Species 

 1. Species are a natural population or a group of populations of individuals which resemble one another in all essential morphological and reproductive characters so that they are able to interbreed freely and produce fertile offspring.

2. Characteristics of a species

(a) All the members whether present in one or more populations found in different and remote areas of the earth are derived from a common ancestor. 

(b) The members resemble one another more than they resemble individuals of other species.

(c) There is a complete anatomical similarity

(d) All the members of a species have similar karyotype.

(e) There is a broad similarity in their morphological characters, 

(f) There is a broad similarity in their morphological characters, There is a similarity in their biochemistry Le. of proteins, enzymes, hormones, etc.

(g) The members are able to interbreed freely and produce fertile offspring

3. Biological concept of species

(i) A systematist and biogeographer from Harvard University, Ernst Mayer (1942) proposed the

biological species concept.

 (ii) It defines a biologic species as a group of individuals with the potential to interbreed and produce fertile offsprings.

(iii) Once the species have formed they do retain their identity which may be grouped into two categories:

(a) A species having two or more varieties or subspecies is known as polytype species. 

(b) The one which cannot be divided into races, varieties, or subspecies is termed monotypic

species, eg. Ginkgo biloba. 

4. Polytypic species can be divided into different types:

(a) Race. It is a group of individuals within a species forming a permanent breed. Eg Human beings belong to a subspecies, Homo sapiens sapiens. There are seven human races: negroid, caucasoid, mongoloid, australoid, Polynesian, bushman and Americans. Except for geographical or psychological barriers, members of races can freely interbreed.

 (b) Cline. The transient population between two ecotypes, biotypes, or breeding groups constitutes a cline.

(c) Biotype. It is a group of individuals of a species that have the same type of genotype. 

(d) Ecotype. A genetically different population of a species colonizing a different specific habitat. Different ecotypes are, however, Inter-fertile

(e) Deme. It is an assemblage of closely related interbreeding Individuals of a species that shows one to numerous demes.

(f) Sub-species. It is a distinct breeding subgroup of species consisting of one or more populations having certain distinguishing characteristics that separate its members from other members of the species. Though from separate branding groups, members of different subspecies are only partially isolated reproductively because they can Interbreed and form fertile offspring. The term subspecies is often replaced by the term variety in the case of plants. Eg. Corvus splendens splendens - Indian Crow

Corvus splendens protegatus - Sri Lankan Crow

Brassica oleracea var. capitata - Cabbage

Brassica oleracea var. botrytis-Cauliflower

5. Mechanism of speciation

Speciation (= Cladogenesis) is the phenomenon of the development of one or more new species from an existing one by a buildup of reproductive isolation between them.

6. Essential steps for speciation

(i) For reproductive isolation to occur gene flow between populations must be reduced to the extent to which 'foreign' genes entering one population from the other by hybridization can be eliminated by natural selection.

(ii) Geographic or ecological isolation for a long time. Reducing 

(iii) selection pressure by moving into the new or unoccupied habitat.

(iv) Increased variability within the species due to natural selection and subsequent variation of gene frequency.

 (v) Genetic isolation and failure to interbreed, with the main stock produce geographic race or sub-species.

(vi) Failure of subspecies to interbreed when reunited establishes themselves as a new species. 

7. Types of speciation

(A) Allopatric speciation.

(i) It is the formation of new species from spatially isolated populations. 

(ii) The isolated population may appear due to various types of geographical barriers and immigration.

(iii) It is put to different selective pressures, mutations, and genetic drift so that the gene pool of the isolated population ultimately becomes quite different from the gene pool of the parent population. 

(iv) The isolated population will also develop a number of characteristics of its own which would prevent interbreeding with the parent population even if the two are brought together. It, thus, becomes a new species.

(v) Allopatric speciation is a slow process and takes a very long time to form.

(B) Sympatric speciation

(i) it is the formation of new species from a segment of a population due to the sudden appearance of reproductive isolation.

(ii) Sympatric speciation is abrupt and is caused by mutations, interspecific hybridization or polyploidy.

(C) Speciation by adaptive radiation

(i) species often possess several ecotypes.

 (ii) Each ecotype is adapted to its environment.

(iii) With the passage of time, the different ecotypes accumulate distinctive variations and become reproductively isolated to form a new species. 

(iv) Darwin's finches of the Galapagos Islands are example of adaptive radiation.

(D) Speciation by phyletic evolution. With the passage of time, each species accumulates so many variations that it gets transformed new species.

 (E) Gradual speciation. This process of speciation or formation of new species is gradual and takes a long time, hundreds and thousands of years. It is carried out by isolating a segment of the population, accumulation of different variations, genetic drift, and natural selection.

(F) Abrupt speciation (Instant speciation). New species develop suddenly due to interspecific hybridization, differential reproduction, mutations, and polyploidy.

(1) Mutations

(a) They are the main cause of evolution. Major mutations can give rise to new species all of a sudden.

(b)The mutation causing genetic isolation produces sibling species (morphologically similar but reproductively isolated).

 (c) With the passage of time the sibling species accumulate different morphological characteristics so as to become visibly different. 

(2) Hybridisation 

(a) Though members of the two different species are not interfertile, occasional crossing do occur.

(b) Such interspecific hybrids possess two sets of genomes, one from each parent.

 (c) They are sexually sterile because their chromosomes do not have homologues.

(d)  However, rarely both the genomes pass on to a single gamete due to failure of anaphasic separation of chromosomes. The fusion of such gametes gives rise to fertile offspring as the latter comes to have two genomes of each type. The phenomenon is called allopolyploidy. 

(e) Allopolyploidy has been induced artificially as well in plants.

(f)  Allopolyploidy or interspecific hybridization followed by polyploidy immediately gives rise to a new species. It has traits of both the parental species as well as some new characters. There is an increase in the size of the gene pool which allows the hybrids to adapt to different habitats and environments. The members of the new species will not mate with members of any of the two parent species.

Eg.-Hybridisation followed by doubling of chromosomes has occurred in nature during the evolution of wheat as well as American cotton. Artificial production of allopolyploid interspecific hybrids includes the synthesis of Raphanobrassica (Raphanus sativus x Brassica oleracea) and Triticale (Triticum durum x Secale cereale; Triticum aestivum X Secale cereale).

(3) Polyploidy

(a) It is the phenomenon of having more than two sets of chromosomes or genomes. Tetraploidy is more common.

(b) Polyploids have increased gene dosage, gigas effect and wider range of distribution.

(c) They are initially less different from the diploid parents which do not freely interbreed with them and, therefore, soon become reproductively isolated.

8. Origin of species occurs through reproductive isolating mechanisms which can be divided into two types: A. Prezygotic mechanisms and B. Post-zygotic Mechanisms.

9. Reproductive Isolating Mechanisms

A. Prezygotic barriers prevent fertilization. Some of the examples are as follows: 

a) Geographical isolation = spatial isolation where populations who live in different habitats do not meet during the breeding season.

(b) In ecological isolation the populations live in different habitats and do not meet; there may be no habitat suitable for their hybrids.

(c) Temporal isolation occurs when populations reproduce at different seasons or different times of the day; individuals do not meet when mating.

(d) Ethological or behavioral isolation occurs when individuals do not find one another attractive as mates.

(e) In mechanical isolation: structure of genitalia or flowers prevent successful copulation or pollen transfer. (Prevention of gamete fusion: male and female gametes do not fuse.

B. Postzygotic mechanisms

(i) Hybrid Inviability. hybrid zygotes fail to develop to sexual maturity.

(ii) Hybrid sterility. hybrids fail to produce viable gametes.

(iii) Hybrid breakdown. hybrids have lower chances of surviving up to reproductive age.