natural selection and adaptation

A British naturalist, Charles Darwin proposed the theory of biological evolution by natural selection. Before Darwin, it was believed that species are unrelated and unchanged since the moment of their creation. In the 1850s, he wrote a book called The Origin of Species, in which he put forth two very important ideas of evolution and natural selection.

Learning Objectives

In this lesson, we will learn about

• Darwin and his observations about the finches at Galapagos islands
• Species descend with modifications over many generations
• Survival and reproduction of species within their immediate environment
• Whether natural selection and adaptation mean the same
• Four components of natural selection

Darwin and his observations

During an expedition, Darwin observed interesting patterns in the distribution and features of finches at Galapagos Islands off the coast of Ecuador. He found that there are similar but non-identical species of finches living on the nearby islands in the Galapagos. He observed that each finch species was well-suited for its environment and role, for example, species that ate insects had thin, sharp beaks while the species that ate large seeds have large, tough beaks. He argued that the existence of distinct species on each island is because over many generations and a long period of time the finches would have adapted to local conditions.

Evolution

Darwin proposed that all species come from a common ancestor but over very long periods of time their heritable (genetic) traits change. This process is known as ‘descent with modification’.

Natural Selection

Natural selection is the mechanism by which species evolve to survive and thrive in their immediate environments. As the resources are limited, only those with heritable traits will survive and reproduce, leaving more offspring than their peers. This causes the traits to increase in frequency over generations.

Natural selection can change a species in small ways, causing a population to change color or size over the course of several generations. This is called “microevolution”.

Over longer periods of time, enough changes accumulate to create entirely new species. This is known as “macroevolution”. This is responsible for the evolution of humans from the ancestors of apes.

Another form of natural selection described by Darwin was ‘sexual selection’ which says that natural selection depends on an organism’s success at attracting a mate. Traits such as the antlers of male deer and the colorful plumage of peacocks evolve under sexual selection.

Darwin’s process of natural selection has four components:

1. Variation. Organisms within populations exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or a number of offspring. On the other hand, some traits show little to no variation among individuals – for example, the number of eyes in vertebrates.
2. Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.
3. A high rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality.
4. Differential survival and reproduction. Individuals possessing traits well-suited for the struggle for local resources will contribute more offspring to the next generation.  

Adaptation

An adaptation is a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states. It is a characteristic that has evolved by natural selection. Members of a population become better suited to some features of their environment through the change in a characteristic that affects their survival and reproduction.