8 Process of Speciation

Dr. Felix Bast

1.Learning outcomes

1.1     To learn about various species concepts and species problem

1.2     To learn various modes of speciation, including allopatry, parapatry, peripatry and sympatry.

1.3     To learn the process of speciation at the molecular level

2.    Concept map

3.      Description 

3.1. Introduction

 Species is the basic unit of biodiversity, as well as biological classification and taxonomic hierarchy; therefore, it occupies a central position in understanding the biological diversity of the world. In earlier days, species is thought as fixed entities, the so-called ‘fixed species concept’, in which static entities of species are thought to have existed ever since the beginning (or the creation). Swedish scientist Carl Linnaeus, widely regarded as the father of taxonomy, based his system of classification on this fixed species concept. He stated “Species are as many as were created in the beginning by the Infinite.” (Linnaeus, 1758). However, ever since Charles Darwin’s Theory of Evolution by Natural Selection, species is regarded as dynamic, ever-evolving entities, and therefore, defining what construes a species has become such a profoundly complicated endeavour. As per the theory of evolution, global biodiversity is not thought to be encompassing of random, static species, but ever-dynamic populations evolving continuously. In other words, the entire biodiversity is related by means of a giant tree, the so-called tree of life. Life on planet earth is thought to have originated approximately 4 billion years ago through the process of abiogenesis, in a hydrothermal vent. The first organism ever formed is now known in acronym LUCA, the Last Universal Common Ancestor, the organism from where the whole biodiversity came from. This LUCA forms the very root of the tree of life. New species form through the process of speciation, and existing species disappear through process of extinction. These two processes, speciation and extinction, are the major factors determining the global biodiversity at any specific time. In the tree of life or any phylogenetic tree, each branch represent an evolving population of species. When this branch (population of species) splits into two, an internal node is formed. These internal nodes represent speciation events, formation of new species from pre-existing population. For example, consider the common ancestor of human and chimpanzee that lived approximately 13 million years ago. This contiguous population of an ancestral species that had no name (neither human, nor chimpanzee) split into two via a speciation event, forming an internal node, thereby paving way for the formation of two daughter species of human and chimpanzee. The term ‘species’ often corresponds to what lay people treat as the different basic kinds of organisms, for example Cat, Dog, Horse, Lion, etc. (but not Butterfly, Monkey, Camel, Deer, Turtle etc., see the discussion below).

3.2. Species concepts

There are a number of contrasting species concepts popular in biology. The three most widely used species concepts are evolutionary or phylogenetic species concept, biological species concept and morphological species concept.

As per the Evolutionary or Phylogenetic species concept, species is defined as “a group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space”. In other words, if all individuals of a certain population do not have a common ancestor without the inclusion of members of other such populations, then this population do not construe a species, as per phylogenetic species concept. This concept is based entirely on Darwin’s theory of evolution, and is currently the most popular species concept, universally adopted by all taxonomists across the disciplines including plants, animals and microorganisms. After the advent of rapid and cost efficient DNA sequencing over last two decades, a whole discipline of DNA Taxonomy have emerged which is entirely based upon phylogenetic species concept.

Biological Species Concept is still popular across zoologists, and in this concept, species is defined as “a group of interbreeding populations which are reproductively isolated from other such groups”. Ability to interbreed is the hallmark of a population as per this concept. For example, cat encompasses one species (Felis catus), as all the members of this taxa can interbreed. However, camel could not be considered as a species, as there is distinct reproductive compatibility within what we call ‘camel’ in everyday life; it encompasses two distinct species (one-humped Arabian camel and two-humped Bactrian camel. Similarly, Deer (Family Cervidae, 62 species), Turtle (Order Testudines, 250 species), Butterfly (Order Lepidoptera, 20,000 species) and Monkey (Suborder Haplorrhini, 260 species) are not considered as a species, they belong to their respective higher taxonomic levels.

As per morphological species concept popular in the discipline of paleontology, species is defined as “the smallest groups that are consistently and persistently distinct, and distinguishable by ordinary means”. The field of paleontology processes its own unique challenge that the fossils oftentimes have no features other than gross morphology and their judgements have to be based upon the morphology. In some exceptional cases such as the soft-body fossil of mammoth discovered from Siberia, the fossil tissues contains amplifiable DNA such that phylogenetic concept can be applied.

3.3. The Species Problem

At the heart of the problems with species concept is philosophical spectra of realism and nominalism. As per the realism, abstract objects and universals, for example love, green color, species etc., do exist. However, as per the nominalism, these abstractions and universals are thought to be mental and linguistic constructs. Nominalists deny the very existence of species or any of the higher taxonomic levels; for them consider the existence of individuals, nothing else. Famous evolutionary biologist JBS Haldane said once: “The concept of a species is a concession to our linguistic habits and neurological mechanisms”. Biological species concept, although popular in the beginning, can only be applied for sexually reproducing organisms, as sexual compatibility is the defining trait of the species. Therefore, this concept can’t be applied to asexually reproducing organisms, that include all microorganisms, vast majority of plants. This concept can’t be applied for fossils and other preserved specimens (including museum specimen, herbarium voucher etc) for obvious reasons. Another issue with species concept is the phenomenon of Horizontal Gene Transfer (lateral gene transfer), the transfer of genes between two unrelated organisms. This phenomenon is very common in the case of bacteria. All definitions of the word “species” assume that an organism gets all its genes from one or two parents that are very much like that organism, but horizontal gene transfer makes that assumption false. Therefore, in a strict sense, assuming the evolution of life on earth in a tree like fashion is partially wrong; the tree has several reticulate (net-like) elements between branches. Another issue is demarcation between species and intraspecific (within species) varieties; i.e., when to call varieties as separate species? This issue can be compared with analogous situation in languages, when to call separate dialects of a language as individual languages? Darwin once famously said “It all comes, I believe, from trying to define the indefinable. I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other”.

3.4. Modes of speciation

There are four major modes of speciation documented till date. These are allopatry, parapatry, peripatry and sympatry.

3.4.1.    Allopatry

Allopatry is the most common form of speciation in which new species are formed when a population is divided through geographical barrier. In allopatry, speciation occurs through reproductive isolation of populations caused by geographical isolation, or vicariance (geographical separation of species). Allopatric speciation occurs when the initial block to gene flow is a physical barrier that isolates the population; physical barriers could be emergence of mountain ranges, movement of glaciers, formation of land bridges, subsidence of large lakes and so on. For example, before 3 million years ago North and South Americas were unconnected continents, allowing terrestrial species in these two continents evolve separately, an example of allopatric speciation. This allowed intermixing of Atlantic and Pacific Oceans, so the marine organisms could freely interbreed and no allopatry would happen in ocean. However, since 3 million years ago, an isthmus (Isthmus of Panama) emerged, connecting these two continents. This isthmus acted as a reproductive barrier for marine organisms, allowing the organisms (for example, shrimp species) in Atlantic and Pacific divulge into different species, allopatric speciation indeed. However, formation of isthmus prevented further allopatry in terrestrial organisms, as land-dwellers like rodent species of these two continents now can interbreed. After opening up of Panama Canal, the situation have once again reversed; the canal serve as a barrier for terrestrial organisms.

Another form of allopatry occurs when a previously contiguous population gets fragmented; for example a large forest cleared for agriculture leading to isolated patches of forests. Tree species in one such isolated anthropogenic ‘island’ now might not be able to exchange pollen with trees from other such patches (of course, it depends on how far a pollen can travel, mode of pollination and so on). The extent of barrier needed to isolate populations depends on the ability of the organism to disperse (mobility in animals, pollen, seed and spore movement in plants). For example, both rims of the Grand Canyon are populated by the same species of birds, but different, unique species of rodents. In Death Valley, isolated springs are remnants of a historical river network that once existed. Currently, each isolated spring contains its own species of pupfish (Cyprinodon spp.) which is found nowhere else in the world (i.e, endemic species). It is very probable that these endemic species were derived from a single ancestral species whose range was fragmented when the region became arid.

When populations become allopatric, speciation can occur as isolated gene pools diverge genetically. A small, isolated population is more likely to change substantially enough to become a new species than a large one. The geographic isolation of a small population usually occurs at the fringe (edge) of the parent population’s range, and therefore, peripheral isolates are good candidates for allopatric speciation. In addition, the gene pool of the peripheral isolate probably differs from that of the parent population since fringe inhibiters usually represent the extreme genotypes.

3.4.2.    Peripatry

Peripatric speciation is a sub-form of allopatric speciation where in an isolated peripheral group gets severed from the parent population, and divulge into a separate species. In severed peripheral population, rare genes move to fixation (i.e., increase in its frequency such that all individuals of a population has it). If these genes are associated with reproduction, sexual selection takes over, then speciation has occurred as they may not mate with the original population. For example, consider brown bear; the peripheral (fringe) inhabitants of this population near the Arctic got somehow severed from the rest (perhaps due to glaciation). This peripheral population evolved into Polar Bears.

A form of peripatric speciation occurs in founder events, when a few individuals (or a single pregnant female) is introduced to an isolated habitat (an island, a lake etc.) that was previously uninhabited by this species. For example, a few birds that got introduced to a remote island, as in the case of finches in Galapagos islands-a tropical archipelago in the Pacific Ocean. Consequence of such an introduction is adaptive radiation, where in many diversely adapted species divulge from a common ancestor. Multiple events of colonization, adaptation, speciation and recolonization lead to the formation of 13 finch species from just one common ancestor that got introduced to the islands in the span of millions of years. However, note that the adaptation to the new environment through adaptive radiation was not the cause, but the consequence of speciation; the cause was vicariance (reproductive isolation caused by geographical barrier) in the case of allopatry, or founder event in the case of peripatry,

3.4.3.   Parapatry

In parapatric speciation there is no specific extrinsic (geographical) barrier to gene flow. Niches can differ along an environmental gradient, hampering gene flow, and thus creating a cline. The population is continuous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range. An example for parapatry is the case of grass species Anthoxanthum odoratum. This grass population growing on mine waste evolved to have a different flowering time from those that grow on the uncontaminated soil. Evolution of different flowering times ensures reduced gene flow between populations, thus leading to formation of different species. A special form of parapatry is ring species; a connected series of neighboring populations. Each of these populations can interbreed with its neighboring populations, but there exist at least two “end” populations in the series, which are too distantly related to interbreed. For example, consider Larus gulls. Larus population of Norway can breed with its neighbors; Scotland and Finland; Finland population can breed with that of Norway and Russia,; Alaskan population can breed with that of Russia and Canada. However, Canadian Larus cannot breed with that of Scotland; by then these two populations have evolved to have sexual incompatibility. Another example of ring species is the case of greenish warbler (Phylloscopus trochiloides) birds. This bird species once occupied much of Central Asia. However, during the last glacial maximum (“ice age” around 30,000 years ago) when much of the Central Asia was covered in thick ice sheets except few valleys of Himalayas, these birds remained in those glacial refugia (areas where populations can survive during unfavorable situations, as in the case of glaciation here). Since 20,000 years ago onwards when the glaciers started retreating, these birds recolonized uninhabited areas once again. During this recolonization, one bird population migrated towards east (Eastern Siberia), while the other towards west (Mongolia and Western Siberia). Both of these populations can breed with the original refugia population (Himalayas). However, populations at these two extremes (East and West) lost the ability to interbreed.

Ring species possess a unique challenge to biological species concept; where does the species delineation occurs in this fine gradation? Can we call Larus gulls of Canada and Scotland (or East and West Warbler birds) as separate species?

3.4.4.   Sympatry

Sympatry is the least common and most controversial among the four speciation modes. In this mode, new species forms within the range of the original species. A mutation might originate in one individual, and spread to descendants; after many generations, a significant subsection of the original population might have this mutation (i.e, its allele frequency have increased). This mutation might lead to assortative mating, where in members of population develops clear preference on whom to mate (non-random mating). Eventually, sexual incompatibility between two populations arises and members of these two populations divulge into two separate species. Sympatric speciation occurs when disruptive selection (selection of extreme phenotypes over intermediate phenotypes) results in adaptations to different ecological niches in two populations within reproductive distance. For example, consider rabbit populations in a valley. The original population encompassed mostly of brown rabbits, with rare white and black variants. After a major natural calamity (like landslide), landscape of this valley consisted mostly of red and black rocks. Now, black rabbits can seek refuge on black rocks, and white on white rocks. Black and white extremes are now selected over intermediate brown phenotype. After several generations, black and white rabbits would have reduced between population gene flow, and might become sexually incompatible, thereby paving way for speciation. A group of animals may become isolated in the range of a parent population due to resource utilization, or due to adaptation to a new niche. Wasp that pollinate figs mate and lay their eggs in the figs. A genetic change which causes certain wasps to select different fig species will segregate mating individuals and divergence can occur after such an isolation. Cichlid fishes in Lake Victoria (Africa) and Lake Apoyo (Nicaragua) have probably evolved numerous species due to exploitation of different food sources and other resources. In the case of Lake Apoyo, the common ancestor was introduced to this lake approximately 22000 years ago. Since then, cichlid populations of pelagic zone developed a preference for planktonic insect larvae at the surface layers, while that at the bottom, the profundal zone, developed preference for snails that live in benthic zone. After many generations, these populations evolved into two distinct species through sympatric speciation, without any geographical barrier. The profundal cichlids are now distinctly larger (Midas Cichlids, Amphilophus citrinellus), while the pelagic ones are slender and smaller (Arrow Cichlid, Amphilophus zaliosu). Sympatric speciation can also occur from a balanced polymorphism combined with assortative mating e.g. finches that are dimorphic for beak size. Sympatric speciation most commonly occurs through polyploidy, in which an offspring or group of offspring will be produced with twice the normal number of chromosomes. Since it is so much easier for plants to self-fertilize than it is for animals, both polyploidy and sympatric speciation are common for plants than animals. As a tetraploid individual cant mate with diploid individual, it must find another animal of the same species but of opposite sex that has also randomly undergone polyploidy with exact sets of chromosomes, which is highly unlikely for the animals.

3.5. Speciation at molecular Level

Evolutionary forces that decrease variation within population and increase variation between populations favors speciation. Mutation increases variation within and between populations, therefore effect is mixed for speciation. Migration prevents speciation-as it exchanges mutations between populations. It (immigration) also increases variation within population. Effect is negative. Genetic drift increases variation between populations while decreases variation within population. Therefore, the effect is positive.

A simplified illustration of various species concepts and speciation is presented in Fig. 1. Based on the initial barrier to the gene flow, four modes of speciation can be demarcated. Once the population divulges into two daughter populations, various biological evidences accumulate, that could serve as an evidence to support new species discovery by the taxonomists. For example, different allele frequencies of two divulging population would alone serve as an evidence to call these populations two separate species as per genotypic cluster concept, and populations occupying different habitats would serve as an evidence for species discovery as per ecological species concept.

Figure 1. Various species concepts and corresponding biological evidence. From Leliaert, F., Verbruggen, H., Vanormelingen, P., Steen, F., López-Bautista, J. M., Zuccarello, G. C., & De Clerck, O. (2014). DNA-based species delimitation in algae. European journal of phycology, 49(2), 179-196.

4. Summary

4.1. Since the advent of Darwin’s theory of evolution, fixed species concepts have gone out of favour for dynamic species concepts

4.2. Three most important specie concepts are Evolutionary/Phylogenetic, Biological and Morphological, amongst which phylogenetic concept is almost universally followed.

4.3. The main problem with biological species concept is that as it is based on sexual compatibility, it can’t be used for asexually reproducing organisms or for dead specimens

4.4. As species is an abstract concept, proponents of nominalistic philosophy believes that species is an artefact, a mental construct, and only individuals exist in nature

4.5. A precise boundary between species and intraspecific varieties is hard to define, as species are dynamic entities, evolving continuously.

4.6. Species are formed by four major modes, out of which allopatry through vicariance is the main mode.

4.7. Peripatry can be regarded as a subtype of allopatry when peripheral population gets severed from the main population and evolve to a new species. Speciation through founder events are a special case of peripatry

4.8. In parapatry, niches differ along an environmental gradient, hampering gene flow, and thus creating a cline. This would lead to assortative mating and ultimately speciation. A special case of parapatry is ring species.

4.9. In sympatry, disruptive selection results in adaptations to different ecological niches in two populations within reproductive distance. This mode occurs commonly in plants via polyploidy.

4.10.Evolutionary forces that decrease within population variation and increase between-population variation tend to favour speciation.

Further e-resources and learn more

1. YouTube videos: https://www.youtube.com/watch?v=LMzgP0-tq9c https://www.youtube.com/watch?v=sE6vf1AuPRI https://www.youtube.com/watch?v=8yvEDqrc3XE https://www.youtube.com/watch?v=Q2vsG77PZ80 https://www.youtube.com/watch?v=udZUaNKXbJA
2. Speciation resource page of University of California, Berkeley https://evolution.berkeley.edu/evolibrary/article/evo_40
3. Bush, G. L. (1975). Modes of animal speciation. Annual Review of Ecology and Systematics, 6(1), 339-364.
4. Losos, J. B., & Glor, R. E. (2003). Phylogenetic comparative methods and the geography of speciation. Trends in Ecology & Evolution, 18(5), 220-227.
5. Platnick, N. I. (2000). A defense of the phylogenetic species concept (sensu Wheeler and Platnick). Species concepts and phylogenetic theory: a debate, 185-197.