(ECO) Darwin's Theory Lesson
Darwin's Theory Lesson
Charles Darwin embarked on a five-year voyage around the world in 1831 on the British ship, the HMS Beagle. Darwin was a naturalist and his job was to collect and study the many living things he saw on the expedition. Off the coast of the Galapagos Islands in South America, Darwin observed about 14 different species of a bird called finches. The main disparity or differences among the finches were the size and shape of their beaks.
The finches lived on different islands and their beaks are examples of physical adaptations. The finches probably derived from a common ancestor on the mainland. However, separated by water, the type of food and habitat on the various islands may have led to different genetic changes among the finches. Genetic changes take a long time to occur. The Warbler finch has long, narrow pointed beak that is used to eat insects. Medium ground finches had larger, wider, and stronger beaks that were used to crack open seeds. Over time, the beaks of different finch species on the Galapagos Islands have become adapted for obtaining and eating different kinds of foods. Darwin also observed similarities and differences among other species of organisms that lived on the mainland and the Galapagos Islands.
The iguanas on the mainland had small short claws that were very useful for climbing trees. Their diet consisted of eating leaves. The iguanas on the Galapagos Islands ate seaweed. They had larger claws that enabled them to clutch and hang on to slippery rocks near the ocean.
Living things have bodies that are adapted for the places they live and the things they do. Fish have gills so that they can remove oxygen that is dissolved in water. Most plants have green leaves that contain chlorophyll so that they can make food. Jellyfish have stinging cells to capture prey. Birds have hollow spongy bones so that they will be light enough to fly. Arctic animals have layers of fat and thick coats of fur to keep warm in the frigid Arctic climate. There are hundreds of examples of ways that organisms are adapted for a successful lifestyle.
Humans, too, are adapted for the things they do. One of our adaptations is our hand. Humans, as well as monkeys, gorillas, and other primates, have a hand that can grasp objects.
Natural Selection
Darwin's observations gave him ideas on how evolution occurs. Darwin proposed that evolution occurred by a process called natural selection. It states that organisms that are best suited and better adapted to their environment are more likely to survive and reproduce compared to other members of the same species. Darwin published his theory of evolution in a book, The Origin of Species, in 1859. A scientific theory is a well-supported explanation that is acquired through the scientific method, and repeatedly confirmed through observation and experimentation. Darwin's theory of natural selection can be summarized in the following statements.
- Organisms tend to produce more offspring than can survive. For example, a single salmon can lay thousands of eggs at one time. Not all of the offspring will survive to maturity. Only a few will live to be adults.
- Individuals within a species have slight variations. If you have ever visited a pet store, you may have noticed a slight variation in the appearance of the fish in the tanks. A population of fish of the same species may differ slightly in color, length, and fin size.
- Competition for resources occurs among individuals. Organisms within the same species struggle to survive and those best suited for the environment are more likely to survive. For example, individuals can compete for food and shelter. Individuals that can't find food or shelter will most likely not survive. Also think about a herd of gazelle grazing in Africa's savanna. The gazelle that can run fast enough to escape a predator can pass that beneficial trait to its offspring.
- The population gradually changes when individuals who are better suited to survive and reproduce pass their genes to their offspring. After many generations, the desirable or helpful traits will be present in the majority of individuals in the population.
Some of the variation in characteristics makes some individuals better at surviving and, more importantly, better at reproducing than others. Other variations may not improve the fitness of the individual but do help the population as a whole to be more fit. A patch of wildflowers that exhibits a variety of colors may be more likely to catch the attention of more pollinating insects or birds than a patch that is all one color. Certain fish tend to school together, making it more difficult for predators to single out a single fish for capture, so all of the population benefits.
Darwin proposed that natural selection is responsible for how favorable variations are passed from one generation to the next generation. Over a long period of time these favorable variations will accumulate in a species while the undesirable or unfavorable traits will eventually disappear. Natural selection is also known as "the survival of the fittest". It means that the "fittest" organisms have adapted to their environment much better than other organisms and are able to pass their genes or traits to their offspring.
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Fitness leads to natural selection that results in population level changes, not just changes in an individual. If the adaptation only benefits the current generation and does not remain in succeeding generations, it will have no effect on the population as a whole and will not cause "evolution".
Evolution occurs when advantageous genetic variations within a species are expressed at a high frequency because they have been passed on to subsequent
generations. Evolution by natural selection is a gradual process that takes place over many, many generations. Some adaptations will cause speciation right away, while others simply increase the diversity of a population. A certain warbler is identical to another in every way except the "mating song." A mutation in one warbler's song led to speciation because warblers with one song attract only other warblers with the same song. They will not mate with warblers who do not have the same mating song. This one trait separates these warblers into two different species.
Survival of the Fittest
Watch the following presentations to learn more.
How Does Evolution Work? Natural Selection
Evolution occurs through the process, or mechanism, of natural selection. Natural selection is a process that occurs when living things with favorable (inherited) characteristics survive to reproduce, thereby passing on those beneficial characteristics.
Watch the following presentations about natural selection.
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