The Emergence of New Animal Species
A Lengthy Dive Into the Various Modes of Speciation
Introduction
One of the fundamental objectives of evolutionary biology has been to provide a coherent explanation of how new species arise from ancestral forms. A notable challenge, as previously argued by Bush (1975), is that speciation appears to be an ad hoc science — no one (during his writing) has observed the emergence of a new species. This line of argumentation lies at the heart of those who deny the actuality of evolution. Something along the lines “if evolution is true then why aren’t modern monkeys transforming into humans?”. Decades earlier, one of the biggest contributors to evolutionary biology, Ernst Mayr (1949), had acknowledged that the essential bit of the theory of evolution concerned with the emergence of new species remained mysterious. Since then, a number of experiments — some described by Rundle (2005) and Schluter (2009) — have been conducted to cement concepts of the emergence of new species.
Irrespective of these developments, a persistent hurdle is the lack of consensus among biologists on what should define a species, aptly called the species problem and discussed in a previous article of mine. The most common definition is Ernst Mayr’s Biological Species Concept (BSC). This concept defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups (Nosil, 2008). The obvious drawback of this definition is it being inapplicable to asexually reproducing organisms, a problem that Mayr himself was well aware of. Regardless, the biological species concept is perhaps one of Mayr’s most notable contributions to evolutionary biology, as it tremendously helped in the understanding of a number of evolutionary phenomena.
It is of necessity that distinction is made between the two ways in which speciation is viewed, i.e., distinguishing the Lamarckian perspective of speciation — named after the French naturalist Jean-Baptiste Lamarck — from the Darwinian perspective of speciation. The former, also known as vertical evolution, discusses speciation as being the transformation of one species into another (Mayr, 1982). This would be akin to questions such as “what species will humans turn into next?” that some people tend to ask. This perspective fosters a linear view of evolution, with populations evolving towards certain improvements. Vertical evolution appears to be intuitive and rather easier to work around, especially when seen under the lens of Darwinian natural selection, but it does not explain how two species can arise from the same parental population. The more difficult task, therefore, is ascertaining modes of horizontal evolution, an aspect concerned with the multiplication of species (Mayr, 1982). How do we explain the divergence of an ancestral population into modern humans and chimpanzees?
Modes of Speciation
Under the light of the Biological Species Concept, the standard of speciation demands that two populations become reproductively isolated. Reproductive isolation between two populations of the same species can be intrinsic or extrinsic, and includes i) the development of different mate-recognition systems, ii) the novel occupation of different ecological niches, iii) a subtle disparity in the breeding period, iv) the infertility of the offspring, and v) the separation of populations by a physical barrier — say a rift separating two populations of wild boars. These barriers to reproduction result in the inability of the two populations exchanging genetic information, in turn aiding to their genetic divergence, and potentially leading them to becoming two different species (Nosil, 2008). Essentially, speciation demands that the two populations develop intrinsic barriers to gene flow.
Mayr’s allopatric speciation has served as the benchmark for which most of speciation events in animals are explained. Other proposed modes of speciation include sympatric speciation and ecological speciation. Interestingly, and somewhat ironic for the Biological Species Concept, is hybridization being a potential mode of speciation.
I. Allopatric speciation
The term allopatry means existing in nonoverlapping geographic areas. Therefore, allopatric speciation is a mode of speciation that relies on geographic barriers to catalyze the separation of two, initially homogenous, populations into separate species. A geographic barrier extrinsically ensures that two populations of the same species are not exchanging any genes. Logically, each population would end up accumulating its own genetic mutations. Mayr (1982) highlights that during this period of geographic isolation, a population can acquire intrinsic reproductive barriers that, upon reuniting with the parent population, will diminish their chances of successfully interbreeding.
While very potent at conceptualizing speciation, the most challenging aspect of allopatric speciation is establishing when two populations have intrinsically become reproductively isolated, and should be considered as two different species. Wiens (2004) points out that, by virtue of a geographic barrier, two populations can be reproductively isolated but, in principle, still be able to interbreed. Allopatric populations, therefore, cannot be considered true species until there is secondary contact between the two population. The secondary contact would test if the two populations have intrinsically become reproductively isolated (James et al, 2010), an encounter that may not occur in some diverged populations for hundreds of years. How can one boldly designate allopatric populations as being distinct species?
II. Sympatric speciation
At the other end of allopatric speciation lies sympatric speciation. The term sympatry means occurring in the same geographic area, or areas that are overlapping. In this mode of speciation, a population breaks away into two species via the development of reproductive barriers without the need for the populations to initially be separated. Both White (1968) and Mayr (1982) were not fond of sympatric speciation, arguing that a [genetic] reproductive barrier cannot selectively develop among a few individuals in a population without the alleles spreading within the entire population. In his paper, however, Bush (1975) argued that the mechanism of reproductive isolation in sympatric populations is more ecological than it is behavioral. There now exist data showing that speciation can occur even in the face of gene flow (Nosil, 2008).
One elaborate mechanism of sympatric speciation has been described in the study of European corn borers. It has been shown that this insect exists in different ‘races’— based on differences in their chemical secretions (pheromones) — and, during reproduction, each population responds to a specific cue of pheromones. It can be suggested that polymorphism (existing in multiple forms) within a population is a precursor for sympatric speciation. Theoretically, different individuals in a polymorphic population may have a mating preference for one morph over the other, and this may eventually lead to divergence.
III. Ecological speciation
Perhaps the most convincing and highly supported mode of speciation, since Mayr’s allopatric speciation, is ecological speciation. Ecological speciation is defined as the evolution of barriers to gene flow between populations as a result of ecologically-based divergent natural selection. Organisms tend to have different ‘occupations’ within their environment. They make their living in different ways. Think of the African Savannah where leopards are solitary and cannot fend off scavenging hyenas, so they evolved the ability to have their meals in the safety of trees. Lions, on the other hand, feast in prides and can hold their ground against the pillaging hyenas, hence have no need to feast in trees.
The distinguished Mayr (1947) himself, though considering it secondary to geographic speciation (Nosil, 2008), appreciated the role that ecology plays in speciation. Under ecological speciation, the populations adapt to different environments, eventually leading them to being reproductively isolated. We can conjecture that the ancestral population that branched into the African lions and leopards arrived at different solutions to the same environmental problem, leading to reduced contact, and eventually evolving independently. Arguing for the importance of ecology in speciation, Mayr (1982) further elucidates that the mere development of reproductive isolation is not enough to allow the coexistence of these diverged populations. The development of a distinctive niche ensures that the populations coexist without outcompeting each other to the death.
While not overtly discussed, catastrophes tend to contribute to speciation through adaptive radiation — the rapid diversification of organisms. The occurrence of catastrophes, which sometimes lead to extinctions of some species, carves new ecological niches in the environment, leading to the accelerated diversification of populations to occupy these new niches.
IV. Hybridization
The idea of hybridization leading to new species is very conflicting. In fact, hybridization is an easier way to confer species status between two population. Given the Biological Species Concept, donkeys and horses are different species because their offspring — the mule — cannot reproduce. Interestingly, in 2020, the Economist reported a peculiar case where a new species emerged as a product of hybridization.
The case was based on the works of evolutionary biologists Rosemary and Peter Grant who, in 1981, observed a large cactus finch (Geospiza conirostris) mate with a medium-sized finch (Geospiza fortis). This mating produced hybrids that were both fertile as well as reproductively isolated from the parent species. Their mating was a result of the large cactus finch being blown from a part of the archipelago approximately 100 km away. A fascinating single-step speciation leap that Mayr and colleagues heavily criticized. It was Richard Goldschmidt who conceptualized single-step leaps in what he called hopeful monsters. Perhaps this single-step hybridization event leading to a new species is what he meant. Who said nature is any easier?
Mechanisms of Reproductive Isolation
Essentially, reproductive isolation mechanisms are grouped into prezygotic (before mating) and postzygotic (after mating). The genetic basis of postzygotic isolation, as evident in interspecific hybrids, is easier to understand. The genetic incompatibilities between genomes that are expressed when present in hybrids are an example of a postzygotic isolating mechanism (Rundle and Nosil, 2005).
Prezygotic isolations can result from both extrinsic and intrinsic factors. The phenomenon of allopatry presents the case of extrinsic isolation, which is easily reversed when the two populations occur in sympatry. Natural selection sometimes penalizes populations migrating from one area to another. This is observed in instances where the migrants are not well adapted to this new habitat, reducing their viability and successful integration with the resident population.
While there are many unknowns on the genetic basis of reproductive isolation, a closer look at the driving force of sexual selection may offer clues to how these isolations happen. In essence, unearthing genes that potentially contribute to the ethological or physiological basis of mate choice among organisms may provide the answer. However, Vaneechoutte (1997) provides a counterargument to the genetic approach to reproductive isolation. He argues that the challenge of a genes-eye mechanism of reproductive isolation is that it requires the establishment of an equally new mate-recognition system in the other sex. That is, if a male bird develops a new singing pattern as a product of a mutation in particular loci, it would require parallel mutations in the loci of the female bird to be able to recognize such a mating call.
With reference to an extensive study of bird songs, Vaneechoutte hypothesizes a non-genetic, cultural approach, that may lead to reproductive isolation. Evidently, regional dialects of bird songs are complex traits and cannot be easily traced back to the genes. The hypothesis is especially powerful in organisms that have some form of cultural learning. Hence, deviation from the standard mating behavior can be learnt by the offspring of these deviants and easily passed on to future generations.
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