NEET Practice Questions (MCQs) with Answers & Solutions

The text states, 'Another example is Australian marsupials. A number of marsupials, each different from the other (Figure 6.6) evolved from an ancestral stock, but all within the Australian island continent.'

The NCERT text mentions, 'When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution. Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial.'

The text states, 'Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial).' This similarity between distantly related groups in similar niches is convergent evolution, which is seen in conjunction with adaptive radiation within each group.

The text states, 'There he observed an amazing diversity of creatures. Of particular interest, small black birds later called Darwin’s Finches amazed him. He realised that there were many varieties of finches in the same island. All the varieties, he conjectured, evolved on the island itself. From the original seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches.'

Adaptive radiation involves diversification into different habitats (ecological niches). An isolated geographical area with available, varied niches allows for less competition and more opportunities for a single ancestral stock to diversify and adapt.

Adaptive radiation implies diversification into numerous distinct forms occupying varied ecological niches from a single ancestral stock in an isolated geographical area. While humans have adapted to diverse environments, human evolution is generally considered a more linear or branching evolution within the Homo genus, rather than a broad 'radiation' into many distinct, niche-specific 'species' from a single recent ancestor in an isolated region in the same way as Darwin's finches or marsupials.

The text explicitly mentions, 'Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial).' This similarity between two distinct evolutionary lineages adapting to similar ecological roles is a classic example of convergent evolution.

The context states the general order of filling as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s... After 3p, 4s is filled next. This is also consistent with the (n+l) rule where (n+l) for 3p is (3+1)=4 and for 4s is (4+0)=4. When (n+l) values are equal, the orbital with the lower 'n' value has lower energy. However, in the given sequence, 4s (n=4, l=0) has higher n but is filled before 3d (n=3, l=2). The general order provided in the NCERT text specifies 4s before 3d, making 4s the correct answer. (Reference: 'However, following order of energies of the orbitals is extremely useful: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s...')

The Pauli Exclusion Principle states that 'only two electrons may exist in the same orbital and these electrons must have opposite spin.' Therefore, the statement that electrons in the same orbital must have the same spin is incorrect. (Reference: 'According to this principle : no two electrons in an atom can have the same set of four quantum numbers. Pauli exclusion principle can also be stated as : “ only two electrons may exist in the same orbital and these electrons must have opposite spin.”')

The context states: 'pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.' It further clarifies, 'pairing of electrons will start in the p, d and f orbitals with the entry of 4th, 6th and 8th electron, respectively.' Since d-subshell has 5 degenerate orbitals, each orbital will receive one electron (total 5 electrons) before pairing starts. The 6th electron will be the first one to pair. (Reference: 'pairing of electrons will start in the p, d and f orbitals with the entry of 4th, 6th and 8th electron, respectively.')