Darwin's Theory of Natural Selection: Overview

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  • 0:03 Natural Selection
  • 0:41 Darwin's Assumptions
  • 2:35 The Elements of…
  • 3:59 Natural Selection Updated
  • 4:53 How Darwin Changed the World
  • 6:09 Lesson Summary
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Lesson Transcript
Instructor: Robert Turner
In this lesson, you'll be introduced to Charles Darwin and his theory of natural selection. You'll also learn a bit about how his ideas impacted science, popular understandings of human origins, and the debate his theories have sparked.

Natural Selection

The theory of natural selection can be summed up in this way: Conditions of a habitat will naturally select individuals who are best adapted to that specific environment. Those individuals are more likely to grow to adulthood and reproduce. In short, sexual selection will vary according to environmental conditions. Charles Darwin is credited with this theory, though you may be amazed at how much work preceded his revolutionary ideas about the origin of species. Darwin chose the term natural selection to differentiate it from artificial selection, or the intentional breeding of animals to favor particular phenotypic traits.

Darwin's Assumptions

A species is a breeding population of any plant or animal that shares common physical traits. In particular, two members of the same species can mate and produce fertile offspring, while members of different species cannot. For example, blue jays and cardinals are both birds because they have a similar anatomy. However, their markings, nesting habits, and vocalizations (songs) are unique. Further, a female cardinal and a male blue jay may admire each other, but they could not mate to produce offspring. And so we say, blue jays and cardinals represent two different species of birds.

All species evolve from common ancestors over very long periods of time. For example, modern science has revealed that humans and chimpanzees have a common ancestor that separated into two distinct breeding populations, or species, some 8 million years ago. Even more amazing to consider, humans, chimpanzees, and whales all have a common ancestor that lived 60 million years ago.

Darwin didn't have access to the findings of modern science. Nor did he or his colleagues understand that Earth is about 4.5 billion years old. They were also unaware of the genetic science underlying our present-day understandings of evolution. However, Darwin and others were increasingly aware that the fossil record revealed a progression from simple and primitive creatures to ever more complex and diverse plants and animals. So the general idea that species have evolved from primitive ancestors was already making the rounds among the scientifically curious - including Charles Darwin.

One basic step in science is carefully and meticulously gathering data. Darwin's theory of natural selection definitely passes the data test. Over a period of five years, partly on shore and partly aboard the specially outfitted sailing ship, HMS Beagle, Darwin gathered information about plants and animals found around the world. The hypothesis he came up with while there shook the world.

The Elements of Darwin's Theory

We noted that blue jays and cardinals are species of birds. However, since most of us are not ornithologists, we are not likely to note that there is always variation within a species. For example, consider the coat markings of a litter of pure bred Siamese cats. You will notice that while the kittens' markings are similar, they are not identical. That's an example of Darwin's idea of natural variation within a species. For another example, consider the case of identical twins. Identical twins develop from a single zygote (fertilized egg). And even though the twins might be very hard tell apart, there will be slight variations between the two.

Another element of Darwin's theory is heritability. Most species' traits are passed on to their offspring. But, due to variation, some offspring will be better adapted to a species' habitat.

Population Dynamicsmeans that more offspring of a species will be born than are likely to survive long enough to reproduce. In general, population growth exceeds the resources of any habitat - or what we would now call an ecological niche.

The resources of a habitat - edible plant leaves, beetle grubs, or acorns - are finite. At the same time, because of excess offspring and natural variation, Darwin put forth the concept of competition, or the idea that some offspring are more likely to survive than others. And so we can say, as Darwin did, that nature features a struggle for survival.

Natural Selection Updated

In terms of the modern science of genetics, a species exhibits a unique genotype. A genotype is an organism's genetic map, which is its genome. The exterior, observable traits of an organism are called its phenotype. In terms of genetics, each species shares a common genotype that produces a common phenotype. So, in fact, we now know that chimp and human genotypes are 99 percent identical. In short, modern genetic theory can be used to support, modify, and extend Darwin's theory of natural selection. For example, psychologists can apply natural selection principles to the evolution of the brain and nervous system.

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