Concept of Species and Speciation: The concept of a species and speciation is intuitive on certain levels. It is not necessary to be a zoologist to group species such as humans, giant pandas, or sunflowers into categories based on their appearance. When the species in question have highly diverse appearances, this strategy works well like a monkey and a sunflower. But, when it comes down to it, what truly distinguishes a species? Similar-looking organisms are often members of the same species, but this isn’t always the case.

On the other hand, organisms belonging to the same species might have drastically distinct appearances. Dogs, for example, exist in a variety of sizes and shapes, ranging from a bulldog to a terrier to a golden retriever, all of which belong to the same species: Canis familiaris, the domestic dog, they can interbreed. Let us look at how species are defined in further depth in this post. We’ll also look at the meaning, types, and factors affecting the concept of species and speciation.

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History of Concept of Species

The species is the fundamental unit of the taxonomic hierarchy, as we all know. In biological taxonomy, Davis (1978) termed them “building bricks.” The biosystematics concept is the earliest in biological phenomena. It is the lowest level of hierarchy that all botanists utilise and recognize. According to Stebbins (1977), it is the fundamental unit of development.

It begins with Plato, who created the concept of eidos, or species, and argued that all objects are shadows of the eidos. Variations in species are detected and reported on the typological species idea, according to Mayr (1957).

Major Concept of Species

Morphological Concept of Species

1. Classic phenetic species concept or Linnaean or classical species concept is another name for morphologic species or Morphospecies concept.
2. Species are the smallest groups that are continuously and persistently unique and distinct from one another.
3. Species are easily recognisable individuals, and their identification in macroscopic plants and animals should be based on basic observation.
4. In the opinion of a competent systematist, a species is a community of several related communities with distinguishing morphological features.
5. This concept was expressed in Aristotle’s and Plato’s philosophies. Cain referred to the Typological species concept as the morphospecies concept.

The Biological Species Concept

What does consistently define a species, if not its appearance? Scientists define a species in most eukaryotes, such as animals, plants, and fungus, based on reproductive compatibility. That is, if organisms can successfully reproduce with one another, they are usually considered to be members of the same species.

1. A species is a group of organisms that can possibly interbreed, or mate, with one another to produce healthy, fertile offspring, according to the most generally used species definition, the biological species concept.
2. In this definition, members of the same species must be able to interbreed.
3. However, that does not imply that they must be members of the same interbreeding group in real life. For example, a dog in Australia and a dog in Africa are unlikely to meet, but if they did, puppies could result.
4. Different species of organisms can mate and have healthy offspring in some situations, but the offspring are infertile and unable to reproduce.
5. The biological species concept ties the conception of a species to the evolution process.
6. Because members of a species can interbreed, they share a gene pool, which is a collection of gene variants.
7. Genes, on the other hand, are not passed down between species. Even if multiple species’ DNA is combined to produce offspring, the offspring will be infertile and unable to pass on their genes. As a result of the limited gene flow, each species evolves as a unique group from the others.

Example

Mules are hybrid offspring produced when a female horse and a male donkey mate. A mule, like a figure below, is completely healthy and can live to a ripe old age, but it is infertile and unable to reproduce. As a result, we classify horses and donkeys as different species.

Fig: Mule (Hybrid) Female Horse × Male Donkey

Phylogenetic Species Concept

1. The concept of a species is an irreducible group whose members are all descended from a common ancestor and share a set of distinguishing, or derived, characteristics.
2. As a result, a species is defined as a group of organisms with a shared and distinct evolutionary history.
3. Breeding between members of different species is not difficult. Hence it is less limiting than the biological species concept.

Evolutionary Species Concept

1. Evolution in genetic species keeps them sharply separate. The evolutionary species concept was advocated by Simpson (1961), who suggested that “An evolutionary species is a lineage” (a population’s ancestral descendant sequence), which has evolved independently of others and has its own unitary evolutionary role and tendencies.
2. This concept avoids the problems of determining actual or possible amounts of interbreeding as gene flow while still allowing for some interspecific hybridization.
3. Van Valen (1976) defined it as “A species is a lineage occupying an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range.”

Types of Species

Grant, along with the Evolutionary species of Simpson (1961), recognized 5 types of species:

1. Taxonomic species: (Morphological species, phenetic species) The taxa, group of
morphologically similar individuals.
2. Biological species: (Genetic species): Sexually reproducing population system.
3. Microspecies: (Agamospecies): Population is a uniparental organism.
4. Successional species: (Paleospecies): Phyletic lineage.
5. Biosystematic species: (Ecospecies, Coenospecies): Fertility group.
6. Evolutionary species: Combined sexually reproducing populations uniparental groups and phyletic lineages.

What are the Factors that keep Species Distinct?

The biological species concept defines organisms as belonging to the same species or not based on their ability to interbreed and produce fruitful offspring. Why, then, are distinct species unable to interbreed successfully? This question may appear absurd for very different species (such as a plant and an animal), but it’s less evident for others, such as the horse and donkey mentioned above.
In general, obstacles known as mechanisms of reproductive isolation prevent distinct species from interbreeding and producing healthy, viable offspring.
These barriers are classified as prezygotic or postzygotic, depending on when they act. Between species, there are prezygotic and postzygotic barriers that hinder mating and the production of healthy, viable offspring.

1. Prezygotic Barriers

Members of different species are prevented from mating and forming zygotes, which are single-celled embryos, by prezygotic barriers. Some example scenarios are below:

Fig: Prezygotic Isolation Mechanism

i. Habitat isolation-Two species may prefer different habitats and so have a low chance of meeting.

ii. Temporal isolation-Two species that reproduce at different intervals of the day or year are unlikely to encounter each other when looking for a mate.

iii. Behavioural isolation-Two species may have different courtship behaviours or mate preferences, making them “unattractive” to each other.

iv. Gametic isolation-Even if two species meet through mating, their egg and sperm cells may not be able to fertilise one other.

v. Mechanical isolation-It’s possible that the bodies or reproductive structures of two species are incompatible.

vi. These are all examples of prezygotic barriers, as they inhibit the formation of a hybrid zygote.

2. Postzygotic Barriers

Hybrid zygotes—one-celled embryos with parents from two different species—can not develop into healthy, fertile adults because of postzygotic barriers. Postzygotic barriers are frequently linked to the mixed set of chromosomes seen in hybrid embryos, which may or may not line up correctly or contain a complete set of information.
In certain circumstances, the chromosomal mismatch kills the embryo or results in an unhealthy individual who survives. In other circumstances, a hybrid can live to adulthood but is sterile because its mismatched chromosomes can’t be evenly split into eggs and sperm. This form of mismatch, for instance, explains why mules are sterile and unable to breed.
Prezygotic and postzygotic barriers not only keep species separate but also play a role in the development of new species.

Speciation Definition

The process by which new species develop is known as speciation. When an original species separates into two or more descendent species that are genetically distinct and cannot interbreed, this is known as speciation. For speciation to occur, two separate populations must emerge from a single original population, and they must evolve in such a way that interbreeding between the two populations becomes impossible.

Speciation Types

Based on reproductive or geographic isolation, among other reasons and environmental factors, four types of natural speciation can occur.

Fig: Mode of speciation

1. Allopatric Speciation

i. Allo meaning other and Patric meaning homeland involves geographic separation of populations from a parent species and subsequent evolution.

ii. The process of speciation in which the original population is divided into two by a barrier, resulting in reproductive isolation, is known as allopatric speciation.

iii. Genetic divergence may occur once the groups have been reproductively isolated. That is, over several generations, their genetic makeup and heritable characteristics may grow increasingly distinct. Natural selection, which may favour various features in different environments, and other evolutionary mechanisms such as genetic drift cause genetic divergence.

iv. Mayr presented his model for allopatric speciation.

v. It is based on the concept that new species form when a physical geographic barrier splits a species’ large population into two or more small populations. Because of their physical isolation, these isolated groups are unable to interbreed.

vi. Physical isolation can be caused by physical obstacles such as extensive ocean stretches, high mountains, glaciers, deep river valleys, wide rivers, or deserts, or by a significant distance due to a larger geographical range.

vii. Each isolated population begins to adapt to its own environment, collecting differences and evolving into new species on its own.

viii. Even when the barrier allows some individuals to cross the barrier to mate with members of the other groups, allopatric speciation can occur.

ix. For speciation to be considered “allopatric,” gene flow between the soon-to-be species must be considerably reduced but not fully abolished.

Examples

Over millions of years, the Colorado River gradually carved out the Grand Canyon. There was just one species of squirrel in the area before it formed. Squirrels found it increasingly difficult to traverse between the north and south sides of the canyon as it grew deeper.

Fig: The Grand Canyon in Arizona

The canyon eventually got too deep for the squirrels to cross, isolating a subgroup of squirrels on each side. Due to the deep canyon barrier between the north and south sides of the canyon, the squirrels on both sides eventually separated into different species.

Fig: Harris’s antelope squirrel & white-tailed antelope squirrel Allopatric Speciation is divided into two subtypes: Peripatric and parapatric speciation.

2. Peripatric Speciation

i. Peripatric speciation is a type of allopatric speciation in which the isolated group is quite small. A small population at the very edge of the species’ range is isolated in this way. In this scenario, genetic drift is relevant in addition to geographic separation because genetic drift occurs more quickly in small populations.

ii. Once they reach the new geographical site, the small isolated subpopulation may have certain uncommon genes that get fixed over the course of a few generations as a result of genetic drift. As a result, these rare genes are found throughout the whole population of the new region.

iii. Natural selection and new genetic traits cause the survival of people who are better suited to the environment and food of the new place over time.

iv. Finally, new species emerge as a result of the interaction of all of these variables.

v. However, explaining the role of genetic drift in the divergence of the two populations is complex, making accumulating evidence to support or disprove this approach controversial.

Examples

A form of the mosquito Culex pipiens and the Australian bird Petroica multicolor have both invaded the London Underground.

3. Parapatric Speciation

i. The populations that are diverging during parapatric speciation maintain a zone of contact and do not cease the exchange of genes completely, unlike allopatric speciation. It is characterized by a small overlap in their ranges as well as significant gene flow amongst the populations.

ii. This is a type of allopatric speciation in which the species are not separated by a physical barrier, but the population’s large geographic range encourages individuals to mate with their neighbours rather than with people from other parts of the range.

iii. This is caused by a drastic change in the environment.

iv. Despite the fact that people in these places can interbreed, they develop distinct personalities and lifestyles.

v. The population is continuous in this case, but it does not mate at random.

vi. Reduced gene flow within the population and varied selection pressures over the population’s distribution cause genetic variation in this population.

vii. Within the existing population, new species could emerge after a few generations.

Examples

Some species of the grass Anthoxanthum odoratum that dwell near mines have become tolerant to heavy metals; however, other plants that do not reside near mines are not. However, because the plants are so close together, they may fertilise one another, resulting in the emergence of a new species.

4. Sympatric Speciation

i. “Sym” meaning “same” and “Patric” meaning “homeland”, involves speciation occurring within a parent species remaining in one location.

ii. Sympatric speciation refers to the emergence of new species from a population that is not geographically isolated.

iii. It is based on the development of new populations of a species in distinct ecological niches, as well as the reproductive isolation of the new population’s founders from the individuals of the parent population.

iv. Intrinsic variables like chromosomal alterations and non-random mating are thought to prevent gene flow between the daughter and parental populations during sympatric speciation.

v. Individuals who are pursuing a new niche may experience less gene flow than those who are exploring a different niche.

vi. When herbivore insects begin feeding and mating on a new plant or when a new plant is introduced within the species’ geographical range, this mechanism of speciation is prevalent.

vii. The gene flow between species that specialise in a particular plant is then decreased, potentially leading to the emergence of new species.

viii. For the population to diverge, the selection resulting in specialisation must be extremely strong.

ix. As a result, sympatric speciation in multicellular organisms or randomly mated populations is a rare process.

Example

Fig: Example of Sympatric Speciation

The North American apple maggot fly is a classic example in this the ancestors of apple maggot flies laid their eggs only on hawthorns — but today, these flies lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to America by immigrants and bred). This reduces gene flow between segments of the population that mate on different types of fruit, and certain genetic variations between these two groups of flies have evolved in less than 200 years.

Adaptive Radiation

The evolution of distinct species in a certain geographical area, starting from a common ancestor and radiating to other geographical areas with little variation in their molecular, morphological, and anatomical traits, assists them in surviving and blending in with their surroundings. In other circumstances, a single species’ population disperses throughout a large area, with each finding its own niche or isolated environment. Because several adaptations emerge from a single point of origin, we call this adaptive radiation.

Example

Adaptive radiation is the Hawaiian honeycreeper. Numerous species have evolved from a single species, the founder species, including the six shown in Figure.

Fig: The Honeycreeper Birds Illustrate Adaptive Radiation.

Summary

A species is defined as a group of individuals residing in one or more populations who can potentially interbreed to generate healthy, fertile offspring, according to the biological species concept. Prezygotic and postzygotic barriers keep species separate from one another. These barriers, which work before and after the formation of a zygote, prevent organisms of different species from mating to generate fruitful offspring. These barriers keep species from reproducing together.

New species form by speciation, in which an ancestral population splits into two or more genetically distinct descendant populations. Speciation involves reproductive isolation of groups within the original population and accumulation of genetic differences between the two groups. Due to a geographical barrier, populations become reproductively isolated and diverge in allopatric speciation. Reproductive isolation and divergence occur without geographical barriers in sympatric speciation, for example, due to polyploidy.

Frequently asked questions (FAQs)

Q.1. What defines a species?
Ans: A biological species is a group of organisms that can reproduce with one another in nature and produce fertile offspring.

Q.2. What does speciation mean?
Ans: Speciation is how a new kind of plant or animal species is created. Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics.

Q.3. What are the four types of speciation?
Ans: There are four types of speciation: allopatric, peripatric, parapatric, and sympatric.

Q.4. What are the factors that affect speciation?
Ans: Geographical Isolation, natural selection, genetic drift, reproductive isolation, and hybridisation are the factors affecting speciation.

Q.5. What are the types of species?
Ans: Taxonomic species, biological species, microspecies, successional species, biosystematic species & evolutionary species.

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