In 1831, Charles Darwin left England on board the HMS Beagle. Darwin's important observations included the diversity of living things, the remains of ancient organisms, and the characteristics of organisms on the Galápagos Islands. Darwin saw many different species. A species is a group of similar organisms that can mate with each other and produce fertile offspring. Darwin saw the fossil bones of animals that had died long ago. A fossil is the preserved remains or traces of an organism that lived in the past.
In 1835, the Beagle reached the Galápagos Islands in the Paciﬁc Ocean. Darwin was surprised that many of the plants and animals on the Galápagos 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 Galápagos 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 or animals that arrived on the Galápagos Islands faced conditions that were different from those on the mainland. Perhaps, Darwin hypothesized, 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 well-tested concept that explains a wide range of observations.
In his book The Origin of Species, Darwin proposed 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. Darwin proposed that, over a long period of time, natural selection can lead to change. Helpful variations may gradually accumulate in a species, while unfavorable ones may 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.
Modern-day organisms can provide clues about evolution. Fossils, patterns of early development, and similar body structures all provide evidence that organisms have changed over time. By comparing organisms, scientists can infer how closely related the organisms are in an evolutionary sense. Scientists compare body structures, development before birth, and DNA sequences to determine the evolutionary relationships among organisms.
Scientists make inferences about evolutionary relationships by comparing the early development of organisms. An adult opossum, chicken, salamander, and ﬁsh look quite different; however, during early development these four organisms are similar. These similarities suggest that these vertebrate species are related and share a common ancestor.
An organism's body structure is its basic body plan, such as how its bones are arranged. Fishes, amphibians, reptiles, birds, and mammals, for example, all have a similar body structure-an internal skeleton with a backbone. This is why scientists classify all ﬁve groups of animals together as vertebrates. Presumably, these groups all inherited these similarities in structure from an early vertebrate ancestor that they shared. Similar structures that related species have inherited from a common ancestor are called homologous structures. Sometimes scientists ﬁnd fossil evidence that supports the evidence provided by homologous structures.
Scientists infer that species with similar body structures and development patterns inherited many of the same genes from a common ancestor. Recall that genes are made of DNA. By comparing the sequences in the DNA of different species, scientists can infer how closely related the species are. The more similar the sequences, the more closely related the species are. Recall also that the DNA bases along a gene specify what type of protein will be produced. Therefore, scientists can also compare the order of amino acids in a protein to see how closely related two species are.
Scientists have combined the evidence from DNA, protein structure, fossils, early development, and body structure to determine the evolutionary relationships among species. In most cases, DNA and protein sequences have conﬁrmed conclusions based on earlier evidence. Scientists use such combined evidence to construct branching trees. A branching tree is a diagram that shows how scientists think different groups of organisms are related.
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.
Most fossils form when organisms that die become buried in sediments. Sediments are particles of soil and rock. Layers of sediments cover the dead organism. Over millions of years, the layers harden to become sedimentary rock. Some remains that become buried in sediments are actually changed to rock. These fossils are called petriﬁed fossils. Sometimes shells or other hard parts buried by sediments are gradually dissolved. A hollow space in sediment in the shape of an organism or part of an organism is called a mold. Sometimes a mold becomes ﬁlled in with hardened minerals, forming a cast. Organisms can also be preserved in ice.
Scientists can determine a fossil's age in two ways: relative dating and radioactive dating. Scientists use relative dating to determine which of two fossils is older. In a sequence of rock layers, the top layers are usually younger than the lower layers. Therefore, fossils found in top layers are younger than fossils found in bottom layers. Another technique, called radioactive dating, allows scientists to determine the actual age of fossils. Rocks near fossils contain radioactive elements, unstable elements that decay, or break down, into different elements. The half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay. Scientists can compare the amount of a radioactive element in a sample to the amount of the element into which it breaks down to calculate the age of the rock.
The millions of fossils that scientists have collected are called the fossil record. Despite gaps in the fossil record, it has given scientists a lot of important information about past life on Earth. Almost all of the species preserved as fossils are now extinct. A species is extinct if no members of that species are still alive. Scientists have calculated the ages of many different fossils and rocks. From this information, they have created a "calendar” of Earth's history that spans more than 4.6 billion years. This calendar of Earth's history is sometimes called the Geologic Time Scale.
Two unanswered questions about evolution involve the causes of mass extinctions and the rate at which evolution occurs. A mass extinction occurs when many species become extinct at the same time. Scientists are not sure what causes mass extinctions. There are two theories about the rate of evolution. According to one theory, called gradualism, evolution occurs slowly but steadily. Tiny changes in a species gradually add up to major changes over very long periods of time. According to another theory, called punctuated equilibria, species evolve during short periods of rapid change. Species evolve quickly when groups become isolated and adapt to new environments. Most scientists think that evolution can occur gradually at some times and fairly rapidly at others.
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