Evolution (biology)

Arguments in favor of the evolution.

Strategy of reasoning

The current species seem immutable with the eyes of the majority among us. It is one of the mental barriers which makes difficult to accept the idea of evolution. Thus if the evolution is a reality, it should initially be admitted that it takes place very slowly. The strategy of reasoning thus consists in going up far in the past, to prove the existence of a species, which would be the Ancêtre of another species, quite different.

The witnesses of this remote past that are the species Fossile S, are often quite different from the current species; but this simple report is not enough to prove the evolution. Moreover Georges Vat (1769-1832), father of the modern Paleontology, explained these differences by a succession of divine creations and extinctions catastrophic. It should be shown that there is a family ties i.e. a bond Génétique between these various species. This bond can be highlighted by the division of nature coming from a common ancestor, whom one calls Homologie.

If two different species has and B divide a Homologie, then these two species derive from a common Ancêtre (which can be has, B or another disappeared species); one is obliged to deduce that there was acquisition of new characteristics, leading to the formation of a new species: there was evolution well.

Thus of many indices of relationship between 2 species are detectable, as well on the level of morphology, as at the molecular level; and sometimes even, for very close species, on the level of the behavior:

Morphological indices

the Whale S, animals adapted to the aquatic life keep a trace of their ancestors quadrupeds by the presence of bone vestigials corresponding to the basin (girdles pelvic);

There are vestiges of legs at some Serpent S (Boa S);

By observing the wing of a bird or a bat, one easily finds the osseous structure of the forelimb of all tétrapode (see Tetrapoda);

the defenses with continuous growth of the elephant S are in fact counterparts of the Incisive S of the others Mammifère S, of which the man;

the appendices masticateurs of the arthropods are at the origin of the reduced locomotor appendices (the same applies apparently for the Onychophores);

the members of the Tétrapode S come from the fins of Poisson S;

in the vegetable world, the presence of a double membrane around the plastids and the presence of a circular D.N.A. inside those Ci betray a procaryotic endosymbiotic origin.

Molecular indices

- the hereditary data carrier is always the D.N.A. for the whole of alive (sometimes a.R.N for certain viruses);

- The genetic Code, code of correspondence between the DNA and the proteins is almost the same one at all the living beings;

- the sequencing of the D.N.A. revealed many areas narrowly related (genes paralogists) which can have given proteins with functions or structures different (hemoglobins, myoglobins…) ;

- the elements transposable (let us transpose - to see Élément transposable) seem closely related to the virus being integrated in the genome of their host.

behavioral Indices:

- See also: Theories of the evolution

Intermediate species

August 1st

Towards a evolutionary classification of the alive one

Of these relationships between species or the group description of species, makes it possible to trace between them ramifications which meet and to establish Arbre of evolution or Phylogenetic tree.

By traversing the tree of alive established by the phylogenetic Method, since his base, one can represent the history of the evolution: appearance of first the vertebrate terrestrial ones, the plants with flowers…

Evolution today.

Evolution, although slow and seldom observable on a human scale can sometimes be detected at the species with fast reproduction. Thus, the mosquitos sometimes locally became resistant to insecticides. The same applies to the bacteria responsible for human pathologies, often multirésistantes with antibiotics. Lastly, one can note the work of a recent divergent evolution in the case of the crab colony discovered under Rome:

Mechanisms of the evolution

The evolution results from the action of several evolutionary mechanisms likely to modify the frequency of the Génotype S in a population, and consequently to modify morphological, physiological or behavioral characters. The process is in all the identical cases, in the same order:

Appearance of new a Hereditary character at one or some individuals.
This new character is spread in the populations, either because it is adapted more to the Environnement, or because of an effect of Hasard.

Appearance of new and hereditary natures

With each generation of new variable genetics appear which can transmit their genetic inheritance to them-even to the moment of the reproduction. These sources of genetic Diversité are:

the specific change S in the sequence of DNA of genes;
the chromosomal rearrangements ;
the genetic recombinations or mixing which occurs at the time of the sexuée Reproduction and, at the Bactéries, during transfers of DNA between cells.

One can add to this list another source of genetic diversity that are the migration S by which genetic stock in a given population is seen renewed by the arrival of other members of the species carrying a different genetic pool.

The change

The change S result from errors at the time of the replication of the Gène S. These modifications accidental can be transmitted to the descent, giving rise to individuals carrying new genes, that none of their parents carried.

There exist several types of changes (specific changes, duplication S of genes, breaks of the Chromosome S, insertions or délétions of sequences). These various modifications are enough to explain the genetic diversity observed in nature.

It can to it be added changes by exchange of genetic material between species, by various natural mechanisms:

the Bactérie S are able to integrate and use genetic material (Plasmide S) exchanged between two bacteria, even simply present in the medium. Generally that does not lead immediately to the definition of a new species, but the phenomenon can contribute to it and it contributes in an important way to the natural selection (for example, of many resistances to the Antibiotique S diffuse this way).
the Virus can be used as vectors of transfer of genetic material of a species towards another, even very different; generally, the viral infection leads to dead cell victim, but it is not necessarily the case, and the target cell can take part in the reproduction (in particular for the plants). One of the examples the best known ones (and the most used by research) is that of the Tabac and of the mosaic S.
the plants can hybrider, leading to new species (the Blé being a typical example).

Finally, it is necessary to mention the strict Symbiose S, which are creations of new species from old, but without exchange nor fusion of the genetic material (at least initially). The Lichen S and, according to the Assumption of the endosymbiose, the organoids known as “autonomous” inside the cells (Chloroplast S, Mitochondrion S) concern this type of association.

The changes increase genetic variability within a population. However, they are seldom beneficial with the individual who carries them because they can destroy the activity of an essential gene.

Migration

The migration is a force of the evolution which increases the genetic variability of a population. New arrivals coming from a distant population bring new alleles, increasing the genetic pool of the population of reception.

The new character is spread in the populations

The mechanisms of the evolution suppose that a whole of individuals, gathered in a Espèce on the basis of shared nature, evolve/move “jointly. ” Thus the difficulty of speciation arises, i.e. of the evolution of the species beyond the simple individuals.

Random drift

Natural selection

It decreases the genetic variability of a population by removing certain unfavourable genetic combinations in the medium where they occur. However it can also maintain at a stable frequency, new variable genetics, without eliminate the old versions. It is the case for the allele of the Drépanocytose. The natural selection thus takes part in the increase in the genetic Diversité.

To become inherited changes and characters

the changes are always made randomly , and are generally lethal. Most of the time, they disturb the operation of the mutant organization seriously. Of course, it also happens that they are neutral (by modifying a character without to make it less functional) or advantageous; the case is much rarer, but the changes relate to million individuals during tens of thousands of generations or more, which ensures a multiplicative factor of 10¹⁰ each one of these random events.
It is the natural selection , highlighted by Darwin, who undertakes “to make the sorting” between the various variations. It is about a purely mechanical process: the underprivileged mutants will have tendency is to die young people than the others, is to be found with difficulty a companion of reproduction. In a case as in the other, they will less leave (even at all) descendants. The neutral mutants will die on average at the same age that the others, will have as many children overall and will be able to thus spread their new characters in a population under the result of the chance. As for the favoured mutants, they will be more competitive and will thus have on average more descendants. A change which makes it possible to better escape a predator, to be better directed, better to allure the opposed sex, to have more descendants, to better resist the diseases, to better benefit from a life in society will tend cause a drop in death rate at the mutants or to improve their reproductive success.
Generation after generation, the natural selection supports the changes which are transmitted more, in the populations (the genetic drift, which is done randomly, also makes it possible some neutral changes to be fixed in the Génome of the species). The adaptation of the populations thus tends gradually to improve; the emergence of bodies as complex and as functional as the human eye or the radar of the bat is explained by such a selection process cumulative. On the other hand, the existence of characters without apparent adaptive utility can randomly be explained by fixing neutral changes. The assumption known as of “hitch-hiking”, or “ hitch hiking ” in English, explains the fixing of the neutral changes by the change on the same chromosome of two embarrassments, one being neutral and the other being positive which will be transmitted simultaneously.

Appearance of new species

Changes at the origin of the great evolutionary modifications

Certain tiny changes make it possible the evolution to very quickly act. One knows genes regulating, known as genes homeotic , which determine the broad outlines of the development and the plan of organization of each part of the body. When these genes (which control themselves a few thousands of other genes) transfer, the individual obtained is often carrying natures definitely different from those from his parents. Certain “evolutionary jumps” are explained by this type of “macro-changes” (why Goldschmidt had discovered by proposing its theory of the “promising monster”). For example, the appearance of fingers and the disappearance of the rays at the end of the fin of the fish Crossoptérygien S (i.e. formation of the first outlines of legs) are explained by a simple shift of the activity of two genes homeotic: Hox-a and Hox-b. This evolutionary-there stage did not require thousands, nor even tens of different changes: the genes homeotic allowed the fast emergence of radically new natures. Even if the “macro-changes” generally give “monsters” unable to survive and even if the process, slow and sure, of cumulative selection of tiny thousands of changes apparently played a more important part in the evolution, it is in fact that certain “promising monsters” made it possible the evolution to make great jumps ahead.

Other changes modify the chronology of the development: one speaks about heterochronies . They can be at the origin of the appearance of a more youthful species (paedomorphic) or, on the contrary, more adult (Hypermorphique) that its ancestor. One knows several examples of evolutionary stories which utilized heterochronies. The Axolotl, a Mexican Urodele which passes its whole life in a larval state without never metamorphosing itself, is paedomorphic compared to its ancestor the ambystome. A simple heterochrony allowed the appearance of a completely different species. In the same way, it would seem that the man is paedomorphic compared to his ancestors simiens, because he keeps all his life of the youthful characters that the chimpanzee and the gorilla lose. Moreover, the general phases of its development are slowed down compared to those of the large monkeys.

The appearance of new genes is explained mainly by the preexistent gene duplication. One knows several “families of genes” from which the various members come apparently from several duplications of the same ancestral gene. Thereafter, each “duplicated” specimen could transfer and evolve/move independently of the others. Several multigenic families obviously appeared thanks to this process.

In the same way, of duplications of the together of the genome (polyploidisation S) also could play. For example, it is estimated that two episodes of polyploïdisations occurred since the separation of the deutérostomiens (are primarily the echinodermatous S and the Chordé S of which the vertebrate ones) and of the Protostomien S (invertebrates) on the branch of the vertebrate ones. One finds like many genes, like HedgeHog implied in the development of the polarities in the drosophila, exist into three or four specimens at the vertebrate ones (HedgeHog, Indian HedgeHog and Sonic HedgeHog Serves) for similar functions.

Does the evolution tend towards an increase in complexity?

There does not exist consensus on the definition even of complexity. It is often estimated by the number of standard cellular identifiable at an organization, but this estimate is not, or, correlated little with the size of the genome or the number of genes (complexity of the genome) and could not in no case to be an indisputable measurement. The increase in complexity is a marginal and rather exceptional phenomenon in the evolution. The majority of the species are still at the unicellular stage (more 99 % of the living beings are Bactérie S), only some rare lines evolved to a complexification.

In certain cases, however, the natural selection could support the most complex organizations. At the Mammalian S, one observes a tendency to the increase in the size of the Cerveau; one generally explains it by a “arms race” between preys and predatory (see the Théorie of the red queen). In both cases, a more powerful central nervous system can save the life of the animal. In the same way, of the “federations of cells”, pluricellular outlines of organizations, could in certain circumstances being more effective than the bacteria isolated to enable them to survive. However, it is plausible that the increase in complexity could, in certain cases, to correspond to a neutral character and to be fixed by the genetic Dérive.

An image given by Richard Dawkins

To be thousand different ancestors, it is necessary to go up behind ten generations, which represents a few centuries. However, before the 19th century, half at least the children died in low age: one can thus wonder how much our the last thousand ancestors died in low age, and the answer is not “at least half”, but of course zero, by definition. We are thus by no means representative of humanity passed, because downward of a long line of people who had all the chance to bring an offspring to the age of procreation (i.e. to survive and find a partner, inter alia). A generation can have chance. When ten in undoubtedly blow, one can suppose that this “chance” corresponds in fact to a whole of favorable factors which find one with the other (factors which all are not genetic: they can be cultural, monks, economic, etc) This consideration with it only watch that even on our scale, we took part very small little, over the few last centuries, with the evolution. Moreover our simple choice of a spouse appears, as shows it the biologist Geoffrey Miller ( The Mating Mind ) to obey choices not always conscious that one can bring closer to the Eugénisme.

Richard Dawkins pushes more before his speculations by considering the living being as a gene container (it would be in any case the point of view of genes, if they had one of them!). For him the products of genes - cells, fabrics, bodies, organizations, companies - are used with genes to be retorted and survive. To some extent, with the question: “Of the egg or the hen, which is the first?”, it answers: “ the hen is the means found by egg to make other eggs. ”. This inversion of prospect considers the living being as the puppet (Dawkins uses the term of vehicle-robot ) of its genes.

While recognizing it clever, Stephen Jay Gould warned against a catch too at the letter of this vision. The genes have neither intention , nor project with the direction which we give in these terms. Dawkins used the term only as a speaking Métaphore, but its readers did not always seize the nuance.

Moreover, it is advisable to remember that the factors of survival are not only genetic, which is quite obvious at the man, but what is also the case in many animals (for example, the song of the birds, essence in their reproduction, depends on a training).

Mutual aid, factor of the evolution

Thus it will be developed, in 1902, by Pierre Kropotkine in his book the Mutual aid: A factor of the evolution a clear criticism with respect to the social Darwinism.

In this work, the prince and anarchistic Russian answers, specifically, with the theories of Thomas H. Huxley published in the Fight for the existence in the human society in 1888. Kropotkine, without denying the theory of the evolution of Darwin, specifies there that best adapted are not necessarily most aggressive, but can be most social and interdependent. It provides empirical examples of the animal kingdom, as of what it calls the “Savages”, “Barbarians”, medieval cities, as well as time present. Kropotkine does not deny either the existence of competition, but thinks that the competition is far from constituting the only factor of the evolution, and that the evolution progressist is rather due to socialization and the mutual mutual aid.

A history imagined by Richard Dawkins

This amusing history has of another goal only to fix well an important point of the theory darwinienne.

Two Brontosaure S see a T-Rex advancing in their direction and start to run as quickly as they can it. Then one of two known as with the other: “Why do we tire ourselves with the Juste? We are not likely in any event least to manage to run more quickly than T-Rex! ”

And the other answers him cynically:

“I do not seek to run more quickly than T-Rex . I just seek to run more quickly than you ! ”

The idea is to recall that the process relates to less one competition between species, that a competition inside each species.

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