From the beginning of the times, the Man tried to explain of some form his own origin, and that's why the same diverse ideas were arising along the history. In all the time that passed since we begin to reason up to today, the theories were taking diverse forms.
In the beginning, we did not have any tool to understand our environment, and we begin to look for explanations and fill our knowledge hollows with diverse causative gods of the phenomena. The time and the observation of the nature was filling some of these hollows and eliminating gods of our imaginary one.
Most of the history of the humanity entrusted itself in the creationism, which with diverse tones, implies a god who has created the man, sometimes to his image and resemblance. But to halves of the XIXth century Charles Darwin realized his famous trip for the world, and thanks to many remarks that it did, proposed in his book The origin of the species (that in fact is called The origin of the species by means of the natural selection, or the preservation of the races favored in the struggle for the life) the famous theory of the evolution.
Between many other things, Darwin had observed how the different islands of the archipelago Turtles were populated by finches lightly different, and the difference in the form and size of the peak of each one was offering to him an advantage on the seeds that they were consuming. Although this is the best anecdote on The origin of the species, it seems not to be so according to the facts. But it illustrates the idea of that all the species varieties come from common ancestors, who were changing with the time to adapt themselves to the characteristics of the ambience.
Darwin was telling that between a group of individuals, it was probable that some of them have different characteristics that others: that some of them are higher, more shallows, stronger, with better vision, and the others. And that in a hostile environment, it would be logical to think that the weakest should perish, allowing to reproduce only the strongest. It called to this Natural selection. But he could never tell why it was working, and the science had to wait for a pair of years so that contemporary Gregor Mendel and his experiments with a vetches garden should confirm that the different characteristics of the individuals were determined by the genes, and that these were inherited, and the changes owed to diverse mutations caused by diverse factors.
The evolution of the species is a fact observed straight of the nature, which can be confirmed from different investigation fields, and nowadays it is not subject to doubt for any serious scientist. Simply it is like that, we liked or not. And these disciplines prove it to us:
Paleontology 
Bought anatomy
It is a discipline that is in charge of studying the similarities and differences between the structures of the organisms. From the concepts of the Analogy and the Homología, there can arm themselves the trees that the organisms families compare at present and relate them to fossils of the antigued, like that of the image that dates of 1866.
The Homología alludes to similar structures that every organism gives him different uses, what implies that they had a relatively nearby common ancestor. As the previous extremities of the dolphins and the bats, all formed by similar structures, but the first ones are used to swim and the second ones to fly.
The Analogy alludes to completely different structures that fulfill similar functions, like the moles that use extremities formed by bones for escavar and the crickets extremities formed by an external skeleton.
While more Homologous and Similar organs have in common two different organisms, they are more similar and it means that they evolved from a common ancestor who lived it does less time.
It is the study and the comparison of the embryos of different species. It is observed in some organisms that they are not so similar, yes they have similar embryonic structures. Which indicates again common ancestors from which they separated in different moments of the history. The embryos of the mammals develop bronchial cracks, which disappear quickly. Which indicates that we have some distant kinship with the fish.
Biochemistry
The supposed different evolutionary branches share biomolecules and similar macromolecules. Similar chemical structures that realize different functions, and you structure lightly different that realize similar functions. In the first case this might indicate that in some moment they belonged to the same forefather and then for different reasons they evolved in different organisms, changing the original functions. The mollusks for example possess in his blood a pigment called hemocianina, which contains copper (and it colors the green blood). Another contribution from the biochemistry is that numerous proteins and enzymes have light differences in his composition aminoacídica, in fact, the more related are the species which components it is compared, minors are the differences. These functional and structural resemblances make to think that the species were related more behind in the time.
Geography
The flora and the fauna of different regions is alike the closer they are. If the species had been created of isolated form, this would not have why to be fulfilled, for what contributes one more evidence to that the species evolve over the course of time. This also relates to the theory firstly theory of the continental leeway, and then the badges tectonics, since for example, it indicates that it does not a lot of time, South America and Africa were joined, and in both continents there are similar species, like monkeys. Nevertheless, Australia separated very much earlier, and the flora and the fauna that it possesses is very different. Also species can be observed lightly different in every island of a particular archipelago.
Incompatibility between the language of the Science and the Society
It is very common to listen round there to persons arguing that the evolution is an only one theory. The problem here is that theory does not mean the same for a scientist than for any other person.
Although it usually believe that a theory is a mere speculation, for the science it is everything opposite. A theory is an explanation of an any phenomenon. The humility of the science, and the possibility that always stops opened to that everything is erroneous and should reformularse does not allow to affirm that anything is an unchangeable fact.
Nowadays we bear in mind that the species evolve over the course of time, because it is at what all the evidences point absolutely in all the fields that have been. We even have seen how they evolve and mutan other "smaller" organisms as bacteria that produce illnesses, and virus like that of the Porcine Flu.
Can we be wrong? Yes, porsupuesto. Of that the science talks each other. Of observing the reality, looking for an explanation and proving if this explanation is real. Nowadays the whole scientific community agrees with that the species evolve. Perhaps some another genius comes some day with a better explanation, find evidence, and beech to change all the textbooks. Until that should happen, the Evolution is going to be still current.
Lost links? Obviously, they are and they are always going to be. The organisms are born, live and die in this planet. The majority decomposes and does not leave tracks of his existence. Lucky few ones suffer the fossilization process, that for a cause or other one, the minerals replace his organic components by others that do not decompose. It would be illogical to suppose that we are going to find a well preserved especímen of each and everyone of the organisms who inhabited the Earth. Any of them simply are not going to be, and others will be buried in some place that we them will never find. But this does not discredit for anything to the Evolution of the species.
Does it have any target? We do not know it, and today it is impossible to know it. Some of them risk to say that the Evolution takes as a target to create intelligent beings, but this is again a vision that puts us in the center of the universe, and we have no evidence to determine that this way it is. It would be interesting that the evolution has some end. But since we have observed till now, it is a simple natural process, and not some conciente.
How is it that the organisms evolve? Inevitably this question would come, and the answer is quite complex, the most recognized mechanism that explains the Evolution is called a Neodarwinismo or Modern Evolutionary Synthesis, established in the years '30 and '40. It is based on the Natural selection that Darwin had tried to explain without knowing how it was working for not knowing the genetics:When the organisms reproduce, possibly errors take place in the replicación of the ADN. Thus descent takes place with different characteristics. Possibly, someone will favor it, and others not. It is probable that they are reproduced more and inherit to his children the most favored organisms, and others perish. And voilá, accumulating changes over the course of time organisms obtain it sufficiently different that will not be already compatible to reproduce. And although the term species is not defined perfectly, the fact that there is no reproductive compatibility it is a quite important factor.
Other beginning on which the Neodarwinismo is based is more complex and own than the genetics in particular: the Genetic Leeway and the Genetic Flow.
On the God of the Hollows
Since we were mentioning at first, in the history of the Humanity there were always hollows to refill with the existence of some God. The Evolution and everything what concerns her is a very controversial theory because it goes against the Creationism, the Biblical origin of the life.
That's why, since so many hollows have been refilled by the evidence, the thought and the observation of the nature, many try to do hollows to support in foot his dogmas.
One of the last disciplines that arose was that of the Intelligent Design, in which a "Intelligent Designer" (or God), designed some structures of the organisms. It is considered to be a pseudoscience by the scientific community that studies all the fields about the evolution (not this way for a few scientists of other fields who look for mystical meanings in the things), since it is based on pseudoarguments like:
"the universe is perfected so well that someone had to make it like that"
Although it is true that if the physical laws were lightly different, we might not exist as we know each other, also it is true that simply we could come even here because the things are like that. If they should be lightly different we might not even be wondering this.
"some structures are so complex that there might no be fruit of the evolution"
The complex structure of the eye was putting itself like example, but it was already descripta and clarified by compared anatomy and embryology, and there exist diverse stages of the development of the eye in living animals. At present there is used like example the unyielding complexity of some bacteria, which without real arguments one says that they might not have evolved.
It does not also clarify any mystery really, since he adds the paradox who designed the designer. Other arguments and criticism of this pseudoscientific discipina are in the Wikipedia entry. We speak already that ex-president George Bush tried to do a law so that in the schools one was dedicating the same time to teach the Biological Evolution and this neocreacionismo. And that resulted in the famous letter of complaint of a student who wanted that also there is taught the theory of the Monster of Spagueti Volador.
The Evolution Theist would be one of the last desperate attempts, and of the most rational, to give existence to a God based on the hollows of the science. It is based in accepting all the principles proved by the science, and be carrying back to the hollows that perhaps are never filled by her: the existence of a soul (immaterial and unverifiable, and that does not weigh 21 grams), and other metaphysical concepts.
The first genesis of the life is not contemplated by the Evolution in itself, but yes by other scientific theories that we will analyze further on.
Sources: Based on this note in Facebook, of the group life, Darwin and everything else (very interesting certainly) and the multiple sources quoted along the article.
