According to evolutionary theory, formation of new species is generally due to

Question

$\begin{array}{1 1} (a)\; \text{Sudden creation by nature} \\ (b)\; \text{Accumulation of variations over several generations} \\ (c)\; \text{Clones formed during asexual reproduction} \\ (d)\; \text{Movements of individuals from one habitat to another} \end{array}$