In biology, speciation is the name given to the process through which a population of a determined species gives way to another, or other, species. The process of speciation, throughout millions of years, has sparked the birth of an enormous diversity of organisms, millions of species from all the kingdoms, that have populated, and continue to populate, the Earth ever since the moment that the first seas were formed.
Principal mechanisms of speciation
- Cladogenesis, or bifurcation, comes about with the reproductive isolation of different populations due to the barriers to hybridisation (for this reason, different species cannot reproduce with others). Before an exchange of gametes, there are ecologic, ethologic or mechanical barriers that prevent them from unity, so fertilisation cannot happen (prezygotic barriers). There is also the possibility that even if two distinct species can reproduce, the result will not be viable, be it through spontaneous abortion, sterility of the hybrid, premature death, weak hybrids or illnesses (postzygotic barriers).
- Hybridisation is the reproductive cross between two different species that can produce viable individuals, that may or may not be fertile.
Types of speciation
Geographic or allopatric speciation
This is the gradual speciation that takes place when a species occupies a large geographic area that, due to geographic barriers like seas, mountains or deserts, prevents the individuals that are located far from each other from meeting. This is, therefore, about the geographic separation of a gene pool so that two or more geographically isolated populations are established, and, over time, the absence of a genetic exchange, or gene flow, leads to differences between populations becoming so great that they now cannot be considered to be part of the same species, therefore forming a new species altogether.
Separation among populations can be due to the migration or extinction of populations situated in intermediate geographic positions, or brought on by geologic occurrences, like the formation of rivers or mountains. The barrier can be geographic or ecologic, for example the peaks that separate valleys in mountain ranges, or desert zones that separate wet zones.
Allopatric speciation can be divided into two models:
Model 1 (allopatric vicariance speciation): This is produced by the division of an ancestral species into two relatively large populations that remain isolated, at least until the appearance of evolutionary independence. This separation occurs due to the formation of a geographic barrier, and the differentiation that happens among populations can be down to genetic factors, selective processes, or adaptation.
One example of this type of speciation that we can apply not only to Mashpi, but to the whole Chocó region, is the formation of the Andes mountain range, a geographic barrier that, once it had formed, separated the tropical forests of the Chocó from the Amazon Basin. So in its evolution, the geographic isolation of the Chocó created a unique diversity.
READ MORE: Understanding Mashpi Diversity
Model 2: This model takes into account the type of allopatric speciation applicable to asexual species, where there is no evidence of genetic flow between populations, and where the identity as a species is determined by historic motives (e.g. evolutionary stasis). In these cases, genetic derivative can be the most important evolutionary factor and speciation can be fastest. But if the original populations are subjected to geographic variation, local adaptive pressures can also be an important factor in speciation.
This occurs when, in a population, the individuals that are found on the periphery conquer a new ecologic niche; that’s to say that the new species can emerge when part of the population moves to a marginal habitat, far from the limits of distribution of a central population of a greater size. In this way, the genetic flow between these two populations is cut short and the peripheral populations become a distinct new species. This process generally occurs after dispersion and colonisation phenomena. Just like in the case before, the adaption to these environments can be the trigger of divergence, but when dealing with small populations, stochastic factors can prove more importance.
Perhaps one of the most important colonialisation and dispersion processes for the diversity of the Chocó, and for the whole of South America, was the Great American Interchange. After the formation of the Isthmus of Panama, there were many species of plants and animals from North American that managed to cross the Isthmus and were able to establish themselves in South America, increasing the biome, or community of plants or animals, of the continent today but also causing the extinction of many species.
According to this model, speciation is produced without the complete geographic separation of populations. In fact, the daughter species can share part of the area of distribution with the mother species and hybridise in contact zones. The rest usually diverge due to factors, anything from fate to local selection. Inter-population migration and hybridisation can counteract divergence, but in some circumstances, the diversifying selection can be the more important factor, implying gradual change in the frequency of some characteristics. The classified mating and a reduction in biological efficiency of the hybrids would drive the final development of speciation.
Sympatric speciation implies the divergence of some local populations until they achieve evolutionary independence within the same geographic space. It often entails the new populations using different ecologic niches, with the distribution range of the ancestral species, developing mechanisms of reproductive isolation.
The divergence in sympatric speciation can be encouraged by the ecologic specialisation of some of these small populations, even though there is also the possibility that speciation is produced by hybridisation between very similar species. Another type of sympatric speciation implies the formation of one asexual species from a precursor sexual species.
Competition between individual members of the same species for a limited resource can be considered a valid example to explain this type of speciation. Where some of the members of an ancestral species had to specialise and occupy a different ecologic niche to be able to survive the intense inter-species competition.
What is endemism?
One word that you are certain to hear on the trails of Mashpi is “endemism”, and reference to “endemic” species, like the Mashpi frog. The term is used in biology to indicate that the distribution of a taxon, or a group of organisms like a species, is limited to a reduced geographic area and is not naturally found in any other part of the world. For this reason, when a species is said to be “endemic” to a certain region, it means that it is only possible to find it in this place.
We can look at endemism within a wide range of geographic scales: an organism can be endemic to a specific habitat like a mountain peak or a lake, a mountain range or river system, or even a continent. Normally, the concept applies to species, but it can also be used for subspecies, varieties, groups or families.
Mashpi is located in the middle of two bioregions: the tropical forest of the Chocó and the mountainous forests of the tropical Andes, both areas of the highest importance, or “hotspots”, for conservation (Mittermeier, et al. 1997), due to their great concentration of endemic species.
Table 2.- Total diversity and endemism for some biologic groups found in the tropical forests of the Chocó and the mountainous forests of the tropical Andes. (Mittermeier, et al. 1997).
Hoorn, C., Wesselingh, F., ter Steege, H., Bermudez, M.A., Mora, A., Sevink, J., Sanmartin, I., Sanchez Meseguer, A., Anderson, Cajsa L., Figueiredo, J., Jaramillo, C., Riff, D., Negri, F.R., Hooghiemstra, H., Lundberg, J., Stadler, T., Särkinen, T., Antonelli, A. (2010). Amazonia Through Time: Andean Uplift, Climate Change, Landscape Evolution, and Biodiversity. Science (New York, N.Y.). 330. 927-31. 10.1126/science.1194585.
Mittermeier, R.A., C. Goettsch-Mittermeier y P. Robles Gil. 1997. Megadiversidad: los países biológicamente más ricos del mundo. Cemex-Agrupación Sierra Madre, México.
Instituto Nacional de Estadística y Censos, Cifras de Biodiversidad, http://www.ecuadorencifras.gob.ec/documentos/web-inec/Sitios/pagina%20ambientales/