Of course, the ultimate cause was global climate change, which resulted in multiple episodes of glacial advance and retreat at more or less regular intervals.
The effects of these glacial cycles are clear. Species ranges were regularly shifted, compressed, subdivided, eliminated or greatly expanded. In other words, a wide-ranging species, perhaps a habitat generalist, would be pushed southwards as climate deteriorated cooled. When once-continuous populations are isolated geographically from each other, a situation termed allopatry living in different places , they begin to accumulate mutations unique to their geographic group, and the process of genetic divergence begins.
The process can be driven by natural selection, sexual selection or even genetic drift. If the newly isolated populations are incompletely isolated, i. But if the populations remain isolated they will accumulate enough diverge in their genetics, ecology or morphology that they will become different species Mayr The nature of divergence will determine whether speciation has occurred under different species concepts.
If allopatric taxa exhibit diagnostic character differences, they are species under the PSC. Only if these diagnostic differences also function as reproductive isolating mechanisms will the taxa be considered biological species. In any event, it appears that species evolved regularly throughout the Pleistocene in northern temperate areas as a result of glacial cycles and the habitat changes they caused Klicka and Zink , Study of the distribution of bird species played a prominent role in identifying the importance of geographic isolation in the speciation process.
In particular, analysis of the ranges of newly separated species, or sister species, was instrumental. Thus, the spatial isolation of sister species is a signature of allopatric speciation. There are always potential exceptions to allopatric speciation, such as speciation occurring without geographic isolation, which is termed sympatric speciation. However, there is little if any evidence for this mode of speciation in birds. The fact that sister species have allopatric distributions today is quite compelling evidence of the importance of geographic isolation in the speciation process.
However, to date we have only assumed that allopatry was maintained from the point of initial divergence. Recent developments in the field of niche modeling Peterson et al. I present two case studies that involve two pairs of currently allopatric species, a pair of North American warbler species and a pair of North American gnatcatcher species.
I reconstruct their historical distributions to test whether they have been allopatric throughout their evolutionary divergence. They are extremely similar in size and shape, with the primary difference being that the former has a white throat and the latter a yellow throat.
They are mostly allopatric, but their ranges overlap in a small area in British Columbia, where some hybridization occurs. This is why some researchers consider them the same species under the BSC , whereas others such as the current author finds this irrelevant and consider them two species under the PSC.
Have these warbler species historically been allopatric, or mostly so? To answer this, one can use niche models. These involve taking a set of modern occurrence records, and a set of various climate variables such as temperature and rainfall, and building a model that predicts their current distributions. A species might have a smaller distribution than predicted owing to the presence of competitors sometimes called the Eltonian or realized niche.
If a niche model can predict where a species occurs at present, we then take a set of climate variables from the past and use the model to predict where the species could have occurred at that time Peterson , Peterson et al. This procedure assumes that the niche of the species has not changed over time. Also, the models only predict where the conditions under which the species lives today occurred at some previous time, not whether the species in fact occurred there i.
However, this is a common procedure and many scientists have concluded that the methods are robust. I used niche modeling to determine whether the two species pairs of warblers were allopatric at the Last Glacial Maximum LGM about 21, years before present ybp and at the Last Interglacial LIG , ca.
The reconstructed distribution of the warblers at the present time Fig. The predicted distributions of the warblers at the LGM Fig. In addition, predicted distributions at the LIG Fig. Thus, the warblers have been allopatric for at least , years, reinforcing the role of allopatry in maintaining and initiating their evolutionary divergence. Incidentally, this result also gives a minimum estimate of the time since they became separate species.
The niche models were constructed by entering breeding records from the breeding bird survey accessed at www. Phillips et al. Climatic data 19 layers were obtained from the Worldclim bioclimatic database Hijmans et al.
Distribution maps were coded to show either presence or absence based on a minimum threshold of predicted occurrence. Two other sister species, the California Gnatcatcher Polioptila californica and the Black-tailed Gnatcatcher Polioptila melanura today have allopatric ranges. The gnatcatchers differ from the warblers in at least two important ways, namely they are sedentary the warblers are migratory , and they are very different genetically.
Read about how speciation factored into the history of evolutionary thought. Or explore different modes of speciation , including:. Find additional lessons, activities, videos, and articles that focus on speciation. Defining speciation. Reproductive isolation. Subscribe to our newsletter. Parapatric speciation sometimes happens when part of an environment has been polluted. Mining activities leave waste with high amounts of metals like lead and zinc. These metals are absorbed into the soil , preventing most plants from growing.
Some grasses, such as buffalo grass , can tolerate the metals. Buffalo grass, also known as vanilla grass, is native to Europe and Asia, but is now found throughout North and South America, too. Buffalo grass has become a unique species from the grasses that grow in areas not polluted by metals. Long distances can make it impractical to travel to reproduce with other members of the species. Buffalo grass seeds pass on the characteristics of the members in that region to offspring.
Sometimes a species that is formed by parapatric speciation is especially suited to survive in a different kind of environment than the original species. Sympatric speciation 4 is controversial. Sympatric speciation occurs when there are no physical barriers preventing any members of a species from mating with another, and all members are in close proximity to one another.
A new species, perhaps based on a different food source or characteristic , seems to develop spontaneously. The theory is that some individuals become dependent on certain aspects of an environment—such as shelter or food sources—while others do not. A possible example of sympatric speciation is the apple maggot, an insect that lays its eggs inside the fruit of an apple, causing it to rot. As the apple falls from the tree, the maggots dig in the ground before emerging as flies several months later.
The apple maggot originally laid its eggs in the fruit of a relative of the apple—a fruit called a hawthorn. After apples were introduced to North America in the 19th century, a type of maggot developed that only lays its eggs in apples. The original hawthorn species still only lays its eggs in hawthorns. The two types of maggots are not different species yet, but many scientists believe they are undergoing the process of sympatric speciation.
Artificial speciation 5 is the creation of new species by people. This is achieved through lab experiments, where scientists mostly research insects like fruit flies. Illustration by Ilmari Karonen, courtesy Wikimedia. Holy Anolis! There are nearly species of the small anolis lizard on the islands of the Caribbean Sea, all of which descended from as few as two initial species.
Pretty Fly The Hawaiian islands are home to some of the most stunning examples of speciation. Over species of fruit fly have developed there and are found nowhere else on Earth! 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.
The molecular analysis suggests that i the five populations in this study along with numerous other nominal Hyalella populations represent a polytomy with deep divergence, and ii H. Based on the depth of molecular divergence between populations, and the paraphyletic distribution of populations conforming to the H. The observation of morphological diversity not conforming to an inferred history of shared common ancestry is not unique to Hyalella Faria et al.
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