The Theory of Evolution by Natural Selection

Charles Darwin challenged traditional views of a young Earth inhabited my unchanging species. In 1859, Darwin published convincing evidence that the many species of organisms presently inhabiting Earth are descendants of ancestral species, that is, species evolve. Most importantly, Darwin proposed a reasonable mechanism explaining how species evolve by natural selection.  

Darwin’s Theory of Evolution by Natural Selection

1. Overproduction & Struggle for Existence Individuals produce more offspring than can survive, that is, than the environment can support. Individuals in the population must compete to obtain food, living space, etc. Darwin described this as the struggle for existence.

2. Variation and Adaptation Individuals in a population have natural variations among their heritable traits. Some variations make an organism better suited to life in its environment. Any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment is called an adaptation. Some variations may include: Some adaptations may include: Color of fur speed size of feet

Adaptions may also involve behaviors such as staying still to avoid predation, looking larger and standing your ground, etc.

3. Survival of the Fittest Individuals with the best adaptations are better suited to their environment, as a result they can survive and reproduce. These individuals are said to have greater fitness. Fitness describes how well an organism can survive and reproduce in its environment. The individuals that have the greatest fitness survive and reproduce more offspring. Individuals with low fitness die before reproducing or reproduce few offspring.

4. Natural Selection Over generations, the individuals with the adaptation tend to accumulate in the population. Natural selection will result in species with adaptations that are well suited for survival and reproduction in an environment. Natural selection is the process by which organisms with variations most suited to their environment survive and leave more offspring. The environment (nature) “selects” which organisms will survive.  

Variations in Populations The individuals in a population are the same species, but they are not identical. The individuals in a population vary in their characteristics or traits. A variation is the difference in a physical trait of an individual from those of other individuals in the population. Variation? ___________________ Color and pattern

Adaptation A variation that allows an individual to better survive in its environment.

Example of Natural selection in jaguars. 11,000 years ago, jaguars faced extinction. There were fewer of their usual prey, mammals. There was an abundance of turtles. But eating turtles required large jaws.

Most jaguars in the population had small jaws. But there was variation in the population in the size of the jaw. Some jaguars in the population had larger jaws. Variation? Size of jaw

Those jaguars with larger jaws were able to eat the turtles. Favorable variation? The larger jaw is an Larger jaw Adaptation

The jaguars with larger jaws survived, and produced more offspring. Their offspring inherited the larger jaws and they survived and produced more offspring.

In time, the number of individuals that carry the favorable adaptation will increase in the population. More and more jaguars in the population Will have larger jaws. Simultaneously, individuals that do not have the adaptation will die, or produce fewer offspring. Eventually, the population will consists only of individuals with large jaws. The nature of the population has changed. Evolution has occurred.

Larger jaws became a more common trait in the population. Over many generations, jaguars evolved larger jaws.

Darwin further suggested that organisms differ from place to place because their habitats present different challenges to survival and reproduction. Each species has accumulated adaptations in response to its particular environment resulting in evolution of the species.

Common Descent Every organism alive today is descended from parents who survived and reproduced. Those parents descended from their parents, and so forth back through time.

Darwin proposed that living species are descended with modification, from common ancestors. Darwin also proposed that, over many generations, adaptations could cause successful species to evolve into new species. All species – living and extinct – are descended from ancient common ancestors. A single “tree of life” links all living things.

How is evolution defined in genetic terms? Variation is the raw material for natural selection. Where does this variation come from? Individuals in a population may have many different alleles. These alleles give rise to many different phenotypes.

What are the sources of genetic variation? Genetic Recombination in Sexual Reproduction: New individuals are a random combination of the alleles of two parents.

What are the sources of genetic variation? Genetic Recombination in Sexual Reproduction: Mutations: A mutation is a change in the DNA of a cell. Mutations change genes sometimes resulting in new phenotypes. Ex: Blue eyes