Chapter 14 – Evolution: How Change Occurs

I. Developing a Theory of Evolution

    A. An Early Explanation for Evolutionary Change

        1. Jean Baptiste de Lamarck was among the first scientists to recognize that living things changed over time.

        2. Lamarck thought that organisms change because they have an inborn urge to better themselves and become more fit for their environments.

        3. Lamarck also believed that change occurred because organisms could alter their shape by using their bodies in new ways.

        4.  Included in Lamarck’s reasoning was the belief, shared by many biologists of that time, that acquired characteristics were inherited.

        5. Later discoveries showed that Lamarck’s explanation of evolution was incorrect.

    B. Ideas That Shaped Darwin’s Theory of Evolution

        1. Charles Lyrell demonstrated that the Earth was very old and that it had changed over time.

        2. In order to explain evolution, to even recognize that evolution had occurred, it was essential for Darwin to realize that the Earth was very old.

        3. In artificial selection, the intervention of humans ensures that only individuals with the more desirable traits produce offspring.

        4. The economist Thomas Malthus observed that babies were being born at a faster rate than people were dying.

        5. Malthus reasoned that sooner or later there wouldn’t be enough living space and food, and that the only conditions that would prevent the        endless growth of human populations were famine, disease, and war.

II. Evolution by Natural Selection

    A. Peppered Moths: Natural Selection in Action

        1. England’s peppered moth provides an example of natural selection in action.

        2. The peppered moth spends much of the daytime resting on the bark of oak trees, and in the beginning of the nineteenth century, the trunks of most oak trees in England were light brown speckled with green, and most of the peppered moths of that time were mottled light brown too, and a few dark colored moths existed.

        3. As the industrial revolution began in Europe, soot stained London’s tree trunks mostly brown, and about the same time, biologists noticed that more and more moths with dark coloration were appearing.

        4. Kettlewell found that in unpolluted areas, more light colored moths had survived, and in soot covered areas, more dark colored moths had survived.

III. Genetics and Evolutionary Theory

    A. Genes: Units of Variation

        1. Genes, the carriers of inheritable characteristics, are also the source of the random variation upon which natural selection operates.

        2. Mutations cause some variation, and much additional variations arises during meiosis as the parents’ chromosomes are copied and dealt out to the gametes.

        3. It is important to remember that genetic variation does not occur because an animal needs or wants to evolve.

    B. Raw Material for Natural Selection

        1. Natural selection can operate only on the phenotypic variation among individuals.

        2. The phenotypic variation is produced by a combination of genetic instructions and environmental influences, such as nutrition and exercise.

        3. In nature, organisms show as many variations as humans, although most humans are not aware of this.

        4. Subtle variations provide the raw material for natural selection.

    C. Evolution as Genetic Change

        1. A population is a collection of individuals of the same species in a given area whose members can breed with one another.

        2. A gene pool is a group in which offspring share a common group of genes.

        3. Each gene pool contains a number of alleles, or forms of a certain gene at a given point on a chromosome, for each inheritable trait, including alleles for recessive traits.

        4. The number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur is called the relative frequency.

        5. Evolution is any change in the relative frequencies of alleles in the gene pool of a population.

    D. Genes, Fitness, and Adaptation

        1. Each time an organism reproduces, it passes copies of its genes to its offspring, and thus we can define evolutionary fitness as the success an organism has in passing on its genes to the next generation.

        2. Adaptation can be defined as any genetically controlled characteristic of an organism that increases its fitness.

        3. Fitness cannot be considered an evolutionary adaptation and cannot contribute to evolutionary fitness.

    E. A Genetic Definition for Species

        1. In the past, biologists defined a species as a group of organisms that looked alike.

        2. Species were defined according to precise physical descriptions and differences among individuals were seen as imperfections or mistakes.

        3. We now define species as a group of similar looking organisms that breed with one another and produce fertile offspring in the natural environment.

        4. Because members of a species can breed with one another, hey share a common gene pool.

IV. The Development of New Species

    A. The Niche: How to Make a Living

        1. The combination of an organism’s “profession” and the place in which it lives is called its niche.

        2. No two species can occupy the same niche in the same location for a long period of time.

        3. The more efficient species will survive, reproduce, and drive the less efficient species to extinction.

        4. If two species occupy different niches, however, they will not compete with each other as much.

        5. In the evolutionary struggle for existence, any species that occupies an unoccupied niche will be better able to survive.

    B. The Process of Speciation

        1. Scientists have learned that new species usually form only when populations are isolated, or separated.

        2. This separation of populations so that they do not interbreed to produce fertile offspring is called reproductive isolation.

        3. Once reproductive isolation occurs, natural selection usually increases the differences between the separated populations.

        4. If the populations remain separated for a long time, their gene pools eventually become so different that their reproductive isolation becomes permanent.

        5. When this occurs, the groups of organisms are not longer separate populations, but separate species.

    C. Darwin’s Finches: An Example of Speciation

        1. Darwin’s finches are descendants of a few ancestral finches that found their way to the Galapagos Islands from the South American mainland.

        2. Then, some birds from Island A crossed to another island called Island B.

        3. Over time, the populations on each island became adapted to the needs of their environments.

        4. The two different animals are no longer able to breed with one another.

        5. The different species now had to share the same island, although they were unable to reproduce with each other, creating separate species.

    D. Speciation and Adaptive Radiation

        1. Adaptive radiation is the process in which one species gives rise to many species that appear different externally but are similar internally.

        2. This process is also known as divergent evolution.

        3. During a period of adaptive radiation, organisms evolve a variety of characteristics that enable them to survive in different niches.

        4. Convergent evolution is a phenomenon in which adaptive radiations among different organisms produce species that are similar in appearance and behavior; opposite of divergent evolution.

        5. Convergent evolution has produced many of the analogous structures in organisms today.

V. Evolutionary Theory Evolves

    A. Genetic Drift

        1. With the aid of theories and genetic experiments, biologists have realized that gene pools can change, in other words, evolution can occur, in the absence of natural selection.

        2. When an allele becomes common in a population by chance, it is known as a genetic drift.

        3. Genetic drift occurs most efficiently in small populations because chance events, such as a volcanic eruption, are less likely to affect all members of a very large population.

    B. Unchanging Gene Pools

        1. Modern evolutionary biologists recognize that although natural selection and genetic drift are both powerful forces of change, they do not cause genetic alterations in all species all the time under all conditions.

        2. Because sexual reproduction by itself does not change the frequency of alleles in a population, it is possible for the gene pool of a species to remain the same for a long time.

        3. Every now and then there arises a species, particularly well adapted to an environment, that does not change over time.

    C. Gradual and Rapid Evolutionary Change

        1. The theory that evolutionary change occurs slowly and gradually is known as gradualism.

        2. When plants or animals do not change much during a period of time, it is known as equilibrium.

        3. Many species have vanished in a phenomenon known as a mass extinction, which were caused by changes in global climates that altered many environments.

        4. Punctuated equilibria is used to describe the pattern of long stable periods interrupted by brief periods of change.

    D. The Significance of Evolutionary Theory

        1. Evolutionary theory is, in the minds of many biologists, the foundation on which all biological science is built.

        2. Only because the physiological properties of all multicellular organisms are so similar can we study other animals to learn how our own bodies operate.

        3. “There are no living sciences, human attitudes, or institutional powers that remain unaffected by the ideas… released by Darwin’s work.”