A literal reading of the fossil record suggests that the animal phyla diverged in an "explosion" near the beginning of the Cambrian period. Calibrated rates of molecular sequence divergence were used to test this hypothesis. Seven independent data sets suggest that invertebrates diverged from chordates about a billion years ago, about twice as long ago as the Cambrian. Protostomes apparently diverged from chordates well before echinoderms, which suggests a prolonged radiation of animal phyla. These conclusions apply specifically to divergence times among phyla; the morphological features that characterize modern animal body plans, such as skeletons and coeloms, may have evolved later.Darwin (1) recognized that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection. He suggested that fossils might eventually be found documenting a protracted unfolding of Precambrian metazoan evolution. Many paleontologists today interpret the absence of Precambrian animal fossils that can be assigned to extant clades not as a preservational artifact, but as evidence of a Cambrian or late Vendian origin and divergence of metazoan phyla (2-6). This would make the Cambrian the greatest evolutionary cornucopia in the history of the Earth. Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla (6). Recent geochronological studies have reinforced the impression of a "big bang of animal evolution" by narrowing the temporal window of apparent divergences to just a few million years (4).The evidence for a Cambrian explosion of animal phyla is based on the absence of fossils of triploblastic metazoans from rocks predating the Cambrian. This negative evidence is not entirely convincing. Tiny unskeletonized animals with no possibility of preservation in the fossil record may have existed before the Cambrian (7, 8). Even if larger, soft-bodied animals were present, conditions appropriate for their preservation may not have existed for much of the The authors are in the . In particular, the famous Lagerstatten of the Cambrian (8, 10) resulted from taphonomic conditions that are exceptionally rare at other times in the rock record (9). Nevertheless, some Vendian trace fossils and body fossils suggest that animals with coeloms existed before the Cambrian (6, 8, 11, 12). Calibrated rates of gene sequence divergence provide another avenue for dating divergence times between animal phyla (13). An early study by Runnegar, based on hemoglobin, suggested Precambrian divergences (14) but was criticized for not testing assumptions of rate constancy (15). A more recent study based on 18S ribosomal RNA (rRNA) sought evidence of rapid divergences in the inability of sequence data to resolve phylogenetic relationships (16). The burgeoning database of gene sequences provides an opportunity to examine the divergence times of metazoan phyla from large data sets based on severa...

No abstract

No abstract

No abstract

Abstract Golden-winged Warblers (Vermivora chrysoptera) and Blue-winged Warblers (V. pinus) are small, brightly colored Neotropical migrant birds that breed in eastern North America. Wherever the two species occur together, they hybridize to a limited degree, producing distinctive hybrid phenotypes. In recent decades, chrysoptera has experienced dramatic population declines across much of its range. Those declines have often been correlated with establishment and increase of pinus in the same areas, but it remains uncertain what, if any, role pinus has played in driving the decline of chrysoptera. In a first attempt at molecular genetic analysis of chrysoptera-pinus population dynamics, Gill (1997) reported cryptic, completely asymmetric, and possibly very rapid introgression of pinus mitochondrial DNA (mtDNA) into chrysoptera, causing what he termed “local cytonuclear extinction” of chrysoptera. As Gill (1997) noted, however, those results were based on relatively small samples from a single area in Pennsylvania. To begin to investigate the generality of Gill’s findings and to establish a baseline for long-term genetic and ecological studies, we intensively sampled one new study area (in southern West Virginia) and also sampled more broadly across two other areas (in Michigan and Ohio) that have experienced pinus invasions and chrysoptera declines. In southern West Virginia, introgression of mtDNA appeared to be roughly symmetrical: 15% (11 of 72) of pinus phenotypes possessed chrysoptera mtDNA, and 12% (17 of 137) of chrysoptera phenotypes possessed pinus mtDNA. Results from much smaller samples from Michigan and Ohio also failed to show any evidence of asymmetric mitochondrial introgression. The results we report here, based on mtDNA and plumage phenotype information for 337 birds representing much of the range of the two species, indicate that previous genetic results and inferences from Pennsylvania may not be broadly applicable to the many areas of contact between chrysoptera and pinus in eastern North America.

The use of bowl traps to sample bee diversity has become common in recent years. To provide guidance in optimizing bee bowl sampling effort, we used several independent datasets from North America (Pacific Northwest golf courses, suburban Maryland, and Chihuahuan Desert) to estimate the number of bowls in a sampling event at which additional bowls added little to estimates of species richness or to cumulative species lists. We examined changes in the value of a nonparametric richness estimator (Chao2) as a function of bowl number by randomly resampling (without replacement) the data for each sampled transect. We also carried out rarefaction (interpolation of expected species richness for all numbers of bowls up to the actual number). Using as our criterion of ''optimal'' number of bowl samples per transect the number of samples at which five additional bowls added \1 additional species to the Chao2 estimate, we find that 30-bowl transects would have met this standard for nearly three quarters or more of the sampling events in any of our three datasets (for the suburban Maryland and Chihuahuan Desert data sets, at least three quarters of sampling events met this criterion with just 20 and 25 bowls, respectively). Results from rarefaction analysis for all sampling events indicate that 30-bowl transects would have met our ''five additional bowls'' standard for more than 80 % of our suburban Maryland and Chihuahuan Desert sampling events, but for only about 61 % of our Pacific Northwest sampling events. For the Pacific Northwest sampling events, there was notable among-site heterogeneity in required sampling intensity. Thus, although 30-bowl transects appear to be adequate for assessing local bee species richness in most instances, investigators may wish to increase (or decrease) this number based on their particular goals and specific knowledge about local bee diversity.

ABSTRACT.• Golden-winged Warblers (Vermivora chrysoptera) and Blue-winged Warblers (V: pinus) are small, brightly colored Neotropical migrant birds that breed in eastern North America. Wherever the two species occur together, they hybridize to a limited degree, producing distinctive hybrid phenotypes. In recent decades, chrysoptera has experienced dramatic population declines across much of its range. Those declines have often been correlated with establishment and increase of pinus in the same areas, but it remains uncertain what, if any, role pinus has played in driving the decline of chrysoptera. In a first attempt at molecular genetic analysis of chrysoptera-pinus population dynamics. Gill (1997) reported cryptic, completely asymmetric, and possibly very rapid introgression of pinus mitochondrial DNA (mtDNA) into chrysoptera, causing what he termed "local cytonuclear extinction" of chrysoptera. As Gill (1997) noted, however, those results were based on relatively small samples from a single area in Pennsylvania. To begin to investigate the generality of Gill's findings and to establish a baseline for long-term genetic and ecological studies, we intensively sampled one new study area (in southern West Virginia) and also sampled more broadly across two other areas (in Michigan and Ohio) that have experienced pinus invasions and chrysoptera declines. In southern West Virginia, introgression of mtDNA appeared to be roughly symmetrical: 15% (11 of 72) of pinus phenotypes possessed chrysoptera mtDNA, and 12% (17 of 137) of chrysoptera phenotypes possessed pinus mtDNA. Results from much smaller samples from Michigan and Ohio also failed to show any evidence of asymmetric mitochondrial introgression. The results we report here, based on mtDNA and plumage phenotype information for 337 birds representing much of the range of the two species, indicate that previous genetic results and inferences from Pennsylvania may not be broadly applicable to the many areas of contact between chrysoptera and pinus in eastern North America.

No abstract