Chapter 24, Problem 1IQ

Summary Introduction

To indicate: The pre-zygotic and post-zygotic barrier and name the type of reproductive isolation.

Introduction: The species in reproductive isolation impedes the members of both species from interbreeding to the species that produce fertile offspring. These barriers stop the gene flow between species and thus limit the formation of hybrids. Such barriers can be classified into two types namely; pre-zygotic and post zygotic. Pre-zygotic barrier includes temporal isolation, habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation; whereas, post zygotic barrier includes reduced hybrid fertility, reduced hybrid viability, and hybrid breakdown.

Answer to Problem 1IQ

Pre- or Post-	Type of isolation	Example
a. Post-zygotic	Reduced hybrid fertility	Two species of frogs mate in a laboratory setup and produce viable but sterile offspring.
b. Pre-zygotic	Gamatic isolation	Two species of sea urchins release gametes at the same time, but the sperm fail to fuse with eggs of a different species.
c. Pre-zygotic	Mechanical isolation	The genital openings of two species of land snails cannot line up because their shells spiral in opposite directions.
d. Pre-zygotic	Temporal isolation	Two species of short-lived mayflies emerge during different weeks in spring.
e. Post-zygotic	Hybrid breakdown	Two species of salamanders mate and produce offspring, but the hybrid's offspring are sterile.
f. Pre-zygotic	Behavioral isolation	Two similar species of birds have different mating rituals.
g. Post-zygotic	Reduced hybrid viability	Embryos of two species of mice bred in the laboratory usually abort.
h. Pre-zygotic	Habitat isolation	Peepers breed in woodland ponds; leopard frogs breed in swamps.

Explanation of Solution

1. In a laboratory setup, two frogs’ species mate and produce viable but sterile offspring. It is an example of reduced hybrid fertility. In this example, the hybrids are vigorous, sterile, and the chromosomes of the two parent species differ in numbers or structures. Therefore, during meiosis in the hybrids may fail to produce normal gametes in case of frogs.
2. Two species of sea urchins release gametes at the same time, but the sperm fail to fuse with eggs of a different species. In this example, the sperm of the sea urchin is unable to fertilize the egg of another species. This is an example of gametic isolation in which closely related species of aquatic animals fertilize.
3. The genital openings of two species of land snails cannot line up because their shells spiral in opposite directions is an example of mechanical isolation. In mechanical isolation, the two species attempted to mate but due to morphological differences, the successful completion of mating process stopped.
4. There are two species with short-lived mayflies emerge during spring weeks. This is an example of temporal isolation. These species breed during different times of the seasons, different times of the day, or different years.
5. There are two salamander species that mate and produce offspring, but the hybrid's offspring are sterile. These species are the examples of hybrid breakdown isolation, in which first generation offspring are fertile and viable when they mate with another species offspring, the next generation will be feeble or sterile.
6. There are two similar species of birds that have different mating rituals; this is the example of behavioral isolation. “Behavioral isolation” is a type of “reproductive isolation” where courtship rituals include attraction of mates and other exclusive behaviors to a species, resulting in development of reproductive barriers, yet in closely related species.
7. There are two embryos of two species of mice bred in a laboratory that usually abort. This is an example of reduced hybrid viability. In this isolation, the genes of two different species may interact in various ways that impair the development of the hybrid or the survival of the hybrid in the environment.
8. Peepers breed in woodland ponds; leopard frogs breeding in swamps are the example of habitat isolation.  Habitat isolation is one of the prezygotic reproductive barriers that kept the species separate to interbreed. However, in captivity, this barrier vanishes and species reproduce successfully.