence of minor alleles at the Es-2 and Es-3 loci. The failure of the musculus and domesticus gene pools to fuse despite long-standing hybridisation argues that genetic isolation cannot be equated with reproductive isolation. Selection against introgression of the genes studied (or the chromosomal segments that they mark) is presumed to involve reduced fitness in backcross generations caused by disruption of co-adapted parental gene complexes.

House Sparrows: Rapid Evolution of Races in North America

Conspicuous adaptive differentiation in color and size has occurred in the house sparrow (Passer domesticus) in North America and the Hawaiian Islands since its introduction in the middle of the 19th century. Patterns of geographic variation in North America parallel those shown by native polytypic species, in conformity with Glogers and Bergmanns ecogeographic rules. Racial differentiation of house sparrow populations may require no more than 50 years.

GENETIC VARIATION IN THE HORSESHOE CRAB (LIMULUS POLYPHEMUS ), A PHYLOGENETIC “RELIC”

To many early evolutionists, particularly those of the mutationist school, the existence of evolutionarily stable forms (living fossils or phylogenetic relics) posed a difficult problem, and, even among contemporary biologists, the feeling that a long-standing stability of organization seems antithetical to the concept of evolution may persist (Volpe, 1967:140). In the early part of this century, it was popularly believed that stable or slowly evolving (bradytelic) lines lacked the genetic variation necessary for more rapid evolution as a result of having abnormally low mutation rates; as Mayr (1963) puts it, a lack of variation in time and space was interpreted as a reflection of genotypic stability. Largely because of the influence of Simpson (1944), Stebbins (1944), Schmalhausen (1949), and Dobzhansky (1951), the mutationists interpretation of bradytely has now been all but abandoned by evolutionists in favor of the view that the principal factors controlling rates of evolution are to be found in the organism-environment relationship. Additionally, Mayr (1963 :304) has suggested that phenotypic uniformity in space and time is due to a highly perfected buffering system, in other words, to genetic homeostasis. The ecological hypothesis is well expressed in Simpsons (1944:141) conclusions that the final and probably the most fundamental factor in the relationship is that bradytelic groups are so well adapted to a particular, continuously available environment that almost any mutation occurring in them must be disadvantageous and that bradytely results from the equilibrium of large breeding populations of animals specifically adapted to a continuously available environment that is relatively invariable . . . . Yet, as Mayr (1963) notes, there is no evidence to refute (or to support) the hypothesis that bradytely results from loss of mutability. The recent development of electrophoretic techniques for demonstrating allelic variation at loci controlling enzymes and other proteins in natural populations (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966) permits a test of the mutationists thesis. The purpose of this study is to compare degrees of protein polymorphism and underlying genic heterozygosity in a classic phylogenetic relic, the horseshoe or king crab (Limulus polyphemus), and in several representatives of horotelic lines (those evolving at standard rates) for which data are already available. Evidence of a level of genetic variation in Limulus similar to that in horotelic forms will support the modern ecological and homeostatic interpretations of bradytely. But if the mutationists thesis is valid, Limulus should show an unusually low degree of polymorphism at its genetic loci. Limulus polyphemus is one of five living species of the Subclass Xiphosura, a group of marine arthropods usually placed with the Subclass Eurypterida (extinct giant water scorpions) in the Class Merostomata of the Subphylum Chelicerata. Following a period of adaptive radiation in the early and middle Paleozoic, the Merostomata became much less prominent after the Permian. Several Mesozoic forms are known in the Family Mesolimulidae, but by the Tertiary the fossil record is all but absent. There are presently three living genera, all belonging to the Family Limulidae, only one of which, Tachypleus, is known from the fossil record (Stormer,

Biochemical Polymorphism and Systematics in the Genus Peromyscus. III. Variation in the Florida Deer Mouse (Peromyscus floridanus), a Pleistocene Relict

Electrophoretically demonstrable variation was analyzed in proteins encoded by 41 structural gene loci in 71 individuals of the Florida deer mouse ( Peromyscus floridanus ) representing four sample areas. Of the 39 loci that were scorable in all individuals, 15 (38 per cent) were polymorphic in one or more populations. The average number of alleles detected per polymorphic locus was 2.01 (range 2 to 3). Average individual heterozygosity was 5.3 per cent. Populations of P. floridanus are less variable genetically than sympatric populations of the old-field mouse ( P. polionotus ), but similar in this regard to populations of the latter species from t0he Florida Panhandle and from South Carolina and Georgia. Coefficients of genetic similarity for paired geographical populations of P. floridanus ranged from 0.96 to 0.98, reflecting little geographical variation in allele frequencies. A comparison of P. floridanus with P. polionotus from five regions yielded coefficients of genetic similarity varying from 0.31 to 0.33.

Biochemical polymorphism and systematics in the genus Peromyscus. VI. The boylii species group.

An analysis of electrophoretic variation in proteins encoded by 21 genetic loci in 275 individuals belonging to the Peromyscus boylii species group yielded the following systematic conclusions. Populations of P. boylii rowleyi and P. b. levipes in four Mexican states and four states in the southwestern United States, separated by up to 3000 kilometers, share common alleles at virtually all loci, and thus show no evidence of representing more than one species. P. (b.) attwateri from Arkansas differs from P. boylii in allelic composition at several loci, and in all probability represents a distinct species, as suggested by other authors on the basis of morphology and karyotype. P. stephani from the Gulf of California is very similar genically to P. boylii, and, on the basis of this and other evidence, should be removed from the subgenus Haplomylomys and placed in the boylii species group of the subgenus Peromyscus. Populations referable to P. pectoralis from Ciudad Victoria, Mexico, Ranger, Texas, and Big Bend, Texas, show considerable allelic differences from one another but form a single cluster in a dendrogram of biochemical similarities. The nature of the allelic differences suggests that two or more species currently are classified as P. pectoralis.

Age-Dependent Allozymic Variation in a Natural Population of Lizards

An analysis of allozymic variation at 17 loci in a population of the sagebrush lizard (Sceloporus graciosus) in southern Utah yielded an estimate of genic heterozygosity of 0.028. Seven of the loci were variable, but only one, Est-1, was strongly polymorphic. The observation that the frequency of the common genotype (MM) at the Est-1 locus declined monotonically in successive age classes from 0.74 in hatchlings to 0.58 in adults 4 years old or older suggests that allele frequencies at this locus are not independent of selective influences.

The incubation patch of the house sparrow, Passer domesticus Linnaeus.

Development of the incubation patch in wild female house sparrows involves loss of down feathers from the ventral apterium of the breast and abdomen, a 5.2-fold increase in thickness of the epidermis of the integument, a 7.2-fold increase in number of dermal blood vessels, a 5.9-fold increase in diameter of the larger vessels, a 3.8-fold increase in nonfat dry weight of the integument, and a 12.4-fold increase in weight of water in the integument. Approximately 74% of the total increase in water content occurs in a period of three or four days in the early part of the incubation period; and, in the fully developed patch, water accounts for 85% of the wet weight of the integument, which shows a 9.0-fold increase over the non-breeding condition. House sparrows given continuous estradiol treatment showed complete defeathering but only slight increases in integumentary growth and edema. The addition of prolactin (50 IU injected on alternate days) augmented the effect of estradiol, producing more rapid defeathering and significantly greater dermal vascularity and edema. The addition of progesterone to estradiol increased epidermal thickness and edema but had no effect on dermal vascularity. Patches induced by treatment with exogenous hormones were not fully developed, being at best equivalent in weight and histology to those of wild females in the laying period.