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More Details Evo Examples Teaching Resources Read about how speciation factored into the history of evolutionary thought. This adaptive speciation may be particularly common where there is partial spatial separation between habitats, such as on the steep environmental gradients that characterise sea-shore habitats.
The marine snail Littorina saxatilis, which shows evidence of progression towards speciation, was used as a model. In particular, the two morphs E and S of L.
Scientists examined how genetic differentiation and phenotypic plasticity can contribute to species adaptation and reproductive isolation between these two morphs. Using genetic tools such as amplified fragment length polymorphism AFLP markers and morphometric analysis, scientists wished to analyze transects on two separate small islands as estimates of gene flow and selection.
Additionally, they tested the possibility that enforced mating between the two morphs in the hybrid zone may affect local adaptation.
Adaptive speciation provided important insight into the field of evolutionary biology and more specifically into the role of reproductive isolation in driving speciation.
This assertion is supported by al lack of evidence for niche partitioning among sympatrically occurring populations of Hyalella Dionne et al. The presence of cryptic lineages, variation in the degree of geographic isolation among lineages, and the evidence of local adaptation in the narrowly distributed lineages makes Hyalella an ideal system for quantifying the factors that contribute to the evolution of reproductive isolation.
This study used five populations Table 1 belonging to Hyalella —a widespread and abundant taxon of freshwater amphipods distributed across North America. Two of the populations studied herein fit the expectations of the H. SMS Hyalella sp. We compared morphological and genetic variation, in combination with attempted mating experiments and study of biogeographic distributions, in an attempt to explain factors contributing to reproductive isolation.
Collection localities and count of dorsal mucronation for each Hyalella population. Stock cultures of Hyalella were established to provide a continuous source of live animals for experimentation and to control for the possibility of morphological differentiation due to phenotypic plasticity in situ. Cultures of Hyalella spp. All amphipods were collected from source localities using dip nets, turkey basters, or a Ponar grab sampler.
Each bucket was given a sand substrate and filled with artesian water with water changes twice monthly. All cultures were fed the same diet of Amblystegium sp.
Great care was taken when handling cultures to ensure that organisms did not get moved between cultures. As conditions in all stock cultures described above were maintained under the same conditions, we anticipated that five generations would be sufficient to control for environmental or maternal effects; a lack of variation in neonate size across all the populations and generations in captivity suggests that maternal effect was not a factor Glazier, ; Table S3.
Therefore, after at least five generations of raising amphipods in stock cultures, morphology was compared between cultures. Morphometric characters total length, longest mucronation length, and head length; see Figure S1 for explanation were estimated from these photographs using Digimizer software www. We also counted the number of dorsal mucronations and calculated the ratio of the length of the longest mucronation to total length.
A molecular phylogeny based on the mitochondrial cytochrome C oxidase subunit I COI locus was constructed in order to analyze the relationship between morphological similarity, geographic factors, and a history of shared common ancestry.
Of these Hyalella sequences, geographic data were available for ; therefore, only these sequences were retained for further analysis.
Additional sequences belonging to amphipods in the families Chiltoniidae, Gammaridae, Gammarellidae, Ischyroceridae, Lysianassidae, Metacrangonyctidae, Niphargidae, and Talitridae were compiled into an alignment with Hyalella sequences to serve as outgroups and to provide context for the depth of divergence within Hyalella.
This pairwise matrix was used to infer the geographic distributions of each haplotype of Hyalella , as well as to remove redundant sequences of each haplotype before further analysis sequences with 0. Phylogenies were estimated using MrBayes Ronquist et al. Saturation of nucleotide substitutions was estimated by plotting uncorrected pairwise distances against the evolutionary model adjusted pairwise sequence divergence i. Saturation was assessed by comparing the resulting slope of the regression with the theoretical slope of 1.
Pairings were established using stock cultures by selecting one female from one population source and selecting one male from the same control groups or one male from a different population experimental groups.
Only females that were not brooding eggs or young in their marsupia were selected for the experiments. This is a conservative approach to estimating reproductive isolation because reproduction is rarely successful between pairs where males are smaller than females.
Some combinations could not be achieved because it was difficult to find suitable males i. Replication of male—female pair combinations of Hyalella by population source and sex. Each count represents one pair. An experimental replicate consisted of one male and female pair.
Each container was given the same sand substrate and fed a diet consisting of Amblystegium sp. Mating trials were run for 8 weeks and were checked once weekly for the production of offspring.
After 8 weeks had elapsed, any pairs that had not reproduced were considered to represent unsuccessful crosses. If free swimming neonates were observed, the adults were removed. Estimated age was used to estimate the date that hybrid offspring had hatched and the date at which they would become 8 weeks old since 8 weeks is the age at which most Hyalella species are thought to have finished most of their ontogenetic growth; Strong, These pairings were allowed to run for 8 weeks and were checked once weekly for the production of offspring.
To evaluate potential factors that might explain the occurrence of reproductive isolation, the results from the reproductive isolation experiment were arranged into a matrix. To assess the possibility that the relative degree of geographic isolation may potentially lead to reproductive isolation, each population was scored as either reproductively isolated 1 or not 0. Factors analyzed in ANOVAs to determine if geography can account for variation in the occurrence of reproductive isolation.
Pairwise comparison of Hyalella sequences yielded 97 unique Hyalella haplotypes; three of the populations we sequenced had only one haplotype while the other two had two haplotypes each Table S5. Appreciable molecular divergence was detected within Hyalella with evidence of saturation Figure 2.
To facilitate discussion of the phylogeny, haplotypes are grouped into clades Figure 1 , Table S5. Terminal nodes represent unique haplotypes.
Haplotypes were grouped into clades where applicable. Bayesian posterior probabilities are given at all major nodes. COI saturation plot. The solid line has a slope of 1 and is a theoretical representation of sequence data that would occur if there was no saturation Jeffroy et al. The observed departure from this theoretical slope which occurs at around 0. Populations segregate with some degree of overlap. Phylomorpho plot of population centroids with phylogenetic relationship.
Genetic similarity is not related to distribution of centroids in principal components analysis space. Decimals along branches represent the Bayesian model inferred number of substitutions. After 8 weeks, all conspecific controls had successfully produced offspring while only three of the potential crosses successfully produced offspring Table 6.
Despite amplexus being observed in all treatment groups, none of the heterospecific pairings involving H. This observation is consistent with those two populations being completely isolated reproductively from all other tested populations. Among the replicates that successfully produced offspring, there was noticeable resistance by the heterospecific pairs to mate. Conspecific control pairs produced offspring as early as 2 weeks into mating trials while none of the successful heterospecific pairs produced offspring until after at least 4 weeks Figure 5.
This result is consistent with interfertile heterospecific populations having some degree of prezygotic reproductive isolation. After rearing hybrid offspring to adulthood, all hybrid offspring successfully produced offspring suggesting that hybrids are fertile. Cumulative proportion of successfully reproducing pairs across time. By the second week, conspecific pairs had produced offspring; however, none of the heterospecific crosses produced offspring until at least 4 weeks had elapsed.
Only the heterospecific crosses that successfully produced offspring are depicted. None of the heterospecific pairings including H. However, geography was found to be an important factor Figure 6 as the number of populations of each clade and the length of reach occupied by each population was both found to significantly explain the occurrence of reproductive isolation Figure 6 , Table 7. Geographic distribution of clades inferred through genetic analysis for which reproductive isolation was assessed.
Note that the populations found to be reproductively isolated occur at only a single locality each while the interfertile populations belong to widely distributed clades. Geography in both size of distribution and number of known localities for each haplotype was found to significantly explain the occurrence of reproductive isolation. Appreciable morphological and molecular differentiation were observed in the five populations of Hyalella in this study Figure 1 , Figure 3.
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