Neo-Darwinism or Modern Concept or Synthetic Theory of Evolution:

 

 The detailed studies of Lamarckism, Darwinism and Mutation theory of evolution showed that no single theory is fully satisfactory. Neo-Darwinism is a modified version of theory of Natural Selection and is a sort of reconciliation between Darwin’s and de Vries theories.

Modern or synthetic theory of evolution was designated by Huxley (1942). It emphasises the importance of populations as the units of evolution and the central role of natural selection as the most important mechanism of evolution.

The scientists who contributed to the outcome of Neo-Darwinism were: J.S. Huxley, R.A. Fischer and J.B.S. Haldane of England; and S. Wright, Ford, H.J. Muller and T. Dobzhansky of America.

  Postulates of Neo-Darwinism:

1. Genetic Variability

2. Natural Selection and

3. Reproductive isolation

 1. Genetic Variability:

Variability is an opposing force to heredity and is essential for evolution as the variations form the raw material for evolution. The studies showed that the units of both heredity and mutations are genes which are located in a linear manner on the chromosomes.

The change in gene pool is called genetic variability.

The sum total of all genes present in all individuals of a species form the gene pool of that species.

 Various sources of genetic variability in a gene pool are:

 (i) Mutations:

These are sudden, large and inheritable changes in the genetic material. On the basis of amount of genetic material involved, mutations are of three types:

(a) Chromosomal aberrations:

These include the morphological changes in the chromosomes without affecting the number of chromosomes. These result changes either in the number of genes (deletion and duplication) or in the position of genes (inversion).

These are of four types:

1. Deletion (Deficiency) involves the loss of a gene block from the chromosome and may be terminal or intercalary.

 2. Duplication involves the presence of some genes more than once, called the repeat. It may be tandem or reverse duplication.

 3. Translocation involves transfer of a gene block from one chromosome to a non-homologous chromosome and may be simple or reciprocal type.

 4. Inversion involves the rotation of an intercalary gene block through 180° and may be paracentric or pericentric.

 (b) Numerical chromosomal mutations:

These include changes in the number of chromosomes. These may be euploidy (gain or loss of one or more genomes) or aneuploidy (gain or loss of one or two chromosomes). Euploidy may be haploidy or polyploidy.

Among polyploidy, tetraploidy is most common. Polyploidy provides greater genetic material for mutations and variability. In haploids, recessive genes express in the same generation.

Aneuploidy may be hypoploidy or hyperploidy. Hypoploidy may be monosomy (loss of one chromosome) or nullisomy (loss of two chromosomes). Hyperploidy may be trisomy (gain of one chromosome) or tetrasomy (gain of two chromosomes).

 (c) Gene mutations (Point mutations):

These are invisible changes in chemical nature (DNA) of a gene and are of three types:

1. Deletion involves loss of one or more nucleotide pairs.

2. Addition involves gain of one or more nucleotide pairs.

3. Substitution involves replacement of one or more nucleotide pairs by other base pairs. These may be transition or transversion type.

These changes in DNA cause the changes in the sequence of amino acids so changing the nature of proteins and the phenotype.

 (ii) Recombination of genes:

Thousands of new combinations of genes are produced due to crossing over, chance arrangement of bivalents at the equator during metaphase – I and chance fusion of gametes during fertilization.

 (iii) Hybridization:

It involves the interbreeding of two genetically different individuals to produce ‘hybrids’.

  (Iv) Genetic drift:

It is the elimination of the genes of some original characteristics of a species by extreme reduction in a population due to epidemics or migration or Sewell Wright effect.

The chances of variations are also increased by non-random mating.

 (V) Mutagens:

 A mutagen is a physical or chemical agent that permanently changes genetic material, usually  DNA in  an organism and thus increases the frequency of mutations.

Physical agents: 

·         Heat and radiation

Chemical agents:

·         Base analogs  . Alkylating agents 

·         Deaminating agents   . Metal ions .

Biological agents:

·         Viruses 

·         Bacteria 

2. Natural Selection:

Natural selection of Neo- Darwinism differs from that of Darwinism that it does not operate through “survival of the fittest” but operates through differential reproduction and comparative reproductive success.

Differential reproduction states that those members, which are best adapted to the environment, reproduce at a higher rate and produce more offsprings than those which are less adapted. So these contribute proportionately greater percentage of genes to the gene pool of next generation while less adapted individuals produce fewer offsprings.

If the differential reproduction continues for a number of generations, then the genes of those individuals which produce more offsprings will become predominant in the gene pool of the population as shown in Fig.

Due to sexual communication, there is free flow of genes so that the genetic variability which appears in certain individuals, gradually spreads from one deme to another deme, from deme to population and then on neighbouring sister populations and finally on most of the members of a species. So, natural selection causes progressive changes in gene frequencies, i.e., the frequency of some genes increases while the frequency of some other genes decreases.

 Which individuals produce more offsprings?

(i) Mostly those individuals which are best adapted to the environment.

(ii) Whose sum of the positive selection pressure due to useful genetic variability is more than the sum of negative selection pressure due to harmful genetic variability?

(iii) Which have better chances of sexual selection due to development of some bright coloured spots on their body e.g. in many male birds and fish.

(iv) Those who are able to overcome the physical and biological environmental factors to successfully reach the sexual maturity.

 So natural selection of Neo-Darwinism acts as a creative force and operates through comparative reproductive success. Accumulation of a number of such variations leads to the origin of a new species.

 3. Reproductive isolation:

Any factor which reduces the chances of interbreeding between the related groups of living organisms is called an isolating mechanism. Reproductive isolation is must so as to allow the accumulation of variations leading to speciation by preventing hybridization.

In the absence of reproductive isolation, these variants freely interbreed which lead to intermixing of their genotypes, dilution of their peculiarities and disappearance of differences between them. So, reproductive isolation helps in evolutionary divergence.

 

Types of Isolating Mechanisms

The three common types of isolation are called behavioral isolation, geographic isolation, and temporal (time) isolation.

Behavioral isolation happens when some kind of behavior changes within a population, like how males court females with songs. If half the population uses one song to attract females and the other half is using a different song, the two parts will not mix, and they become isolated.

Geographic isolation is easier to visualize. It happens when a population is somehow divided by geography, like back when the continents separated. On a smaller scale, populations can become separated by rivers, mountains, or bodies of water. A type of squirrel actually became two separate species because the grand Canyon divided the population.

Temporal isolation means that the population was divided by time. Temporal is a fancy way of saying time. If half of a population mates in January and the other half mates in June, they will eventually become different species. Because they mate in different months, the two parts of the population can't breed with each other.

 

Thus, by combined effect of these three factors (processes) new species are formed.