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Emergence of Reproductive Isolation and Speciation

Discuss about the Learning on Sexual Selection and Speciation.

Speciation is an evolutionary process via which the a specific group of populations evolve in order to become distinct species. The concept of speciation and the difference between the species was first elucidated via Orator F. Cook who coined the term cladogeneisis in the year 1906. Cladogenesis means splitting of lineages and is different from that of anagenesis which deals with phyletic evolution within the lineages (Abbott et al., 2013; Rabosky et al., 2013). The concept of speciation furthered gained a new meaning with the publication of the book, The Origin of Species by Charles Darwin. It is Darwin who highlighted the concept of natural selection and sexual selection which further defined the concept of speciation (Abbott et al., 2013; Rabosky et al., 2013). Speciation is the central topic of evolutionary science (Butlin et al., 2012). This is because; studying speciation helps to track the path of the evolution of organisms. However, according to Butlin et al. (2012), the debate upon the concepts of species is often considered as unproductive as biological species concept is now predominantly accepted is the term for defining speciation. The following essay aims to highlight the main factors responsible for speciation. Analysis of these factors will help to get a detailed insight about the process of gradual progression of the concept of speciation. At end, the report also aims to highlight the future implication of the study in the domain of speciation.

In order to conduct this research I employed a thematic analysis approach based on the search of the literary articles. The articles were searched in the electronic databases like Medline and PubMed. The main keywords which are used to search literary articles include “speciation”, “speciation barriers”, “speciation AND evolution”. On the basis of the keyword search I found few relevant articles which I will use in the body of the assignment to backup my arguments. These references will be helpful because it will help to draw a critical insight about the factors contributing towards speciation with insightful examples.

As per the biological species concept, understanding the concept of speciation demands proper insight about the process of breakdown of gene flow (Butlin et al., 2012). The process of gene flow can be critically analysed upon discussion of the reproductive isolation. The primitive scheme used for elaborating the origination of the reproductive isolation is whether the flow of gene is restricted before (“prezygotic”) or after (“postzygotic”) the formation of hybrid zygote. Butlin et al. (2012) have described prezygotic isolation as either crucial or previous evolving barrier towards flow of the gene in comparison to that of post zygotic isolation. Thus, it can be regarded as an important process behind the concept of speciation. The view stated by Butlin et al. (2012), stems from numerous examples of speciation showcasing significant evidences against prezygotic isolation (frequently behavioural mate differentiation among the animals or difference in pollinator in plants) in comparison to postzygotic isolation. According to Sobel et al. (2010) prezygotic isolation is crucial at any stage of speciation. This is because an organism is required to mate outside its own herd before it generates a hybrid yet sterile progeny. However, this ideas does not correctly responsible for extrinsic postzygotic isolation due to the process of differential adaptation that at times considered as the first step of speciation (Butlin & Ritchie, 2009). In the domain of reproductive isolation, Riesch et al. (2012), is of the opinion that evolution of human has been stringently shaped by the interaction between gene and culture and there is an ever rising evidence that identical processes can also cast a huge impact on the population of non-human primates. According to the theoretical studies, the culture can be regarded as important evolutionary mechanisms due the ability of the cultural traits to spread rigorously in both vertical, horizontal and oblique direction (Schluter, 2009). This results in decrease in variance within the group and increase in variance between the groups. In their study Riesch et al. (2012) have accumulated extensive literature over the population divergence in killer whales (Orcinus orca). They have argued that they are in a process of ongoing ecological speciation arising out of dietary specialization. Although they failed to overlook the chances that cultural divergence pre-dates ecological divergence. They proposed that cultural difference coming as learned behaviours in between the ecologically divergent population of killer whales have resulted in the achievement of significant reproductive isolation leading to the generation of incipient speciation (Riesch et al., 2012). Thus from my understanding from the above discussion, I think that proper understanding of the contributing component of the reproductive isolation is crucial in order to understand the detailed process of speciation.

Impact of Genetic Drift on Speciation

According to Uyeda et al. (2009), chance events and genetic drift like mutation, colonisation, chromosomal rearrangement and polyploidy are regarded as strong contributors towards the process of speciation. However, the conditions under which the drift is the principal contributor of speciation is limited this is primarily because of the huge impact of gene flow. According to Tazzyman and Iwasa (2010), genetic drift might initiate the process of speciation via bestowing the preliminary divergence upon which the selection acts and this said point of overlap is responsible towards the evolution of reproductive isolation under the presence of significant flow of gene. Servedio and Noor (2003) further stated that genetic drift might also effect during the process of reinforcement via generating preliminary linkage disequilibrium in between the selected traits and mating traits. Moreover, it can be stated that speciation occurring via divergent selection is influenced via the founder events and this must often be the scenario of colonisation of new habitats (Butlin et al., 2012). According to Martin et al. (2013) the difference in the pattern of colours in between the population of poison-dart frog Oophaga pumilio across Bocas del Tro archipelago arise out of sexual selection as two other non-sexually selecting species of Dendrobatic found in the identical habitat do not showcase this variation. Martin et al. (2013) theoretically tested this assertion via employing quantitative genetic sexual selection model. They incorporated random drift and aposematic coloration. Martin et al. (2013) elucidated that sexual selection might have caused the observed variation through novel process interlinked with genetic drift. Under this novel process, sexual selection forces the colour of the frog to strictly follow the evolution of female genotype. Any in between population divergece arising out of genetic drift is transferred on to colour. Thus genetic drift lead to the development of sexual selection resulting in speciation if O. pumilio. The results elucidated by Runemark et al. (2010) upon their experiment on females of Skyros wall lizard, Podarcis gaigeae showed that interaction between genetic drift and selection pressure cause divergence even at phenotypic level and this further leads to speciation. As per my understanding from the above discussion I feel that rate of gene flow between the diverging population readily influence the role of genetic drift in the process of speciation. I feel proper evaluation of the impact of drift on the rate of gene flow during the process of divergence is the major challenge in the speciation research.

Reinforcement and Speciation

Reinforcement remains a contentious factor contributing towards the process of speciation (Pfennig, 2003). It is defined as a process of evolution arising out of increased prezygotic isolation that results out of selection occurring against inter-specific mating (Bank, Hermisson & Kirkpatrick, 2012). Yukilevich (2012) have further opined that reinforcement is one of the most widely discussed matters in the domain of evolutionary biology under the mechanisms of evolution. Although the process of speciation is assumed as the by product of evolutionary process, the mechanism of reinforcement specifically posits that the theory of natural selection directly favours the process of speciation. However, the main problem in quantifying the nature of reinforcement in the process of divergence is that the important pattern highlighted by the concept of reinforcement (that is increase isolations in the habitats of sympatry in comparison to habitats of allopatry). This crucial pattern might collapse over time as a result of active flow of gene or can also result as an outcome of the other ecological processes like ecological character displacement (Pfennig, 2003). According to Bank, Hermisson and Kirkpatrick (2012) whether total closure of the genetic introgression via reinforcement takes place can vary on the size of effect which the mutations have over the sensory system used in the choice of mate. On the basis of this method, Yukilevich (2012), proposed that assymetrical pattern of speciation that is common in drosophila is uniquely supported by the process of reinforcement.  Their study concluded that there is 60 to 83% of impact on reinforcement on the sympatric Drosophila and this impact enhance pre-mating by 18 to 26% thereby causing speciation. Another study conducted by Matute, (2010) further provided a detailed insight about the process of reinforcement and its subsequent effects on the evolutionary process of speciation. According to Matute, (2010), reinforcement might overcome the flow of gene during the speciation in Drosophila and thereby providing valuable evidence towards the contribution of reinforcement on speciation.  Thus from the above review of literature, I think the process of reinforcement can be regarded as the major contributing factors behind the process of speciation. I think the study of reinforcement of Drosophila have helped the researcher to get a detailed insight about the process of reinforcement.  However, the direct impact of reinforcement on the process of speciation is difficult to study because of its divergence pattern on sexual selection.

According to Wood et al. (2009), hybridization is a crucial process in the history of plant speciation. Evidence behind the fast speciation in plants especially by allopolyploidy is strong on the other hand, speciation centring polyploidy is comparatively less in animals (Wood et al., 2009). Barriers that putatively decrease the process of speciation through this mechanism of hybridization have been discussed by Clarkson et al. (2010). According to Clarkson et al. (2010), the principal problem in hybridization is to overule is the lack of reproductive isolation from the parental population. Gompert et al. (2006) provided solution to this problem via including the concept of significant ecological separation from parental species (for example transgressive variation or novel niche occupation). In contrast to the homoploid hybridisation under which new linkage is close to equal mixture ration of the parent population, hybridisation can highlight be an important source of genetic diversity. This genetic diversity arising out of hybridisation allows one or more than one few selectively favoured genetic traits to introgress from a associated species and simultaneously contribute towards the success of the novel linkage in the absence of general amalgamation of the parental genomes (Clarkson et al., 2010). As per my understanding about the concept of hybridization in the domain speciation, there are an urgent requirement for elucidation of advanced analytical techniques that will help to distinguish between the past and the current process of hybridization from other genetic effects that have resulted in the generation of shared variation in the cross taxa.

Research on the process of speciation demands which genes and network of genes contribute towards the process of speciation. However, other researchers have used different criteria of defining or establishing this association. It has currently been critiqued that only the changes occurring at the genetic level which contributes towards an increase in the reproductive isolation must be accepted as the process of speciation genes (Nosil & Schluter, 2011). However, I think distinguishing such speciation contributing genes can be difficult. I also feel that the concept in extremely relevant for the genes that contribute towards early and ongoing process of speciation. Nevertheless, employing more constrained definition will enable to give proper guidance towards future work process. According to Presgraves (2010), the genes of speciation or the speciation genes can be linked with any various types of reproductive isolation barrier but majority of the information is available in the domain of hybrid dysfunction. Lack of compatibility between the mitochondrial genes and nuclear genes which are associated with the process of reproductive isolation is well-established in plants but are recently been established in relation to the human genome (Ellison, Niehuis & Gadau, (2008); Gibson et al. (2010). Recent works undertaken by Johnson (2010) has indicated that genetic changes which are responsible for the process of hybrid incompatibility can be constant for the non-adaptive reasons (that is: co-evolution of meiotic drivers along with their suppressors and duplicate gene silencing). As per my understandings, this highlights the requirement for the future work-process to remain connected with the speciation genes alongside with the process of that has lead to the generation of species divergence. I also feel that more and more genes are required to be elucidated in the wider range of taxa for extra components of isolation along with the proper view of the interactions of gene-environment and proper evidence stating their distinct roles in the process of evolution of the reproductive isolations. I think only then it will be possible to make generalised view about the types of gene that are responsible to incompatibility leading to sexual isolation or divergence and thereby causing speciation.

Apart from studying the factors that are responsible for the process of speciation, I feel it is also important to note that the cause behind the variation of the rate of speciation and duration. According to (Yoder et al. 2020), duration and rate of speciation might be influenced by the same factor. For example, the success rate of adaptive divergence must co-vary with the ecological opportunity. Moreover according to Funk, Nosil and Etges (2006), the process of natural selection can increase the process of divergence and this has been linked with the extent of reproductive isolation. Thus barren environments might cause both frequent and rapid isolation. However, this might generate to more frequent initiation of speciation but the same remains incomplete because either the flow of gene persists or because the population which is divergent do not persist or due to certain environmental change.  So I feel resolving this problem will demand both advanced yet comparative method for better understanding of the mechanisms of speciation and extinction.

Understanding the process of speciation is extremely crucial towards the process of building a comprehensive macro-evolutionary synthesis. A number of theoretical constructs have been opined in order to explain numerous differential patterns viewed in speciation like fossil records or higher rates of speculations among the generalist taxa versus the specialist taxa. However, the majority of these explanations mostly rely on one or two explanatory variables and these may lead to the generation of overly simplistic narration of the complex process of speciation (Stigall, 2015). Development of a comprehensive understanding about the process of speciation demands broadness in synthesis of explanatory factors. The major explanatory factors that can employed in order to fully understand the concept of speciation include the role of reproductive isolation in the mechanism of speciation, significance of genetic drift in the process of speciation, importance of natural selection and sexual selection in the process of speciation and the environmental and genetic factors contributing towards speciation (Stigall, 2015; Verzijden et al., 2012).

Proper understanding of the speciation based on above mentioned factors will in turn help to get a detailed insight about the process of biogeography (Wiens, 2011). Biogeography can be defined as the process of distribution of ecosystem and species within the within the geographic space throughout the geological time (Wiens, 2011). Thus the understanding about the biogeography will further help to get a detailed insight about the niche matters of biogeography, biogeographic pattern of species migration, habitat change and subsequent niche conversation (Shafer & Wolf, 2013). Thus I think, getting a detailed aspect of numerous factors of biogeography via an active study of speciation and this in turn will help to study the process of evolution.

Conclusion

Thus from the above discussion it can be concluded that, there are numerous factors that contributes towards the process of speciation like reproductive isolation (resulting in sexual selection and subsequently natural selection), genetic drift, reinforcement and hybridization. However, in future a detailed insight is required to study the influence of environmental factors towards the process of speciation. This analysis will lead to elucidations of the external factors which are responsible for the incompatibility and thereby promoting sexual isolation, genetic drift and divergence and thereby causing speciation. Clear concept about the ongoing process of speciation will help to get a clear picture of evolutionary biology and thereby leading to advancement in biogeography.

References

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Bank, C., Hermisson, J., & Kirkpatrick, M. (2012). Can reinforcement complete speciation?. Evolution, 66(1), 229-239.

Butlin, R. K., & Ritchie, M. G. (2009). Genetics of speciation. Nature

Butlin, R., Debelle, A., Kerth, C., Snook, R. R., Beukeboom, L. W., Castillo, R. C., ... & Hoikkala, A. (2012). What do we need to know about speciation?. Trends in Ecology & Evolution, 27(1), 27-39.

Clarkson, J. J., Kelly, L. J., Leitch, A. R., Knapp, S., & Chase, M. W. (2010). Nuclear glutamine synthetase evolution in Nicotiana: phylogenetics and the origins of allotetraploid and homoploid (diploid) hybrids. Molecular Phylogenetics and Evolution, 55(1), 99-112.

Ellison, C. K., Niehuis, O., & Gadau, J. (2008). Hybrid breakdown and mitochondrial dysfunction in hybrids of Nasonia parasitoid wasps. Journal of evolutionary biology, 21(6), 1844-1851.

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