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As discussed in Darwinism,
The formation of new species from an existing one due to reproductive isolation of a portion of the population is known as speciation.
The process of speciation can be explained clearly with the below illustration.

The illustration of beetles and crows used to explain variations is an example of micro-evolution as the changes were minor, even though they were significant.
The evolution in which the variations are small but significant is known as micro-evolution.
In such a type of evolution, the typical characteristics of a particular species change. However, it does not result in the formation of new species.
In this case, a new species is said to be formed if the group of beetles (it can also be any organism) divide into two different populations that cannot reproduce with each other. In such a case, the organisms are known to be two independent species. Thus the phenomenon of speciation can be explained by the illustration of beetles.
Let us consider that the bushes on which the beetles feed are widely spread over a range of mountains. As a result of this, the beetle population expands. On the other hand, individual beetles spend much of their lives eating only a few neighbouring bushes. They don't go far. As a result, there will be sub-populations in each neighbourhood in this massive population of beetles. Because male and female beetles must meet to reproduce, most reproduction will occur among these sub-populations.

But there are chances that certain beetles may go from one site to another, or the beetle may be picked up by a crow from one site and dropped on the other side without being eaten.

The migrating beetle will breed with the native population in any situation. The migrant beetle's genes will spread to a new population as a result of this. This type of gene flow is unavoidable amongst partially isolated populations.

However, if a huge river forms between two such sub-populations, the two will become even more isolated. There will be much less gene exchange between them.

Important!
Genetic drift will accumulate more variants in each geographically isolated sub-population after a few generations. Furthermore, natural selection may function differently in these geographic regions.
Crows, for example, are wiped out in the region of one sub-population by eagles. However, this does not apply to the other sub-population, which has a large number of crows.

As a result, the green variant will not be chosen at the first location but will be firmly chosen at the second. These two isolated sub-populations of beetles will become increasingly divergent as a result of genetic drift and natural selection working together. Even if they happen to meet, members of these two groups will eventually be unable to reproduce with each other.

The above condition can happen in a many different ways. If theDNA changes are substantial enough, such as a shift in the number of chromosomes, the two groups' germ cells will eventually be unable to unite.

Alternatively, a novel variety may evolve in which green females will only mate with green males rather than red males. Natural selection for greenness is compelling as a result of this. If a green female beetle encounters a red male from the other group, it will act so that cannot  reproduce. In effect, new beetle species are being created.
The geographically divided sub-populations become increasingly differentiated as a result of this. Individuals from these two groups will eventually be unable to interchange genetic material as they cannot reproduce with each other, resulting in reproductive isolation. Natural selection will then operate on the genetic diversity within that population until it has evolved into a new species.
Types of speciation:
Based on the geographical mode, speciation is of $$4$$ types:
1. Allopatric speciation
2. Peripatric speciation
3. Parapatric speciation
4. Sympatric speciation
Types of speciation

1.  Allopatric speciation:

In this type of speciation, the population splits into two geographically isolated regions, and each population undergoes genotypic and/or phenotypic divergence.

Allopatric speciation
Example:
Darwin's finches
2.  Peripatric speciation:

In this type of speciation, new species emerge in isolated small peripheral groups that cannot exchange genes with the main population.
Peripatric speciation
Example:
Underground mosquito is a variant of Culex pipiens.
3.  Parapatric speciation:

In this type of speciation, two populations are partially separated geographically so that each species may come into touch with one other from time to time. Still, the heterozygote's reduced fitness prohibits breeding between the two species.
Parapatric speciation
Example:
Ensoltina salamanders
4.  Sympatric speciation:

In this type of speciation, two or more of two or more descendant species from a single ancestral species all occupying the exact geographical location are formed.

Sympatric speciation
Example:
Cichlids in East Africa
Reference:
https://commons.wikimedia.org/wiki/File:Speciation_modes.svg
https://commons.wikimedia.org/wiki/File:Allopatric_Speciation_(Process_diagram).png
https://commons.wikimedia.org/wiki/File:Peripatric_Speciation_(process_diagram).png
https://commons.wikimedia.org/wiki/File:Parapatric_Speciation_(process_diagram).png
https://commons.wikimedia.org/wiki/File:Sympatric_Speciation_(process_diagram).png