Isolation mechanisms

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In the theory of evolution, isolation mechanisms describe properties of organisms of different species or populations which, during sexual reproduction, prevent the formation of offspring among one another that contribute to the gene pool of the species concerned. This so-called reproductive isolation is a prerequisite for species remaining separate from one another when they are in contact with one another, especially because they occur together in the same habitat. The formation of isolation mechanisms is therefore essential for the process of speciation .

Basics

There are usually no barriers between individuals in the same population that restrict reproduction from one another. In the context of sexual reproduction, individuals continually exchange genetic makeup with one another. This tends to level out all genetic differences within the population. This homogenizing effect is known by the technical term gene flow . In an ideal population, the frequency of all variants of the various genes, known as alleles in the technical term, remains constant. This condition is known as the Hardy-Weinberg equilibrium . In the state of Hardy-Weinberg equilibrium, no evolution takes place.

A prerequisite for speciation is not only a change in the genetic makeup itself, but that the genetic makeup of one population changes in a certain direction, that of another population of the same species in another direction. For this, the gene flow between these populations must be restricted or completely interrupted. Otherwise, any changes that take place in one of the populations would be immediately carried over to the other. Then the characteristics of the species can change ( called anagenesis ), but a new formation of species ( called cladogenesis ) would be impossible.

The simplest way to isolate two populations is to cut off contact between them because they are geographically separated. For example, if individuals of the species live on two separate islands that they cannot switch between, each population can develop genetically in a different direction. This best understood model of speciation is called allopatric speciation . Since the isolation is purely geographical here, no isolation mechanisms are initially required. In this model, however, these come into play when the separated (emerging) species come into contact with each other again. If there were no isolation mechanisms between them, they would immediately merge again into one species and the differences would be lost. Isolation mechanisms must therefore arise here after the species have been separated. In the simplest case, this can simply happen by accident; called genetic drift in connection with genes .

If species arise without geographical isolation, either in the same area ( sympatric speciation ) or in neighboring areas without a barrier between them ( parapatric speciation ), the isolation mechanisms must precede the formation of the separate species. For a long time, this appeared implausible in classical population genetics, which is why the theory focused on allopatric speciation. However, interruption of the gene flow and subsequent speciation can in many cases be evolutionarily advantageous. It is a prerequisite for populations to optimally adapt to two different ecological niches (known as niches ).

The only decisive factor for speciation is the isolation that precedes the splitting and thus the speciation. Genetic differences that emerge later, although of course biologically interesting, are not essential to the process of speciation. Two populations can be reproductively isolated, i.e. separate species, which under certain circumstances do not differ morphologically or ecologically at all, these are called cryptic species ( cryptospecies ). Normally, however, two species will also differ in their morphology, biology and way of life, especially if they occur together (sympatric).

Genetic level

At the level of the genes themselves, isolation mechanisms arise primarily through the action of two factors.

  • Pleiotropy . If characteristics that evolve within one of the two emerging species (for example for ecological reasons) simply accidentally lead to incompatibility and thus act as isolation mechanisms, this is a case of pleiotropy. In this case, the insulation comes about by chance as a by-product.
  • Epistasis . However, genes do not evolve independently of one another, but rather together form an individual with certain characteristics. In many cases, this requires that numerous genes are finely tuned to one another. Such a gene ensemble, which enables an individual to occupy an ecological niche, is disrupted in its structure if a non- co-evolved allele intervenes. If different populations specialize in different directions, mixed breeds ( hybrids ) between them may have a lower fitness. This connection is called (interspecific) epistasis.

Prezygotic and postzygotic isolation mechanisms

According to their mode of action, isolation mechanisms are divided into two groups. Those that act before the germ cell ( zygote ) is formed are called prezygotic, those that take effect afterwards are called postzygotic. In most cases, prezygotic mechanisms work before a possible mating or prevent it. Even with extrinsic postzygotic mechanisms (see below), offspring (hybrids) may be produced that can also be viable, at least in the laboratory or in captivity, but cannot establish themselves in the natural population. Such hybrids can even be quite common in the wild, for example, when two parapatric species or subspecies form a hybrid zone where their ranges are adjacent. In a study of 20 hybrid zones, a lower fitness of the hybrids was demonstrated in 11 of them (in the rest of the cases, the data basis was mostly insufficient for a decision). The formation of hybrid offspring alone is therefore not yet proof that no isolation is effective. In addition, isolation mechanisms can also collapse secondarily, often after human influence, which, for example, brings previously ecologically separated species back into contact with one another by creating new habitats.

Prezygotic Mechanisms

  • Phenological isolation: No contact due to different periods of activity or life cycles
  • ecological isolation: no contact due to different habitats (also hosts or food plants).
  • Isolation due to sexual selection . Mating partners are sexually unattractive (wrong coloring, wrong behavior, etc.) and are avoided.

In the past, due to the extremely different mating organs between closely related, otherwise very similar species, it was assumed that species could simply be mechanically isolated because they no longer fit together (“key-lock principle”). This is no longer considered significant today.

Postzygotic Mechanisms

Postzygotic mechanisms are often further subdivided into extrinsically acting mechanisms, which are based in their effect on environmental effects (e.g. reproductive success in the habitat) and intrinsically acting mechanisms, which are independent of the habitat (e.g. death of hybrids as embryos due to genetic incompatibility). Intrinsically acting postzygotic isolation mechanisms bring about perfect isolation under all conditions, but evolve as a rule only emerge late after the others have been effective for a long time.

  • Hybrids do occur, but are less fit than individuals of the parent species
  • Hybrids do occur, but are sterile (sterile). If only one gender is sterile, it is almost always the heterogametic one (mostly the male): Haldane's rule .
  • Hybrids can no longer be formed or are not viable.

Accumulation of isolation mechanisms

If one compares two actually existing, sympatric species, there are usually a number of different isolation mechanisms between them (but there are also cases known in which two separate species are apparently isolated from each other only by a single factor). These can have different strengths and may only lead to an effective isolation of the species if they work together. In evolution, however, such complex mechanisms can only have been acquired step by step. The isolation mechanisms that are currently most effective do not necessarily have to be the ones that emerged first. Logically, a mechanism that acts early has a stronger effect than one that acts later - if mating does not occur in the first place, it is irrelevant whether the hybrids would be fertile or not. A comparison of two types of juggler flowers (genus Mimulus ) shows, for example, that the fertility of the hybrids is only about 60 percent compared to individuals of the two species themselves. However, since both species colonize separate habitats and therefore rarely come into contact with each other, i.e. there is a prezygotic reproductive barrier, this strong (postzygotic) isolation contributes less than one percent to the actual isolation of the species. In addition to the absolute strength of an isolation mechanism, its relative strength must also be determined in order to understand evolution.

Reinforcement

It is often observed that between different species that only occur together in parts of their range (are sympatric), the isolation mechanisms have a stronger effect in the overlapping part of the area. This can only be observed when individuals from the other areas are artificially brought into contact with one another. This effect is known as reinforcement.

Introgression

Even where functioning isolation mechanisms normally genetically isolate two species from each other, it can happen that, occasionally or regularly, hybrids are formed which, although actually endowed with less fitness, occasionally successfully produce offspring with one of the parent species. This then has the consequence that hereditary traits of the second type are crossed into the first, even after the types were actually already separated. This mechanism is called introgression . Introgression can be very significant in evolutionary terms. For example, genes of the brown bear were crossed into the genome of the polar bear by means of hybrids , after the lineages had separated from one another for several hundred thousand years. Neanderthal genes were also crossed into the human genome in the same way and can still be detected today. Such introgression is a problem in cladistic analyzes of relationships based on genes.

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Individual evidence

  1. ↑ cf. Michael Turelli, Nicholas H. Barton, Jerry A. Coyne (2001): Theory and speciation. Trends in Ecology & Evolution Vol.16, No.7: 330-343.
  2. ↑ Jerry A. Coyne and H. Allen Orr (1998): The evolutionary genetics of speciation. Philosophical Transactions of the Royal Society London Series B 353: 287-305.
  3. ^ NH Barton, and GM Hewitt (1985): Analysis of Hybrid Zones. Annual Review of Ecology and Systematics Vol. 16: 113-148 doi: 10.1146 / annurev.es.16.110185.000553
  4. ↑ John P. Masly (2012): 170 Years of “Lock-and-Key”: Genital Morphology and Reproductive Isolation. International Journal of Evolutionary Biology Volume 2012, Article ID 247352, 10 pages. doi: 10.1155 / 2012/247352
  5. ↑ Ole Seehausen, Roger K. Butlin, Irene Keller, Catherine E. Wagner, Janette W. Boughman, Paul A. Hohenlohe, Catherine L. Peichel, Glenn-Peter Saetre (2014): Genomics and the origin of species. Nature Reviews Genetics Vol.15: 176-192.
  6. ↑ Justin Ramsey, HD Bradshaw Jr., Douglas W. Schemske (2003): Components of reproductive isolation between the monkeyflowers Mimulus lewisii and M.cardinalis (Phrymaceae). Evolution 57: 1520-1534. doi: 10.1111 / j.0014-3820.2003.tb00360.x
  7. ↑ cf. Maria R. Servedio and Mohamed AF Noor (2003): The role of reinforcement in speciation: theory and data. Annual Review of Ecology, Evolution and Systematics 34: 339-364.
  8. ↑ cf. Mohamed AF Noor and Jeffrey L. Feder (2006): Speciation genetics: evolving approaches. Nature Reviews Genetics Vol. 7: 851-861.