Explain Charles Darwin's theory of evolution?

Answer 1

The nonrandom survival and reproductive success of randomly varying organisms.

That is, The Theory of Evolution By Means Of Natural Section, and it is no longer Darwin's theory.

Answer 2

The Theory of Evolution is basically that organisms change. They are not static. One species can evolve into another. In better terminology, all species share a common ancestor and all currently existing species at any point in time are descended from pre-existing species. One population of a species may diverge (genetically and thus also morphologically) if two parts of that population become separated from one another geographically. Genetic divergence may take place to such an extent that the subsequent separated populations (originally of the same species) can no longer interbreed and should be considered separate species. Life, by common ancestry (all life sharing a common ancestor) should exist on a great, branching, tree of life.

Darwin supported his argument for his Theory of Evolution by looking into comparative anatomy and artificial selection among animals such as pigeons and dogs. He saw that morphological divergence could take place: there was some sort of something within organism that could make organisms different from their parents. From the wolf Canis lupus, all domestic breeds of dog Canis lupus familiaris were bred by selecting desired traits. The squashed face of a pug was artificially selected. The spots of a dalmation were artificially selected, as were the size differences between great danes and dachshunds. Look at the variation among horses and artificially bred pigeons. Look at the variation in apples and brocolli. If great variation could occur in nature, could organisms change as dramatically as from a wolf to a dalmation or from a rock pigeon Columba livia to a domestic pouter pigeon or Birmingham roller?

The problem is, the domestic dog only exists in all its forms due to artificial selection, the key word being selection. Many traits from a parent population have to be hidden or lost in order for new traits to fully show themselves. Breed many different artificial breeds of pigeons together for a while and soon the pigeons look like rock pigeons again. That is why I used the example of geographic isolation above. What if the new morphologies of a wild organism could not breed together and their genes and morphologies could not be homogenised back to the looks of the original organism?

I am using allopatric speciation (production of new species via geographically isolated populations) as an example for it is easier to understand. A subpopulation is separated from its parent population. It is impossible for the organisms in the different populations to interbreed once more. An example to illustrate this may be a barrier such an impassable mountain or a river that dissects the two populations. Imagine the common ancestor of a panther like a lion or a tiger is separated in this way. The lion-tiger common ancestor diverges into separated populations. (This is a hypothetical, illustrative example - I do not know the hypotheses of the evolution of lions and tigers in that much detail). Genetic divergence occurs between the two populations and a gene evolves (via a mutation) for stripes, while the common ancestor did not have stripes, in the one population. In the other population, the gene does not mutate and produce a stripe-producing gene (this is hypothetical - I don't know if it is yet hypothesised whether there is such a thing as a stripe-gene - I am just illustrating that the divergence will be genetic of course as the base of all life is genetic) and this population continues to resemble the common ancestor in stripelessness. The common ancestor does not have a mane in my illustration, but a gene for great hair around the head (mane) arises in the stripeless population. There is no breeding between the two populations due to their geographic isolation remember, so stripes and no-stripes cannot homogenise together and produce a no-stripes creature once more (perhaps the no-stripes gene is dominant). So a striped animal evolves (tiger) and a maned animal evolves (from a stripeless, maneless, common ancestor). That is the production of new species explained, called speciation and I have emphasised the genetic origin of the divergence.

But can anything (eg. stripes) just evolve? Yes, from a genetic harbour of variability. And no, not if the environment disallows it. This brings in Natural Selection. Organisms are perfectly adapted to their environments (or almost perfectly if you want to be pedantic). Stripes camouflage a tiger presumably, as does the grass-coloured coat of the lion. They can evolve. The lion and the tiger will be hidden from the prey during hunts and thus will be facilitated in prey catching and thus survive to pass on tawny coats or stripes to their offspring and continue the panther line. Imagine a ridiculous example that a blue lion with green spots is born as lurid as a peacock's tail. I doubt this is possible, just an example. That lion will not catch a single spot of prey at all. The prey will spot the lion and the lion will be unable to get enough to eat. It will surely die and not send on peacock-lion genes to a future generation. Everything is mechanical and genetic. Animals and plants and bacteria look the way they do because that is what the environment allows. Lions and tigers and leopards are camouflaged from their prey. Polar bears have thick, hollow hair which traps warmth enough to survive. Seals living at the poles have thick blubber. Desert animals have long Loops of Henle in their kidneys which reabsorb much more water than short Loops of Henle and thus allow survival in the desert. If a population of animals enter a desert and none produce offspring that can survive a drought in a way such as this, where they have long enough Loops of Henle then all of those offspring will die. That is Natural Selection. What can survive and reproduce does survive and reproduce and what cannot survive and reproduce, does not.

So, in short. Organisms change. They can do this via their genes, which may mutate, producing different morphologies in the offspring. Those morphologies that do survive and are visible today, are the ones that could survive at least until now. Natural Selection (which should not be anthropomorphised in the way that this sentence unfortunately does) kills the variation that cannot survive in the environment in which it finds itself.

Darwin knew of morphological variation. He coined the terms sexual selection and natural selection. He knew nothing of genetics and DNA. The knowledge from DNA and cytology has been added since Darwin's time. Darwin also knew very little of the fossil record. He hypothesised that there should be fossil links between great classes of organisms. If birds and reptiles had a common ancestor and reptiles were more ancient than birds (ancient in the fossil record) then presumably birds evolved from a reptile. And indeed, the transition was found in the fossil of Archaeopteryx. Archaeopteryx had reptilian characters such as a toothful jaw and and a bony tail. But it also had feathers like a bird.

Transitions have been found for many groups of animals. The transition from a pig-like animal or a hippopotamus-like animal (of the Eocene) and whales is seen in fossils like Rodhocetusand Ambulocetus.

In other words, evolution is supported by evidence from many fields. Change has occured in organisms in the past such as the Eocene evolution of whales and the Jurassic evolution of birds from reptiles (dinosaurs), both seen in fossils. There is also evidence from genetics, biogeography and comparative anatomy, comparative behaviour and comparative biochemistry, to name a few. I could go on and on. The point is, life changes.