Biology/Evolution/Darwin

I n 1831, Charles Darwin left England on board the HMS Beagle. On

the ship’s voyage, Darwin was amazed by the tremendous diversity, or

variety, of living things he saw. Today, scientists have identified more

than 1.7 million species of organisms. A species is a group of similar

organisms that can mate with each other and produce fertile offspring.

In 1835, the Beagle reached the Galapagos Islands in the Pacific Ocean.

Darwin was surprised that many of the plants and animals on the

Galapagos Islands were similar to organisms on mainland South America. However, there were also important differences. Darwin inferred that a small number of different species had come to the islands from the mainland.

Eventually, their offspring became different from the mainland relatives. The finches on the Galapagos Islands were noticeably different from one island to another. The most obvious differences were the varied sizes and shapes of the birds’ beaks. Beak shape is an example of an adaptation, a trait that helps an organism survive and reproduce. Darwin reasoned that plants and animals on the islands faced conditions that were different from those on the mainland. Perhaps, Darwin thought, the species gradually changed over many generations and became better adapted to the new conditions.The gradual change in a species over time is called evolution. Darwin’s ideas are often referred to as the theory of evolution. A scientific theory is a welltested

concept that explains a wide range of observations. In his book The Origin of Species, Darwin explained that evolution

occurs by means of natural selection. Natural selection is the process by

which individuals that are better adapted to their environment are more

likely to survive and reproduce than other members of the same species.

A number of factors affect the process of natural selection:

overproduction, competition, and variations. Any difference between

individuals of the same species is called a variation. Some variations

make certain individuals better adapted to their environment because of

helpful traits they possess. Over a long period of time, natural selection

can lead to evolution. Helpful variations gradually accumulate in a

species, while unfavorable ones disappear. Without variations, all

members of a species would have the same traits. Only traits that are

inherited, or controlled by genes, can be acted upon by natural selection.

Isolation, or complete separation, occurs when some members of a

species become cut off from the rest of the species. A new species can

form when a group of individuals remains separated from the rest of

its species long enough to evolve different traits. Geographic isolation

has occurred in the past because of continental drift.