Observation: The members of a natural population show great variation in their traits, and much of the variation is passed on through generations (it has a heritable, or genetic, basis.)
Inference: Some heritable traits are more adaptive than others. They give the individual a competitive edge in surviving and reproducing.
Inference: Over the generations, there is natural selection - a measurable difference in survival and reproduction among individuals that differ from one another in one or more traits.
Inference: Thus the character of the population changes over time - it evolves - as some forms of traits increase in frequency and others decrease or disappear.
After nearly five years, Charles returns to England in 1836. In the years to follow, his writing established him as a respected figure in natural history. Having this good but conventional reputation in natural history, he spent 22 years secretly gathering evidence and pondering arguments - both for and against his theory - because of his meticulousness and fear of a short lived burst of unpersuasive notoriety. (He may have delayed, too, because of his anxiety about announcing a theory that seemed to challenge conventional religious beliefs - in particular, the Christian beliefs of his wife, Emma and the beliefs of his past. Darwin himself quietly renounced Christianity during his middle age, and later described himself as an agnostic only to revert before his death.) All the while, his consuming interest was the "species problem." What could explain the remarkable diversity among organisms? As it turned out, field observations he had made during his voyage enabled him later to recognize two clues that pointed to the answer.
First, while the Argentine coast was being mapped, Charles repeatedly got off the ship, seasickness. During his many exploratory trips inland, he made detailed field observations and collected fossils. For the first time he saw many unusual species, including an armadillo. Among the fossils were the remains of the now-extinct glyptodonts. Glyptodonts were very large animals that bore an inconclusive resemblance to armadillos. If both kinds of animals had been created at the same time, lived in the same part of the world, and were so much alike, why were armadillos still lumbering about but the glyptodonts gone extinct? Nothing else in the world resembled either animal. Although neither Charles nor anybody else had ever seen one species evolve into another, he later wondered whether armadillos were descended from the glyptodonts.
In the same manner this is why not only Charles, but many others over the years still question:
"If man is descendant or a close cousin to the chimpanzee or apes, why is it that they still remain yet we evolved further."Good question, and one that may take eons to answer. Many theories suggest that the evolutionary branch man is attached to was altered in some way on the genetic level. From that basis many people draw there own conclusions, everything from the profound such as the entrance of special mitochondria; to the abstract -- such as aliens altering the genome as a test. Whatever belief you subscribe to one thing is certain, nature is still in control of the animal species, but we are in control of the human species.
Second, Charles had observed the populations of similar kinds of organisms that were confined to different geographic regions often showed pronounced differences in some of their traits. For example, the Galápagos Islands are almost 1,000 kilometers off the coast of Ecuador. Every island or island cluster is home to diverse species, including birds called finches. Although Charles didn't think much about it during his voyage, later discussions with colleagues back in London made him realized that the island were home to more than a dozen closely related species. Perhaps all those species were descended from the same ancestral form and had become modified slightly after they became isolated on different islands as is being researched by Peter and Rosemary Grant.
This raises the question as to, how such modifications could occur? A clue came from an essay by Thomas Malthus, a clergyman (like Charles was going to be) and economist. In Malthus' view, any population tends to outgrow its resources, and its members must compete for what is available. Charles thought about all the populations he had observed during his voyage. He thought about how the individual members of those populations had varied in body size, form, coloring, and other traits. It dawned on him that some traits could lead to differences in the ability to secure resources.
If there were competition within a population, then individuals born with a stronger seed-crushing beak or some other favorable trait might have an edge in surviving and reproducing. Nature would favor individuals with advantageous traits and neglect to the point of elimination others - and so a population could change. Preferred individuals would pass on the useful traits to offspring. Their offspring would do the same. In the passage of time, descendants of the preferred individuals would make up most of the population, and less favored individuals might have no descendants at all.
Later, in 1858, the middle aged Charles received a paper from the naturalist Alfred Wallace, who arrived at the same conclusion! Unlike Alfred Wallace, who was younger and less meticulous, Charles recognized the importance of providing an edifice of supporting evidence and logic. Charles' colleagues prevailed upon him to formally present a paper along with Wallace's. The next year Charles’ detailed evidence in support of the theory was published in book form: The Origin of Species. In which he reveals something that made the book remarkable, it offered a rational explanation of how evolution must occur. Referring to a part of the evolutionary process known as speciation, the genetic changes that sometimes accumulate within an isolated segment of a species, but not throughout the whole, as that isolated population adapts to its local conditions. Gradually it goes its own way, seizing a new ecological metier. At a certain point it becomes irreversibly distinct, so different that its members cannot procreate with the rest. Two species now exist where once upon a time there was one. Charles Darwin called this spitting-and-specializing phenomenon the "principle of divergence."
The Principles of Divergence was an important part of his theory, explaining the overall diversity of life as well as the adaptation of individual species. The evidence, as he presented it, mostly fell within four categories: Bio-geography, paleontology, embryology, and morphology. Bio-geography is the study of geographical distribution of living creatures. Paleontology investigates extinct life forms, as revealed in the fossil record. Embryology examines the revealing stages of development (echoing earlier stages of evolutionary history) that embryos pass through before birth or hatching; at a stretch, embryology also concerns the immature forms of animals that metamorphose, such as larvae of insects. Morphology is the science of anatomical shape and design. Morphology, his fourth category of evidence, was the "very soul" of natural history, according to Darwin.
It should be realized that all of his research and understanding came out of his persistent observation of the world in which he lived. These observations led him to these extraordinary conclusions. And so observing the world in which we live, my observations have led me in the direction of similar qualities but a profoundly different arena of the evolutionary progression.
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