What is Speciation?

The process of speciation involves the formation of new species during evolution. The new species evolve in such a way that both new and old species are not able to interbreed. Thus, speciation occurs when few members of one species get separated from the main species due to geographical, mechanical, or reproductive isolation. These separated members develop new traits that make them different from the main species. In other words, speciation could be defined as the absence of gene flow between two populations that become new species. 

Cladogenesis and Ana genesis

Cladogenesis and Ana genesis are the two processes by which speciation occurs. Ana genesis is the type of speciation where species gradually gather small variation. Then, after millions of years, these variations develop into a new species reproductively isolated from its parent species. For example, the evolution of horses from Eohippus is an example of Ana genesis. On the other hand, "cladogenesis" is the second type of speciation where one species splits up into two or more groups, and each group keeps on evolving until they become a new species. Darwin's finches are an example of cladogenesis.

Modes of Speciation

There are mainly three types of speciation. One is sympatric speciation, the second one is allopatric speciation, and the third is parapatric speciation.

1. Sympatric speciation

This type of speciation occurs when the population of the same species living in the same geographical area becomes separated from one another due to reproductive isolation. The major reason for reproductive isolation is the development of polyploidy in some individuals of the same species. The polyploidy individual could not mate with normal diploid individuals and therefore is reproductively isolated. For example, apple maggot flies lay their eggs only on the fruit of hawthorn trees, but less than 200 years ago, some apple maggot flies began to lay their eggs on apples instead.

2. Allopatric speciation

This type of speciation occurs when a population gets separated from its species due to any physical barrier between the two. This physical barrier could be due to forming a new island or the development of canyons and valleys. For example, two populations of the same species of squirrels get separated by a river, and thus, two species of squirrels emerged from one common species.

3. Parapatricspeciation

This is a very rare type of speciation. It takes place when populations are separated not by a geographical barrier but by a change in habitat. Sometimes, parapatric speciation is a special type of sympatric speciation.

Adaptive Radiation

Adaptive radiation is important in evolutionary biology as it assists an organism to adapt to new environmental conditions and thus enables the organism to radiate into new environmental conditions. Furthermore, s. It forms many species from a single species, and each species develops different traits that help them adapt to their environmental conditions. One famous example is Darwin's finches which are thought to have originated from one common ancestor and radiated to the same Galapagos. Recent examples include adaptive radiation of cichlid fish into 250 species of fishes that are highly diverse in their morphology, behavior, and ecology and are found in Lake Tanganyika of East Africa.

Natural Selection and Speciation: 'Ecological Speciation'

Charles Darwin gave the natural selection theory in 1859 in his book "Origin of Species."  According to this theory, individuals in a population have a genetic variation that helps them adapt according to the changing environmental conditions.

The four postulates of natural selection theory include:

• Each species shows variation between individuals of a population.
• There is a competition between each species for food, habitat, and mate.
• The individuals of the species which are better adapted have more chances of survival.
• The surviving members of the species would pass their better genes to their offspring.

One of the apt examples of natural selection speciation is the diversity of the three-spined stickleback. The stickleback is a marine fish that forms a new species inhabiting freshwater colonies in isolated lakes and streams. After 10,000 generations of this fish, sticklebacks show structural differences in their structure of fins, changes in the number or size of their bony plates, variable jaw structure, and color differences that are large variations found in the initial group of fish.

Factors that Lead to Speciation

• Geographical isolation: Geographical isolation could be any physical barrier that separates a population of the same species. The gene flow between these separated populations is prevented, and thus new species are developed, or speciation occurs.
• Genetic drift: It is the change in the frequency of an allele in a population. The genetic drift results in the alternation of gene pools of a small population. It is a random process that could lead to the formation of new species.
• Natural selection: this is the key mechanism that is responsible for evolution. In this process, the genetic variation within the individual population leads to the gradual formation of new species.
• Reduction in gene flow: Gene flow is the movement of genetic material from one population to another. If any population in a given species has a low gene flow, then that population would not have the same allele frequency compared to the rest population of the species.
• Reproductive isolation: According to this factor, if two populations of species get separated from each other and produce fertile offspring, they would give rise to two different species after millions of years.

 Reproductive isolation could be of two types depending upon when they act. One is the prezygotic barrier, and the other is the postzygotic barrier.

1. Prezygotic barrier: This type of barrier does not allow the members of a species to reproduce and thus prevents the formation of a zygote. The prezygotic barrier exists due to habitat isolation, behavioral, temporal, gametic, and mechanical isolation. It occurs at the beginning of speciation.
2. Postzygotic barrier: This type of barrier blocks reproduction after fertilization. For example, hybrid inviability occurs when mating between two individuals develops a hybrid that would not survive.

Types of the Species Concept

All scientists do not agree with the single definition of species. So, a single definition of species does not exist and is broadly divided into four species concepts. These are biological, morphological, ecological, and phylogenetic.

1. Biological species concept: According to this concept, two organisms would belong to the same species to reproduce and produce fertile offspring. Ernst Mayr gave this definition in the year 1970. This is the most accepted definition of species.
2. Morphological species concept: According to this definition, if two organisms share the same morphological traits like the same form or structure, they belong to the same species. In other words, members of the same species look similar to each other, and different species look different.
3. Ecological species concept: The set of organisms that shared the same niche and adapted to the same particular resources would belong to the same species as per the ecological species concept.
4. Phylogenetic species concept: Paleontologists Niles Eldredge and Joel Cracraft gave a new species concept known as the "phylogenetic species concept." According to this concept, organisms belong to the same species if they share a common evolutionary ancestor.

Artificial Speciation

Humans intentionally develop novel species in the laboratory using animal husbandry or any other laboratory experiments in this type of speciation. Hybridization is also used to produce new species artificially, but sometimes it also leads to hybrid sterility. In addition, sometimes, the cytoplasmic incompatibility (CI) problem arises after hybridization if the sperm and egg cannot form viable offspring. One of the major examples of artificial selection is developing a new species of Drosophila melanogaster by William R. Rice and George W. Salt. They bred D. melanogaster fruit flies using a maze with three different habitats: light/dark and wet/dry.

Dobzhansky-Muller model of Hybrid Incompatibility

Theodosius Dobzhansky, a Ukrainian-American evolutionary biologist, gave a model to perform the genetic study of speciation and know the genetic factors responsible for hybrid sterility. He emphasized that mutation is responsible for the nucleotide changes in the ancestral genotype followed by fixation of new mutations in the offspring that change its genetic constituents not to produce fertile offspring.

Genomic Divergence during Speciation 

In recent years, scientists have tried to know how speciation genes are fixed and arrayed within the genome and how genomes evolve during population divergence. A speciation is an important event in the evolutionary study, and scientists are trying to know the genomic variability between members of different species. Scientists use the term "genomic island" to refer to the parts of the genome that underlie this reproductive isolation or adaptation.

Context and Applications

• For Bachelor of Science in Biology: The study of evolution is an important part of every biology student. It helps the student to understand the process and the mechanism responsible for new species formation.
• For Master of Science in Biology: For post-graduate students, it is very important to study evolution. Various concepts like gene flow, natural selection, mutation, and genetic drift have to be studied.