Speciation: causes, process, types and examples

What is Speciation?

• In evolution, speciation is the process that results in the formation of new and distinct species that are isolated from one another.
• However, biologists have developed two different pathways for the speciation to occur.
• Allopatric speciation, referring to speciation in other homelands,” includes a spatial separation from a parent species and eventual evolution of populations.
• Sympatric speciation, referring to speciation in the ‘same homeland’, includes speciation taking place within a parent species while staying in a same location.

What are the causes of speciation?

• There are various factors that cause speciation. The two main causes for speciation are listed as:

i. Geographical isolation:

• Speciation results from a splitting event in which a parent species is separated into two separate species, often as a result of geographical isolation or some driving force involving population separation.
• Separation could occur either due to physical barriers such as huge ocean expanses, mountain ranges, glaciers, deep river valleys, large rivers or deserts, or a substantial distance due to wider geographical range.
• The free-flow of alleles is prevented when populations become geographically isolated.
• The two species are able to evolve into different trajectories when the separation continues for a period of time.
• Thus, their allele frequencies progressively become more and more different at various genetic loci as new alleles in each population independently emerge through mutation.
• Usually, environmental conditions for the two groups, such as climate, resources, predators, and competitors, will vary, causing natural selection in each group to favor divergent adaptations.

ii. Reproductive isolation:

• The reproductive isolation which is central to the process of speciation takes place because of reproductive barriers, that are formed as a result of genetic, behavorial or physical differences emerging between the new species.
• These are either pre-zygotic processes i.e. differences in courtship behaviors, non-compatible genitalia, or gametes that are unable to fertilize between species.
• On other way, they are post zygotic for instance zygote mortality or the production of sterile offspring.

Types of speciation:

• There are total of 4 types of speciation i.e. allopatric, parapatric, peripatric and sympatric speciation. However, artificial speciation is also included sometimes.
• Allopatric speciation occurs when speciation via geographical separation takes place whereas sympatric speciation occurs when speciation occurs without geographic isolation.
• Peripatric and parapatric speciation, however are similar to allopatric speciation as they occur when populations are isolated. 

1. Allopatric speciation (Speciation via Geographical separation)

• The process of speciation that take place when the members of the population are isolated geographically from each other, where they are unable to mate and hence genetic exchange is prevented or interfered is termed as allopatric speciation.
• There may be a number of ways of isolating populations leading to allopatric speciation: from a river forming a new branch, erosion forming a new valley, or a group of species migrating to a new location without the opportunity to return, such as seeds floating to an island across the ocean.
• The essence of the geographical separation needed to separate populations is entirely dependent on the organism’s biology and its capacity for dispersion.
• If two flying insect populations took up residence in different neighboring valleys, it is possible that individuals would fly back and forth from each population, continuing gene flow.
• If two rodent populations were split by the creation of a new lake, however, continued gene flow would be unlikely; speciation would therefore be more likely.
• Allopatric processes are classified into two groups.
  • If a few individuals of a species migrate to a new geographical area, this is called dispersal.
  • If a natural condition happens to physically separate species, this is called vicariance.

How does allopatric speciation occurs?

• Allopatric speciation occurs due to geographical separation of population.
• The geographical separation of population may be due to geological shifts, such as the formation of a mountain by a volcano, the formation of islands, the division of ecosystems by glaciers and rivers, or the destruction of habitats due to human activity.
• As a result of various selective pressures acting on populations, the separated populations then experience divergence in genotypic or phenotypic traits.
• When mutations occur within species, this allows natural selection to induce genetic drift.
• Over time, because of adaptation to their new environment, the separate populations can evolve morphologically different characteristics.
• The characteristics can become so markedly distinct that there is reproductive isolation, preventing the inbreeding of populations and thereby generating new species.
• If this is the case then it is suggested that allopatric speciation has taken place.
• If populations become sufficiently different to be classified as new species, but not sufficiently distinct for the occurrence of reproductive isolation, the species can return into contact and mate, creating hybrids.

Example of Allopatric speciation; Darwin’s Finches:

• In the Galapagos finches that Charles Darwin studied, a significant example of allopatric speciation occurred.
• On the islands of the Galapagos, there are about 15 different species of finches, each of which looks different and has specialized beaks for consuming various kinds of food, such as insects, seeds and flowers.
• All these finches originated from a single species of ancestor that must have emigrated to the several islands.
• When populations on the islands were created, they were isolated from each other and numerous mutations emerged.
• In their respective habitats, the mutations that caused the birds to be more powerful became more and more common, and several different species evolved over time.
• If several new organisms evolve in a relatively rapid geological period from one common ancestor, this is called adaptive radiation.

2. Peripatric speciation:

• It occurs when the individuals lying on the periphery, or border of a huge population split off from the main group and result to a new species in course of time.
• Differentiating it from allopatric speciation can be hard.
• When the population that branch off enters a distinct biological niche, like feeding on different food or surviving in a different environment, peripatric speciation occurs.
• Often these new populations that split away from the existing one are typically small, so this can have an effect on the proportion of some characteristics in the new population compared with the old one.
• Say for instance, that there is a bird population that is mostly blue, but some are red.
• A smaller group of birds splits out of the main group, and red is the majority of this smaller group.
• It is probable that their descendants will also be mainly red, which is different from the main group.
• This type of change in gene frequency is referred to as genetic drift.
• Many changes can take place over time, and these, combined with the effects of genetic drift, can cause new species to evolve.

Example of peripatric speciation; the London underground mosquitoes

• The London Underground mosquito is a type of mosquito found in the Underground area of London.
• Because of to its edacious biting, biologists called the London Underground mosquito Culex pipiens f. molestus.
• It eventually adapted to human-made underground structures, from being a local above-ground Culex pipiens.
• Recent evidence indicates it is a southern mosquito variety related to C. pipiens that has modified to the warm underground spaces of northern cities.
• The proof for this specific mosquito becoming a distinct species from C. pipiens comes from studies done by Kate Byrne and Richard Nichols.
• The species have very unique features and are particularly difficult to mate.
• More precisely, the C. pipiens f. molestus is cold intolerant and bites rodents,  and humans, and can breed all year round, while the above-ground species is also cold tolerant, but hibernates in the winter and targets only birds.
• The eggs were infertile when these two varieties were cross-bred, indicating reproductive isolation.

3. Parapatric speciation:

• Parapatric speciation occurs when subpopulations of the same species are largely isolated from each other however have a small region where their ranges overlap.
• This could be caused by a partial geographical barrier or an uneven distribution of members of subpopulation.
• It has very less chances to occur.
• It can occur between several neighboring subpopulations where all the neighboring populations can interbreed, but each subpopulation is so slightly different that it would not be possible for the members on the extreme ends to interbreed with each other. This is referred as ring species.
• That means within the group, the population does not mate randomly, but rather individuals mate with their nearest geographical neighbors more generally resulting in unequal gene flow.
• Non-random mating could increase the rate of dimorphism within populations, in which differed morphological aspects of the same species are exhibited.
• Parapatric speciation results in one or more distinct sub-populations (termed as ‘sister species’) that have small continuous overlaps in their biogeographic range and are genotypically dimorphic.

Example of parapatric speciation; Agrostis tenuis:

• In populations of the grass Agrostis tenuis that span mine tailings and natural soils, the best-known example of ongoing parapatric speciation occurs.
• Heavy metal tolerant individuals, a heritable trait, live well on polluted soil, but poorly on soil that is not contaminated.
• For intolerant populations, the reverse happens.
• Gene flow occurs between sub-populations on and off mine tailings, but small variations in flowering time between the two locations inhibit hybridization.

4. Sympatric speciation (Speciation without geographical separation):

• It is the evolutionary process by which organisms are created from a single ancestral species while occupying the same geographical area.
• The distribution ranges of organisms that evolve by sympatry may be similar, or they may only overlap, as contrasted to allopatric speciation.
• Instead of geographical distance causing a reduction in gene flow between populations,  sympatry occurs as members of one population make use of a new niche.
• For example, this could occur if a herbivorous insect starts feeding on a new or noble source of plants with which it is not ancestrally associated, or if a new plant species is introduced into the geographical range of the species.
• As insects normally reproduce or lay eggs within the type of fruit in which they were born, the individuals will specialize in feeding and mating on specific fruits over time.
• As a result, gene flow between populations that specialize in different fruits would be decreased, leading to populations being reproductively isolated.
• As new species emerge from populations living in highly overlapping or even similar environments, sympatric speciation is very distinct from the other forms.
• It may be more prevalent in bacteria than in multicellular organisms because when they split, bacteria may shift genes to each other as well as transfer genes to offspring.

How does sympatric speciation occurs?

• One type of sympatric speciation can start with a chromosomal defect during meiosis or the formation of a hybrid individual with large number of chromosomes.
• A condition in which there is an additional set of chromosomes, or sets, in a cell or organism is termed as polyploidy.
• Polyploidy results from a meiosis defect in which, instead of dividing, all the chromosomes pass into one cell.
• There are two major types of polyploidy that could result in reproductive isolation of an individual in the polyploid state.
• One is autopolyploidy where polyploid individuals will possess two or more complete set of chromosomes from its own species.
• For instance, if a plant species with 2n=6 results in autopolyploid gametes which are also diploid, the gametes now possess twice as many chromosome they should possess.
• These new gametes are incompatible with the usual gametes that this species of plant produces.
• However, they could either self-pollinate or reproduce with several other autopolyploid plants with gametes that have the same diploid number.
• In this way, sympatric speciation will occur rapidly by producing offspring with 4n called a tetraploid.
• Only those of this new kind and not those of the ancestral species will be able to reproduce immediately with these individuals.
• Allopolyploidy is the other form of polyploidy where individuals of two different species reproduce to yield a fertile offspring.
• The examples of allopolyploids are cultivated forms of wheat, cotton, and tobacco plants.
• Sympatric speciation can occur in ways other than polyploidy, as well.
• If we consider a species of fish residing in a lake.
• Competition for food increased as the population grew.
• Under pressure to find food, if we assume that the genetic versatility of a group of these fish was to discover and feed off another resource unused by the other fish.
• If the new food source was discovered at a different depth of the lake then, Those fed on the second food source would communicate more with each other over time than the other fish; they would therefore also breed together.
• The offspring of these fish are likely to act and live in the same area as their parents and feed, keeping them apart from the original population.
• If this group of fish continued to remain separate from the first population, as more genetic differences accumulated between them, sympatric speciation would eventually occur.

Example of sympatric speciation; Cichlid fish

• In Tanzania, cichlid fish that live in a small volcanic crater lake are seen as one such example of sympatric speciation.
• There are two very distinct ectomorphs or forms in the population: a yellow-green one that lives along the shore, and a blue-black one that lives at the bottom of the lake.
• By looking at the DNA of the fishes, researchers could see that the two ectomorphs were genetically very distinct.
• It is assumed that these two forms are in the gradual speciation phase at present.

5. Artificial speciation:

• The type of speciation that can be accomplished through the input of human intervention is termed as artificial speciation.
• Human beings may create new, distinct species by separating populations and thus preventing reproduction, or by purposely breeding individuals with desired morphological or genotypical traits.
• This is also known as ‘artificial selection’; artificial selection has been undertaken for most modern domesticated animals and plants.
• While evolution of our modern crops and livestock has taken thousands of years, it is possible to imagine the process of artificial selection in organisms that have short life cycles.

Example of artificial speciation; Drosophila melanogaster:

• In fruit fly (Drosophila melanogaster) species, artificial selection has been most effectively displayed.
• The changes that occur as flies adapt to each environment are shown by experiments in which flies are placed in environments that contain different resources or habitats.
• The flies are removed from the experimental zone after several generations and allowed to cohabit, although the populations are unable to mate due to the process of reproductive isolation that occurred while in isolation.