1.1.3: Intro to Evolution
- Page ID
- 32049
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Learning Objectives
- Describe the role of evolution in the history of life.
- Describe the role of natural selection in evolution.
The history of life on Earth goes back more than three and a half billion years. This past serves as a key to understanding the diversity of life. A unifying theme that can be employed throughout this course to boost biology from an overwhelming sea of facts to a coherent study of life is the theory of evolution. Evolution explains both the diversity and unity of life. Through time, some species evolved adaptations that increased their ability to survive and reproduce in their environment. This gave them an evolutionary advantage to other species. Natural selection, the most dominant evolutionary force, is a main mechanism that is responsible for the diversity of species on Earth today. Yet, it is through evolution that we also understand the unity of all life. All living organisms today can be followed back to a common ancestor. Scientific evidence from this is extensive and includes data from fossils, genetics, cell biology, and molecular biology, just to name a few. Through descent with modification, our common ancestors gave rise to the species we see on Earth today.
Evolution
Evolution can be defined as the change in the genetic make up of a population over time (or, for short, change over time). It is important to note that the change happens from one generation to the next. Thus, an individual cannot evolve because their genes do not change throughout their lifetime. However, a population, a community, or an ecosystem can change over time, as can a species or a group of species. The time it takes for evolution to occur is better described as generation time. This can be very short (minutes for a bacterium), or very long (a century or more for a sea turtle). It is the time it takes for one generation of individuals to reproduce and pass their genes down to the next generation. Charles Darwin's book "On the Origin of Species" published in 1859 described and popularized the theory of evolution and proposed a mechanism for evolution he called natural selection. Independently, Alfred Russell Wallace also introduced the concept of natural selection.
Natural Selection
There are many forces or mechanisms that cause evolution. However, natural selection is a key mechanism as it is the one that produces adaptive traits. For natural selection to occur, several conditions must be met:
- The population must contain genetic variation in the trait. This means the differences in the traits of the individuals within the population.
- The trait must be heritable. This means the individual receives the genetic information for the specific trait from their parent(s).
- There must be variation in fitness as a result of the trait. This means that some traits result in the individuals with higher reproductive success (they have more offspring) than others.
If these three criteria are met, individuals with the most adaptive trait for the current environment will have more offspring, and thus the adaptive traits will accumulate in the environment.