Chapter 22.4, Problem 1R

Summary Introduction

To review:

On the basis of the data on hybridization of two frog species, Rana berlandieri and R. sphenocephala, and their hybrid offspring at different life stages, answer the following questions:

a. Create pie charts for each life stage of the frog, showing the percentage of hybrids at each stage and the percentage (%) of each species.

b. Give the possible reasons for the differences in the percentage of hybrids at each stage. Also, give postzygotic isolating mechanisms consistent with the given data.

c. The changes expected in the breeding seasons of these species over time in the same pond. In case the predictions are correct, state the way the present pie charts would differ from the ones in part (a).

Given:

The given figure shows the temporal variation of breeding seasons in terms of egg laying in (A) allopatric and (B) sympatric species of frog.

The following table gives the data on hybridization of two frog species and their hybrid offspring at different life stages.

Life stage	Season	R. berlandieri	R. sphenocephala	Hybrids (F1)
Tadpoles (recently hatched)	Spring (year 1)	155	125	238
Tadpoles (late stage)	Summer (year 1)	45	55	64
Froglets (newly metamorphosed)	Fall (year 1)	32	42	15
Frogs (adult)	Year 2	10	15	1

Introduction:

In the given study, two species of frog R. berlandieri and R. sphenocephala have different breeding seasons in areas where sympatric species occur while having overlapping breeding seasons during fall and spring in the area where allopatric species occur.

Reproductive isolation prevents individuals from different species from producing offspring. These mechanisms have been classified into pre- and postzygotic mechanisms. Prezygotic mechanism acts before fertilization and prevents the fusion of gametes from different species to produce offspring, whereas postzygotic mechanisms act after fertilization and prevent hybrid organisms from developing into adult individuals as they have reduced fitness.