摘要:

Despite a diversity of larval forms, remarkably conservative features of asteroid development define a larval body plan that occurs throughout the class. However, recent work on the starfishPteraster tesselatushas documented a highly derived pattern of development. Several features, including radial symmetry, parallel embryonic and adult axes of symmetry, absence of a preoral lobe, and formation of coeloms in the adult orientation from seven separate enterocoels, have not been reported in asteroids before. The complete absence of the larval body plan features that are found in other asteroids, indicates thatP. tesselatusdevelops directly from the embryo to the juvenile and has a pelagic, nonfeeding (lecithotrophic), but nonlarval mode of development. I postulate that direct development evolved over an extended period in a lineage of brooding, deep‐sea velatid (probably pterastcrid) ancestors ofP. tesselatus.Selection for increased developmental efficiency (loss of nonfunctional larval features) in the brooded offspring, could explain the lack of larval settlement structures, the nonlarval arrangement of coeloms, the lack of a preoral lobe, the transverse orientation of the juvenile disc, and the lack of bilateral symmetry. The pattern of coclomogenesis could have been derived from that of other velatids (e.g. solasterids) by relatively simple changes in timing and orientation of entcroeoel formation. Rotation and posterior translation of the coelomic fate map of the archenteron prior to enlerocoel formation would produce the coelomic compartments in the adult orientation that characterizes direct development inP. tesselatus.These unusual developmental features lead to a radically different interpretation for the evolution of the pelagic ‘larva’ ofP. tesselatus:(1) evolution of benthie brooding, (2) extreme simplification of development involving the loss of all larval features from the life cycle, and (3) subsequent re‐evolution of pelagic development. In the case ofP. tesselatus, where all larval structures were lost, there do not seem to be functional constraints preventing the re‐evolution of pelagic development. Analysis of pelagic and benthie larvae, in other asteroids, suggests that major ecological transitions in life historiesneednot be associated with substantia] changes in morphology. The loss of pelagic development should have occurred repeatedly and should be readily reversible. These findings have interesting implications for the loss and evolution of pelagic dispersal in the life histories of marine benthie inve

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1995.tb01040.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

We used hierarchical and pairwiseF‐statistics to describe genetic differentiation and infer gene flow (M) on local and regional scales within and among parapatric European butterfly taxa in thePieris napi(L.) group. Within‐population allozyme variability is consistently high, and local effective population sizes are inferred to be in the thousands of individuals. The pairwise analysis yields an average neighbourhood area of radius 3.5 km. Among populations within most regions, differentiation is low andM>2 effective individuals population‐1generation‐1. Pairwise comparisons within thebrilannicagroup show a disjunction indicating that it is out of equilibrium, perhaps as a result of secondary contact between highland and lowland groups. Comparison betweenmeridionalisgroups on mainland Italy and Corsica yieldsM>12; this is surely loo high and lack of equilibrium resulting from initial colonization is suspected. The hierarchical analysis indicates that 23 ≤0020M≤ 88 among the taxanapi, bryoniaeandmeridionalisthat meet in hybrid zones; no effective gene flow barrier exists among them. This high estimate could also result from recent primary contact, but such a genetic barrier should produce the ‘edge effects' seen in population genetic simulations, and no evidence of this was found among geographically close samples ofnapiandbryoniaepopulations from Switzerland. Studies of gene flow among geographic regions are greatly limited by the equilibrium assumption, though studies of local differentiation are much less so. Population studies of gene flow on local scales at regional boundaries provide limited means of testing the equilibrium assumption, and both regional and local analyses provide testable predictions about local population structure. When the equilibrium assumption is not upheld, local patterns at regional boundaries can provide historical information about primary vs. seco

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1995.tb01041.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The importance of body mass prediction from several cranial, dental and appendicular variables in living Equini are studied. Relationships between the body mass changes and the ecogeographic picture of Equini evolution are also analysed. The metapodial and phalanx variables, particularly antero‐posterior diameters, are better correlated with body mass than cranial variables in living Equini. Large sized species are correlated with cold climates, open habitats and/or soft soils; small ones are correlated with warm climates, more closed habitats and/or hard soils. Pleistocene horses from Europe and Africa follow an evolutionary trend opposite to their North American counterparts, from larger sized species to smaller ones. In South America the pattern of body size is different to those of the other continents. Species ofHippidionreaching large body mass, whereas some species ofEquus, E. andium, follow a diminishing tren

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1995.tb01042.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

In order to be elaborated by sexual selection, sexual ornaments must vary perceptibly and genetically among individuals in natural populations. Rather little is known about ornament variation in monogamous species, in which sexual selection should act more weakly than in polygynous species. We report phenotypic variation in feather ornament size (elongated tails and pectoral tufts) and body size in the scarlet‐tufted malachite sunbirdNectarinia johnstoni, a monogamous, sexually dimorphic nectarivore of East African alpine zones. Fully‐expressed male ornaments are highly significantly more variable (CVs = 12–29%) than are skeletal and wing measures primarily affected by natural selection (CVs = 2 4%). Female sunbirds have pectoral tufts which are significantly (22–25%) smaller than those of adult males, but more variable (CVs= 21–22%, CVs= 12–15%), and more variable than body size. Among males with fully‐grown ornaments, those with longer tails tend to have longer wings and wider tufts. The high variation in fully‐grown ornaments in malachite sunbirds is consistent with the view that the ornaments are condition‐dependent sexual signals. Finally, we review studies of feather ornament variation to date, and show that ornaments are much more variable in monogamous than non‐monogamous species, apparently due to the relatively weak pressure

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1995.tb01043.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Fishes in the genusXiphophorus(swordtails and platyfishes) are well known for the influence of the pituitary (P) locus on variation of male size at maturity both within and among species. We report the discovery of large male size (>29 mm SL) in several populations of the swordtailX. pygmaeus, a species previously thought to consist of only small males (<29 mm SL). Large size is geographically restricted, and average male size varies significantly by site and year sampled in a pattern suggesting a recent origin and slow spreading of the large male phenotype. However, large male size is not strongly paternally inherited in this species, as it is in its two closest relatives,X. nigrensisandX. multilineatus, showing that large size does not result from the same genetic (Plocus) mechanism. LargeX. pygmaeusmales do not court, can exhibit the gold morph, do not possess swords and have slender body shape. In these traits they resemble small conspecific males and small males ofX. nigrensisandX. multilineatusrather than large males of these latter two species. This shows that correlations between morphological and behavioural traits that occur inX. nigrensisandX. multilineatusare absent inX. pygmaeus.

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1995.tb01044.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY