Final answer:
A large lake can lead to the formation of a new species of lizard through allopatric speciation, where the lake serves as a geographical barrier that isolates populations and disrupts gene flow. Birds, however, can typically fly over such barriers, maintaining gene flow, which decreases the likelihood of speciation.
Step-by-step explanation:
The impact of a geographical barrier, such as a large lake, on the formation of a new species can be significant, particularly for organisms that are not adept at overcoming such obstacles. For lizards, a large lake might represent an insurmountable barrier, leading to the isolation of populations on either side. This geographical separation can disrupt gene flow between the two groups, allowing for allopatric speciation. Over time, differences in selection pressures and genetic drift can lead to the populations diverging sufficiently to become distinct species.
In contrast, for birds which generally have a high potential for dispersal due to their ability to fly, a large lake may not constitute a significant barrier. Birds can easily cross over the water, allowing for continued gene flow between populations, thus reducing the likelihood of speciation due to geographical isolation.