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Before Darwin the general consensus was that species were created independently by some extranatural force, and that they died out without any major change. So, which were the steps needed for a theory of evolution of the biological species to be accepted in the XIX century?

• Accepting the idea that species can change during their life span.
• Accepting the idea that species can change so much that they can even generate new species, either by diversification (cladogenesis) or by a global transformation of one whole species (anagenesis).
• Accepting the idea (that looks like an inevitable result of the previous one) of a common descent, i. e., that all species are related, have a kinship, and they all come from a common ancestor.
• Accepting the idea that the Earth is old enough for these huge changes to have enough time to occur... so slowly that can't even be perceived during a human life span.
• And most notably: knowing of any acceptable mechanism by which these types of changes can occur.

Considering that not so long ago, the Anglican bishop James Ussher had stated that the Earth had been created the night preceding 23 October 4004 BC, the idea of an old Earth was, quite possibly, the necessary precondition for all the others to be even borne in mind by anyone. And this revolutionary idea was just there in the right moment, when Darwin started his five years long journey in the Beagle, and was given the first volume of Charles Lyell's Principles of Geology, which set out the idea of masses of land slowly rising or falling over immense periods of time to finally yield the geological features that can be observed in present time. According to Lyell, the Earth was much older than what was thought by that time, probably even millions of years old (some 4,560 years old, to be more precise, and as we know today).

The concept of a gradual change of biological species, now that Darwin knew that there was enough time for it, arose from the observation of different races of tortoises, finches and others, adapted to the particular environments that the Galápagos islands had to offer. And more particularly, to the different nutritional niches available; while some finches had a beak specialised in cracking hard nuts, others were the perfect weapon for chasing insects, etc. And considering that those islands are some 1,200 km away from the continent, it was clear to Darwin that all those finches (or tortoises) had had to come from one (or a few) small group of ancestors that, departing from the mainland, happened to make their way to the islands. The Galápagos finches had evolved on-site, as it was highly unlikely that all those varieties could have possibly reached the islands from the continent, one by one.

That panorama was clearly speaking about gradual transmutation of those animals by adaptation to different environments (species can change), and with it, Darwin's mechanism of evolutionary change had started to develop.

But was that slow and gradual change powerful enough to produce new species, or were all those differently adapted finches simple varieties of the same species? Friends are to come to the rescue when needed, and so it was with Darwin's mate John Gould, an ornithologist, who announced that the specimens of finches that Darwin brought back to England after his trip belonged, in fact, to three different species. In Darwin's mind, that meant that gradual adaptation to the environment can actually produce new species.

Finally, how do species get to adapt to each one of the many specific environments that Nature provides? How can a population gradually reach its ecological niche, one that allows it to survive and thrive for generations? That was the last and greatest obstacle to overcome, and recent History showed that clearly. Jean-Baptiste de Lamarck had already come up with similar thoughts to Darwin's a couple of decades before the Beagle departed: species evolve, and do it by gradual adaptation to their environment. But Lamarck could never demonstrate that acquired-on-life traits can be passed on to the offspring, and we know nowadays that that just can't happen: the genes of your eggs or sperms will not change because of you dying your hair. And this is how Lamarck passed to History as an unsuccessful attempt to explain the how and the why of the evolutionary process.

But not Darwin. It turns out that among the many different types of individuals that are randomly produced every generation in every species, some happen to be better suited to their environment than others: the former feed better, grow faster, survive longer and as a result... reproduce more than the latter. And as the traits that make them the fittest were inherited, they will also pass those traits to their children. As a final result, those traits will be more present in the next generation than those that conferred a lesser success. This was what Darwin (and Alfred R. Wallace) elucidated, and that's why everyone is now celebrating his 200^th anniversary. Or almost everyone.

Natural Selection is all about a central event: some individuals reproduce more than others. And this has a cause and a result. Step by step:

• There are many different types of individuals in any biological population. We all can see that. And it is due to mutation and recombination. Mutation increases the number of possible alleles for every loci throughout time, and even creates new loci, or gets rid of loci or alleles. But in the long run mutation tends to increase the number of genes (either alleles or loci) for every species. Mutation takes place mostly due to errors during the duplication of DNA prior to cell division. Also, the recombination of loci during meiosis multiplies the number of different possible gametes that any individual can produce.
• Different types of individuals interact differently with their environment. Some of them are stronger, faster, bigger, smarter... and so some of them grow faster and reach fertility sooner, hide better from predators and manage to sort the threats out better, are more efficient absorbing nutrients or chasing the prey, are more convincing when looking for someone of the opposite sex to mate...
• The individuals that have more successful interactions with their environment leave a greater offspring. No way it can be otherwise. If you reach fertility faster, survive longer, and your nutritional efficiency leaves you more free time, chances are that you are going to have a greater reproductive success.
• Many of the traits that make these individuals more successful are inherited. Although in some species (us!) some "rules for success in Life" can be learned, capability for learning is itself an inherited trait. And, although your genes don't determine the weight that you'll have when you are 25, they do determine a certain range or weights you are likely to fall within.
• The alleles of the individuals with a greater reproductive success will be present with a higher frequency in the next generation, and vice versa. As a result, the phenotypic traits coded by the successful alleles will be more present in the next generation, and this noticeable phenotypic change improves the overall fitness of the population gradually, generation after generation.

In short, Natural Selection takes place through the following sequence of events:

1. Variation due to mutation and recombination.
2. Differential fitness.
3. Differential reproduction.
4. Heritability of phenotypic traits.
5. Change in the frequencies of the alleles of a population.
6. Overall phenotypic change in a population.

The formation of new species or speciation is one of the main outcomes of the evolution of Life, the opposite to extinction, and the most visible result of adaptation.

Sexually reproducing species (virtually all that exist, except prokaryotes) are defined as those groups of individuals that can leave fertile offspring by sexual reproduction, and thus share a common gene pool: the genome of species. This is so because sexual reproduction can be considered as a way to produce new combinations of genes (new genotypes) by means of sharing half of the genes of each one of the two partners. Sexual reproduction is a bit like dealing sets of, say, five cards after having shuffled the deck. In the analogy, each card would be a gene, each set of five cards the genotype of a new individual, and the deck of cards is the genome of the species.

The members of the same species also share the same ecological niche, this is, they dwell on the same habitat, they feed the same way, they prefer a similar amount of humidity or sunlight, the are prey to the same predators, etc.

If two individuals of opposite sex that can leave fertile offspring by sexual reproduction belong to the same species, then, speciation must be the process by which two populations, that originally belonged to the same species, split apart and, after a time of adaptation to different environments (or ecological niches), evolve differently to the end that eventually they become unable to leave fertile offspring by sexual reproduction. This end point of speciation is called reproductive isolation.

This process can take place by different ways. Sometimes the appearance a new geographical barrier is the decisive event. That is the case of the Galápagos tortoises, presumably descendants of a common ancestor, that after colonising the different islands of the archipelago, the sea between the islands posed an almost invincible obstacle for them to meet and mate. This way, the different populations settled down in the different islands, couldn't share a common gene pool anymore, and natural selection made them to evolve differently by improving their adaptation to each particular island. Eventually, they changed so much that if individuals of these different populations happened to meet due to the sea currents or the action of men, they were so different that they would not able to leave fertile offspring by sexual reproduction. Those different populations were now different species.

But speciation does not always need huge geographic barriers. In the case of Galápagos finches, different species appeared in the same islands because when this virgin territory was colonised by their ancestors, a variety of ecological niches were at their disposal, and while some of the finches specialised in feeding off insects, other preferred grains, and so forth. They shared a common habitat, but occupied different places in it and performed different ecological roles. The enhancement of that specialisation (the improvement of their adaptation) to each specific ecological niche by means of natural selection did the rest, and, like the tortoises, with time enough took them to be so different that one day they were reproductively isolated.

But how different two individuals of the opposite sex have to be, to be reproductively isolated? The answer is: either...

• So different that the male wouldn't be able to fertilise the female, because...
• They don't attract each other sexually anymore: courting does not work for them;
• Their genitals have become incompatible;
• The sperms are unable to make their way towards the egg, in the case of mammals;
• etc.
• Or so different that even if the male can fertilise the female, they can't leave fertile offspring, because...
• The zygote can't divide by mitosis (consider a sperm with 23 chromosomes and an egg with 24: this makes a total diploid number of 47, which is odd, and mitosis cannot take place with an odd number of chromosomes);
• The embryo degenerates soon;
• The children are weak and die before sexual maturity;
• The offspring is sterile, as in the case of the mules;
• etc.