Evolution and Biological Adaptation

The presence of populations of a species in distant localities preventing the gene flow between them leads to geographical isolation. Incompatible differences in reproductive structures refer to as mechanical isolation. Different periods of attaiment of sexual maturity lead to temporal isolation. Hybrid sterility is a post zygotic isolation. Incompatible premating behaviour leading to no attraction between groups for mating is termed as behavioural isolation. Thus, the correct answer is C.

Evolutionary changes occur at the level of populations rather than individuals. A population is the smallest unit that can undergo evolutionary change over generations through shifts in allele frequencies.

Saltation is an abrupt variation in the appearance of an organism, species, etc, usually caused by a genetic mutation (single step large mutation).  

Heredity and variation are the fundamental pillars of evolution. Heredity ensures traits are passed on, while variation provides the raw material for selection to act upon.

The modern synthetic theory of evolution is the result of the work of a number of scientists namely T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Swall Wright, Ernst Mayr, and G.L. Stebbins. Stebbins in his book, Process of Organic Evolution, discussed the synthetic theory.
It includes the following factors 

Environmental change and isolation of groups of organisms play an important role in evolution. Isolation means that organisms of the same species are separated, and happens when there is something between the organisms that they can't cross. Organisms become isolated as a result of environmental change. The cause of isolation can be gradual, like when mountains or deserts form, or continents split apart. It can also be quick, such as organisms being blown to different places by a storm or tsunami (tidal waves).
When organisms become isolated the two groups are also not able to reproduce together, so variations and mutations that occur in one group are not necessarily found in the other group. The longer the groups are isolated, the more different they are. They eventually become different species. Moreover, if there is a change in the environment of one group it does not necessarily occur in the environment of the other. So they will evolve and adapt differently. So, the role of isolation in evolution is maintenance of species. Thus, option B is correct.
Differentiation of species or evolutionary divergence or extermination of species are not the roles of isolation in evolution. Thus, other options are wrong.

The modern synthetic theory of evolution integrates Darwinian natural selection with Mendelian genetics, including mutation, recombination, and reproductive isolation as key drivers.

Late in the 1930s, Dobzhansky discovered "isolation mechanisms", i.e., finely-tuned sex traits, other genetic factors and to him physical barriers in the environment which impede gene flow between populations.  Without these mechanisms incorporated into evolutionary theory, it would under-explain the level of species-diversity found in nature. The theory of origin of isolation which stresses that nature is not in favour of hybridization was given by Dobzhansky. Thus, option C is correct. 

The modern synthetic theory of evolution is the result of the work of a number of scientists namely T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Swall Wright, Ernst Mayr, and G.L. Stebbins. Stebbins in his book, Process of Organic Evolution, discussed the synthetic theory. It includes the following factors - Gene mutations, Variation (Recombination), Heredity, Natural selection and Isolation. In addition, three accessory factors affect the working of these five basic factors. Migration of individuals from one population to another as well as hybridization between races or closely related species both increase the amount of genetic variability available to a population. The effects of chance acting on small populations may alter the way in which natural selection guides the course of evolution. Thus, option D is correct.

• The Modern Synthetic theory of Evolution explains the evolution of life in terms of genetic changes occurring in the populations that leads to the formation of new species. 
• It also explains the genetic population or Mendelian population, gene pool and the gene frequency
  
• Hence, According to the modern synthetic theory, the unit of evolution is a/an Population.
• So, the correct answer is 'Population'.

• The theory of evolution by natural selection, first formulated in Darwin's book "On the Origin of Species" in 1859, is the process by which organisms change over time as a result of changes in heritable physical or behavioural traits.
• Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation.
• Evolution occurs when evolutionary processes such as natural selection and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population. 
• Hence, Modern evolution theory involves Natural selection, Genetic variations and Isolation. 
• So, the correct answer is 'All of the above'.
  

Postmating isolating mechanism is any structure, physiological function, or developmental abnormality that prevents organisms of two different populations, once mating has occurred, from producing vigorous, fertile offspring. Postmating isolating mechanisms includes, gametic. Gametic incompatibility  (Sperm transfer takes place, but egg is not fertilized), Zygotic mortality (Egg is fertilized, but zygote does not develop), Hybrid inviability (Hybrid embryo forms, but of reduced viability), Hybrid sterility (Hybrid is viable, but resulting adult is sterile) and Hybrid breakdown (First generation (F$ _{1}$) hybrids are viable and fertile, but further hybrid generations (F$ _{2}$ and backcrosses) may be inviable or sterile). Thus, option D is correct and other options are wrong.

Geographic isolation: In the fruit fly example, some fruit fly larvae were washed up on an island and speciation started because populations were prevented from interbreeding by geographic isolation. Scientists think that geographic isolation is a common way for the process of speciation to begin. Rivers change course, mountains rise, continents drift, organisms migrate and what was once a continuous population is divided into two or more smaller populations. So, a new species is formed from existing population when it is geographically isolated and not by different size of genitalia, different mating time and seasonal variations.