Which of the following occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region?

Speciation, Theories of

Hope Hollocher, in Encyclopedia of Biodiversity (Second Edition), 2013

Parapatry and Speciation through Hybridization

Parapatric speciation occurs when new species evolve in contiguous, yet spatially segregated habitats. Unlike allopatric speciation, the populations that are diverging during parapatric speciation maintain a zone of contact and do not cease the exchange of genes completely. In this case, a balance is achieved between continual gene flow and strong natural selection to maintain divergent populations at the two ends of the contiguous habitats. The zone of contact between the two diverging populations, where hybridization between the differently adapted types takes place, is called a hybrid zone (for more general information on hybrid zones, see Harrison, 1993; Hewitt, 2001). There can be a single zone of contact along a linear environmental gradient or several points of contact between habitats that are more patchily distributed. In both cases, the basic dynamics of hybrid zones are similar. In practice, it is generally impossible to distinguish between the situation in which the two populations continually maintained contact during the process of divergence (in which case the hybrid zone would then be considered a primary zone of contact) and the scenario in which the populations were actually allopatric at some point during divergence and then more recently came back into contact (in which case the hybrid zone would be considered a secondary zone of contact).

In either event, hybrid zones are particularly compelling to population geneticists because of the opportunity they present for examining the rate of exchange of genes that may be under different selection pressures in the two habitats. For most stable hybrid zones, the point of contact between the two species represents a semipermeable barrier to gene flow. Examining the distribution of different alleles sampled along a transect through the two habitat types can provide insights into the nature of the selective forces operating during divergence. The distribution of alleles at loci that have absolutely no effect on fitness will be governed entirely by migration distances and rates of exchange between the two species. Genes that are important for maintaining adaptive differences will show entirely different dynamics relative to these more neutral genes, revealing different patterns depending on the number of genes involved in the adaptations and the strength of selection operating in the two different habitats. Examination of the fates of alleles that shows differential movement across the hybrid zone is also a powerful method for actually mapping the genes responsible for adaptation (see Barton article in Harrison, 1993), which has been expanded to include alleles responsible for early species differentiation even outside the context of hybrid zones (Wu, 2001; Via, 2009).

In addition, hybrid zones represent natural laboratories for determining whether there is speciation by reinforcement. If two species coming into contact exhibit some degree of postzygotic isolation, it is then theoretically possible for selection to act on mating behavior (increasing assortative mating) to eliminate the production of hybrids, thus reinforcing the divergence that has already occurred and completing the speciation process (Dobzhansky, 1940; Servedio and Noor, 2003; Servedio, 2004). Reinforcement is a very appealing concept because it allows postzygotic isolation to play an active role in driving speciation rather than simply being a pleiotropic consequence of divergence. Although evidence of reinforcement occurring in nature has been presented (e.g., Noor, 1995; Nosil et al., 2003), theoretical arguments narrowly limit the range of circumstances under which it is likely to occur and it remains unclear how prevalent the operation of reinforcement may actually be in hybrid zones (reviewed in Coyne and Orr, 2004). The ability of reinforcement to cause complete cessation of gene flow between diverging species is highly dependent on the genetic architecture of mate preference mechanisms, potentially limiting the overall efficacy of reinforcement even further (Bank et al., 2012).

Hybridization is generally perceived as creating zones of tension between the operation of natural selection and gene flow. Speciation is thought to be proceeding through the constant action of natural selection serving to increase adaptation at the two extremes of the zone in the face of a continuous influx of genes that hamper adaptation. Intermediate types that form at the zone of contact are often inferior in fitness and disfavored by natural selection. The actual situation is far more complicated than this simple scenario suggests, even for hybrid zones that generally operate in this fashion (see Harrison, 1993). More importantly, it has become increasingly clear that hybridization does not always play such a negative role in speciation. Instead, it has been shown repeatedly that hybridization can actually provide an important arena for evolutionary innovation (Gross and Rieseberg, 2005; Arnold et al., 2011). In this case, hybridization presents the opportunity for the formation of unique genetic combinations through the mixing of different gene pools. In certain circumstances, these unique gene combinations end up performing better than either parental type in particular habitats and can result in the establishment of novel independent evolutionary lineages and the formation of new species (Donovan et al., 2010).

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Species Concepts and Speciation

D. Ortiz-Barrientos, in Encyclopedia of Evolutionary Biology, 2016

Sympatric and parapatric speciation, as well as reinforcement of reproductive isolation, are difficult because gene flow antagonizes the forces that create divergence (such as drift and natural selection). Gene flow can be present since the beginning of sympatric and parapatric speciation (primary contact), or during the completion of allopatric speciation (when populations come into secondary contact). Although primary contact may never lead to divergence of populations, and secondary contact can fuse them, theoretical models suggest that strong divergent selection, physical linkage between genes responsible for adaptation, and stable genetic associations between prezygotic and postzygotic reproductive isolation can facilitate speciation with gene flow in both cases. Sympatric and parapatric speciation are likely to always lead to the origin of Darwinian species, but perhaps less likely to the origin of biological species.

Which of the following occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region?

Figure Box 1. Speciation with gene flow during primary and secondary contact. (a) Populations showing strong associations between phenotype and habitat often form clearly defined clusters in multivariate trait space (clusters 1 and 2), and are likely to be reproductively isolated. (b) However, some populations may partially overlap in morphology, either because they are in the early stages of speciation, or because they have come back into contact after a previous period of allopatric divergence (clusters 4–6). Natural selection can resolve these stages and lead to the formation of populations clearly separated by complete extrinsic or intrinsic reproductive isolation. (c) Examination of variability in multiple genetic markers is expected to reveal the history of their divergence, although patterns of gene flow can distort relationships amongst populations. m represents migration rate between populations, and its strength is denoted by the thickness of the inverted arrows connecting a population pair.

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Speciation, Process of

Jeffrey L. Feder, ... Peter J. Meyers, in Reference Module in Life Sciences, 2021

Parapatric Speciation

Parapatric or clinal speciation occurs when populations partially geographically overlap during speciation (Fig. 1). Parapatric speciation may be caused by populations adapting to different ecological habitats that vary across their ranges or accumulating different mutations by selection or drift in different portions of their ranges, despite the populations being connected in part of their distributions such that a degree of gene flow was ongoing during their divergence (Fisher, 1930; Endler, 1977; Gavrilets, 2003). Thus, the overall rate of gene flow is higher in parapatric than allopatric speciation, but lower compared to sympatric speciation (see below). Although in principle parapatric speciation may be common, it is perhaps the most difficult mode to verify (Coyne and Orr, 2004). Historical data of past species ranges are often unknown, making it difficult to determine if currently overlapping taxa represent cases of parapatric divergence versus secondary contact. Certain cichlid fish in Lake Victoria in Africa may represent, through the evolution of different opsin genes to see at different water depths, examples of parapatric divergence (Seehausen et al., 2008). Other cases may include the grass Anthoxanthum adapting to different levels of heavy metal contamination in soils surrounding abandoned mines (Gregory and Bradshaw, 1965), Martinique’s anole (Thorpe et al., 2012), and the Gyrinophilus Tennessee cave salamander (Niemiller et al., 2008).

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Species and Speciation

C.L. Boggs, in International Encyclopedia of the Social & Behavioral Sciences, 2001

3.2 Parapatric Speciation

‘Parapatric’ derives from ‘para’ meaning ‘near’ and ‘patria’ meaning ‘country.’ Parapatric speciation thus occurs when a smaller population is isolated, usually at the periphery of a larger group, and becomes differentiated to the point of becoming a new species. Gene flow may remain possible between the two populations during the speciation process, and hybrid zones may be observed at the interface between the two populations as a result.

The best-known example of incipient parapatric speciation occurs in populations of the grass Agrostis tenuis which span mine tailings and normal soils. Individuals that are tolerant to heavy metals, a heritable trait, survive well on contaminated soil, but poorly on non-contaminated soil. The reverse occurs for intolerant populations. Gene flow occurs between sub-populations on and off mine tailings, but hybridization is inhibited by slight differences in flowering time between the two locations (McNeilly and Antonovics 1968, McNeilly and Bradshaw 1968).

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Evolution and Geography

Annalisa Berta, ... Kit M. Kovacs, in Marine Mammals (Third Edition), 2015

6.8 Summary and Conclusions

Understanding the identity of a species and its geographic distribution involves knowledge of both their evolutionary biology and ecology. New species form in three major ways: allopatric, parapatric, and sympatric speciation. The ecological requirements of a species limit where it can live, but past events can have a dramatic influence on where a species is actually currently found. For marine mammals, ocean temperature patterns and the distribution of primary productivity have influenced their past distributions and continue to influence their present distributions. Two major patterns of present-day marine mammal distribution can be identified: widespread or cosmopolitan distributions and disjunct or antitropical distributions. Two major historical processes have influenced the geographic distribution of species, dispersal (movement of a species into an area), and vicariance (the formation of a barrier that splits the range of a species). An area cladogram shows the phylogenetic relationships between species inhabiting different areas. If geographic distributions are determined mainly by vicariant events, then the area cladograms of a taxon should match the geologic history of an area. Phylogenies have also been used to study the evolution, biogeography, and feeding ecology of manatees. The pattern of lineage diversification through time is sometimes rapid with diverse morphological changes as species radiate into new habitats (adaptive radiation) or slow with relatively little morphological change (stasis).

The historical biogeography of pinnipeds suggests the following major patterns. The early evolution of walruses took place in the North Pacific approximately 18 Ma. The modern walrus lineage dispersed through an Arctic Ocean route, although which direction was taken is still a subject of debate. Otariid evolution took place largely in the North Pacific. Fur seals and sea lions crossed into the southern hemisphere by 6 Ma and rapidly diversified during the Pleistocene. The evolutionary history of phocids apparently began in the North Atlantic, although the common ancestor of phocids likely migrated there from the North Pacific earlier, following a southern route through the Central American Seaway sometime before 18 Ma. Subsequent climatic cooling resulted in monachine seals retreating southward whereas the phocines adapted to colder climates in the north. Monachines evolved in the North Atlantic and apparently diversified later in the colder waters of the southern hemisphere to produce the lobodontine seal fauna present in the Antarctic today. The early biogeographic history of phocine seals centered in the Arctic and North Atlantic. Subsequent dispersal of phocines into the Paratethys and the Pacific likely occurred during the Pleistocene; speciation in the Pacific and North Atlantic was affected by glacial events.

Both cetaceans and sirenians had a Tethyan origin. The earliest baleen and toothed whales are from the southern hemisphere. The evolution of filter feeding in mysticetes has been linked to initiation of the Antarctic Circumpolar Current and associated increases in zooplankton productivity. Recent diversification of cetaceans has been related to sea-level changes that promoted isolation and speciation in some cases and extinction in other cases. Dugongids were considerably more diverse in the past but have been reduced to a single genus confined to a tropical distribution in the Indo-Pacific Basin. The lineage that led to Steller’s sea cow had a North Pacific distribution and occupied the Bering Sea during the late Pleistocene; until it was hunted to extinction by man.

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Speciation

A.J. Moehring, in Brenner's Encyclopedia of Genetics (Second Edition), 2013

Speciation with Gene Flow

Although certain theoretical aspects remain contentious, enough well-documented examples of speciation with gene flow have been found in recent years to confirm that this type of speciation does occur, albeit far less frequently than geographical speciation. In these cases, speciation occurs without a geographical barrier, providing the opportunity for interbreeding between populations, and thus new genetic variants arising in one population can spread into the other population. This form of speciation is categorized as that involving free gene flow between populations (sympatric speciation), or limited gene flow between populations (parapatric speciation). Speciation is less likely to occur in the former of these two cases since complete gene flow arguably prevents the maintenance of separate evolutionary paths, and thus genetic divergence is unlikely to occur. Indeed, almost all of the documented cases of speciation with gene flow involve some initial barrier that limits the amount of gene flow between populations, allowing for genetic divergence and speciation. For example, if an individual of a host-specific insect shifts to another plant host and establishes a population, this founder population might become a new species, equally host-specific on the new host; the two species exist on different hosts and so rarely come into contact. Likewise, sexual selection for males with different coloration in different habitats could cause divergence in mate preference, leading to speciation. A genetic anomaly can also act as a barrier, as in cases where an inversion (flipped sequence) in part of the genome of one population prevents it from aligning and mixing with the genome of another population, leading to ‘genomic islands’ that do not experience gene flow and hence can evolve in separate directions.

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Birds

Jorge L. Pérez-Emán, ... Elisa Bonaccorso, in Biodiversity of Pantepui, 2019

Future prospects and conservation

Current knowledge of the Pantepui avifauna has come a long way since the first biogeographical analyses were published. As our knowledge of the current distribution of its avifauna improves (as more basic surveys are done) and their phylogenetic relationships become available, more thorough and informative analyses could be done. Global evaluation of the origin of the Pantepui avifauna, including an assessment of potential mechanisms of dispersal, the description of patterns of elevational segregation in the Pantepui avifauna and further testing of different evolutionary hypotheses about its origin are just some of the questions that need to be addressed in the future. The last question has the potential to evaluate the likelihood of parapatric speciation or secondary contact in explaining such population replacements, a process associated with the buildup of biodiversity in montane ecosystems (Cadena et al., 2019).

Ecological studies in Pantepui have largely been neglected. These studies should range from ecosystem-level studies, assessing ecological services provided by birds (e.g., dispersal, pollination, prey regulation), to community-level studies that focus on the spatial and temporal dynamics of avian communities or assemblages. Migration studies are lacking and could include the likelihood of species altitudinal migration, the importance of these habitats/ecosystems to latitudinal migrants, and short-range migratory behavior (from northern Cordilleras to the tepuis). Nearctic–Neotropical migratory birds occur in low abundance on Pantepui, but the high number of observed species suggests that the region is an important area for migrants that has yet to be evaluated.

Population-level studies that provide details on the demography and natural history of the species are of vital importance to understand the vulnerability (or not) of these ecosystems. There is sparse information on the phenology of molt and reproduction published as a result of specimen collection, but only Willard et al. (1991) provided some analyses. Moreover, available information is biased temporally, as most tepui expeditions have been conducted during the dry season. An interesting pattern has been observed in lowland species distributed at different elevations on the tepui slopes. Individuals of Xiphorhynchus pardalotus that inhabit the lowlands start molting a month later than individuals at higher elevations, potentially in response to regional rainfall patterns. Questions such as how general these patterns are, what the association could be with differential responses of species to climate change, and what factors are associated with differentiation along elevation ranges are only some of the topics that could be addressed with these studies. Even in the face of all potential limitations to studying these ecosystems, a clear key to success is the logistics necessary to conduct ecological studies in the area, particularly in the eastern region, due to the availability of roads, their easy accessibility, and the large regional area crossed by them.

The conservation status of the Pantepui avifauna needs to be evaluated. The avifauna of the region is distributed in areas that are not currently threatened by human influence (though tourism activity might require some evaluation). However, the impact of mining and habitat degradation in the lowlands (and lower slopes) adjacent to the tepuis might have an effect on the ecological dynamics of these birds. Additionally, an important conservation aspect to consider is the impact of climate change on Pantepui bird populations. Careful evaluation and descriptions of avian distribution along elevational gradients, spatial connectivity in the region, and the potential impact of climate changes could identify threatened populations and/or species and highlight critical aspects to consider for the conservation and management of these ecosystems (Rull and Nogué, 2007; Nogué et al., 2013).

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Fungi

Eva H. Stukenbrock, in Advances in Botanical Research, 2014

2.1 Isolation with migration

Allopatric populations are geographically separated from each other, while parapatric or sympatric populations coexist in the same habitat. Parapatric populations occupy distinct niches in their environment and are thus separated by ecological factors, while sympatric species share the same ecological niche. In natural habitats, heterogeneity in the landscape can generate spatial variation between populations, and this spatial fragmentation can provide the starting point for genetic divergence and parapatric speciation. Over time, spatially fragmented metapopulations can adapt to local environmental conditions and thereby independently accumulate distinct mutations. In the case of fungal plant pathogens, metapopulations may be the product of host jumping or adaptation to heterogeneously distributed host species (see review by Stukenbrock and Mcdonald (2008)). Under both scenarios, the diverging populations can still coexist in the same environment but be separated according to their host niches. Gene flow will be the genetic constraint counteracting local adaptation and divergence by repeatedly breaking down coevolved gene combinations in the distinct populations. Most fungal species produce enormous numbers of propagules that are dispersed by air, soil or water. Consequently, levels of gene flow locally and even more distantly can be significant. Still, divergence of fungal plant pathogens can occur in the presence of gene flow as documented by the finding of many closely related species inhabiting the same geographic location but occupying distinct niches. How reproductive barriers are established in the presence of gene flow is poorly understood but may result from strong divergent selection on metapopulations.

However, natural selection can directly promote the formation of reproductive barriers between sympatric or parapatric species, a phenomenon called reinforcement. This implies that particular genes involved in mating or progeny formation and fitness are affected by natural selection to work only in certain genetic combinations. Reinforcement of reproductive isolation can thereby allow the divergence of coexisting species even in the presence of gene flow between diverged populations or species. As mentioned in the preceding text, such reinforcement of reproductive barriers has been clearly demonstrated in sympatric lineages of A. mellea; however, the loci responsible for reproductive isolation have so far not been identified (Anderson et al., 1980). A recent study of N. crassa and its sister species N. intermedia allowed the identification of 11 QTL loci responsible for hybrid abortion in crosses of sympatric N. crassa and N. intermedia isolates (Turner et al., 2011). The genomic footprints of reinforcement in Neurospora are signatures of positive selection at loci that are strongly associated with mating and abortive hybrid fruiting body development.

Genetic divergence between allopatric populations accumulates as a consequence of prolonged independent evolution. Between such physically isolated species, gene flow is prevented or strongly reduced by geographic barriers allowing neutral and nonneutral variations to accumulate. Because allopatric species never or rarely encounter each other, reproductive isolation is not a trait selected for by natural selection. Genomic divergence between allopatric species will reflect the adaptation to local environmental conditions, but will not bear traces of reinforcement selection.

Coalescence theory and coalescence models have been widely applied to study the divergence of species. In its simple form, coalescence theory assumes divergence of populations in the absence of gene flow, thus resembling scenarios of allopatric speciation (Wakeley, 2009). However, more recent models allow for the presence of gene flow during isolation of species (Gavrilets, Li, & Vose, 2000; Nielsen & Wakeley, 2001). One of these is the “isolation with migration” (IM) model, which models the divergence of an ancestral population into two or more descendant populations (Hey, 2010; Nielsen & Wakeley, 2001). Migration, modelled as rates of gene flow, can occur between these descendant species in spite of their divergence as isolated lineages. Thereby, IM models allow for more biologically relevant scenarios of species divergence. Depending on the particular IM model, several parameters can be inferred, such as ancestral and present-day population sizes, rates and times of gene flow and speciation times. Together, these parameters can provide insight into the species dynamic at early stages of population divergence or speciation. Previous studies applying IM models have used relatively short sequences sampled from population data. For example, speciation histories in model systems of fungal plant pathogens were inferred using IM models and population genetic data sampling (Gladieux et al., 2011; Stukenbrock et al., 2007). Recently, the IM model has been modified for whole-genome data, providing a very powerful approach to infer divergence times and migration rates across whole-genome sequences (Mailund et al., 2012).

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Sea Plants

Alejandro H. Buschmann, ... Daniel A. Varela, in Advances in Botanical Research, 2014

6.4 Kelp Genetics and Strain Selection

There are relatively few published studies regarding kelp breeding and genetic improvement, most of which are focused on Saccharina japonica (for a review see Robinson, Winberg, & Kirkendale, 2013). In China and neighbouring countries, where the species is extensively cultivated, the main strategy has been to perform repetitive breeding generations of a few inbred lines (Robinson et al., 2013). This has led either to improved biomass production, or the increase of specific traits/molecules. To date, knowledge of kelp genetics in Chile is focused on evolutionary and ecological topics: (1) global phylogeography and parapatric speciation; (2) regional phylogeography and post-glacial recolonisation; and (3) local genetic diversity and structure following natural and human perturbations. The first published studies were aimed at understanding the effects of natural (i.e. ENSO, Martínez et al., 2003) and anthropogenic disruptions (i.e. copper mining wastes, Faugeron, Martínez, Correa, & Billot, 2005) on the spatial and temporal dynamics of the kelp L. nigrescens in northern Chile. Both studies characterised population genetic structure, within and between continuous stands, separated by the impacted areas. They showed that the disruption of an otherwise continuous distribution of this kelp coincided with increased genetic differentiation between stands occurring northward and southwards from the impacted area. These authors demonstrated that gene flow occurred mainly between populations separated by short distances (i.e. a few km at most) as a result of their relatively short dispersal capacity. Indeed, after the 1982/83 ENSO event, even after 20 years L. nigrescens had not recolonised more than 50 km of coastline. This is a slow rate considering that the event killed extensive kelps beds along 600 km of coastline, leaving only sparse relic populations in upwelling centres (Martínez et al., 2003). These results were further confirmed by phylogeographic studies embracing most of the species distribution (Tellier, Meynard, Correa, Faugeron, & Valero, 2009). Based upon nuclear, mitochondrial and plastidial markers, a low genetic diversity was observed in each sampled locality, and shared only with neighbouring populations. In addition, strong genetic divergence was observed in the 30° S biogeographic transition zone described by Camus (2001) and Thiel et al. (2007). This genetic divergence corresponded to a speciation process (Tellier, Tapia, Faugeron, Destombe, & Valero, 2011), as a result of both the reduced dispersal and the ecological divergence (López-Cristoffanini, Tellier, Otaiza, Correa, & Contreras-Porcia, 2013; Oppliger et al., 2012, 2011). Overall, these results showed that specific traits (in this case, traits related to temperature and desiccation tolerance) could evolve in response to pressures of natural selection in wild populations.

Genetic studies in M. pyrifera, along the coast of Chile, focused on global phylogeography. As for L. nigrescens, Macaya and Zuccarello (2010) showed poor local (i.e. intrapopulation) genetic diversity and a strong genetic differentiation between the populations of M. pyrifera, and/or between regions along the Chilean coast north of the 42° S. This was surprising for a species capable of long-distance dispersal through rafting (Macaya et al., 2005; Thiel & Haye, 2006), and again suggested that local, rather than global, processes were shaping the genetic diversity predominantly because of restricted dispersal. On a global scale, the reduced genetic diversity was emphasised by the relatively recent arrival of the genus to the southern hemisphere, as a product of trans-tropical dispersal, from the northern hemisphere, during late Pleistocene (see Coyer, Smith, & Andersen, 2001; Macaya & Zuccarello, 2010). The situation is different south of 42° S where post-glacial colonisation led to the presence of a single haplotype in the region of the Magellan Strait (Macaya & Zuccarello, 2010). There, the importance of long-distance dispersal has been a determining factor in shaping the genetic diversity of such a large geographic region. Indeed, this single haplotype is shared with other sub-Antarctic regions, including south New Zealand, a pattern very similar to the one reported for the bull kelp, Durvillaea antarctica (Fraser, Nikula, Spencer, & Waters, 2009). However, in persistent populations, for which equilibrium is reached between the respective influence of gene flow, genetic drift and mutation, it seemed that restricted dispersal of spores was the dominant process shaping local and regional genetic diversity (see Alberto et al., 2010, 2011). This was a seemingly common pattern in most kelp species (see Valero et al., 2011).

One interesting feature of the phylogeographic studies was that they reported sharing haplotypes between all known morphotypes, previously considered as different species (e.g. Macrocystis integrifolia, Macrocystis angustifolia and Macrocystis laevis sensu Demes, Graham, & Suskiewicz, 2009), which suggested that they shared a very recent ancestor. However, this result is still under discussion, as suggested by Astorga, Hernández, Valenzuela, Avaria-Llautureo, and Westermeier (2012) who used nuclear markers (ITS-2) and representatives of Macrocystis from Canada, USA, South Africa, Australia and Chile. The authors observed a strong phylogenetic signal of morphological differences, which indicates that historical processes associated with diversification may have determined the current differentiation of the morphotype. Recent evidence of heterosis in the giant kelp (Westermeier et al., 2010, 2011) was an additional indication that phenotypic differentiation existed between genetically differentiated populations in Chile, and that local diversity could be re-arranged by controlled crosses into new genotypes, producing valuable phenotypes (Westermeier et al., 2011). This contrasted strongly with the paradigm of a high phenotypic plasticity underlying Macrocystis diversity, as suggested by Graham et al. (2007) and Demes et al. (2009). In addition to morphological plasticity, plastic phenotypic responses in terms of physiological and life history strategies were observed in different coastal habitats (Buschmann et al., 2006, 2014, 2004). Therefore, the relative roles of plasticity and selection in shaping kelp phenotypes are still a debate that requires dedicated scientific attention.

Future prospects for the establishment of a breeding program in Macrocystis should involve a detailed analysis of the phenotypic (morphology, chemical composition and other relevant production traits) differentiation between natural populations, together with an estimation of the genetic contribution (i.e. heritability) to trait values. It is important to pursue research on natural populations because the domestication process of this species is incipient, and up to now, most algal production has been based on spores obtained from natural stands. On the other hand, genetic interaction of cultivated and natural stocks will require attention in order to avoid potential genetic modifications of natural stocks through gene flow from modified cultivars, when cultivated outdoors, which has also been suggested for algae such as G. chilensis (Guillemin et al., 2008). Finally, kelp life cycle management is an issue that requires special attention for its potential to use parthenogenesis to produce pure genetic lines (i.e. fully homozygous genotypes) in a single generation, as suggested by recent studies showing that parthenogenetic sporophytes can produce single-sexed (e.g. 100% females) gametophytic progenies (Shan, Pang, & Gao, 2013). Pure genetic lines are essential for the quantitative genetics of specific traits (e.g. QTL analysis) and the definition of genetic markers to assist breeding strategies.

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Theory and speciation

Michael Turelli, ... Jerry A. Coyne, in Trends in Ecology & Evolution, 2001

The ‘isolation by distance’ necessary for parapatric speciation depends on the strength of selection acting during population divergence. If strong selection is involved (either causing adaptation to local conditions, or maintaining alternative adaptive peaks), then divergence of traits leading to reproductive isolation can occur over small spatial scales (σ2s, where σ measures average per-generation dispersal distance and s measures selection); this is true whether divergence is driven by drift or by selection 73. If divergence is ‘quasi-neutral’, then larger spatial scales might be involved, although they might still be small compared with the range of the species. Parapatric divergence of reproductive compatibility seems most difficult when it is based on alleles that are favorable everywhere against the ancestral genetic background but incompatible with each other. Such divergence requires that two or more alleles be established in different parts of the range before any one allele has spread over the whole range 74.

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What results when a small group of individuals establishes a new population far from existing populations?

In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall Wright.

What is it called when individuals leave one population to join another population and breeds in that new population?

gene flow, also called gene migration, the introduction of genetic material (by interbreeding) from one population of a species to another, thereby changing the composition of the gene pool of the receiving population.

What occurs when individuals migrate into or away from a population?

Gene flow is when alleles either enter or exit a population. Entering is called immigration, and exiting is called emigration. When new, genetically unique individuals immigrate to a preexisting population, they bring along new alleles with them.

What occurs when small groups of a population become isolated through extinction or migration?

Allopatric speciation (1) occurs when a species separates into two separate groups which are isolated from one another. A physical barrier, such as a mountain range or a waterway, makes it impossible for them to breed with one another.