Stabilising selection favours average sized individuals whereas eliminates small sized individuals. It reduces variation and thus does not promote evolutionary change. But, it maintains the mean value form generation of generation. In directional selection, the population changes towards one particular direction. It favours small or large sized individuals and more the mean size of the population changes. Disruptive in eliminates most of members with mean expression, so may lead to developemnt of two different individuals. it is important in bringing about evolutionary change.

According to Oparin 's hypothesis, early protocell could have been a coacervate. Such a structure could have consisted of a collection of organic macromolecules surrounded by a film of water molecules. This arrangement of water molecules, although not a membrane, could have functioned as a physical barrier between the organic molecules and their surroundings.

In England, before industrialisation in 1860, it was observed that there were more white-winged moths on tree trunks than dark-winged or melanised moths. But after industrialisation in 1920, there were more dark-winged moths in the same area. Before industrialisation, the tree trunks were covered by white-coloured lichens. In that background, the white-winged moths survived but the dark-winged moths were eaten by predators. During post industrialisation period, the tree trunks became dark due to industrial smoke and soot. Therefore, the white winged moths could not survive due to predators but dark-winged moths survived, i.e., the moths that were hidden in background survived. Thus, industrial melanism supports evolution by natural selection.

Hardy Weinberg punciple describes a theoretical situation in which a population is undergoing no evolutionary change. It states that allele frequencies in a population are stable and constant from generation to generation. Them are five factors that affect Hardy-Weinberg Pricniple. These are - mutation, gene flow, genetic drift, genetic recombination and natural selection pressure. 

When more individuals of a population acquire a mean character value, it is called stabilising selection. No change in allelic frequency in a population will lead to Hardy-Weinberg equilibrium.

When the population is at decline, the number of individuals may reduce to a extent that the small group of population constituting the population becomes isolated and restricted on distribution. These are then exposed to random genetic drift resulting in the fixation of certain genes. Thus, the population re-estacblishes its former nichness. Such reduction in allele frequencies is called a genetic bottle-neck effect which often prevents the species from extinction.

The random changes in gene frequencies in a population occurring by chance alone rather than by natural selection are called genetic drift or Sewall wright effect. The effects of genetic drift are most marked in very small isolated population, although it occurs in all populations.

Reproductive isolation is the prevention of interbreeding between the populations of a species present in the different environments. It maintains the characters of the species but also leads to origin of new species (speciation).

In a stable population, for a gene with two alleles, 'A '(dominant) and 'a '(recessive), if the frequency of 'A 'is p and the frequency of 'a 'is q, then the frequencies of the three possible genotypes (AA, Aa and aa) can be expressed by the Hardy-Weinberg equation:
p²   + 2pq + q² = 1
where p² = Frequency of AA (homozygous dominant) individuals
q² = Frequency of aa (homozygous recessive) individuals
2pq = Frequency of Aa (heterozygous) individuals
so, p = 0.6 and q = 0.4 (given)
∴2pq (frequency of heterozygote)
= 2 ×0.6 ×0.4
= 0.48

The development of melanin pigment in moth occurred due to mutation. The light coloured form of peppered moth gave rise to the dark coloured form by  mutation.

Once inter-breeding population of finches, now have failed to produce fertile F₁ hybrids, this means they have now formed two distinct species.

Isolation is the prevention of mating amongst inter breeding groups due to physical (e.g., geographical, ecological) and biotic (e.g., physiological, behavioural, mechanical, genetic) barriers. Reproductive isolating mechanisms are the biological properties of individuals which prevent the interbreeding of naturally sympatric populations. It maintains the characters of the species but can lead to the origin of new species. Also, Darwin considered that useful variations are transmitted to the offspring and appear more prominently in succeeding generations. After some generations these become continuous and gradual variations in the possessor would be so distinct that they form a new species.

In allopatric speciation, a part of the population becomes eographically isolated from the main population. The population becomes entirely separated and finally consititutes a new species. Formation of Darwin 's finches that formed separate species in Galapagos islands in an example of allopatric speciation.