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Evolution can be described as a change in species over time. Dinosaur fossils are significant evidence of evolution and of past life on Earth.
Evolution of Life The diversity of life on Earth today is the result of evolution. Life began on Earth at least 3.5 to 4 billion years ago, and it has been evolving ever since. At first, all living things on Earth were simple, single-celled organisms. Much later, the first multicellular organisms evolved, and after that, Earth’s biodiversity greatly increased. Figure below shows a timeline of the history of life on Earth. You can also find an interactive timeline of the history of life at the link below. |
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Today, the most accepted theory of life on Earth is evolution, and there is a vast amount of evidence supporting this theory. However, this wasn’t always the case.
As you view Introduction to Evolution and Natural Selection, focus on these concepts:
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Darwin and the Theory of Evolution
The idea of evolution has been around for centuries. In fact, it goes all the way back to the ancient Greek philosopher Aristotle. However, evolution is most often associated with Charles Darwin. Darwin published a book on evolution in 1859 titled On the Origin of Species. In the book, Darwin stated the theory of evolution by natural selection. He also presented a great deal of evidence that evolution occurs.
Evolution is a change in the characteristics of living things over time. As described by Darwin, evolution occurs by a process called natural selection. In natural selection, some members of a species, being better adapted or suited to their environment, produce more offspring than others, so they pass "advantageous traits" to their offspring. Over many generations, this can lead to major changes in the characteristics of the species. Evolution explains how living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth. As living things evolve, they generally become better suited for their environment. This is because they evolve adaptations. An adaptation is a trait that helps an organism survive and reproduce in a given environment.
Despite all the evidence Darwin presented, his theory was not well-received at first. Many people found it hard to accept the idea that humans had evolved from an ape-like ancestor, and they saw evolution as a challenge to their religious beliefs. Darwin had actually expected this type of reaction to his theory and had waited a long time before publishing his book for this reason. It was only when another scientist, named Alfred Russel Wallace, developed essentially the same theory of evolution that Darwin put his book into print.
Although Darwin presented a great deal of evidence for evolution in his book, he was unable to explain how evolution occurs. That’s because he knew nothing about genes. As a result, he didn’t know how characteristics are passed from parents to offspring, let alone how they could change over time.
Evolutionary Theory After Darwin
Since Darwin’s time, scientists have gathered even more evidence to support the theory of evolution. Some of the evidence comes from fossils, and some comes from studies that show how similar living things are to one another. By the 1930s, scientists had also learned about genes. As a result, they could finally explain how characteristics of organisms could pass from one generation to the next and change over time.
Using modern technology, scientists can now directly compare the genes of living species. The more genes different species share in common, the more closely related the species are presumed to be. Consider humans and chimpanzees. They share about 98% of their genes. This means that they shared a common ancestor in the not-too-distant past. This is just one of many pieces of evidence that show we are part of the evolution of life on Earth.
This text was adapted from _CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)
The idea of evolution has been around for centuries. In fact, it goes all the way back to the ancient Greek philosopher Aristotle. However, evolution is most often associated with Charles Darwin. Darwin published a book on evolution in 1859 titled On the Origin of Species. In the book, Darwin stated the theory of evolution by natural selection. He also presented a great deal of evidence that evolution occurs.
Evolution is a change in the characteristics of living things over time. As described by Darwin, evolution occurs by a process called natural selection. In natural selection, some members of a species, being better adapted or suited to their environment, produce more offspring than others, so they pass "advantageous traits" to their offspring. Over many generations, this can lead to major changes in the characteristics of the species. Evolution explains how living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth. As living things evolve, they generally become better suited for their environment. This is because they evolve adaptations. An adaptation is a trait that helps an organism survive and reproduce in a given environment.
Despite all the evidence Darwin presented, his theory was not well-received at first. Many people found it hard to accept the idea that humans had evolved from an ape-like ancestor, and they saw evolution as a challenge to their religious beliefs. Darwin had actually expected this type of reaction to his theory and had waited a long time before publishing his book for this reason. It was only when another scientist, named Alfred Russel Wallace, developed essentially the same theory of evolution that Darwin put his book into print.
Although Darwin presented a great deal of evidence for evolution in his book, he was unable to explain how evolution occurs. That’s because he knew nothing about genes. As a result, he didn’t know how characteristics are passed from parents to offspring, let alone how they could change over time.
Evolutionary Theory After Darwin
Since Darwin’s time, scientists have gathered even more evidence to support the theory of evolution. Some of the evidence comes from fossils, and some comes from studies that show how similar living things are to one another. By the 1930s, scientists had also learned about genes. As a result, they could finally explain how characteristics of organisms could pass from one generation to the next and change over time.
Using modern technology, scientists can now directly compare the genes of living species. The more genes different species share in common, the more closely related the species are presumed to be. Consider humans and chimpanzees. They share about 98% of their genes. This means that they shared a common ancestor in the not-too-distant past. This is just one of many pieces of evidence that show we are part of the evolution of life on Earth.
This text was adapted from _CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)
Other Evolution Mechanisms:
How do a population's genes change?
Remember, without change, there cannot be evolution. Together, the forces that change a population's gene frequencies are the driving mechanisms behind evolution.
We can infer factors that cause allele frequencies to change. These factors are the "forces of evolution." There are four such forces: mutation, gene flow, genetic drift, and natural selection. Natural selection will be discussed in the "Natural Selection" concept.
Mutation
Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act.
Gene Flow
Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. The allele frequencies of both the population they leave and the population they enter may change.
During the Vietnam War in the 1960s and 1970s, many American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Was the gene pool of the American population also affected? Why or why not?
Genetic Drift
Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies in the parents.
This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads or tails. In a small population, you may also, by chance, get different allele frequencies than expected in the next generation. In this way, allele frequencies may drift over time.
There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder effect.
How do a population's genes change?
Remember, without change, there cannot be evolution. Together, the forces that change a population's gene frequencies are the driving mechanisms behind evolution.
We can infer factors that cause allele frequencies to change. These factors are the "forces of evolution." There are four such forces: mutation, gene flow, genetic drift, and natural selection. Natural selection will be discussed in the "Natural Selection" concept.
Mutation
Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act.
Gene Flow
Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. The allele frequencies of both the population they leave and the population they enter may change.
During the Vietnam War in the 1960s and 1970s, many American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Was the gene pool of the American population also affected? Why or why not?
Genetic Drift
Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies in the parents.
This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads or tails. In a small population, you may also, by chance, get different allele frequencies than expected in the next generation. In this way, allele frequencies may drift over time.
There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder effect.
- Bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire. By chance, allele frequencies of the survivors may be different from those of the original population.
- Founder effect occurs when a few individuals start, or found, a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left. An example is described in the Figure below.
This text was adapted from _CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)
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