EFFECTS ON POPULATION DIVERGENCE OF WITHIN‐GENERATIONAL LEARNING ABOUT PROSPECTIVE MATES

Although learned mate preferences are suspected to have important effects during speciation, theoretical models have largely neglected the effects on speciation and population divergence of within‐generational learning, that is, learning based upon prior experience with potential mates. Here, we use population genetic models to address this deficit. Focusing on the situation of secondary contact between populations that still hybridize, we consider models of learning by females and by males under polygyny. We assess the effects of learning to prefer conspecifics from previous conspecific encounters, learning to avoid heterospecifics from previous heterospecific encounters, and learning to prefer familiar types. We examine the amount of population divergence that results from learning in these models. We also assess the effect of learning on the spread of an allele that strengthens assortative mating in both models. We find that learning can have counterintuitive, but logical and understandable effects that differ with the version of the model assessed. In general, population divergence is expected to increase most consistently when females learn to strengthen their preferences for conspecifics from previous encounters with conspecifics. Our results also suggest that within‐generational learning will generally inhibit the spread of alleles strengthening assortative mating.     

THE RATE TEST OF SPECIATION: ESTIMATING THE LIKELIHOOD OF NON‐ALLOPATRIC SPECIATION FROM REPRODUCTIVE ISOLATION RATES IN DROSOPHILA

Among the most debated subjects in speciation is the question of its mode. Although allopatric (geographical) speciation is assumed the null model, the importance of parapatric and sympatric speciation is extremely difficult to assess and remains controversial. Here I develop a novel approach to distinguish these modes of speciation by studying the evolution of reproductive isolation (RI) among taxa. I focus on the Drosophila genus, for which measures of RI are known. First, I incorporate RI into age‐range correlations. Plots show that almost all cases of weak RI are between allopatric taxa whereas sympatric taxa have strong RI. This either implies that most reproductive isolation (RI) was initiated in allopatry or that RI evolves too rapidly in sympatry to be captured at incipient stages. To distinguish between these explanations, I develop a new “rate test of speciation” that estimates the likelihood of non‐allopatric speciation given the distribution of RI rates in allopatry versus sympatry. Most sympatric taxa were found to have likely initiated RI in allopatry. However, two putative candidate species pairs for non‐allopatric speciation were identified (5% of known Drosophila). In total, this study shows how using RI measures can greatly inform us about the geographical mode of speciation in nature.     

PARALLEL EVOLUTION OF LAKE-STREAM PAIRS OF THREESPINE STICKLEBACKS (GASTEROSTEUS) INFERRED FROM MITOCHONDRIAL DNA VARIATION

Three drainage systems in British Columbia, Canada, contain divergent parapatric lake-stream pairs of threespine sticklebacks (Gasterosteus aculeatus): Drizzle and Mayer Lakes on Graham Island, Queen Charlotte Islands, and Misty Lake on northeastern Vancouver Island. Ecological and morphological differences between members of all three lake-stream pairs are strikingly similar; lake fish are melanistic and slim bodied with smaller mouths and more gill rakers than the mottled-brown and robust-bodied stream sticklebacks. We estimated the level of genetic divergence between lake and stream fish in Misty Lake and tested hypotheses of single versus multiple origins of the pairs by assaying mitochondrial DNA (mtDNA) restriction site variation in samples from the three lake systems. MtDNA analysis revealed the existence of two highly divergent lineages differing by 2.7% in sequence. One lineage predominated in Misty stream fish (73%), whereas the other lineage predominated in Misty Lake samples (96%). Comparable forms (lake or stream) in the different lakes did not cluster together in terms of mtDNA nucleotide divergence, suggesting that the pairs have had independent origins. We concluded that: (1) divergent mtDNA lineages in North Pacific sticklebacks stem from historical isolation in the two major glacial refugia proposed for the North Pacific (Beringia and Cascadia); (2) the stream and lake pair in Misty Lake are distinct gene pools; (3) the divergence between parapatric lake and stream Gasterosteus represents parallel evolution having occurred at least twice in the North Pacific; and (4) different scales of evolutionary divergence exist in North Pacific Gasterosteus, that is, a relatively ancient divergence of mtDNA clades as well as recent (i.e., postglacial) divergence of ecotypes within major clades.     

HYBRIDIZATION,NATURAL SELECTION,AND EVOLUTION OF REPRODUCTIVE ISOLATION: A 25‐YEARS SURVEY OF AN ARTIFICIAL SYMPATRIC AREA BETWEEN TWO MOSQUITO SIBLING SPECIES OF THE Aedes mariae COMPLEX

Natural selection can act against maladaptive hybridization between co‐occurring divergent populations leading to evolution of reproductive isolation among them. A critical unanswered question about this process that provides a basis for the theory of speciation by reinforcement, is whether natural selection can cause hybridization rates to evolve to zero. Here, we investigated this issue in two sibling mosquitoes species, Aedes mariae and Aedes zammitii, that show postmating reproductive isolation (F1 males sterile) and partial premating isolation (different height of mating swarms) that could be reinforced by natural selection against hybridization. In 1986, we created an artificial sympatric area between the two species and sampled about 20,000 individuals over the following 25 years. Between 1986 and 2011, the composition of mating swarms and the hybridization rate between the two species were investigated across time in the sympatric area. Our results showed that A. mariae and A. zammitii have not completed reproductive isolation since their first contact in the artificial sympatric area. We have discussed the relative role of factors such as time of contact, gene flow, strength of natural selection, and biological mechanisms causing prezygotic isolation to explain the observed results.     

SPECIES ISOLATION,GENITAL MECHANICS,AND THE EVOLUTION OF SPECIES-SPECIFIC GENITALIA IN THREE SPECIES OF MACRODACTYLUS BEETLES (COLEOPTERA,SCARABEIDAE, MELOLONTHINAE)

The question asked was why male genitalic structures have diverged in three syntopic species of Macrodactylus beetles. Four hypotheses were evaluated: 1. The ways in which male genitalia mesh with internal female structures indicate that selection for species isolation via mechanical exclusion (“lock and key”) is unlikely to explain the genitalic differences. 2. The specific mate recognition hypothesis also clearly fails to explain genitalic differences due to the implausibility of postulated environmental effects on genitalia, and lack of postulated coevolution of male and female morphologies. 3. Selection for species isolation via differences in genitalic stimulation (sensory lock and key) is unlikely due to relatively infrequent cross-specific pair formation and intromission in the field, and “excessive” numbers of species-specific genitalic structures and male courtship behavior patterns which nevertheless occasionally fail. It also fails to explain the frequent failure of intraspecific copulations to result in sperm transfer. This hypothesis cannot, however, be rejected as confidently as the previous hypotheses. 4. Conditions under which sexual selection by cryptic female choice could take place are common. Females frequently exercise their ability to prevent sperm transfer by conspecific males even after intromission has occurred, and females generally mate repeatedly, probably with different males. Males behave as if cryptic female choice is occurring, courting assiduously while their genitalia are within the female. Sexual selection by female choice could thus contribute to the divergence in genitalic structures.     

IDENTIFICATION OF THE MINUS MATING‐TYPE SPECIFIC GENE MTD1 FROM GONIUM PECTORALE (VOLVOCALES,CHLOROPHYTA)1

Gonium pectorale O. F. Müll. (Volvocales, Chlorophyta), a colonial 8‐ or 16‐cellular alga, is phylogenetically important as an intermediate form between isogametic unicellular Chlamydomonas and oogamous Volvox. We identified the mating‐type specific gene GpMTD1, from G. pectorale, the first homologue of Chlamydomonas reinhardtii MTD1 (CrMTD1). The GpMTD1 gene was found to be present only in the minus mating‐type locus and was expressed specifically in the gametic phase as is the case for CrMTD1, suggested to participate in development of the minus gametes. This gene is useful as a probe in analyzing the bacterial artificial chromosome (BAC) library for resolving genomic structures of the mating‐type loci in isogamous and oogamous colonial volvocaleans.     

A SKULL OF THE GIANT BONY‐TOOTHED BIRD DASORNIS (AVES: PELAGORNITHIDAE) FROM THE LOWER EOCENE OF THE ISLE OF SHEPPEY

Abstract: The first substantial skull of a very large Paleogene bony‐toothed bird (Pelagornithidae) is described from the Lower Eocene London Clay of the Isle of Sheppey in England. The specimen is assigned to Dasornis emuinus (Bowerbank), based on a taxonomic revision of the large London Clay Pelagornithidae. Very large bony‐toothed birds from the London Clay were known previously from fragmentary remains of non‐comparable skeletal elements only, and Dasornis londinensis Owen, Argillornis emuinus (Bowerbank), A. longipennis Owen, and Neptuniavis miranda Harrison and Walker are considered junior synonyms of D. emuinus. The new specimen allows a definitive assignment of Dasornis to the Pelagornithidae and documents that this taxon closely resembles other bony‐toothed birds in cranial morphology. It is hypothesized that giant size (i.e. a wingspan above 4 m) evolved only once within Pelagornithidae and that Dasornis emuinus is the sister taxon of the giant Neogene bony‐toothed birds, which share a derived wing morphology.