Does character displacement initiate speciation? Evidence of reduced gene flow between populations experiencing divergent selection

Character displacement – trait evolution stemming from selection to lessen resource competition or reproductive interactions between species – has long been regarded as important in finalizing speciation. By contrast, its role in initiating speciation has received less attention. Yet because selection for character displacement should act only where species co‐occur, individuals in sympatry will experience a different pattern of selection than conspecifics in allopatry. Such divergent selection might favour reduced gene flow between conspecific populations that have undergone character displacement and those that have not, thereby potentially triggering speciation. Here, we explore these ideas empirically by focusing on spadefoot toads, Spea multiplicata, which have undergone character displacement, and for which character displacement appears to cause post‐mating isolation between populations that are in sympatry with a heterospecific and those that are in allopatry. Using mitochondrial sequence data and nuclear microsatellite genotypes, we specifically asked whether gene flow is reduced between populations in different selective environments relative to that between populations in the same selective environment. We found a slight, but statistically significant, reduction in gene flow between selective environments, suggesting that reproductive isolation, and potentially ecological speciation, might indeed evolve as an indirect consequence of character displacement. Generally, character displacement may play a largely underappreciated role in instigating speciation.     

鸟类鸣声行为对其物种分化和新种形成影响

鸟声和鸟类的形态特征一样,具有物种的特异性,在鸟类分类和野外识别方面是有意义的,从生物种的基本概念出发,新种的形成有赖于两个亲缘种群间不能相互婚配和繁殖隔离的产生。鸟类主要的有关特征是在配对形成过程中的听觉和视觉特征,如果出现鸣声特征的差异,而且这种差异已经超出了种间“语言通讯”的变化范围,那么它就有可能与群内其它异笥无法配对,从而被排除到该种群或种的范围之外,然而鸣禽的鸣唱有时在不同种群间变化很大,而在同一种群内比较稳定,从而形成种群的方言,在方言种群之间,如果长期隔离,就会在种群之间形成障碍乃至遗传上的隔离,从而使种群间失去了相互配偶的机会,新的亚种或种可能由此形成,文章最后假设出了基于鸣声行为的鸟类新种形成机理图解。     

Proteomic evidence of a paedomorphic evolutionary process within a marine snail species: a strategy for adapting to extreme ecological conditions?

The exposed and sheltered ecotypes of the marine snail Littorina saxatilis from European rocky shores are considered a key model system to study adaptation and ecological speciation. Previous studies showed that two ecotypes (RB and SU) of this species in NW Spain have adapted differently to different shore levels and microhabitats. In order to understand how this divergent adaptive process has been accomplished, we followed a quantitative proteomic approach to investigate the proteome variation in a number of different biological factors, that is, ecotype, ontogeny and their interactions. This approach allowed testing the hypothesis that one of the ecotypes has evolved by paedomorphosis, and also whether or not the molecular mechanisms related to ecotype differentiation are set up in early developmental stages. Additionally, the identification of some candidate proteins using mass spectrometry provides some functional insights into these evolutionary processes. Results from this study provided evidence of higher ontogenetic differentiation at proteome level in the RB (metamorphic) than in SU (paedomorphic) ecotype that point to the possibility of juvenile stage retention in this latter ecotype. The level of protein expression (proteome) differences between ecotypes maintained nearly constant from late embryonic stages to adulthood, although some proteins involved in these changes considerably differed in embryonic compared to other ontogenetic stages. Paedomorphosis may be the evolutionary response of the SU ecotype of solving the trade‐off during sexually immaturity that is caused by the evolution of small size arising from adaptation to the wave‐exposed habitat. Some potential candidate genes of adaptation related to energetic metabolism have been identified, providing a promising baseline for future functional analyses.     

Familiarity adds to attractiveness in matters of siskin mate choice

There is currently considerable controversy in evolutionary ecology revolving around whether social familiarity brings attraction when a female chooses a mate. The topic of familiarity is significant because by avoiding or preferring familiar individuals as mates, the potential for local adaptation may be reduced or favoured. The topic becomes even more interesting if we simultaneously analyse preferences for familiarity and sexual ornaments, because when familiarity influences female mating preferences, this could very significantly affect the strength of sexual selection on male ornamentation. Here, we have used mate-choice experiments in siskins Carduelis spinus to analyse how familiarity and patterns of ornamentation (i.e. the size of wing patches) interact to influence mating success. Our results show that females clearly prefer familiar individuals when choosing between familiar and unfamiliar males with similar-sized wing patches. Furthermore, when females were given the choice between a highly ornamented unfamiliar male and a less ornamented familiar male, half of the females still preferred the socially familiar birds as mates. Our finding suggests that male familiarity may be as important as sexual ornaments in affecting female behaviour in mate choice. Given that the potential for local adaptation may be favoured by preferring familiar individuals as mates, social familiarity as a mate-choice criterion may become a potential area of fruitful research on sympatric speciation processes.     

Vireo song repertoires and migratory distance: three sexual selection hypotheses fail to explain the correlation

Previous studies have found a relationship between migrationand the degree of elaboration of sexually selected traits,but investigators have differed in the mechanisms they proposedto account for this association. We examined the relationshipbetween song repertoire size and distance migrated among birdsin the genus Vireo. There is a strong positive relationshipbetween migratory distance and repertoire size in this genus,but our data do not support the specific predictions of anyof the three proposed mechanisms (the "rapid pairing," "goodmigrations," and "territory lottery" hypotheses). Migrationdistance is presumably correlated with other life-history characteristicsthat influence the development of sexually selected traits.     

Reinforcement of mate preference among hybridizing Heliconius butterflies

Recent models of mate preference evolution suggest that direct selection on alleles at preference loci and correlated evolution of preference with locally adapted mating cues are more likely to drive the evolution of assortative mate preference than reinforcement. Mate preference evolution in mimetic Heliconius butterflies has been attributed to all three forms of selection, but here we show that reinforcement has been critical. By examining geographical variation in assortative mating and male mate preference among seven populations of three hybridizing Heliconius species from Costa Rica, we found pronounced character displacement of preference such that sexual isolation was enhanced in areas of interspecific contact. Of the different explanations for the evolution of assortative mate preference, only reinforcement is dependent on interspecific contact in this system. Thus, the observed pattern of reproductive character displacement of mate preference is best explained as a product of indirect selection generated by natural selection against nonmimetic hybrids.     

Notes from a snail island: Littorinid evolution and adaptation

The most successful study systems are built on a foundation of decades of research on the basic biology, ecology and life history of the organisms in question. Combined with new technologies, this can provide a formidable means to address important issues in evolutionary biology and molecular ecology. Littorinid marine snails are a good example of this, with a rich literature on their taxonomy, speciation, thermal tolerance and behavioural adaptations. In August 2017, an international meeting on Littorinid evolution was held at the Tjärnö Marine Research Laboratory in Western Sweden. In this meeting review, I provide a summary of some of the exciting work on parallel evolution, sexual selection and adaptation to environmental stress presented there. I argue that newly available genomic resources present an opportunity for integrating the traditionally divergent fields of speciation and environmental adaptation in Littorinid research.     

Further acoustic diversity in Hawaiian forests: two new species of Hawaiian cricket (Orthoptera: Gryllidae: Trigonidiinae: Laupala)

The genus Laupala is endemic to the rain forested islands of the Hawaiian archipelago. In his 1994 study Otte described 35 Laupala species, each restricted to a single island, and sometimes a single volcano. All Laupala species produce male calling songs consisting of slow trills. Two new species described here, one from the Koolau Mountains of Oahu and another from Kipahulu Valley of East Maui, produce the fastest and one of the slowest pulse rates, respectively, in the genus. Both species are found with sympatric congeners. The discovery of these species increases the acoustic diversity locally (within their respective communities), within their respective islands, and across the archipelago. This enhances the opportunity for the investigation of signal interference and character displacement in a relatively youthful assemblage of closely related species of crickets.     

Differentiating the evolution of female song and male–female duets in the New World blackbirds: Can tropical natural history traits explain duet evolution?

Female bird song and combined vocal duets of mated pairs are both frequently associated with tropical, monogamous, sedentary natural histories. Little is known, however, about what selects for duetting behavior versus female song. Female song likely preceded duet evolution and could drive apparent relationships between duets and these natural histories. We compared the evolution of female song and male–female duets in the New World blackbirds (Icteridae) by investigating patterns of gains and losses of both traits and their relationships with breeding latitude, mating system, nesting pattern, and migratory behavior. We found that duets evolved only in lineages in which female song was likely ancestral. Both female song and duets were correlated with tropical breeding, social monogamy, territorial nesting, and sedentary behavior when all taxa were included; however, correlations between duets and these natural history traits disappeared when comparisons were limited to taxa with female song. Also, likelihood values supported stronger relationships between the natural history traits and female song than between these traits and duets. Our results suggest that the natural histories thought to favor the evolution of duetting may in fact be associated with female song and that additional selection pressures are responsible for the evolution of duets.