Speciation

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The speciation ( speciation . English speciation ) - the emergence of new biological species - is one of the most important consequences of the evolution and one of the central questions of the theory of evolution . The counterpart to speciation is the extinction of a species.

Charles Darwin placed species formation at the center of his theory, which was also reflected in the title of his major work " The Origin of Species ". With Gregor Mendel and the synthetic evolution theory formulated by Ernst Mayr in particular in the 20th century , attention was then drawn more to the individual. Today there is a tendency to deal more with the development of species again.

The question of speciation is particularly important because the species is the only precisely defined taxon in biological systematics - at least for most eukaryotes . According to this, a species includes all living beings and populations that produce reproductive offspring among themselves without artificial intervention ( reproductive community ). However, this definition is strictly only applicable to recent living beings. The term chronospecies is often used for fossils . Phenotypic variation is an essential prerequisite for the emergence of new species .

In medium-sized mammals, complete reproductive isolation generally lasts at least 1.4 million years.

The classic model of allopatric speciation

The model of speciation developed by Theodosius Dobzhansky in 1937 was used by Ernst Mayr to define the biological species. According to the model, the central process of speciation is the splitting of a population into two reproductively isolated populations by physiological isolation mechanisms. In simplified terms, it can be imagined as the following steps:

  1. Two populations of the same species are separated ( separation = geographical isolation ). The separation takes place through geographical barriers caused by climatic factors ( e.g. ice ages ), geological factors (e.g. rift fractures , volcanism , plate tectonics , land uplifts and subsidence with collapse or drying out of inlets), possibly also by human interventions (e.g. . Isolation of formerly connected biotopes through destruction of the connecting surfaces). Isolation due to migrations is also possible , e.g. B. by the repopulation of islands and separated inland waters. All of these mechanisms abolish the reproductive community and separate the populations into two gene pools ( allopatric speciation ).
  2. The populations develop apart through mutations in their gene pool. Some of them are useful and advantageous for the respective population and are selected (they are adaptive ), but in many cases the allele frequencies differ simply by chance ( genetic drift ). Both factors reduce the genetic match. More and more genes form different alleles .
  3. This results in different phenotypes that differ from one another anatomically, in metabolism and / or in behavior . In part, these differences are adaptive ; This means that the populations are exposed to different selection pressures due to different ecological conditions in the two areas and thus differ in their ecological niche .
  4. Irrespective of this, there are incompatibilities in morphology and anatomy (e.g. different forms of genital organs), ecology (different symbionts , different types of insects for pollination and adaptation of the flower structure), genetics (e.g. different chromosome numbers or chromosome sizes), which cause problems of meiosis (genetic separation) or in the behavior (for example, different mating behavior ) are justified and prevent a mixing of the populations in a lifting of the barrier. So-called reproductive isolation has taken place, and with it two different biological species have emerged.

Isolation mechanisms

As long as two populations develop geographically separated from each other in different areas (adaptive or random) in different directions, it is not possible according to the biological species concept to make a prediction whether separate species will emerge from them.

This is due to the fact that differently adapted and also morphologically distinguishable local populations very often develop within a species. In many cases, species show a sequence of genetically fixed adapted traits (called: a kline ) in an ecological gradient (e.g. a sequence from acidic to basic soil ), which is maintained by selection even with reproductive contact. It has long been known that a very weak selection pressure is sufficient for this. These local populations may or may not develop into separate species. In many cases, reproductive contact persists. It is then a polymorphic species.

Whether it really is a question of two separate species can only be determined upon renewed contact between the previously separated populations or by specifically bringing together individuals from both populations. The decisive factor is whether joint fertile offspring can still emerge from both populations or whether isolation mechanisms have developed. Without isolation mechanisms, the populations could merge again to form an undivided species at this point in time.

If the two species are sufficiently different due to the formation of different ecological niches, it can happen that they coexist together in the same area in the later course of evolution.

In addition to the frequently occurring allopatric speciation, for which spatial separation is a prerequisite, there is also, in rare cases, sympatric speciation , which is based on genetic isolation without geographical separation. The mechanisms of sympatric speciation are not fully understood and are controversial.

Isolation mechanisms are differentiated according to whether they work before fertilization (pre-zygotic) or only afterwards (post-zygotic). Research has identified different isolation mechanisms and tried to assess their effects in natural populations.

1. Prezygotic isolation mechanisms

  • Ecological separation: The species colonize different habitats and do not come into contact with each other.
  • Behavior: The species have e.g. B. different courtship calls , chants o. Ä. Developed.
  • Reproduction time: The species mate and reproduce at different times .
  • Pollinators: Different types of plants are pollinated by different flower visitors .
  • Gametic incompatibility: mating or pollination occurs, but not fertilization. In plants, the pollen tube does not grow or grows too slowly, in animals the sperm are incorrectly adapted or too slowly.
  • Sexual selection: Partners of the other kind are sexually unattractive or are no longer recognized as sexual partners.

2. Postzygotic isolation mechanisms

  • Hybrids are not viable or weaker.
  • Hybrids are viable, but sterile (the sex with heterogametic genetic material is almost always sterile, mostly the male: Haldane's rule ).
  • Hybrids are viable and fertile, but have a lower ecological fitness (characteristic expression “between” those of the niche-adapted parent species).

In rare exceptional cases, species can be reproductively isolated by a single, suddenly acting factor without the need for an accumulation of isolation mechanisms. The most important examples are infection with incompatible, endosymbiotic bacteria (especially of the genus Wolbachia ) in insect species, and speciation through polyploidization v. a. in plant species.

There was no complete agreement among the founders of the synthetic theory of evolution about the formation and importance of the isolation mechanisms. For Dobzhansky, the random accumulation of isolating, mutated alleles was sufficient as a mechanism. Mayr, on the other hand, emphasized the interactions within the genome, through which co-evolved gene ensembles can arise that would be broken up during hybridization (“coherence” of the genome).

Isolation as the first step can, in rare cases, also result from a different ecological orientation of two populations ( e.g. different microhabitats due to different food, change of host for parasites ). Basically, there can also be a genetic mutation at the beginning (compare: sympatric speciation ) that creates incompatibility, for example through polyploidy or a profound mutation that affects several characteristics and genes at once, for example through mutation of master genes and on the way to a change of alternative splicing .

Hybridization and speciation

Hybrids between separate populations are due to missing or imperfect isolation mechanisms. Hybridization can cause separate local populations (or even separate species) to merge again. An abolition of natural isolation mechanisms is observed particularly often after human interventions, through which geographical or ecological separations are abolished, behavioral adaptations and associated signals are decoupled or completely new habitats and locations are created. In fact, it is quite common that individuals from various generally accepted (“good”) species then have fertile offspring. For example, it is known that 19% of all known butterfly species occasionally form hybrids in nature. In 33% of these cases the hybrids are fertile and viable. The fact that they nonetheless remain separate in nature is due to non-physiological (prezygotic) isolation mechanisms between them. Population geneticists measure the exchange of imperfectly separated populations: this quantity is known as the gene flow between them. The evolutionary period up to which separate species can no longer produce hybrids is estimated at around two to four million years for mammals and insects. In the case of amphibians and birds, the process seems to take place even more slowly, the order of magnitude of the times required here is around 20 million years; hybrids between species of different genera are also by no means rare; B. between geese ( Anser ) and ducks ( Anas ).

Especially with plant species, but also with animal species more often than previously assumed, new species can arise through hybridization. In many cases this process is associated with a doubling of the chromosome set, which is called polyploidization . Polyploidy is very common in nature, for example 70% of all vascular plant species are polyploid; however, it is rare in animal species. Polyploidization in connection with the union of alien gametes is called allopolyploidy (in contrast to autopolyploidy, in which the genome of the same species doubles). Numerous plant species are of allopolyploid origin, such as the important crops cotton , maize and wheat (in the case of wheat, one even assumes three species of origin). Polyploidization can lead to immediate reproductive isolation because mostly gametes with odd numbers of chromosomes (e.g. three after fertilization of a diploid species and a tetraploid offspring) are barely viable. Hybridization can create individuals with unusual characteristics that are missing in both parent populations in one step; this can violate the otherwise almost universal principle of gradualism in evolution, and sometimes this can create ecological niches that did not exist before . This process can also be observed particularly frequently in habitats newly created or heavily influenced by humans, but it also occurs naturally. A case in which a (non-polyploid) hybridization of two animal species resulted in the colonization of a habitat that had been changed by human influence was observed in a new population of bullheads in the Lower Rhine . The hybrid between Rheingroppe and Scheldegroppe , which was created as a result of a newly built canal, is better able to colonize the river, which has been greatly altered by humans, than both original species and represents a new species (rapidly expanding upstream).

Species formation with existing gene flow

If there are no or incomplete isolation mechanisms between two populations, i. That is, if the gene flow between them is not interrupted, speciation is unlikely due to the purely random accumulation of different alleles; this also applies to spatially separated populations, provided that individuals who migrate (“ migrating ”) are not exchanged between them too rarely . A separation into separate species is still possible in these cases if the populations are subject to disruptive selection. Selection is disruptive if, in the case of continuous characteristics, typically determined by the cumulative small influence of numerous genes (geneticists call this “quantitative characteristics”), the extreme values ​​cause greater fitness than the mean value, for example when especially large and small individuals are fitter with a size gradient are as medium-sized. Disruptive selection seems to be widespread in natural populations; it is possibly not less common than stabilizing selection, which disadvantages extreme characteristics. When disruptive selection affects a population, it tends to increase the population's variability. This can lead to the development of differently adapted forms or local races - but only if other factors are added. Either gene ensembles are created with co-adapted genes which, viewed individually, increase fitness, but reduce it when they occur together. In this case, corresponding hybrids between the forms are selectively disadvantaged. Or individuals with similar characteristic attributes prefer each other in pairs so that sexual selection increases the adaptive selection ( " assortative mating "; Engl .: "assortative mating"). Once such polymorphic species with differently adapted subpopulations have arisen, isolation mechanisms can also arise between these subpopulations. These are even adaptive if the hybrids are less fit and should therefore be encouraged through selection (“adaptive speciation”). Such models of speciation from a polymorphic parent species have been proposed both from an evolutionary perspective and from a population-genetic point of view. Although they have received a lot of attention in research over the past 20 years, their relevance is still controversial.

In the specific course of speciation, two difficultly distinguishable cases are possible here. Either the splitting takes place on the spot ( sympatric speciation ) or in spatially directly adjacent areas ( parapatric speciation ). Or the original split is still “classically” allopatric, but the differences in the characteristics intensify as soon as the populations come into secondary contact again. This process is of evolutionary side reinforcement (English "reinforcement".) Called; Ecologists call the same characteristic shift (Engl. "Character displacement"). Since speciation is a one-time (historical) process, it is not easy to differentiate between these cases. Often they are considered together in research under the name "ecological speciation".

Species formation through macromutations

In the pioneering days of evolutionary research, geneticists such as William Bateson and Richard Goldschmidt proposed models according to which new species could be caused by individual mutations with a very large effect, so-called macromutations. These ideas were fiercely opposed by the founders of the synthetic theory of evolution, especially Ronald Fisher , and found no entry into the synthetic theory of evolution. A major reason for the rejection was that in sexually reproducing species it was hardly conceivable how a favorable macromutation in the inevitable backcrossing with carriers of the previous gene (after all, others are not available as potential mating partners) and in recombination would be retained and in the population could prevail. Fisher's criticism is also based on the conclusion that it would be extremely implausible that a single mutation would happen to hit an ecological optimum. There is a high probability that their effect would either be too small or too great and in either case would not be of any benefit to the wearer. On the other hand, achieving an optimum in numerous small steps is very plausible and almost inevitable.

In the past few decades, the theory of evolution has begun to deal again with macromutations (or mutations with a large effect) as the cause of speciation on a changed basis. However, these questions are highly controversial within evolutionary theory.

Some evolutionary biologists have observed that, in some cases, individual populations do indeed differ from their relatives by a key trait that is apparently due to a single gene. Typically, the corresponding trait for the carrier of the corresponding gene is actually more or less far from the optimum. It is then observed that the cumulative effect of numerous genes with little effect means that the trait is more or less precisely controlled. It can then be assumed that the original mutation gave its carrier a great advantage despite the imperfect result. This could then have been further optimized in small steps. Such a pattern (different characteristics based on a gene with a large effect and fine control by numerous genes with a small effect) were observed in the transition from herbivorous insects to new host plants and in the formation of wing patterns on butterfly wings that cause mimicry .

Investigations into the wing patterns of butterflies produced other interesting results. In some cases it was found that several genes were linked to form modular units (“super genes”), which can produce different patterns in different ways. These are probably due to gene duplication with subsequent divergent development. Gene duplications could thus be the main macromutations. This theory is currently being intensively researched in numerous other applications. Very rare duplications of the entire genome could play a special role, which may have played a decisive role in promoting the radiation of vertebrate species.

The pattern formation in the butterfly wing is regularly controlled by modularly organized development paths in which individual control genes, which each encode transcription factors, control numerous effector genes. There is thus the possibility that mutations in such control genes (i.e. actually not in the genes themselves, but in the regulatory sequences controlled by them) could bring about major changes in the body plan in one step. This mechanism was used to explain evolutionary novelties in the context of evolutionary developmental biology . For such co-evolved genetic networks, which can be switched on or off by individual genes acting as switches, Sean B. Carroll coined the image of the “genetic toolbox”. However, whether corresponding macromutations actually occur during speciation is, although theoretically plausible, highly controversial, because similar shifts in characteristics can be explained by numerous small steps.

Species formation in non-sexually reproducing species

This model of speciation applies - since it presupposes the ability to reproduce sexually - primarily to eukaryotes. In bacteria and archaea similar mechanisms for splitting different shapes are possible, but the biological species definition in these organisms due to the separation of sexual processes and proliferation is not fully applicable. Numerous animal and plant species also reproduce non-sexually. Species consisting of such individuals are called agamo species . Agamospecies are in principle clones , if one disregards the secondary variation generated by new mutations.

Agamosperm animal or plant species can reproduce purely vegetatively . Much more common, however, is propagation via apomixis . In the process, eggs or seeds are formed, but the usually preceding fusion of different germ cells is suppressed in various ways; with some, the stimulus of fertilization is even necessary, even if the male gamete no longer contributes any genetic material (“pseudogamous” reproduction). Apomictic species reproduce in the usual way, but their offspring are genetically identical to the maternal organism. In the vegetable kingdom of particular importance is the formation of apomictic new species in groups that usually reproduce pseudogamously, but in which sexual reproduction is still possible. Occasionally and exceptionally, fertilization still occurs here. The resulting offspring are therefore genetically different from the maternal organism. If they prove to be ecologically successful, they can then spread asexually and thus establish a new agamosperm "species". Many taxonomically difficult and extremely species-rich plant genera v. a. of the families rose plants , sweet grasses and composites such as Rubus , Taraxacum or Potentilla owe their biodiversity (from numerous, extremely similar species, so-called " small species ") to this mechanism. Apomictic plant species are usually more northerly and at higher altitudes than their sexually reproducing relatives, they are also strikingly common in habitats disturbed by humans. The success of apomictic plant species is mostly explained by their ability to colonize free habitats and niches very quickly, e.g. B. after the glaciation of Central Europe in the ice ages.

For clonal, non-sexually reproducing organisms with no shared gene pool , the only force that can create what could be called a “species” is extinction. The offspring z. B. a bacterial line will be distinguished from its ancestor by differences that go back to random mutation events. Individual individuals can thereby acquire traits that have a favorable effect in the respective environment, and thus multiply faster, while other lines with less favorable traits are less successful. The extinction of the less successful lines leaves groups of more similar individuals who appear to be separated from other lines by a feature jump. These are called tribes; even if they differ morphologically, species. (It makes sense to consider only lineages that clearly differ in their ecological demands as species, because otherwise all influencing factors affect both groups of lineages to the same degree and direction, which means that they cannot behave independently of one another in the same habitat then have a look as if it were a single undivided group.) the model of morphological / ecological defined species in asexual reproduction was except for prokaryotes, also on a group asexually propagated species, the bdelloid rotifers successfully applied.

This species model can also be extended to viruses which, according to many definitions, are not even living beings, because the underlying factors (variation through mutations and selection) have the same effect on them. Viruses "species" often arise in new environments (with new selection factors), e.g. B. when transitioning to a new host species. In a recent study, it was even shown that lytic bacteriophages of a phytopathogenic bacterium ( Pseudomonas syringae pv. Aesculi ) of the horse chestnut were more infectious than bacteria from the same tree. Here, bacteria inside the leaves were more likely to be infected by specialized strains (45%) than those on the leaf surface (3%), which are much more populated by random bacteria spread by the wind. This means that a long-lived organism such as a single tree is environment enough for a specialized phage strain to evolve. The very high number of generations and effective population density of the phages is therefore sufficient for the evolutionary development of a detectable difference in the phages (but apparently not yet in the bacteria).

The picture is complicated by horizontal gene transfer between bacteria. The genetic material is mixed, although not completely indiscriminately, between morphologically and physiologically very dissimilar bacterial strains that are conventionally referred to as different species. The evolution here is reticulate, i.e. H. instead of a “family tree” there is a “network of life”.

Biogeographic methods

Species formations are historical processes that can take hundreds of thousands or even millions of years to complete and are therefore difficult to observe directly. An important method for studying previous species divisions uses distribution patterns; H. the biogeography of groups of species that are believed to have originated from splitting a parent species. The distribution patterns can be compared with model predictions that would result from different speciation processes; this allows their plausibility to be checked. A frequently observed phenomenon is e.g. For example, related species have ranges that are separate from one another and do not overlap. In biogeographical research, two models for the development of such distribution images have been discussed for a long time:

  • Dispersion: At the edge of the range of distribution of a species - through the long-distance distribution of a few individuals - small populations are established that are not in direct contact with the original population. These develop into new species as a result of the founder effect and unusual combinations of ecological factors. This model was especially preferred by Ernst Mayr.
  • Vicariance: In this model, the distribution area of ​​a widespread species is subsequently split up, e.g. B. by mountain formation or plate tectonic processes. The now separated populations then develop into new species.

Recent research suggests that the importance of the two models varies from case to case. For example, for the animal world in the southern hemisphere, the vicariance model seems to best explain the distribution pattern, and for the plant world, the dispersion model.

Patterns and Sequences

If one looks at higher taxa living today (e.g. genera or families), they show very different numbers of species. According to the fossil record, the duration of species also seems to be very different, and there are references to certain epochs in which speciation was faster (e.g. adaptive radiations ). Statistical analyzes and models try to explain whether such differences are subject to biological laws or whether it is simply a matter of chance. So could z. For example, it can be shown that Old World monkeys form new species at a significantly higher rate than New World monkeys, lemurs or great apes. Within the birds, the songbirds and walking birds formed new species faster than the other lines. In general, however, it is difficult to find laws and then to interpret them. So could z. For example, the assumption that newly emerging higher taxa would split up more at the beginning than later cannot be confirmed.

On the basis of extinction rates or survival times of species that can be derived from fossil records, John Sepkoski estimates the average rate of speciation in the history of the earth: Since around 98% to 99% of the species that have ever lived are extinct today, the average lifespan of a species is around 4 Million years and the current number of species is in the order of millions, an average of around 2.5 species must have become extinct per year. The rate of new species should be in the same order of magnitude, but (for obvious reasons) somewhat higher. It comes to a value of around 3 species, which on average would have to be newly formed every year on earth.

Examples of speciation

See also

literature

  • Michael Turelli, Nicholas H. Barton and Jerry A. Coyne: Theory and speciation. In: Trends in Ecology & Evolution. Volume 16, No. 7, 2001, pp. 330-343.
  • Jerry A. Coyne & H. Allen Orr: Speciation. Sinauer Publishers (Sunderland) 2004.

Web links

Individual evidence

  1. ^ Charles Darwin On the Origin of Species by Means of Natural Selection (or The Preservation of Favored Races in the Struggle for Life) (1866)
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