Speciation- Definition, causes, process, types, examples

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Speciation Definition

Speciation is the process of formation of a new genetically independent group of organisms, called species, through the course of evolution.

The process of splitting of genetically homogenous population into two or more populations that undergo genetic differentiation and eventual reproductive isolation is called speciation.
The entire course of evolution depends upon the origin of new populations (species) that have greater adaptive efficiency than their ancestors.

Speciation occurs in two ways.

Transformation of old species into new species over time.
Splitting of a single species into several, that is the multiplication of species.

Speciation Causes

Speciation occurs as a result of several factors which are:

Natural selection

As explained by Charles Darwin, different individuals in a species might develop specific distinct characteristics which are advantageous and affect the genetic makeup of the individual.
Under such conditions, these characteristics will be conserved, and over time, new species might be formed.
However, in this case, the essential aspect of this factor is that speciation occurs only when a single species splits into several species resulting in the multiplication of species.

Genetic drift

Genetic drift is the change in the allele frequencies in a population as a result of “sampling error” while selecting the alleles for the next generation from the gene pool of the current population.
It has been, however, argued that genetic drift doesn’t result in speciation and just results in evolution, that is, change from one species to another, which cannot be considered speciation.

Migration

When a certain number of species from a population migrate from one geographical region to another, the species might accumulate characteristics which are different from that of the original population.
Migration usually results in geographical isolation and ultimately leads to speciation.

Chromosomal Mutations

Chromosomal mutations have the potential to serve as (or contribute to) isolating mechanisms, and the locking up and protection of a particularly favorable gene complement through a chromosomal mutation.
These mutations, when preserved from one generation to another, might result in the formation of new species.

Natural causes

Sometimes, natural events imposed by the environment like a river or a mountain range might cause the separation of what once a continuous population is divided into two or smaller populations.
These events result in geographical isolation of the incipient species followed by reproductive isolation leading to speciation.

Reduction of gene flow

Speciation might also occur in the absence of some extrinsic physical barriers.
There might be a reduced gene flow over a broad geographical range where individuals in the far east would have zero chance of mating with individuals in the far western end of the range.
In addition, if there are some selective mechanisms like genetic drift at the opposite ends of the range, the gene frequencies would be altered, and speciation would be ensured.

Speciation process (how does speciation occur?)

Classically, speciation has been observed as a three-stage process:

Isolation of populations.
Divergence in traits of separated populations (e.g. mating system or habitat use).
Reproductive isolation of populations that maintains isolation when populations come into contact again (secondary contact).

Recent research shows that steps one and two may take place simultaneously in the same place, and often the third step does not occur.
The process of speciation begins with the isolation of subpopulation of a species which could either occur through physical isolation (allotropic speciation) or genetic isolation (sympatric speciation).
Once the population is separated, a gradual accumulation of small genetic changes results in a subpopulation of a species that eventually accumulate so many changes that the subpopulations become different species.
Over time, the subpopulation now becomes genetically independent and will continue to diverge by mutation, selection, and genetic drift.
The genetic differentiation might cause a slight change in the mating dance or even a small change in the shape of the male genitalia or some changes in the habitat or feeding habits of the subpopulation, which results in reproductive isolation.
Eventually, the genetic differentiation between the subpopulation becomes so high that the formation of hybrids between them would be physiologically, developmentally, or behaviorally impossible even if the modes of the separation were abolished.

Types of speciation/Modes of speciation

Image Source: Wikipedia (Ilmari Karonen)

The classification of the modes or types of speciation is based on how much the geographical separation of the original population contributes to the reduced gene flow and ultimately, the formation of new species.

The modes of speciation are:

Allopatric Speciation

Allopatric speciation is the mode of speciation in which the original population is divided into two by a barrier resulting in reproductive isolation.
The model for allopatric speciation was presented by Mayr.
It is based on the concept that new species arise when some physical geographic barrier divides the large population of a species into two or more small populations.
The individuals of these isolated populations cannot interbreed because of their physical isolation.
Physical isolation might occur either due to physical barriers like vast expanses of ocean, high mountains, glaciers, deep river valleys, wide rivers or deserts, or a considerable distance due to a larger geographical range.
Each isolated population starts to adapt to their separated environments while accumulating differences and evolving independently into new species.
Allopatric speciation can occur even in cases in which the barrier allows some individuals to cross the barrier to mate with the members of the other groups.
For speciation even to be considered “allopatric,” gene flow between the soon-to-be species must be significantly reduced—but it doesn’t have to be entirely reduced to zero.

Examples of Allopatric speciation

The classic example of allopatric speciation is that of Darwin’s finches. The divergent populations of finches inhabiting the Galapagos Islands were observed to have differences in features such as body size, color, and beak length or shape. The differences resulted because of the different types of food available in various Islands.
Another example is of Grand Canyon Squirrels which were separated during the formation of the Grand Canyon and resulted in two different species of squirrels.

Peripatric Speciation

Peripatric speciation is a special condition of allopatric speciation which occurs when the size of the isolated subpopulation is small.
In this case, in addition to geographic separation, genetic drift also plays an important as genetic drift acts more quickly in small populations.
The small isolated subpopulation might carry some rare genes which upon reaching the new geographical region become fixed over the course of a few generations as a result of genetic drift.
As a result, the entire population of the new region ends up having these rare genes.
Over time, new genetic characters, as well as natural selection, cause the survival of individuals which are better suited to the climate and food of the new region.
Finally, under the influence of all these factors, new species are formed.
However, it is very difficult to explain what role genetic drift played in the divergence of the two populations, which makes gathering evidence to support or refute this mode very challenging.

Examples of Peripatric speciation

The Australian bird Petroica multicolour and London Underground mosquito, a variant of the mosquito Culex pipiens, which entered in the London Underground in the 19th century are the examples of Petripatric speciation.

Parapatric Speciation

Parapatric speciation is a mode of speciation in which there is no extrinsic barrier between the population but, the large geographic range of the population causes the individuals to mate with the neighboring individuals than with the individuals in a different part of the geographical range.
In this case, the population is continuous, but the population doesn’t mate randomly.
Here, the genetic variation occurs as a result of reduced gene flow within the population and varying selection pressures across the population’s range.
This occurs in population which is distributed over a large geographical range. Thus, the individuals in the far west region cannot mate with the individuals in the far east region.
Through a few generations, new species might be formed within the existing population.

Examples of Parapatric speciation

The grass species Anthoxanthum odoratum where some species living near the mine have become tolerant to heavy metals; however, other plants that don’t live around the mines are not tolerant.
But because the plants are close together, they could fertilize each other and result in a new species.

Sympatric Speciation

Sympatric speciation is the process of the formation of new species from an original population that are not geographically isolated.
It is based on the establishment of new populations of a species in different ecological niches and the reproductive isolation of founders of the new population from the individuals of the source population.
Gene flow between daughter and parental population during sympatric speciation is postulated to be inhibited by intrinsic factors, such as chromosomal changes and non-random mating.
Exploiting a new niche might automatically reduce gene flow with individuals exploiting a different niche.
This mode of speciation is common in herbivore insects when they begin feeding and mating on a new plant or when a new plant is introduced within the geographical range of the species.
The gene flow is then reduced between the species that specialize in a particular plant which might ultimately lead to the formation of new species.
The selection resulting in specialization needs to be really strong for the population to diverge.
Thus, sympatric speciation is a sporadic event in multicellular organisms or randomly mating populations.

Examples of Sympatric speciation

Sympatric speciation is observed in apple maggot flies which 200 years ago laid eggs and bred only on hawthorns but now lays eggs on both hawthorns and domestic apples.
As a result, gene flow between parts of the population that mate on different types of fruit is reduced, and in fewer than 200 years, some genetic differences between these two groups of flies have evolved.

References

Verma PS and Agarwal VK (3005). Cell Biology, Genetics, Molecular Biology, Evolution, and Ecology. Multicolored Edition.
Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Process of speciation.Available from: https://www.ncbi.nlm.nih.gov/books/NBK22000/
https://biologydictionary.net/speciation/
https://evolution.berkeley.edu/evo101/VC1bAllopatric.shtml
https://evolution.berkeley.edu/evo101/VC1dParapatric.shtml
https://evolution.berkeley.edu/evo101/VC1cPeripatric.shtml
https://evolution.berkeley.edu/evo101/VC1eSympatric.shtml

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