Question

All of the following are examples of postzygotic reproductive isolating mechanisms EXCEPT Select one: a. two plant species flowering at different times. b. the hybrid offspring of two species dying before birth. c. a sheep adapted to mountains mating with a sheep adapted to desert producing offspring maladapted to either habitat. d. the hybrid offspring of two species being fertile but their offspring being inviable or sterile. e. the hybrid offspring of two species being sterile.

Answer 1

Answer:

a. two plant species flowering at different times.

Explanation:

The only option that is not a post-zygotic reproductive isolation mechanism would be option a, when two plant species flower at different times.

Post-zygotic reproductive isolation mechanisms are those mechanisms that act to ensure that the product of fertilization remains invalid. This is as opposed to pre-zygotic isolation mechanisms that act to prevent fertilization and hence, formation of zygotes.

In option b, c, d and e; the zygote has been formed already and all the forces are just acting to ensure that it remains invalid. whereas in option a, there is no fertilization yet.

Two plant species flowering at different times will ensure that there is no cross pollination between the two species. Without pollination, there cannot be a zygote formation.

Hence, the correct option is a.

Answer 2

Answer:

B) The hybrid offspring of two species dying before birth.

Explanation:

The hybrid offspring of two species dying before birth.

There are two things which can prevent the

interbreeding between two or more species. The two things are pre-zygotic

reproductive isolation, which occurs before fertilization and post-zygotic

reproductive isolation, which occurs after fertilization. These kinds of reproductive isolations deduce the gene flow. Pre-zygotic reproductive isolation happens when:

a.Species are adapting in different habitats.

b.Species adapt to a particular mating behaviour in order to choose a mating partner.

c.Incompatibility of sex organs.

d. A difference in chemotactic factors that cause fertilization events.

e.A time window when species mature at different time frames in the year or period.

Post-zygotic reproductive isolation happens when:

a.There is a premature death involving hybrids.

b.Offsprings which can live and develop may not be fertile due to random mixing of chromosomes.

c.Some hybrids may be fertile but they can as well display a hybrid

breakdown which produces weaker and less fertile

offsprings from generation to generation.

The odd option (The hybrid offspring of two

species dying before birth) is a subtopic under Post-fertilization

reproductive isolation.