An organism in the mainland experiences two competitions namely inter and intraspecific. Hence due to the pressure on the organism it has to evolve more differently from its parents to survive if it doesn’t it will have to go through a tough time but still, certain organisms in the mainland do not evolve much as they have evolved a stable character which its competitors have not and hence doesn’t need a variation but most organisms in mainland need it and hence migrate to escape dangers or to find food hence if they adapt they survive if not they perish. Formation means to evolve under two of the above pressures but one might guess that diversification can be faster in the mainland but it is not due to the interspecific competition which hampers diversification but helps in the formation.
Also as the population is large one may expect more variation which is again hampered by interspecific competition. In the above image, the right represents an organism in Island with the same in the mainland after ‘t’ years the formation of the mainland species is ‘h'[the black vertical line on the left] and the formation of island species is ‘x'[the black vertical line on the right]. Hence the Difference of formation between mainland species and formation of island species is h-x which is always positive. The Red lines represent diversification. The smaller lines indicate extinct species [extinct due to pressure and competition]. We can see that more red lines are seen in the right due to a lack of interspecific competition. The red lines in the right represent Diversification[B] and diversification[A] to the left.
An easy example to prove Diversification[B] is faster than [a] is to take say 1000 bacteria A in a medium When we introduce another species of bacteria the bacteria A are reduced to 500. It is a well-known fact that in a small population only a small amount of variation is possible and hence when there is no interspecific competition they diversify more. In the above graph I take the no.of species in the X-axis and the land area the species cover divided by no.of species. From the graph, it is visible that lemurs have better diversified and tarsiers are the poorest as they are primitive and live in the mainland.