STABILITY OF THE NORTHERN FLICKER HYBRID ZONE IN HISTORICAL TIMES: IMPLICATIONS FOR ADAPTIVE SPECIATION THEORY

The hybrid zone between the Red- and Yellow-shafted Flickers has been stable on the United States Great Plains in historical times. This conclusion is based on multivariate comparisons of historical and contemporary collections from 18 locales. Adaptive speciation theory predicts that the hybrid zone should either become broader or narrower as a result of introgressive hybridization or reinforcement of premating isolating mechanisms. Neither of these predictions was borne out. Despite 10,000-13,000 years of hybridization, mating between subspecies remains indiscriminate. The data are also inconsistent with a dynamicequilibrium hypothesis wherein narrow hybrid zones are maintained by hybrid unfitness. According to this hypothesis, the hybrid zone would probably “flow” unless it was trapped by a population density trough. The hybrid zone does not appear to be associated with such a feature. The data are consistent with a bounded hybrid superiority theory of a hybrid zone, but this is more a question of survival in a process of elimination than a resounding corroboration.     

A MODEL FOR THE EVOLUTION OF ASYMMETRICAL MALE HYBRID STERILITY AND ITS IMPLICATIONS FOR SPECIATION

Chromosomal analysis of several cases of asymmetrical male hybrid sterility in Drosophila has implicated the X- or the Y-chromosome and one or more autosomes. Here, I develop a model for the evolution of this phenomenon. An autosomal locus is assumed to affect viability and to interact with a Y-linked or an X-linked locus to determine male fertility. In a new environment, selection for viability favors a new allele at the autosomal locus, but incompatibility of this new allele with the sex-chromosome-linked gene generates male sterility. The incompatibility can be resolved if a new allele at the sex-linked locus invades the population. This results in nonreciprocal male hybrid sterility, the direction of the nonreciprocity being determined by the dominance or recessiveness of the new autosomal gene in its effect on fertility. It is shown that stable polymorphism for the autosomal locus is possible and that, if the equilibrium frequency of the new allele is above a critical value, the population will be constantly at the verge of speciation, “waiting” for the sex-linked mutation to occur. The appearance of this mutation causes a runaway process leading to rapid fixation of the new autosomal and sex-linked alleles. If the equilibrium frequency of the new autosomal allele is less than the critical value, deterministic speciation is impossible, but random drift may increase the frequency above the critical value and predispose the population to the invasion of the new sex-linked allele. Thus, both deterministic and stochastic modes of speciation are possible. Because deterministic speciation requires large selection coefficients, which impose a severe genetic load on the population, and because stochastic speciation requires repeated population bottlenecks, it is concluded that relative to the number of successful speciation events there will be many more events of deme extinction.     

THE BIRDS OF TASMANIA: ECOLOGY AND EVOLUTION

SUMMARY Tasmania, about the size of Ireland, separated from the Australian mainland by 140 miles of sea, and isolated for about the last 12,000 years, has 104 species of native breeding land-birds, with the addition of ten introduced species. The environment is described; in particular the vegetation is classified into nine natural types and three produced by European man; and the distribution of the bird species among these is defined and discussed. The other vertebrates are briefly considered. No extinction is definitely known to have taken place as a result of European settlement except of the Tasmanian Emu (and of course Tasmanian Man). An attempt is made to reconstruct the Late Pleistocene history of Tasmania and its vegetation, with special reference to the Last Glaciation, when the island would have been joined to the mainland. When the avifauna is divided into categories, water-birds, raptors, etc., it is found to have much the same proportional composition as the Australian mainland avifaunas with which it is compared, though it consists of many fewer species. The vegetation types of the colder and wetter areas of Tasmania house far fewer species of birds than the drier and warmer habitats. The 104 breeding species include 14 endemics, which are considered in detail, and 27 endemic subspecies. As shown by comparison with other islands, the total proportionate endemism is extraordinarily high for a recent continental island (though it is actually lower than that in the remote ecological island formed by the sclerophyll of southwestern Western Australia). A contributory cause may be that Tasmania is not regularly visited by land-birds from the continent (though at least one-fifth of the Tasmanian species are partial or total migrants in winter). The most noteworthy endemics are two monotypic genera, Lathamus and Acanthornis, and the Native Hen Tribonyx mortierii, which has become flightless apparently in the face of a formidable array of local predators. Considerations of climate and habitat suggest that at least half the avifauna, including 19 of the endemic subspecies and six of the members of superspecies, arrived in Tasmania some time after the amelioration of the Last Glaciation began, some 18,000 years ago. Geographical considerations suggest that four of those six members of superspecies existed in their present form when the land-bridge to the mainland was cut, 12,000 years ago. Certain habitats, widespread over southeastern Australia during the glaciation, are now virtually confined to Tasmania where they form the stronghold of certain species, such as the Pink Robin Petroica rodinogaster, which occur only as relicts on the mainland. The endemic Scrub Tit Acanthornis magnus is practically confined to such habitats, where its ecology suggests it would have been well adapted to glacial conditions. Among the local endemics there is a strong tendency for colouration to be more saturated than on the continent of Australia (Gloger's rule) but there is less consistency in tendency to greater size (Bergmann's rule). A noteworthy proportion of the Tasmanian species have duller plumage than their mainland relatives, usually with an assimilation to female or juvenile plumage—unexpected in an island as big as Tasmania with so considerable an avifauna.     

MITOCHONDRIAL INHERITANCE PATTERNS ACROSS A COTTONWOOD HYBRID ZONE: CYTONUCLEAR DISEQUILIBRIA AND HYBRID ZONE DYNAMICS

In this study we examine the cytoplasmic inheritance patterns of an interspecific hybridizing population of Fremont and narrowleaf cottonwoods, using mitochondrial DNA. Three mitochondrial probes showing polymorphisms were used to distinguish between trees of known nuclear inheritance. Every tree screened had only one cytoplasmic genotype, either Fremont or narrowleaf. Thus, these results demonstrate that mitochondria are uniparentally inherited in these trees. Previous studies of the nuclear inheritance of this interspecific hybridizing population of cottonwood trees indicated an asymmetry in the frequency of parental genes. Using mitochondrial markers we tested one hypothesis potentially responsible for this asymmetric distribution (i.e., trees of mixed genotypes will be sterile or will not survive if their cytoplasm is derived from one or the other parent). Our results, however, show that both Fremont and narrowleaf mitochondrial markers are found in trees with mixed nuclear genotypes. Thus, nuclear-cytoplasmic incompatibilities do not appear to account for the asymmetric distribution of nuclear genotypes within the hybrid swarm. An alternative explanation for the observed asymmetric distribution of nuclear genotypes is advanced. Although nuclear-cytoplasmic incompatibilities do not appear to explain the asymmetric distribution of nuclear alleles within the hybrid zone, nonrandom associations between nuclear and cytoplasmic genotypes do exist. For example, all F₁ hybrids had Fremont mitochondrial genotypes. Furthermore, backcrosses between F₁ hybrid and narrowleaf trees have a higher than expected proportion of heterozygous loci and a higher than expected proportion of Fremont mitochondria. We propose that seeds, seedlings, or trees with high proportions of heterozygous loci are at a disadvantage unless they also have the Fremont mitochondrial genotype. While it is generally difficult to infer dynamic processes from static patterns, studies such as ours enable one to gain new insights to the dynamics of plant hybrid zones. A hybridization pattern of decreasingly complex backcrosses as one proceeds from higher to lower elevation within the hybrid swarm, a residue of Fremont cytoplasmic DNA within the pure narrowleaf population, and the unidirectional nature of these crosses suggest that the narrowleaf population may be spreading down the canyon and the Fremont population receding. The eventual fate of the hybrid zone, in relation to these processes, is discussed.     

MALE MATE CHOICE AND THE EVOLUTION OF FEMALE PLUMAGE COLORATION IN THE HOUSE FINCH

The house finch (Carpodacus mexicanus) is a sexually dichromatic passerine in which males display colorful plumage and females are generally drab brown. Some females, however, have a subdued version of the same pattern of ornamental coloration seen in males. In previous research, I found that female house finches use male coloration as an important criterion when choosing mates and that the plumage brightness of males is a reliable indicator of male nest attentiveness. Male house finches invest substantially in the care of young and, like females, stand to gain by choosing high-quality mates. I therefore hypothesized that a female's plumage brightness might be correlated with her quality and be the basis for male mate choice. In laboratory mate choice experiments, male house finches showed a significant preference for the most brightly plumaged females presented. Observations of a wild population of house finches, however, suggest that female age is the primary criterion in male choice and that female plumage coloration is a secondary criterion. In addition, yearling females tended to have more brightly colored plumage than older females, and there was no relationship between female plumage coloration and overwinter survival, reproductive success, or condition. These observations fail to support the idea that female plumage coloration is an indicator of individual quality. Male mate choice for brightly plumaged females may have evolved as a correlated response to selection on females to choose brightly colored males.     

DISPERSAL AND PLANT MATING SYSTEMS: THE EVOLUTION OF SELF-FERTILIZATION IN SUBDIVIDED POPULATIONS

Intermediate rates of self-fertilization can be evolutionarily stable when the progeny of self-fertilization events are less successful migrants than those of outcrossing events, unless self-fertilization reduces an individual's contribution to the pollen pool by an amount equal to the rate at which it self-fertilizes. This result holds regardless of whether pollen or diaspores are more widely dispersed. The differential migration of selfed and outcrossed progeny may be a result of differential establishment with comparable rates of dispersal, or it may be a result of differential dispersal rates. In the first case, detailed predictions concerning the evolutionarily stable selfing rate can be made. In the second case, only qualitative predictions are possible in the absence of specific assumptions about how the migration rate is affected by the average selfing rate in each subpopulation.     

VARIATION IN MATING CALL ACROSS THE HYBRID ZONE BETWEEN THE FIRE-BELLIED TOADS BOMBINA BOMBINA AND B. VARIEGATA

Three components of mating call (pulse duration, cycle length, and fundamental frequency) were measured and six diagnostic enzyme loci scored across the hybrid zone between the toads Bombina bombina and B. variegata. All three call components differ significantly, but only cycle length is diagnostic. The clines in call coincide with those for enzymes, and have similar widths. This suggests that there is no strong selection on any of these characters. There are significant correlations between electrophoretic markers and call components, but these are no stronger than would be expected if the electrophoretic loci and the genes causing mating call were neutral. The selection differential on the call is no greater than 6% of the difference in mean cycle length between the two taxa. There is a substantial increase in the variance of cycle length in the center of the zone, suggesting that a small number of loci are involved (≈ three). Recombination between these loci will hinder the evolution of reinforcement and may partly be responsible for the lack of premating isolation between B. bombina and B. variegata.     

STRUCTURE AND DYNAMICS OF HARDWOOD SWAMPS IN THE NEW JERSEY PINE BARRENS: CONTRASTING PATTERNS IN TREES AND SHRUBS

The biomass, density, age structure, and dominance patterns of the tree and shrub populations in hardwood swamps of the New Jersey Pine Barrens are described and contrasted with each other. The total biomass of individual swamps ranges from 92,000 kg/ha to 194,000 kg/ha, depending on the age of the stand. These biomass values are similar to those reported for southern hardwood swamps. The Pine Barrens swamps are distinguished by very large populations of ericaceous and other shrubs, which range up to 13% of the total above-ground biomass. The tree populations, composed principally of Acer rubrum, Nyssa sylvatica and Magnolia virginiana, are even-aged; Acer is strongly dominating, with a minimum of 73% of the biomass, and species richness is associated with the availability of different microhabitats. The shrub populations, by contrast, are all-aged, with the dominance patterns among the species and the stand biomasses varying with the degree of flooding of the stand. Differences in size and growth rate among the shrub species parallel their evident response to the flooding gradient. The shrub stratum has lower species richness than the comparable southern swamps and floodplains, but ranges up to higher biomass values.     

PATTERNS AND PROCESSES OF DIVERSIFICATION: SPECIATION AND HISTORICAL CONGRUENCE IN SOME NEOTROPICAL BIRDS

This paper documents congruence in geographical patterns of speciation for four clades of birds having taxa endemic to the same areas within the Neotropics. Two genera, Pionopsitta parrots and Selenidera toucans, corroborate a well known biogeographic disjunction in which taxa endemic to southern Central America and the Chocó region of northwestern South America are the sister-group to a radiation within the Amazon basin. These two genera, along with two lineages within the toucan genus Pteroglossus, also document a pattern of historical interrelationships for four well known areas of endemism within Amazonia: Guyanan + (Belém-Pará + (Inambari + Napo)). These generalized historical patterns are interpreted to have arisen via fragmentation (vicariance) of a widespread ancestral biota. A review of the paleogeographic evidence suggests that these vicariance events could have originated as a result of several different mechanisms operating at various times during the Cenozoic. The inference that diversification of the Neotropical biota is primarily the result of the most recent of these possible vicariance events, namely isolation within Quaternary forest refugia, is unwarranted, given present data. These patterns of historical congruence are also interpreted as direct evidence against the hypothesis that diversification of the forest biota was a consequence of parapatric differentiation along recently established ecological gradients.     

EVOLUTION OF ALTRUISM IN KIN-STRUCTURED AND RANDOM SUBDIVIDED POPULATIONS

A. population structure favorable to the evolution of an altruistic trait is studied by Monte Carlo simulation. The model is based on a small-scale nonindustrial human society but seems generalizable to other highly social mammals. Three hierarchical levels are recognized: 1) the ecologically isolated local group (hamlet) which may be composed of kin and/or unrelated individuals; 2) the deme (settlement) comprising several such groups which interbreed; and 3) the set of demes (metapopulation) among which gene flow occurs. The first two levels of the model are based on D. S. Wilson's structured deme concept; the third allows for gene flow among demes in the metapopulation and for the structured diffusion of alleles across a wider area than might be included within the scope of a single deme. The simulation models genetic drift by a process of hamlet formation which may be random, or variously kin-structured. Hamlets may then become extinct based on a probability function of their gene frequencies. Individual selection within settlements is modeled deterministically, and gene flow among settlements is modeled as two-dimensional steppingstone migration of random or kin-structured groups. Results of the simulations show that, with realistic values for group sizes, moderate extinction rate, and high rates of migration (m > 27%), disadvantageous alleles (s = 10% and 25%) may increase markedly due to differential hamlet extinction over the course of 50 generations. The greater the degree of kin-structuring of founder groups, the higher the variance among hamlets and the faster the rate of increase of the allele for altruism. Nonetheless, even in some randomly founded groups, a clear increase in the altruism gene frequency occurred. It is also notable that kin-structured group selection by hamlet extinction may be effective when the initial frequency of altruism genes is very low (average of one per deme) and among a relatively small number of demes (25). Thus the process of group extinction in a hierarchically structured population allows rapid increase of an allele for altruism under plausible demographic conditions.     

DEVELOPMENTAL CORRELATES OF GENOME SIZE IN PLETHODONTID SALAMANDERS AND THEIR IMPLICATIONS FOR GENOME EVOLUTION

We present an analysis of the evolutionary relationship between genome size (C-value, mass of DNA per haploid nucleus) and developmental rate using observations of limb regeneration in salamanders of the family Plethodontidae. Rates of growth and differentiation of regenerating limbs are reported for 27 plethodontid species whose C-values range from 14 to 76 picograms. A phylogenetic analysis employing Felsenstein's method of independent contrasts indicates that rate of differentiation is inversely proportional to genome size, although we have not identified any statistically significant association between genome size and the growth rate of regenerating tissue. Our results are consistent with an interpretation that genome size may place a limit on the maximum rate of regeneration attainable in plethodontid salamanders. The implications of our findings for the “junk DNA,” “nucleotypic DNA,” “selfish DNA,” and “skeletal DNA” hypotheses of genome evolution are discussed.