REPRODUCTIVE ISOLATION: MECHANISMS, CONSEQUENCES (EXAMPLES) - BIOLOGY - 2024

The reproductive isolation or reproductive isolation includes various mechanisms that result in sterility between two populations of individuals. In other words, the crossing of two reproductively isolated species does not produce offspring or the offspring are not viable.

Isolation can occur before zygote formation, because populations do not share habitats, because they have different preferences, or because their reproductive organs are not compatible; or after the formation of the same, where the zygote can die or develop in a sterile individual.

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The speciation process - the formation of new species - is usually divided into three successive steps: first, a population isolation stage occurs, then the divergence of certain characters or traits occurs, and lastly, reproductive isolation occurs.

Once gene flow between these two populations has been eliminated, evolutionary isolation occurs.

Reproductive isolation mechanisms

Depending on when the reproductive isolation barriers act, they can be classified as prezygotic and postzygotic. The former act before the formation of the zygote.

Prezygotic barriers include any event that prevents copulation between two species, call it temporary isolation, isolation by habitat or resource differentiation, and isolation by behavior or ethology.

In this category are also the physiological or mechanical incompatibility of the sexual organs of the species that are trying to reproduce.

In contrast, postzygotic barriers encompass all events that prevent hybrid zygotes from developing a normal life, since they have low biological or fitness efficacy.

Temporary prezygotic barriers

An example of temporary isolation occurs in insects of the genus Magicicada. In these cicadas, there is a species with a life cycle of 13 years and another species whose cycle extends up to 17 years.

The imagos of the species emerge from the ground, every 13 or 17 years, depending on the species. As there is no time synchronization, there is no mating opportunity between the two species.

Ethological prezygous barriers

It is this same genus, there is prezygotic isolation of the ethological type. The sound that each species produces is unique to this species and cannot be recognized by others.

Although the meeting of two individuals of different sexes occurs, they will not be recognized as potential sexual partners.

Mechanical prezygotic barriers

Mechanical isolation occurs due to incompatibility between the genitals. Sex organs resemble a lock and key mechanism, where they must fit together perfectly. In the event that they do not fit, the copulation is not successful.

Prezygotic barriers due to habitat differentiation

This type of barrier occurs when two species exhibit a marked preference for a certain resource. The barrier is accentuated when copulation events occur in that area.

For example, salamanders of the genus Ambystoma have members that breed in ponds, and these do not interbreed with individuals that breed in streams.

Postzygotic barriers: mortality, infeasibility and sterility of hybrids

If any of the above prezygotic barriers fail, the hybrid may suffer the consequences of reproductive isolation.

The zygotes product of the crossing of two different species are known as hybrids and these may not develop or die in the course of their life.

Role of selection and gene drift

From the point of view of genetics, barriers to reproduction can be based on: genetic divergence, cytoplasmic incompatibility or cytological divergence.

For the evolution of reproductive barriers to occur, the following forces must be present: natural selection and genetic drift. These act when gene flow has been reduced in two populations of a species.

Gene or genetic drift

Gene drift is an evolutionary force that randomly fixes certain alleles, while others - for the same stochastic reasons - disappear from the population. This mechanism has more pronounced effects when it acts in small populations (with few individuals).

When two populations are isolated, gene drift acts in different ways: first, the "portion" of the population that remains isolated is a non-random sample, that is, the alleles are not represented in equal proportions. Then, the random fixation and loss of alleles enhances the divergence between populations.

Natural selection

For the speciation process to continue, it is necessary that there are very marked genetic differences between the populations studied. Natural selection has a major effect on the development of this divergence if populations occupy a new environment.

A classic example to illustrate the role of natural selection is the speciation of the apple and hawthorn fly. Populations are separating as selection is acting on their preferences when choosing food.

This species performs almost all the steps of its life cycle with the tree from which it feeds. For this reason, a group of researchers wondered if the flies that parasitized apple trees belonged to the same population as the hawthorn flies.

To test this hypothesis, the researchers applied a technique called "protein electrophoresis" and were able to conclude that there were statistically significant differences between the flies that lived in different trees.

This occurs because the flies show a significant preference for their type of fruit. Furthermore, mating occurs on the tree, preventing gene flow with the population of the other fruit.

Sexual selection

Sexual selection refers to the characters involved in the process of obtaining a mate. The way or the key elements that an individual uses to choose her partner seem to be key to the differentiation between populations and function as a barrier.

The songs in amphibians are an essential characteristic for the choice of mate and in some species the frequency of the song acts as a reproductive barrier. Likewise, coloration plays a fundamental role in the reproductive isolation of a certain class of fish.

Consequences

The consequence of reproductive isolation is speciation - formation of new species. Reproductive isolation barriers occur after the separation of two populations occurs and these evolve through natural selection or gene drift.

In turn, the consequence of speciation is the enormous diversity in the different lineages of living organisms. In taxa that have sexual reproduction, each branch of their phylogenetic tree represents a speciation event, where each population has been reproductively isolated.

Thus, speciation is considered as the bridge between microevolution and macroevolution.

References

1. Freeman, S., & Herron, JC (2002). Evolutionary analysis. Prentice hall
2. Futuyma, DJ (2005). Evolution. Sinauer.
3. Gallardo, MH (2011). Evolution. The course of life. Editorial Médica Panamericana.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw-Hill.
5. Ridley, M. (2004) Evolution. Third edition. Blackwell publishing.
6. Soler, M. (2002). Evolution: the basis of Biology. South Project.