• Species: It is a group of organisms which can interbreed and produce fertile offspring. A species shares a common gene pool.
  • Speciation: It is the formation of new species and diversification of species. It occurs when the common gene pool shared by a species is interrupted by isolating mechanisms.

Modes of Speciation:

Phyletic Transformation:

  • (= Autogenous Speciation; Vertical Evolution; Phyletic Gradualism) (Phyletic means line of descent; Autogenous means arising from within.)
    • It is simply the gradual transformation of one species into another, over a period of time by accumulation of slow changes in gene pool.
    • Neither it involves splitting of gene pool nor evolution of reproductive isolation.
    • Gene pool of parental species could get modified due to- changes in Gene frequency, Chromosomal inversions or other structural changes in chromosomes. These changes gets accumulated over period of time in response to following factors-
      • Adaptations (to a shifting environment)
      • Improved Adaptations (in constant environment)
      • Increasing specialization (for a particular environment).
    • When evolution occurs by this mode, it is usually through a process called anagenesis (gradual evolution of a species that continues to exist as an interbreeding population & there is a steady transformation of a particular species into a new one).
    • A species’ evolutionary transformation does not necessarily leads to multiplication of species. 

Example: A population of species “a” on an isolated island changes into species “d” gradually, through changing into species “b” & “c” one after the other without splitting into more than one species.

Summing up phyletic gradualism is the continuous genetic & evolutionary change within populations of species without generating reproductive isolation.

Fusion of Species:

  • A new species may arise through fusion of two already existing species due to breakdown of their reproductive isolation.
  • It may lead to reticulate evolution i.e. origin of a lineage through partial merging of two ancestor lineages.
  • According to Ernst Mayr, fusion of species leads to reduction in number of species.
  • Fusion of species can occur in following ways:
    • Hybridization e.g. many new plant species are formed by hybridization.
    • By breakdown of reproductive isolation in previously existing sympatric species e.g. as in case of Pipilo erythrophthalamus and Pipilo ocai in Mexico.

As “pure species” they are widespread in following regions:-

Pipilo erythrophthalamus: Widespread in North America & extends south as far as Chiapas & Guatemala.

Pipilo ocai: occurs from Oaxaca to Jalisco.

In Oaxaca, the two species live side by side, no intermixing.

The breakdown causes of isolating mechanisms among organisms may involve mode of fertilization, nature of mating bond, rarity of one parental species, disturbance of habitat etc.

True Speciation:

It involves origin of two or more species from one parental species.  

It requires formation of new gene pool(s) from parental gene pool & also establishment of reproductive isolation between them.

Instantaneous Speciation (=Sudden Speciation; Quantum Speciation):

This refers to production of single individual or offspring that is reproductively isolated from parental species along with being capable of reproducing & establishing itself as a new species. It can occur by chromosome aberrations which include Inversion, Translocation, Polyploidy, Autopolyploidy & Amphiploidy.

Chromosome Aberration simply means abnormal structure or number of chromosomes.

The chromosomal or chromosome aberrations which may lead to speciation…

  • Inversion: When a chromosome segment is detached, flipped 180and reattached to rest of the chromosome. It can be of two types-
    1. Pericentric: Inversion includes the centromere.
    2. Paracentric: Inversion does not include centromere.
  • Translocation:  A segment of a chromosome detaches & further gets reattach to a non-homologous chromosome. Types of translocations…
    • Reciprocal Translocation: Chromosome segments are interchanged between two non-homologous chromosomes. No net loss of genetic material occurs in it. Reciprocal translocation is of further two types…
      • Homozygous reciprocal: Both homologues of one chromosome pair exchange segment with another non-homologue pair.
      • Heterozygous reciprocal: Only one member of homologue pair exchange segment with one member of another non-homologue pair.
  • Non-Reciprocal Translocation: It involves one way transfer of chromosome segment from one chromosome to another non-homologous chromosome.
  • Types of Non-reciprocal translocation are:
  • Simple Non-Reciprocal Translocation: Segment from homologue pair of chromosomes breaks off & gets attached to some other non-homologue pair of chromosomes.
  • Shift Non-Reciprocal Translocation: Interstitial segment of one chromosome breaks off & gets attached interstitially in some other non-homologous chromosome.
  • Polyploidy: When an organism possesses more than normal two sets of chromosomes, like Triploidy having 3 sets of chromosomes (3n) same for Tetraploidy (4n), Pentaploidy (5n) etc. It is caused when DNA replication occurs but subsequent nuclear division doesn’t occur. Following types of polyploidy based on source of chromosome are said to give rise to new species…
    • Autopolyploidy: When an organism possess more than two sets of chromosomes and both are derived from same parental species.
    • Amphiploidy: When an organism possess at least one diploid set of chromosomes from each parental species.

Significance of Chromosomal Aberrations in speciation: The above listed chromosomal aberrations may lead to reproductive isolations with parental species causing isolation of gene pool, thus leading to formation of new species.

Gradual Speciation:

It involves origin of new species by accumulation of many minor gene differences over time in daughter populations under influence of environment in which they reside, involving natural selection. It may be of following types-

1) Allopatric Speciation-

(Allo= other; patric= fatherland)

Allopatric speciation occurs when a parental population diverge & become new independent species due to emergence of physical/geographical barrier in between them, preventing gene flow. It is said to be by two ways:

  • By Vicariance:

(Vicariance= geographical separation of a population, especially by physical barrier like mountain etc.)

In Allopatric Speciation by vicariance, a population gets divided/separated by some external physical barrier like mountain, river etc. such that it causes genetic isolation between them and gradually reproductive isolation gets build up, thus forming new species overtime.

  • By Founder Effect (Peripatric Speciation):

Allopatric speciation by founder effect or Peripatric speciation occurs when a small part of a population that could be an individual or limited no. of individuals migrate to some other geographical area & overtime due to natural selection operating on them of the new environment causes new gene combinations to occur as compared to their parental population, resulting into new species formation.

2) Parapatric:

Parapatric speciation occurs when a population develops reproductive isolation while continuing some unequal gene flow in between them. In parapatric speciation there is non-random mating and the population can exist in continuous or discontinuous geographic area.

3) Alloparapatric:

It occurs when initially a population is under allopatric effect due to separate geographical areas but later on become parapatric.

4) Sympatric: (Involves same geographical area)

It involves development of some biological barrier like mutations in between a previously randomly mating population leading to restrictive interbreeding and gradually forming a new species.  

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