Genetic Basis of Evolution q Species q Sibling

Genetic Basis of Evolution q. Species q. Sibling species q. Deme q. Speciation q. Variations

• • Species ? Morphological Concept of Species Genetic Concept of Species Biological Concept of Species 1. Morphological Concept: • “A group of individuals that resembles each other in most of their visible or morphological characters & differing only marginally”. • But this criterion is not applicable in all cases & very often greater morphological differences seen among individuals of a single population.

2. Genetic Concept: • According to Losy (1918) “a species is a group of genetically identical individuals”. • This definition is found to be incorrect because no two individuals are genetically similar. 3. Biological concept: • This has been proposed by Dabzhansky in (1937) & Mayr in (1942). • “species are the most inclusive Mendelian population which share in the common gene pool: . • Mayr has given a most complete & practical definition of a species.

• “Species is an array of actually or potentially inbreeding natural population that are reproductively isolated from other such group under natural conditions. • These are converted into separate species only when they are unable to interbreed & then take a independent path.

• General characters of species: 1. Members of a species exhibit distinctive features in common, in which these are different from all other groups. 2. Species are reproductively isolated. 3. Members of separate species do not usually interbreeds of species are totally absent or rare. 4. Allied species usually have separate but adjoining territories. 5. Each species fills an ecological niche not exactly utilized by other species.

6. Each species remains in process of continually adjusting to its environment. 7. Each species possesses a constellation isolating mechanism that directly or indirectly prevent exchange of genes with related species. 8. Each species has the capacity to give rise to new species. 9. A species is a population of individuals with similar structural & functional characteristics, which have common ancestry & in nature breed only with each other.

Sibling Species: • These are the members of the population that are morphologically similar if not identical but are reproductively isolated i. e. these species are morphologically indistinct. • These species can not be classified as separate species but are called : biological races”. • Mayr (1942) for the first time used the term ‘Sibling species’ to these biological races. • Sibling species occur in a almost all animal groups but they are more common in insects. • Sibling species exhibit differences in their habits, ecology, breeding period & sexual behavior.

• These are of great practical importance in applied biology, medical entomology & in agriculture. • Sibling species help in understanding the process of speciation.

• Deme: • A deme is a community of potentially inbreeding individuals at a given locality. • These are genetically similar. • It is a smallest population of unit, which is able to inbreed with other demes of the species. • These usually share a large number of basic features of similarity & the differences are so incipient. • Deme lives in an area where conditions are most suitable for life & the areas of habitat are separated by regions of unsuitable conditions.

• Speciation: • The process of splitting a genetically homogenous population into two or more populations that undergo genetic differences & eventual reproductive isolation is speciation. • Types of Speciation: 1. Phyletic speciation 2. Quantum speciation 3. Gradual speciation.

1. Phyletic speciation: • Transformation of a species from one to other due to slow & steady change or changes in time. • This occurs due to… (i) Adaptations to a shifting environment. (ii) Increasing specialization for a particular environment. (iii) Improved adaptations in a constant environment.

2. Quantum speciation: (i) Represents accelerated pace of phyletic speciation. (ii) It involves accumulation of small genetic changes in relatively rapid succession. (iii) These evolutionary events occurs suddenly & occasionally. (iv) May occurs because of chromosomal aberrations.

3. Gradual speciation: (i) It is a micro evolutionary event resulting from the gradual accumulation of many minute gene differences over a long period of time under the influence of natural selection. (ii) In this type of speciation one species gives rise to one or more species (sub-species).