One of the best studied cases in fossil record concerns evolution of horses.The evolution of horse started with Eohippus during Eocene period.It involved the following evolutionary stages:
(i)Gradual increase in body size
(ii)Elongation of head and neck region
(iii)Increase in the length of limbs and feet
(iv)Gradual reduction of leteral digits
(v)Enlargement of third functional toe
(vi)Strengthening of brain and sensory organ
(vii)Increase in the complexity of teeth for feeding on grass
 Eqquus(Plestocene period)
 \(\uparrow \)
 Pliohippus(Pliocene period)
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Merychippus(Miocene period)
\(\uparrow \)
  Mesohippus(Oligocene period)
\(\uparrow \)
Eohippus(Ecocene period)

Yes,a natural disaster can be cause for extinction of species.As new species evolved to fit ever changing ecological niches,older species fade away.
But,the rate of extinction is far constant.In last 500 million years,50-90% or more of all species on earth have disappeared in a geological blinks.the most studied mass extinction between the cretaceous and palaeocene peroid about 65 million years the cause is suspected to be volcanic eruptions and impacts of large asteroids or comets striking the earth.

(i) 250 million ; pangaea 
(ii) seas ; barriers
(iii) six ; Ethiopian
(iv) birds ; all animals and plants
(v) Charles Darwin ; organic evolution.

The earth originally had a strongly reducing atmosphere. There was no oxygen in molecular form in the primitive atmosphere.

1.Population is large enough and there is free flow of genetic material among individuals.
2. Mating takes place at random.
3. Mutation does not take place or occurs at same rate in both directions.
4. There is either nil or negligible immigration or emigration.

The meaning of the given words are as given below
Race. It is a classification system used to categorise humans into large and distinct populations or groups
by anatomical, cultural, linguistic, geographical, historical and for religious relationship.
Breed. It is a specificgroup ofdomestic animals or plants having homogenous appearance and behaviour
and other characteristics that distinguish it from other animals or plants of the same species. They were formed through selective breeding.
Cultivar. It is a plant or grouping of plants selected for desirable characteristics that can be maintained by propagation. 'Cultivar' stands for 'cultivated variety'.
Variety. A variety arises naturally in the plant kingdom and plant grown from its seeds will typically come true to type.

Yes,  we agree. As the gene flow occurs through geographical barriers over generations, by studying specific allelic frequencies in various populations of the world, we can predict the human migratory patterns in pre-historic and historic era.
There have been projects undertaken such as human genographics project. Which uses data from studies on specific genes/chromosomes/mitochondrial DNA to trade the evolutionary history and migratory patterns of humans.

Factors affecting allele frequency in populations are described below:
(i) Mutations: These are sudden heritable changes which are supposed to be the primary source of genetic variation. They are of following two types.
(a) Chromosomal Mutations: They arise due to changes in chromosome number and changes in structure.
(b) Gene Mutations: These are changes in gene structure and expression due to addition, deletion, substitution or inversion of nucleotides.
(ii) Non-random Mating: Repeated mating between individuals of certain selected traits changes the gene frequency, e.g., selection of more brightly coloured male bird by a female bird may increase the gene frequency of bright colour in the next generation.
(iii) Gene Flow (Gene Migration): It is the movement of alleles into and out of a gene pool. Breeding of immigrants with the host population adds new alleles to the gene pool of the host population.

1. We are still not clear whether evolution and natural selection are processes or results of some unknown processes.
2. On one hand, when we describe the world as we see today, we describe evolution a process.
3. On the other hand, when we describe life on earth, we treat evolution as a consequences of the process of natural selection.

With the industrial growth in 1920, the white-winged grey-coloured Biston betularia was rapidly replaced by a dark-coloured melanic Biston carbonaria as the later had more chances of survival and favoured by Natural selection. But after 1956, the coal was rapidly replaced by oil and electricity as energy source in the industries which reduced soot deposition. So the light coloured moth is again replacing the dark-coloured moth by reverse evolution.