Secondly, Darwin proposed that “natural selection” is an evolutionary mechanism for descent with modification. His theory was assembled together from pieces of observations other scientists before him had already described. Darwin made the following 3 pertinent observation:
Individuals in a population vary in their traits, many of which seem to be heritable. These traits are passed on from parents to offspring.
A population can produce far more offspring than can survive to produce offspring of their own. With more individuals than the environment is able to support, competition is inevitable.
Species generally suit their environments or are adapted to their environments. For example, a common adaptation among birds that eat tough seeds as their major food source is that they have especially thick, strong beaks.
From those 3 observations Darwin inferred that individuals with inherited traits that are better suited to the local environment are more likely to survive and reproduce than less well-suited individuals. Over generations, a higher and higher proportion of individuals in a population will have the advantageous traits. Evolution occurs as the unequal reproductive success of individuals ultimately leads to adaption to their environment, as long as the environment remains the same.
The mechanism of evolutionary adaptation is in his own words is “natural selection” because the natural environment “selects” for the propagation of certain traits among naturally occurring variant traits in the population.
English naturalist Charles Darwin developed the idea of natural selection after a five-year voyage to study plants, animals, and fossils in South America and on islands in the Pacific. In 1859, he brought the idea of natural selection to the attention of the world in his best-selling book, On the Origin of Species.
Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits—traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection, favorable traits are transmitted through generations.
Natural selection can lead to speciation, where one species gives rise to a new and distinctly different species. It is one of the processes that drives evolution and helps to explain the diversity of life on Earth.
Darwin chose the name natural selection to contrast with “artificial selection,” or selective breeding that is controlled by humans. He pointed to the pastime of pigeon breeding, a popular hobby in his day, as an example of artificial selection. By choosing which pigeons mated with others, hobbyists created distinct pigeon breeds, with fancy feathers or acrobatic flight, that were different from wild pigeons.
Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.
Darwin did not know that genes existed, but he could see that many traits are heritable—passed from parents to offspring.
Mutations are changes in the structure of the molecules that make up genes, called DNA. The mutation of genes is an important source of genetic variation within a population. Mutations can be random [for example, when replicating cells make an error while copying DNA], or happen as a result of exposure to something in the environment, like harmful chemicals or radiation.
Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait. When mutations occur in germ cells [eggs and sperm], they can be passed on to offspring.
If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection. Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species, a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species.
Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.