Question

Would each of the following examples of reproductive isolation be considered a prezygotic or postzygotic mechanism? A. Horses and donkeys can interbreed to produce mules, but the mules are infertile. B. Three species of the orchid genus Dendrobium produce flowers 8 days, 9 days, and 11 days after a rainstorm. The flowers remain open for 1 day. C. Two species of fish release sperm and eggs into seawater at the same time, but the sperm of one species do not fertilize the eggs of the other species. D. Two tree frogs, Hyla chrysoscelis (diploid) and Hyla versicolor (tetraploid), can produce viable offspring, but the offspring are sterile.

Short answer

A. Postzygotic mechanism (mules are infertile). \nB. Prezygotic mechanism (difficulties in cross fertilization due to timing differences). \nC. Prezygotic mechanism (sperm of one species cannot fertilize eggs of the other). \nD. Postzygotic mechanism (offspring are sterile).

Step-by-step solution

Analyze Example A

Horses and donkeys can produce a hybrid offspring, a mule, meaning that a zygote can form. However, the mule is infertile. Hence, any isolation happening here is after the formation of the zygote, which points to a postzygotic mechanism.

Analyze Example B

These three species of the Dendrobium orchid all produce flowers at different times after a rainstorm and these flowers remain open for only one day. Therefore, they are unable to cross fertilize due to timing differences in their periods of reproductive activity. This inability to mate prevents the formation of a zygote, which points to a prezygotic mechanism.

Analyze Example C

Even though these two species of fish release sperm and eggs into seawater at the same time, the sperm of one species cannot fertilize the eggs of the other. This failure of one species' sperm to fertilize another species' egg prevents zygote formation. Therefore, it is an example of a prezygotic mechanism.

Analyze Example D

In the case of Hyla chrysoscelis (diploid) and Hyla versicolor (tetraploid), they can produce viable offspring, but these offspring are sterile. This means a zygote is able to form, but the result is infertile offspring. This is a characteristic of postzygotic mechanisms.

Key concepts

Prezygotic Mechanisms

Prezygotic mechanisms refer to barriers that prevent species from mating and creating a fertilized egg or zygote in the first place. These barriers can occur in various forms, such as physical differences, behavioral distinctions, or temporal separation.

For example, if two species of fish exist in the same environment but their mating rituals or pheromones do not attract each other, they are experiencing a prezygotic barrier. Similarly, if sperm from one species is not able to fertilize the egg of another because of molecular incompatibilities, this is another form of prezygotic isolation.

Behavioral Isolation

One common type of prezygotic mechanism is behavioral isolation, where differences in courtship rituals or mating behaviors prevent interbreeding. Even if the species live in the same area, they may not recognize each other as potential mates because of specific behaviors or signals.

Temporal Isolation

Another example is temporal isolation, demonstrated by the orchids that bloom at different times. They cannot interbreed because their reproductive timing does not coincide, making it physically impossible for cross-pollination to occur.

Postzygotic Mechanisms

Postzygotic mechanisms come into play after a zygote has formed. These mechanisms result in reduced viability or fertility of the offspring, impairing the hybrid individuals' ability to reproduce or survive. The presence of these barriers suggests that even if two species can mate and produce offspring, they are still distinct species because those offspring are not capable of continuing the species line.

Infertility in hybrids, like mules from horses and donkeys, is a classic example of a postzygotic barrier. Even though these two species can sometimes produce viable offspring, the sterility of the hybrids indicates that long-term gene flow between the species is not possible.

Reduced Hybrid Viability

Along with infertility, there can be reduced hybrid viability, where the offspring may be weak or less adapted to survive in the environment. If these offspring die before they reproduce, the genes from the two different parent species do not mix in the population.

Hybrid Fertility

Hybrid fertility is a specific aspect of postzygotic reproductive isolation. As opposed to cases where hybrids are entirely inviable, here, hybrids may be born healthy but are infertile or have reduced fertility. Such is the case with mules; they are strong and can survive well in their environment but cannot reproduce.

This reduction in fertility can be due to a variety of factors including genetic incompatibilities, differences in chromosome number, or other genetic complexities that arise when two distinct species interbreed. Mules, for instance, end up with an uneven number of chromosomes that cannot pair properly during meiosis, the process that creates sex cells.

Long-Term Implications

Hybrid infertility can have significant implications for species conservation and biodiversity. It affects the genetic flow between populations and can lead to further speciation, as it enforces genetic isolation between groups.

Species Reproductive Timing

Species' reproductive timing plays a crucial role in maintaining reproductive isolation and thus species integrity. When two species reproduce at different times of year or in different seasons, this is referred to as temporal isolation, a prezygotic mechanism.

The case of the orchid species blooming after rainstorms at distinct intervals is a telling example where reproductive timing prevents hybridization. Not only do different pollinators prefer different species, but the fact that these species' flowers do not open simultaneously prevents any chance of cross-fertilization.

Impact on Ecosystems

Temporal isolation is essential for maintaining the diversity of ecosystems. By preventing interbreeding, species are able to coexist in the same habitat without competing for the exact reproductive resources. This kind of separation ensures that species remain adapted to their specific ecological niches.