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            The Latin term for hybrid is ‘hybrida,’ referring to a mongrel or more specifically the offspring of a tame sow and wild boar.  Hybridization has historically been seen as a negative influence on evolution by disrupting already pre-established gene pools. Species hybridization offers an interesting twist in the evolution of species as homoploid speciation or allopolyploid speciation mechanisms allow for the introduction of a novel hybrid species.  Recent studies have shown that novel species have the possibility of introducing novel genotypes as a result of hybrid speciation, which wouldn’t have been observed in normal speciation.  This research article introduces the model of hybrid speciation by breaking down its two main mechanisms and analyzing the evolutionary influences of each.

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            The study of speciation has long been an evolving topic in the world of evolutionary biology.  Models of speciation such as allopatric and sympatric speciation allow for researchers to understand how single lineages of species split into two or more genetically distinct lineages.  Another interesting mode of speciation involves hybridization of genetically divergent populations that may form new evolutionary lineages (Abbott and Rieseberg 2012).   Interestingly enough, the percentage of species that hybridize is low.  On average, around 10% of animal and 25% of plant species are known to hybridize with at least one other species {Abbott, 2012, Hybrid Speciation}(Mallet 2005).  The most studied methods by which hybrid speciation can occur is by duplication of a hybrid’s chromosome complement (allopolyploid speciation) or by the stabilization of a fertile hybrid segregant (homoploid speciation) (Abbott and Rieseberg 2012).  Hybridization has been shown to be a source of genetic anomalies such as chromosomal rearrangements, genome expansion, differential gene expression, and gene silencing as described by (Baack and Rieseberg 2007). The aim of this article is to introduce the term “species hybridization” and describe how it leads to novel evolutionary lineages.


What is a Species? A Brief Overview.

            Prior to answering the question “what is a hybrid species,” it is first necessary to have a working understanding of some of the species concepts and what is meant by the term ‘species’.  Known as the Species Problem, biologists have found difficulty in isolating characteristics that define a species for all organisms, hence a multitude of concepts with different parameters (De Queiroz 2007).  Out of nearly 22 species concepts, some of the more well known species concepts include the biological, ecological, and phylogenetic concepts (Mayden 1997).  Despite inconsistencies between concepts being pointed out by scientists and philosophers, the term ‘species’ needs to be defined as “distinguishable groups of genotypes” that stay isolated, even during situations of hybridization and gene flow (Mallet 2007).  For species hybridization, it is important that the third new species is a genetically isolated group which justifies the authors reinterpretation of what makes up a species.


What is a Hybrid?

            Historically there have been conflicting views about the role that hybridization plays in the evolutionary process.  Crosses between two distinct species interested the earliest scientists who postulated whether hybridization could be an explanation for the existence of unusual creatures (Arntzen 1995).  Nearly 100 years ago, it was stated that “Hybridism…grades into mongrelism, mongrelism into cross-breeding, and cross-breeding into normal pairing, and we can say little more than that the success of the union is more unlikely the further apart the parents are in natural affinity” (Harrison 1993).  In this monumental statement, a scientist by the name of Mitchell points out how hybridism ultimately mixes into normal pairing, suggesting that the sexual reproduction of two distinct species leads to homogenized mixtures.  Some say that the term hybrid can be isolated to the product of interspecific crosses and others would agree with the notion that all crosses are considered hybrids because all individuals have unique genotypes (Arntzen 1995).  Despite either classification, scientists have been well aware that there is a clear continuum of ‘hybrid’ crosses (Harrison 2012).

            Another source of conflicting views can be found between botanists and zoologists.  As early as 1954, botanists have reported plant hybrids as common-place and view hybridization beneficial to evolution (Anderson and Stebbins 1954).  Additionally, botanists find difficulty in acknowledging plants a species, instead of hybrids, because of how ambiguous phenotypic differences are and how plant groups do not fit within distinct categories (Rieseberg and Willis 2007).  In contrast, zoologists have been known to be skeptical of this process in animals due to a lack of evidence.  Only recently have zoologists and evolutionary biologists found evidence for hybrid speciation and these animals (e.g. butterflies, ants, and flies) have become model organisms (Mavarez and Linares 2008).


Allopolyploid Hybrid Speciation

            Polyploidy is a characteristic given to a cell or organism has contains two or more sets of homologous chromosomes. The cause of polyploidy is usually due to mitotic or meiotic catastrophe leading to abnormal gametes with additional chromosome pairs.  Categorically, polyploidy can be split up into distinct groups depending on the composition on the composition of chromosomes and the mode by which they are formed. Allopolyploid speciation is well known as a chromosome doubling event that leads to reproductive isolation due to the different ploidy in the hybrid species compared to either of the parents (Nolte and Tautz 2010).  Another type of polyploidy is autopolyploidy.  It is not considered to contribute to hybrid speciation because the species resulting, also, from a chromosome doubling event happens within the original parent species (Mallet 2007).  Figure 01 illustrates the aforementioned difference between allopolyploid and autopolyploid species by illustrating the crossing on chromosomes between two species (A) and (B).

Figure 01. Three Different Types of Polyploidy. This figure shows how each type of hybrid speciation (homoploid or allopolyploid) is different genetically in terms of ploidy.  Furthermore, this image serves as a visual distinction between allopolyploid and autopolyploid.  The latter is similar to the former but this type of ploidy happens amongst one species, not two.

Source: (Soltis and Soltis 2009)

            Kim et al. (2008) utilizes a low-copy nuclear gene region to show several examples of allopolyploid speciation in Persicaria (Polygonaceae) (Kim et al. 2008).  Persicaria is a clade of Polygonaceae, which is family of weeds that contains approximately 120 species occupying disturbed areas and crop fields.  This species was selected because they found inconsistencies between chloroplast DNA (cpDNA) and nuclear ribosomal internal transcribed spacer (nrITS) phylogenies trees after conduction previous molecular studies (Kim et al. 2008). Additionally, they conducted phylogenetic analysis, using a low copy nuclear gene to necessarily reveal the full extent of observed allopolyploidy (Kim et al. 2008).  Upon further investigation, the use of cpDNA is increasingly common for studying plants as it is the most similar to mitochondrial DNA in mammals.  The findings of this study are particularly relevant because they were able to infer the geographic location of predicted hybridization events between species within the Persicaria clade.

            Analysis of cpDNA assisted Soltis and Soltis (1989) in identifying evolutionary information regarding Tragopogon mirus and T. miscellus, examples of recent allopolyploid speciation (SOLTIS and SOLTIS 1989). In 1950, these hybrid species were shown to have different phylogenetic characteristics such as thicker stems and leaves, larger flower and fruit.  Furthermore, the volume of the pollen grains was shown to be the sum of the mean volumes of the parental species (Ownbey 1950).  Similar plant species have been found to be the result of polyploidy and they have been used to study the genetic consequences of such hybridization events.  Focusing on plant species such as Tragopogon is scientifically beneficial because it has been found to naturally exist and serves as a primary model of experimentation before applying these concepts to synthetic models such as Arabidopsis or Gossypium (Soltis and Soltis 2009).

            Another interesting plant species known to be of hybrid origin includes the tetraploid orchid, Platanthera huronensis.  P. huronensis has morphological features that appear to make it a hybrid of P. dilatata and P. aquilonis and Wallace (2003) provides molecular evidence that supports allopolyploid hybrid speciation.  Morphologically, the hybrid species is said to have a greenish white flower color with dilated or tapering lips (Lisa E. Wallace 2003).  Genetic analysis of nuclear and chloroplast markers was used to conclude the evolutionary novelty of this species.  Identification of the parental species included selecting genetic markers that showed a high degree of intraspecific polymorphism (Lisa E. Wallace 2003), which would indicate the similar phenotypes between the hybrid species and the potential parents.   As with most of the hybrid species research, it is necessary to evaluate any sources of error or any alternate reasoning for the collected results.  One concern for Wallace (2003) is the potential for interspecific hybridization due to the species sharing the same geographic location.


Homoploid Hybrid Speciation

            Homoploid hybrid speciation is the second main type of hybrid speciation, which occurs when distinct species are found within the same geographic regions and hybridization between the two species takes place.  This does not result in ploidy differences from the parent species and is considered to be not as common as allopolyploidy (Abbott et al. 2013).  Similar to allopolyploidy, homoploid speciation is just as equally a creative force by forming new traits unseen in parental species.  Some major examples not only include plants such as the desert sunflowers but also animals such as the yellow-rumped warbler or the Italian sparrow.

            Initially, before species A and species B can create a hybrid species, an environmental change must first occur which will in turn promote the “secondary contact of separated evolutionary lineages” (Nolte and Tautz 2010).  At some point in evolutionary history, the common ancestors of species A and B diverged.  Due to homoploid speciation, these two species are able to now create the hybrid lineage, as represented in figure 02.  After an initial environmental or geographic change, allowing for secondary contact between species, a hybrid swarm is formed.  The formation of this swarm is important because this is the step of homoploid speciation that leads to the formation of a successful hybrid zone and even a third independent species (Nolte and Tautz 2010).  This pattern of speciation is also supported in allopolyploidy in plant speciation.


Figure 02.  Homoploid Hybrid Speciation Process.  This figure visually represents two species (A) and (B) coming in contact, producing a hybrid swarm represented by the cloud, and eventually starting one or more novel hybrid lineages.

Source: (Nolte and Tautz 2010)

The yellow-rumped warbler complex (Dendroica coronata) was most recently studied by Brelsford et al. (2011) to examine homoploid hybrid speciation.  This complex is made up of four forms, one white-throated form and three yellow-throated forms (MilÁ et al. 2007).  The study conducted by Brelsford et al. (2011) investigates the possibility of whether D. auduboni is the result of a successful secondary contact and hybridization between D. coronata and D. nigrifrons.  This hypothesis was supported by mitochondrial DNA analysis suggesting the divergence between D. coronata and D. auduboni to have occurred approximately 1.7 million years ago.  As is the case for allopolypoid hybrid speciation, there are several categories that have been measured and analyzed to strengthen the assurance of a third independent species. 

Some of the most important quantitative variables that were used for data collection and analysis include mDNA, amplified fragment length polymorphism (AFLP), morphology and plumage coloration (Brelsford et al. 2011).  The AFLP analysis produced significant results indicating a clear cluster distinction between the parent species, D. coronata and D. nigrifrons (Brelsford et al. 2011).  Additionally, the D. auduboni cluster was found to be between D. coronata and D. nigrifrons.  Despite the strong genetic evidence supporting this strong model of homoploid hybrid speciation, reproductive isolation between D. auduboni and D. nigrifrons has yet to be demonstrated (Brelsford et al. 2011).  Reproductive isolation, in any situation of possible hybrid species, is crucial and plays an important role in increasing overall genetic diversity.  The lack of reproductive isolation is considered by Brelsford et al (2011), suggesting that a fusion event may be possible.

The importance of reproductive isolation is not lost with the yellow-rumped warbler complex.  An excellent model of hybrid speciation and reproductive isolation has been made by studying the Italian sparrow, a hybrid species whose parents are the Spanish sparrow P. hispaniolensis and the house sparrow P. domesticus.  The authenticity of the Italian sparrow was assured by Hermansen et al. (2011), as their findings suggested great divergence between the parental species and the hybrid, a mixed nuclear genome and no unique haplogroups found at various loci.  The house sparrow has been shown by population genetic inference to have grown in population and expanded its distribution in the middle east about 3000 – 7000 years ago (Saetre et al. 2012).  On the other hand, the Spanish sparrow has long been known to reside in the Mediterranean region (Ericson et al. 1997) and these two parental species came into contact about a few thousand years ago (Hermansen et al. 2011). An assessment known as the hybrid index provided a relative understanding of the contribution of parental loci (Eroukhmanoff et al. 2013). The reported results for the hybrid index values ranged from 0.3 to 0.64, with a theoretically ideal hybrid index being 0.5 (Eroukhmanoff et al. 2013).  These index values, accompanied with heterozygosity values ranging from 0.77 to 0.93, demonstrate that the Italian sparrow is a recently developed hybrid species with a high amount of genetic diversity (Eroukhmanoff et al. 2013).  

Finally, a complete understanding of homoploid hybrid speciation would not be sufficient without an example of known plant models such as the North American sunflower.  Interestingly enough, this is one of the better known examples of homoploid hybrid speciation in plants.  The genus Helianthus, has three hybrid species (H. anomalus, H. deserticola, and H. paradoxus) that arose from the same two parent species, H. annuus and H. petiolaris.  The resulting hybrid species have been shown to have restrictive geographic boundaries despite the parent species being distributed across the continent of North America (Gross and Rieseberg 2005).  Additionally, this hybrid speciation demonstrates hybrid transgressive variations, which are known as extreme adaptations viewed in new hybrid species (Rieseberg et al. 1999; Mallet 2007).  All three of the resulting hybrid species have traits that allow for survival in extreme conditions such as geographic areas that experience drought (Mallet 2007).  Additionally, synthetic hybrids were used to conclude that complementary gene action is the mechanism that caused hybrid transgressive variation in ancient hybrid species dating back nearly 60,000 years (Gross and Rieseberg 2005).  The increased fitness of the species hybrids by means of introduction of extreme traits acts as a unique mechanism of genetic variation amongst the Helianthus genus.   



              Hybrid speciation, once the source of great skepticism, is now being unraveled and investigated further.  Examples such as the Italian sparrow and the yellow-rumped warbler complex should be of great interest to scientists because of the strength of the arguments being provided.  Additionally, genetic and morphologic analysis techniques such as those illustrated in several of the examples provided serve as excellent examples for scientists looking to move forward with identification of hybrid species. Hybrid speciation has been shown to be an incredible source of genetic variation, which ultimately creates species that are able to occupy new niches. When successful, hybrid transgressive variation allows for the genetic information of each parent to continue on, at least in some part, to new local or even global locations.  The coming together of genetic information from two distinct species is an idea that seems rather monstrous, but when viewed under an evolutionary microscope, has applications to all forms of life.  Because speciation is a rather slow process, taking thousands of generations to develop significant measurable differences, one area of hybrid speciation that lacks in research is the time after the secondary contact.  Further points of additional research include a more detailed analysis of synthetic hybrid experimentation as well as understanding any potential mathematical models that may compliment hybrid speciation.

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