Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

Evolution of Protein Molecules

Hybridization has many and varied impacts on the process of speciation. Hybridization may slow or reverse differentiation by allowing gene flow and recombination. It may accelerate speciation via adaptive introgression or cause near-instantaneous speciation by allopolyploidization. It may have multiple effects at different stages and in different spatial contexts within a single speciation event. We offer a perspective on the context and evolutionary significance of hybridization during speciation, highlighting issues of current interest and debate. In secondary contact zones, it is uncertain if barriers to gene flow will be strengthened or broken down due to recombination and gene flow. Theory and empirical evidence suggest the latter is more likely, except within and around strongly selected genomic regions. Hybridization may contribute to speciation through the formation of new hybrid taxa, whereas introgression of a few loci may promote adaptive divergence and so facilitate speciation. Gene regulatory networks, epigenetic effects and the evolution of selfish genetic material in the genome suggest that the Dobzhansky-Muller model of hybrid incompatibilities requires a broader interpretation. Finally, although the incidence of reinforcement remains uncertain, this and other interactions in areas of sympatry may have knock-on effects on speciation both within and outside regions of hybridization.

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Hybridization and speciation

Developmental Plasticity and Evolution. In: Mary Jane West-Eberhard, Editor, Oxford University Press (2003) Pp. xi+794. Price $50.00 paper.

In addition to considerable debate in the recent evolutionary literature about the limits of the Modern Synthesis of the 1930s and 1940s, there has also been theoretical and empirical interest in a variety of new and not so new concepts such as phenotypic plasticity, genetic assimilation and phenotypic accommodation. Here we consider examples of the arguments and counter-arguments that have shaped this discussion. We suggest that much of the controversy hinges on several misunderstandings, including unwarranted fears of a general attempt at overthrowing the Modern Synthesis paradigm, and some fundamental conceptual confusion about the proper roles of phenotypic plasticity and natural selection within evolutionary theory.

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Phenotypic plasticity and evolution by genetic assimilation

We need a new theory of mating systems … [one] that incorporates the conflicting interests of males and females, and the factors determining which sex is in control, in order to predict patterns of male-female pairing. Gross, 1994 Given that females, to one extent or another, subvert male interests by the internal manipulation of ejaculate, it is not inconceivable that males will have evolved little openers, snippers, levers and syringes that put sperm in the places females have evolved (‘intended’) for sperm with priority usage - collectively a veritable Swiss Army Knife of gadgetry! Lloyd, 1979 ABSTRACT The male–female interaction is an asymmetrical, usually obligate mutualism in which there are conflicts of interest whenever multiple potential partners that vary in quality are available for either sex. Understanding male–female confluences and conflicts of interest is required to explain the sexual sequence and how it evolves. Mating interactions involve multiple steps or stages, distinguishable because of differences arising out of changes in selection that occur during the sequence. Sexual selection and competition take several different forms, which must be understood before accurate interpretations can be made of mating events in any particular case. Sexual selection guided primarily by male–female conflicts of interest can result in resolvable evolutionary chases that lead to evolutionary stable strategies but perhaps more frequently lead to chases that tend to be unending (Parker 1979).

The Evolution of Mating Systems in Insects and Arachnids: Evolutionary perspectives on insect mating

Acoustic mate-attracting signals of related sympatric, synchronic species are always distinguishable, but those of related allopatric species sometimes are not, thus suggesting that such signals may evolve to ''reinforce'' premating species isolation when similar species become sympatric. This hypothesis predicts divergences restricted to regions of sympatry in partially overlapping species, but such ''reproductive character displacement'' has rarely been confirmed. We report such a case in the acoustic signals of a previously unrecognized 13-year periodical cicada species, Magicicada neotredecim, described here as a new species (see Appendix). Where M. neotredecim overlaps M. tredecim in the central United States, the dominant male call pitch (frequency) of M. neotredecim increases from approximately 1.4 kHz to 1.7 kHz, whereas that of M. tredecim remains comparatively stable. The average preferences of female M. neotredecim for call pitch show a similar geographic pattern, changing with the call pitch of conspecific males. Magicicada neotredecimdiffers from 13-year M. tredecim in abdomen coloration, mitochondrial DNA, and call pitch, but is not consistently distinguishable from 17-year M. septendecim; thus, like other Magicicada species, M. neotredecim appears most closely related to a geographically adjacent counterpart with the alternative life cycle. Speciation in Magicicada may be facilitated by life-cycle changes that create temporal isolation, and reinforcement could play a role by fostering divergence in premating signals prior to speciation. We present two theories of Magicicada speciation by life-cycle evolution: ''nurse-brood facilitation'' and ''life-cycle canalization.''

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Reproductive character displacement and speciation in periodical cicadas, with description of a new species, 13-year Magicicada neotredecim.

Periodical cicada ( Magicicada spp.) sexual pair formation takes place in dense aggregations and involves intense male-male competition for limited mating opportunities. Pair-forming behavior in these species has been poorly understood because of limited knowledge of sexual communication. We have found that sexually receptive female Magicicada flick their wings in timed response to an individual chorusing male; this previously unrecognized female response is hereafter referred to as a 'wing-flick' signal. We document the nature, timing, and species-specificity of this signal as well as its absence in both immature and mated females. We also document changes in male chorusing and searching behavior in response to wing-flick signals and male responses to the signals' visual and acoustical components. We test the hypothesis that female sensory psychology has shaped the evolution of-decim calls by favoring frequency-modulated male calls that are more readily distinguishable in an intense background chorus. Within mating aggregations, male Magicicada attempt to usurp ongoing courtships and also engage in interference competition by acoustically obscuring the calls of potential interlopers, reducing the likelihood of a female response.

Sexual signaling in periodical cicadas, magicicada spp. (hemiptera: cicadidae)

Periodical cicadas have proven useful in testing a variety of ecological and evolutionary hypotheses because of their unusual life history, extraordinary abundance, and wide geographical range. Periodical cicadas provide the best examples of synchronous periodicity and predator satiation in the animal kingdom, and are excellent illustrations of habitat partitioning (by the three morphologically distinct species groups), incipient species (the year classes or broods), and cryptic species (a newly discovered 13-year species, Magicicada neotredecim). They are particularly useful for exploring questions regarding speciation via temporal isolation, or allochronic speciation. Recently, data were presented that provided strong support for an instance of allochronic speciation by life-cycle switching. This speciation event resulted in the formation of a new 13-year species from a 17-year species and led to secondary contact between two formerly separated lineages, one represented by the new 13-year cicadas (and their 17-year ancestors), and the other represented by the pre-existing 13-year cicadas. Allozyme frequency data, mitochondrial DNA (mtDNA), and abdominal colour were shown to be correlated genetic markers supporting the life-cycle switching/allochronic speciation hypothesis. In addition, a striking pattern of reproductive character displacement in male call pitch and female pitch preference between the two 13-year species was discovered. In this paper we report a strong association between calling song pitch and mtDNA haplotype for 101 individuals from a single locality within the M. tredecim/M. neotredecim contact zone and a strong association between abdomen colour and mtDNA haplotype. We conclude by reviewing proposed mechanisms for allochronic speciation and reproductive character displacement.

Allochronic speciation, secondary contact, and reproductive character displacement in periodical cicadas (Hemiptera: Magicicada spp.): genetic, morphological, and behavioural evidence.

http://magicicada.org/cooley/reprints/Marshall_ea_2008.pdf

John R. Cooley

Papers

The Evolution of Mating Systems in Insects and Arachnids: Evolutionary perspectives on insect mating

Reproductive character displacement and speciation in periodical cicadas, with description of a new species, 13-year Magicicada neotredecim.

Sexual signaling in periodical cicadas, magicicada spp. (hemiptera: cicadidae)

Allochronic speciation, secondary contact, and reproductive character displacement in periodical cicadas (Hemiptera: Magicicada spp.): genetic, morphological, and behavioural evidence.

Steady Plio-Pleistocene diversification and a 2-million-year sympatry threshold in a New Zealand cicada radiation.