Evaluating the Statements

Let’s analyze each statement in the context of current scientific understanding:

A. The different call frequencies may signal incipient speciation

True.
Differences in mating calls can be an early sign of reproductive isolation, which is a key step in the speciation process. When populations develop distinct signals for mate attraction, gene flow between them can be reduced, setting the stage for speciation.

B. The change in call frequency can lead to allopatric speciation

True.
Allopatric speciation occurs when populations are geographically isolated (such as by a river), and divergent evolution in traits like call frequency can reinforce this isolation, eventually leading to the formation of new species.

C. Individuals of one population transplanted to the other population (across the river) may have lower chance of finding mates than residents

True.
If call frequency is critical for mate attraction, migrants with non-local calls may be less successful at finding mates, further reducing gene flow and promoting reproductive isolation.

D. Call frequencies have changed from ultrasound to infrasound across the river

False.
Cricket call frequencies, even when divergent, remain within a tonal range typical for the species (generally between 2–9 kHz). There is no evidence of a shift from ultrasound (above 20 kHz) to infrasound (below 20 Hz) in natural cricket populations. Such a drastic change is not supported by research or the biology of crickets.

Correct Combination

The statements supported by scientific evidence are A, B, and C.

Conclusion

Differences in cricket call frequencies across a river can signal the beginning of speciation (incipient speciation), facilitate allopatric speciation due to geographic and reproductive isolation, and cause migrants to have reduced mating success. However, the idea that call frequencies shift from ultrasound to infrasound is not accurate.

Correct answer: (1) A, B and C