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After graduating from Cambridge University, Darwin took an unpaid position as a naturalist on the HMS Beagle for a 5-year (1831 - 1836) around-the-world voyage. He observed a wide diversity of organisms, especially at the Galápagos Islands. _Vid_Campbell7e/BoobiesCourtship-V.swf

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Charles Darwin published The Origin of Species in 1859, after a five-year voyage on the HMS Beagle, and after receiving an essay from Alfred Russel Wallace describing similar ideas.

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Evolution of drug resistance in HIV. When patients are treated with the HIV drug 3TC, mutations in the virus population allow some to be resistant to the drug and reproduce. Within weeks, 100% of the virus population in each case is 3TC–resistant.

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Artificial selection. These vegetables have all been selected from one species of wild mustard. By selecting variations in different parts of the plant, breeders have obtained these divergent results.

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Inference #2: adaptation. Over time, populations (NOT individuals) accumulate favorable characteristics such as camouflage and evolve by adapting to their environment. These species of insects called mantids have diverse shapes and colors that are adapted to different environments. Different species adapt to unique ecological niches (environmental resources). Video: SeaHorses

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Convergent evolution. The sugar glider is a marsupial mammal (young finish development in a pouch) that evolved in Australia. While sugar gliders superficially resemble the eutherian flying squirrels of North America, the ability to glide through the air evolved independently in these 2 distantly related mammals.

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Descent with modification. Darwin proposed that organisms are related by being descendants of a common ancestor, with modification among the descendants. Based mainly on fossil evidence, this evolutionary tree reveals that manatees and hyraxes are the elephants' closest living relatives.

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Anatomical homologies revealed by comparative embryology. At some stage in their embryonic development, all vertebrates have a post-anal tail (located behind the anus), as well as pharyngeal (throat) pouches. Descent from a common ancestor can explain such similarities.

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Evolutionary tree. Homologous characteristics that are inherited from a common ancestor are strong indications of evolutionary relationships. For example, "Tetrapods" all possess four limbs, presumably because they evolved from an ancestor that had four limbs.

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The Galápagos Islands are home to 14 species of related finches; differing mainly on their beaks, which are adapted for specific diets.

1. The long beak of the cactus finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp.
2. The green warbler finch (Certhidea olivacea) uses its narrow beak to grasp insects.
3. The large ground finch (G. magnirostris) has a large beak adapted for cracking seeds.

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Homologous structures. Even though these anatomical structures have been adapted for different functions, the forelimbs of all mammals are constructed from the same basic skeletal elements: one large bone, attached to two smaller bones, attached to several small bones, attached to five phalanges.

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Comparison of a protein found in diverse vertebrates.

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Darwin formed his hypothesis of natural selection by drawing two inferences based on his observations.

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Overproduction of offspring. A single puffball fungus can produce billions of offspring. If all of these offspring and their descendants survived, they would quickly carpet the surrounding land surface.

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Variation in a population. To the extent that the variation in color and dot patterns among the members of this population of ladybird beetles is heritable, it can be acted on by natural selection.

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Inference #1: struggle for existence. Inherited traits give an advantage to some individuals over others in a struggle for existence.

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Transitional fossils. Whales are mammals that must have evolved from terrestrial ancestors. This hypothesis predicts that their ancestors had four legs. Paleontologists digging in Egypt and Pakistan have identified extinct whales that had hind limbs. Pakicetus and Rhodocetus had ankle bones similar to that of hippos.

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To the extent that the variation in color and banding patterns in this snail population is heritable, it can be acted on by natural selection.