Contingency theory (evolution)

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The contingency theory of evolution is a macroevolutionary theory which states that life on earth is largely dependent on coincidences (contingent events) and would not arise again as it is today.

The contingency theory deals with the long-term geological forms of the origin of life. Its main representative is the New York paleontologist and evolutionary biologist Stephen Jay Gould . Other important contributions come from the American philosopher of science John Beatty. Based on the study of the many blueprints of the Cambrian , it is observed that only a few species made it to adaptive radiation . Those who have survived have not made health reasons and as a result of adaptation to survive, but by accident. Coincidences are less mathematical-stochastic events than unpredictable natural events of various types and dimensions, such as the meteorite impact on the Cretaceous-Tertiary boundary , but also slight climatic temperature fluctuations, etc. This theory does not determine the cause and direction in the development of the species Consistently Darwinist in the sense of the synthetic evolution theory seen in permanent selection-adaptation cycles, but primarily in the influence of unpredictable events. Gould made this clear with the example of a tape recording when he says: The last 500 million years of life on earth could have been very different if we had the opportunity to rewind and replay life like on a tape device.

Furthermore, the contingency theory claims that the predominance of contingency over convergence ( convergence theory (evolution) ) in evolution restricts the framework for the course of life restrictively, because innumerable alternative forms become extinct regardless of fitness. Contingency thus defines the framework for life on earth (external constraints ). As a consequence of the contingency theory, in comparison to the convergence theory, the fact that man could arise in evolution has no pre-determination. There is no inevitable evolutionary way to get there. On the contrary, Gould says: If the Cambrian had taken a slightly different course, there would have been neither the variety of chordates nor the people on earth. Gould thus expresses himself hostile to any immanent progress in evolution.

criticism

The counterprogram to contingency theory is the convergence theory of the Briton Simon Conway Morris . Here, it is assumed that strictly adaptive processes predominate and that certain macroevolutionary developments, such as wings, fins, intelligence, inevitably had to come about in the face of analogous challenges and therefore often emerged convergently, i.e. independently, in many alternative ways. The wing development of birds, bats or hymenoptera or the lenticular eye of the squid, which has a similar structure to the eye of fish, birds, reptiles and mammals, can serve as an example. Squids are not related to these vertebrates, but belong to the molluscs . A strong current in biology supports the idea of ​​convergence today.

A further criticism of the contingency theory is that platitudes can easily arise when it simply means that some changes in the initial conditions can lead to some changes in the end result. The critical question here is: What are the initial conditions? What does slightly different mean? Three degrees temperature difference or five? However, this can be partially controlled in experiments with organisms with a high rate of reproduction.

Jonathan Losos takes a mediating position when he accepts the role of convergence under pressure to adapt (e.g. a living being that moves very quickly in water will probably always have a streamlined construction plan and move by flapping or snaking movements), but explained that with great genetic distance between two species the convergence of their evolutionary development possibilities is limited. For him, contingency also means the dependence of evolution on minimally different original starting conditions or on a randomly varying sequence of the first steps of evolution under pressure to adapt (path dependency), which limits the spectrum of future solutions. Also there are extremely specialized solutions such as the gripping elephant's trunk or the platypus , which arose more than once independently.

Testing

  • Anolis -Echsen on Caribbean islands. The inquiring at Harvard University scientist Jonathan B. Losos has with colleagues Anolis examined versions on four Caribbean islands as a test case for the theory. On each of these islands several live Anolis TYPES side by side (sympatric). One of these species is morphologically adapted in a special way to a specific habitat in which it is superior to the others. Six such ecological specializations can be distinguished (for large species in the canopy, in bushes and grasses, on tree trunks, on tree trunks up to the canopy, on thin branches) that occur on (almost) all islands (one of these is missing on two islands Types). An analysis of the relationships (DNA family tree with mitochondrial DNA) shows that it is not the habitat specialists that are most closely related to one another, but rather the species that live together on the same island. The easiest way to explain this is that the same set of morphological specializations developed independently on each of the islands in such a way that the species correspond exactly in their adaptations. Conclusions: If one looks at the developments on the islands as a whole, the development is strictly deterministic: the same result is achieved regardless of the initial conditions. Since the order of species splitting is different on each island, the process is random when considering only the individual island.
  • Snails in Polish cities. The two researchers are using three populations of the shell snail species Cepaea hortensis in three Polish cities as a natural experiment . The snails were introduced here by humans relatively recently, so they form islands of distribution outside of their natural range. Since presumably only a few animals were responsible, the differences are presumably mainly due to adaptations that have only taken place since the introduction. The scientists are now comparing snail shells of a population in sunny and shady habitats in each city. It is known of snails that animals in sunny habitats have lighter shells than those in shaded ones (meaning for thermoregulation through direct radiation). It has now been observed in each of the three cities that the animals in the shady habitats do indeed have darker shells than those in sunny habitats. This effect was achieved in different ways. Light shells are yellow, dark pink tinted, in addition the housing can carry either none, one, three or five dark ribbons, whereby these forms can live side by side in a population (genetic polymorphism). Although the dark animals in shaded habitats actually predominate in every city, this was achieved through a different combination of color and ribbon morphs. Depending on the random genetic makeup of the founder population, the same goal can be achieved in different ways.
  • Selection in laboratory experiments: E. coli strains in Petri dishes In a series of experiments, microbiologists Richard E. Lenski, Michael Travisano and colleagues examined strains of the intestinal bacterium E. coli , one of the most common genetic model organisms . In one experiment, they established a clone of twelve genetically identical laboratory strains and allowed them to grow for over 2000 generations on nutrient media that were poor in glucose . They then tested the bacteria that evolved as a result of this selection on other nutrient media that were depleted in other sugars (maltose and lactose). Here they showed (in spite of previous identical selection) significant fitness differences which later came closer together with longer culture on this medium. It was found that the selection in the glucose-depleted milieu had caused very different genetic changes in detail, which, however, were very similar in their adaptation value. However, they caused significant differences when they were tested in the new nutrient medium. The similar (but not identical) adaptations the bacteria had achieved after 2000 generations did not converge any further when the experiment was extended to 10,000 generations. Although similar adjustments were made to identical selection factors in all cases, the path taken was completely different from one another in detail, which obviously points to random dependence.

Individual evidence

  1. ↑ Gould, Stephen J .: Chance Man. The game of life in nature. Hanser Verlag 1999
  2. ^ Gould, Stephen J .: Illusion and Progress. The many ways of evolution. Fischer TB 3rd edition 2004
  3. ^ Conway Morris, Simon: The Convergence of Life. In Fischer, Ernst Peter & Wiegandt, Klaus: Evolution. History and future of life. Fischer TB 2003
  4. ^ Powell, Russel: Reading the book of life: Contingency and Convergence in Macroevolution. (Diss. Duke University) 2008
  5. ↑ Jonathan Losos: A stroke of luck man. Is Evolution Predictable? Munich 2018.
  6. ↑ Jonathan B. Losos, Todd R. Jackman, Allan Larson, Kevin de Queiroz, Lourdes Rodrıguez-Schettino (1998): Contingency and Determinism in Replicated Adaptive Radiations of Island Lizards. Science 279: 2115-2118 doi : 10.1126 / science.279.5359.2115
  7. ↑ Małgorzata Ozgo & Michael T. Kinnison (2008): Contingency and determinism during convergent contemporary evolution in the polymorphic land snail, Cepaea nemoralis. Evolutionary Ecology Research 10: 721-733.
  8. ↑ M. Travisano, F. Vasi, RE Lenski (1995): Long-Term Experimental Evolution in Escherichia coli. III. Variation among the Replicate Populations in Correlated Responses to Novel Environments. Evolution Volume 49 Issue 1: 189-200.
  9. ^ RE Lenski & M. Travisano (1994): Dynamics of adaptation and diversification: a 10,000-generation experiment with bacterial populations. Proceedings of the National Academy of Sciences USA vol. 91 no.15: 6808-6814.