Neo-Darwinism

Evolution can be regarded as the effect of natural selection on the continous appearance of mutant forms. It is a modified version of natural selection and mutation.

Postulates

1. Genetic Variability

2. Natural selection 

3. Reproductive Isolation

4. Speciation

1. Genetic Variability

Variations are essential for evolution. There are number of sources of genetic variability :

- Chromosomal aberrations as inversion, translocation.

- Change in chromosomal number as aneuploidy, deletion,duplication,polyploidy.

- Gene mutation as substitution, addition, point, frameshift, gross mutations.

- Mutagens-HCHO, H₂O2, Mustard,5- bromouracil, colchicine, rays, viruses etc. 

- Hybridisation - It is the interbreeding of two genetically different individuals. - - -

- Genetic Drift- It is the elimination of genes of some original characteristics of a species due to migration. It operates only in smaller population ie, error in gene pool sample (sampling error). In a new habitat, the colonisers are generally small, called founders. So there is increase in inbreeding that leads to increase in homozygosity. This produces a change in gene pool and a different population. Such mechanism of speciation is called Founder's effect.

There are certain species which show seasonal flush & crash in their population e.g., Houseflies, mosquitoes etc. Only a small population survive the unfavourable season (crash period) which brings about shrinkage of gene pool. Reduction in allele frequencies during the brief reduction in size of population is called Bottle neck effect.

2. Natural Selection

Natural selection of Neo-Darwinism operates through differential reproduction which states that best adapted organisms mature earlier and leave more progeny than

those which are less adapted. If the differential repro duction continues for a number of generations then the genes of individuals which produce more offsprings will become dominant in gene pool causing the change in gene frequency i.e., well adapted individuals contrib ute more to gene pool.

Examples/Evidences to Support Natural selection

- Peppered Moth/Industrial Melanism-In 19th Selection century, light coloured Biston betularia moth was abundant because it mimicked with the grey lichens that covers the tree trunks. With the advent of industrial revolution, black moth Biston carbonaria became quiet visible against light grey background & were easily killed by predators i.e., birds. As burning of coal became common, population of dark coloured moth and their frequency increased due to mimicry. Gradual be replacement of coal by electricity & oil (less soot production) reduced soot deposition on trees. Dark coloured moths disappeared and light coloured moths flourished.

Light and dark varieties of moth Biston which resembled the open country light coloured and smoked dark coloured tree trunks respectively

This replacement of light coloured moth by dark coloured melanic spp. due to industrial smoke was called. 

Industrial melanism

- DDT Resistant Mosquito/Directional Selection When DDT was used on large scale, only resistant type mosquitoes were able to survive and all sensitive types were killed. Ultimately the entire population now consists of resistant type which make DDT ineffective.

This evolution is because of change in gene frequency in population in response to change in the environment. That is why such insecticides remain useful for a limited time only. 

- Sickle cell Anaemia-It is characterised by sickle shaped RBC as

• Hbs. Hbs - Homozygous (Dies)

. HbA. Hbs-Heterozygous (Carrier)

It mostly occurs in tropical Africa where malaria is common. It was reported that heterozygous (HbA. HbS) individuals kill the malarial parasite as they resist the malarial infection in a much better way than the individuals with normal haemoglobin (HbA, Hb^). 

- Drug Resistance In Bacteria-Colon bacteria is 250 times more resistant to antibiotic than normal bacteria e.g., E. coli for chloramphenicol.

3. Reproductive Isolation

Any factor which reduces the chances of interbreeding between related groups of living organisms is called reproductive isolation. It helps in evolutionary diver gence.

(A) External Barriers/Extrinsic (Occurs First)

- Mechanical Isolation-Differ in morphology (size, structure etc.) of genitalia.

- Geographical Isolation-Because of barriers like sea, mountain, deserts, rivers etc.

- Ecological Isolation-Difference in the habitat.

- Temporal Isolation-Differ in their breeding period.

- Behavioural/psychic Isolation-Differ in behaviour e.g., Nest building, courtship dance etc.

- Physiological Isolation-Functional incompa tability in their mating. e.g., In Drosophila, vaginal mucous membrane swells up after mating. The swelling lasts for few hrs. when mating is between the members of the same species but conti es for days if mating in interspecific. This swelling blocks the passage and eggs fails to come out. 

- Ethological Isolation-Potential mates meet but -do not mate.

(B) Internal Barriers/Intrinsic 

Prezygotic :- 

- Gametic Mortality-Sperms of one species are either unable to survive or survive poorly in genital tract or sperms are transferred but eggs are not fertilized.

-Zygote Inviability-Eggs are fertilized but zygote is inviable.

Postzygotic :-

- Hybrid Inviability-Zygote produces F₁ hybrid of reduced viability. Hybrid sterility-

- Hybrid is produced -but partially or completely sterile.

Post zygotic forms are more costly in terms of time, energy & fitness than the prezygotic forms.

(C) Artificial Selection

When the individuals with desired characters inter breed, the process is called artificial selection.

- It is a fast process.

- selection is man made.

- It can give rise to varieties different from originating generation.

- Process operates in domestic population, resulting in improved varieties of animals and plants.

Most important factor for the development of a new species is extensive outbreeding.