16 Reproductive isolate and the concept of gene pool

Dr. Subhendu K Acharya

Contents:

• Introduction to Reproductive isolate and the concept of gene pool
• What is a reproductive isolate?
• Process of isolation
• Cause of Speciation
• Reproductive Isolating Mechanisms
• Process of isolation
• Cause of Speciation
• Reproductive Isolating Mechanisms

Introduction

After Darwin’s theory of natural selection evolution of species remains a hot topic. Reproductive isolation” is widely considered an important component in defining the word species. As Darwin believed reproductive isolation plays an important role in maintaining distinct populations. In the Origin Darwin explains that species within the same country could hardly have kept distinct had they been capable of crossing freely.” He further says “indeed it is obvious if all forms freely crossed, nature would be a chaos.” The neo-Darwinian theory has also emphasized reproductive isolation as the key to the production and maintenance of distinct forms. So, it is hypothysed that over generations, a great deal of change can happen in a population from its original state. In spite of that all members are considered of belonging to the same species untillreproductive isolation differentiate some members from the original population. Reproductive isolation can happen due to mating differences, sterility or environmental barriers. This reproductive isolation driven differentiation is termed as speciation in which two previously interbreeding populations get rise to two populations that can not produce fertile or viable offpring any longer.

From the level of geographic isolation point of view many theories have been postulated to explain the emergence of new species, a product of reproductive isolation. Recent studies suggest that the early stages of divergence and the evolution of reproductive isolation may highly be affected by natural selection and adaptation. Adaptive speciation explains evolutionary species diversity particularly in case of reproductive isolation.

What is a reproductive isolate?

Reproductive isolate is defined in relatively to an original population. The term isolate is usually applied to a group of organisms that share characteristics with the other species but cannot reproduce offspring. It is believed that the two groups evolved to two separate species from a single one due to developing sufficient difference in terms of their gene pool and characters in the process of evolution to be considered as separate species.

Process of isolation:

Step 1:  interruption in gene flow between two populations

Step 2:  gradual accumulation of genetic differences between the two populations

Step 3: this divergence gives rise to reproductive isolation

Cause of Speciation:

Speciation, or the evolution of reproductive isolation, occurs when genetic changes take place between interbreeding populations of a particular species leading reproductive isolation and further different species. The whole genetic process involved in this process of isolation leading to new species is called reproductive isolating mechanism.

In genetics and evolution, shared gene pool is the basis. A group of interbreeding, or potentially interbreeding, organisms that does not exchange genes with other such groups is considered to be a species. Each species is reproductively isolated from every other species. So whether or not groups of organisms are reproductively isolated from each other is the basis of the genetic approach. It is notworthy that organisms can be reproductively isolated though easily recognized phenotypic characteristics are not there. So such organisms intaxonomy would be regarded as a single species and in genetics and evolution as separate species. Similarly, organisms may not be reproductively isolated but have different phenotypic characteristics. So, preferabily we apply the genetic definition though otherwise for example, with fossilized organisms—we are limited to the taxonomic definition. Reproductive isolation is the key to the genetic definition of a species. Groups of organisms can be reproductively isolated from each other by different mechanisms though they inhabit the same territory.

There have been different mechanisms of reproductive isolation. From genetic point of view, there are genetically controlled different mechanisms of reproductive isolation and it has been demonstrated experimentally that they can evolve in species whose geographic distribution overlaps, called as sympatric speciation of reproductive isolate or as the result of divergence out of adaption resulting allopatric speciation. Ernst Mayr classified the reproductive isolation mechanisms as: act before mating or pre-copulatory and act after mating or post-copulatory. It is also classified as pre-zygotic and post-zygotic mechanisms.

Table 1: Reproductive Isolating Mechanisms

Prezygotic isolating mechanisms prevent the members of different groups from producing hybrid offspring. Postzygotic isolating mechanisms prevent any hybrid offspring that are produced from passing on their genes to subsequent generations. Prezygotic isolating mechanisms operate by preventing matings between individuals from different populations of organisms, or by preventing the gametes of these individuals from uniting to form zygotes. For example, two populations of organisms that inhabit the same area might seek out different habitats within that area. If the habitat preference is strong, the two populations will have little or no contact with each other. Ecological isolation based on habitat preference can therefore prevent the populations from producing hybrid zygotes. Temporal or behavioral factors can also bring about re-productive isolation between populations of organisms. For instance, the organisms might become sexually mature at different times, or they might have different courting rituals. If ecological, temporal, and behavioral isolating mechanisms fail to prevent mating between different organisms, then anatomical or chemical incompatibilities in their reproductive organs or gametes might prevent them from producing hybrid zygotes. The organisms might be unable to copulate successfully, or to exchange pollen, or their sperm or pollen might die in the reproductive tissues of their mates. Any of these prezygotic isolating mechanisms could prevent genes from being exchanged between populations occupying the same territory. Postzygotic isolating mechanisms operate after hybrid zygotes have been formed, either by reducing hybrid viability or by impairing hybrid fertility. The zygotes from matings between different organisms might not survive, or they might not reach sexual maturity. If they do reach sexual maturity, they might not produce functional gametes. Any of these circumstances could prevent populations of organisms that live in the same territory from exchanging genes.

Modes of isolation

The key event in formation of reproductive isolate is the splitting of a population of organisms into one or more subpopulations that become reproductively isolated from each other. The most straight for-ward way for this event to happen is for the subpopulations to become geographically separated so that they evolve independently—that is, geographical barriers keep the subpopulations apart so that they accumulate their own sets of genetic changes over time. Then, if the subpopulations are reunited by the disappearance of the geographical barriers, the genetic changes they have accumulated may make them reproductively isolated from each other.

For example, one subpopulation may have evolved a preference for a particular food source and another subpopulation may have evolved a preference for a different food source. When the two subpopulations are rejoined in the same territory, their distinctive food preferences may limit contact between them to such an extent that interpopulational matings never occur. Another possibility is that during the time the subpopulations were separated, they may have evolved different physiological processes or mating habits. When the subpopulations are reunited, they may not be able to mate with each other, or if they can mate with each other, their hybrids may not be viable or fertile. The process whereby subpopulations evolve reproductive isolation while they are geographically separated is called allopatricspeciation (from Greek roots meaning “in other villages”).Allopatric type of speciation leading to reproductive isolate is prevalently observed type in speciation in animals. Here genetic drift acts as a selection force. The effects of genetic drift occur in all types of populations, and particularly in small populations.Here it is noteworthy that the habitat types of two populations also plays role with respect to effective role of natural selection.

The evidences of allopatric speciation are observed from different types of geographic patterns. When gene flow between populations gets reduced due to physical barriers, the geographic populations of the same species can be seen different from each other genetically, example of which is populations on islands.

Ring species (ensatina) is also a suitable example which explains speciation happeningallopathically leading to formulation reproductive isolate. Thisspecies is believed to be a interbreeding population at one end of the barrier which evolved and spread out to both side of the barrier thus getting isolated both geographically and further genetically.

The process of allopatric speciation.

Example of Allopatric speciation:

Since Darwin’s study of the Galapagos Islands, archipelagos have played a central role in understanding how new species evolve

from existing ones (speciation). Islands epitomize allopatric speciation, where geographic isolation causes individuals of an original species to accumulate sufficient genetic differences to prevent them breeding with each other when they are reunited.

Similarly, taking example from Arizona’s Grand Canyon formation it is observed that after the formation, small mammals for example squirrels could not contact with each other due to the geographic barrier. So due to this gap there was no breeding and which has led to formation of two new species now.

At the same time the findings of a recent study on island lizards showing isolation of even many millions of years has not given rise to separate species questions the established theories of evolution (

These tree lizard species had their such phylogenic structure in the said geographic location for about six to eight million years.Roger Thorpe and his coworkers tried to distinguish the reproductive isolation of the lizards genetically.

The selectively neutral genetic markers showed that there was a freely exchanging genes and therefore noseparate species. This finding rejected the allopatric speciation process of reproductive isolateand suggested the prospect of ecological speciation as the basis of reproductive isolate.

Peripatric speciation leading to development of isolate is defined when the new population is smaller than the original population.

Example of isolates from peripatric speciation based reproductive isolate

Another type of speciation that leads to formation of reproductive isolate is parapatric isolation in which rather than physical barrier the differences in the common environment for the species defines their character.

Example of isolates from parapatric speciation leading to reproductive isolate

The example was taken from the areas of mining activities where lead and zinc like metals are present in the soil. Though it prevents most plants to grow, buffalo grass could grow and adopt the soil type. Such type of grass is found in many parts of the world particularly known from the mining areas.

It is also conceivable that subpopulations might evolve reproductive isolation without being separated geographically. Perhaps the subpopulations become ecologically specialized so that they evolve more or less independently, or perhaps their members mate assortatively so that there is little or no genetic exchange between the subpopulations. The process of evolving reproductive isolation between subpopulations that exist in the same territory is called sympatric speciation (from Greek roots meaning “in the same villages”). It can be said that the difference between sympatric and allopatric speciation based reproductive isolateis with respect to how gene flow occurred.

Because the evolution of reproductive isolation may require hundreds of thousands of years, it is not easily studied. Most investigations of formation of reproductive isolate are done post factum— that is after the species have already formed. Based on data collected from the species, researchers attempt to determine how and why they became reproductively isolated from each other. One issue in these studies is whether the species evolved allopathically or sympatrically. Did they develop reproductive isolating

Figure :Example of sympatric speciation in Myzomela honeyeater from pacific.

mechanisms while they were geographically separated, or did they develop these mechanisms while they inhabited the same territory? Usually this question cannot be answered with certainty. However, most evolutionary geneticists are inclined toward the view that allopatric formation of reproductive isolate is more prevalent than sympatric reproductive isolate, if only because allopatric type is a more straightforward process. For example, imagine that a small number of organisms migrate to a remote oceanic island where they found a population that evolves independently of the main population on the nearest continent. The island population may change significantly over time and eventually become reproductively isolated from its closest relatives on the continent. This scenario—which is allopatric speciation based formation of reproductive isolatepure and simple—may have played out many times on oceanic islands. Indeed, Darwin proposed it as an explanation for the species of plants and animals he observed on the Galapagos Islands off the west coast of South America. It is not too hard to imagine other types of geographic separation that would permit allopatric type to occur. Deserts and mountain ranges can subdivide continents; reductions in rainfall can isolate lakes and river systems; land masses can rise up to separate oceans. Popu-lations that are subdivided by these kinds of barriers have thepotential to evolve into distinct, reproductively isolated species. Although allopatric speciation basedreproductive isolatemay have been the prevalent mode in creating the species that exist today, there is evidence that sympatric speciation based reproductive isolatehas also contributed to species diversity. The strongest case for sympatric speciation comes from the study of cichlid fish in two small crater lakes located in west central Africa. Today these lakes are isolated from other significant bodies of water. However, in the relatively recent past they were apparently colonized by cichlids from the surrounding river systems. These colonists then evolved into the groups of species now present in the lakes. Analysis of mitochondrial DNA sequences indicates that the cichlid species within each lake are derived from a common ancestor and that they are more closely related to each other than to the cichlid species found in the surrounding river systems. There are no obvious geographic barriers within these lakes. Their shorelines are regular, and they do not seem to have been subdivided during their history. Thus, it appears that the crater-lake cichlids evolved into different species sympatrically. Cichlid fish inhabit many of the lakes and rivers in tropical Africa, especially the East African Great Lakes—Lake Victoria, Lake Malawi, and Lake Tanganyika, where over 1500 cichlid species have been identified. The apparent sympatric speciation of cichlids in the small crater lakes of west central Africa raises the possibility that some of the species in these large lakes may also have originated sympatrically. More research is needed to determine how the Great Lake cichlids evolved.

Genetics of formation of isolates

The Genetics of formation of reproductive isolates, one of the fundamental questions in evolutionary biology is what types of genetic changes bring about reproductive isolation between populations of Organisms. This question is difficult to address because if the populations are reproductively isolated, they either cannot be crossed, or if they can be crossed, their hybrids are sterile or unviable.

In precopulatory stage, reproductive isolation takes place when males and females of two species are placed together. Genetic studies on such conditions show that though Drosophila melanogaster and D. simulans are two separate species females and males of Drosophila melanogaster and females and males of D. simulans can cross copulate. It was observed that the difference is more quantitative that qualitative. So in the process of experiment the D. melanogaster were hybridized with D. simulans first. Then the offspring were placed for copulation with D. simulans. It was observed that D. melanogaster carried one gene of isolation in three of eight chromosomes.

Researchers have been able to carry out some analyses of the genetic basis of reproductive isolation by using closely related species of Drosophila. From post copulatory points of view, reproductive isolation of organisms can occur from the beginning or later. For example Drosophila pavani and D. gaucha though separate species but can produce hybrids. Interestingly, their hybrid though can produce gametes and ovules but not viable offspring.

Figure: Dobzhansky and Muller model for the evolution of reproductive isolation

Dobzhansky and Muller argued that the inviability or sterility of hybrids is due to incompatibilities between genes that evolved separately in different populations. For example, if a population with the genotype 244/ BIB/ becomes subdivided by geographical barriers, one of its subpopulations might fix a new mutant allele of the A locus to becoine A2A2 BiBi, and another subpopulation might fix a new mutant allele of the B locus to become ALAI B2B2 (Figur6 27.24). If these subpopulations rejoin after the disappearance of the geographical barriers that separated them, they might cross to produce A/A2 B/B2 hybrids. Such crosses would bring the A2 and B2 alleles together in the same genotype for the first time. Because these alleles evolved in separate populations, they might interact unfavorably with each other and cause death or sterility. Thus, the independent evolution of genes in separated populations might produce genotypes that, when hybridized, lead to unviable or sterile offspring.

Figure: Phylogenetic tree based on mitochondrial DNA sequences showing the relationships among cichlid fish in the genus Tilapia from Lake Bermin, an isolated crater lake, and from surrounding bodies of water in Cameroon

Chromosomally differentiated populations can also give rise to post-copulatory isolation due to chromosomal translocations and inversions. For example, a reduced fertility with chromosomal aberration arises due to the incomplete meiosis when a reciprocal translocation is fixed in a population. Such example of chromosomal changes leading to reduced fertility can be drawn from Drosophila nasuta and D. albomicans which are twin species from the Indo-Pacific region.

Presence of microorganisms in the cytoplasm is also a factor for post zygotic isolation. Taking D. paulistorum case, it is observed that the female D. paulistorum hybrids are fertile while male D. paulistorum are sterile. The males contain a mycoplasma in the cytoplasm that can reduce them to sterile.

Summary

Reproductive isolation is defined in the way that the two groups evolved to two separate species from a single one due to developing sufficient difference in terms of their gene pool and characters in the process of evolution to be considered as separate species. Reproductive isolation is widely considered an important component in defining the word species. As Darwin believed reproductive isolation plays an important role in maintaining distinct populations. The neo-Darwinian theory has also emphasized reproductive isolation as the key to the production and maintenance of distinct forms. There have been different mechanisms of reproductive isolation. Evolving of new species with overlapping geographic distribution are called as sympatric speciation of reproductive isolate; evolving of new species as the result of divergence out of adaption results allopatric speciation. Peripatric speciation leading to development of isolate is defined when the new population is smaller than the original population. parapatric isolation in which rather than physical barrier the differences in the common environment for the species defines their character. Ernst Mayr classified the reproductive isolation mechanisms as: act before mating or pre-copulatory and act after mating or post-copulatory. It is also classified as pre-zygotic and post-zygotic mechanisms.

Chromosomally differentiated populations can also give rise to post-copulatory isolation due to chromosomal translocations and inversions. Presence of microorganisms in the cytoplasm is also a factor for post zygotic isolation. The Genetics of formation of reproductive isolates, one of the fundamental questions in evolutionary biology explain what types of genetic changes bring about reproductive isolation between populations of Organisms. Dobzhansky and Muller argued that the inviability or sterility of hybrids is due to incompatibilities between genes that evolved separately in different populations. Further, different alleles evolved in separate populations, might interact unfavorably with each other and cause death or sterility. Thus, the independent evolution of genes in separated populations might produce genotypes that, when hybridized, lead to unviable or sterile offspring.

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