Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

in the sequencing of all human chromosomes. The Human Genome Project revealed that the human genome contains only 20,000 to 25,000 genes. This estimation, based on the sequence data, is substantially below previous predictions. The sequence data has led to the estimation that only about 1 percent of the human genome actually encodes functional proteins. Once the jigsaw puzzle is completed, the data will undoubtedly help researchers devise new diagnostics and treatments for genetic diseases.

 

In addition to sequencing the human genome, researchers have sequenced the genomes of Drosophila melanogaster (fruit fly), Arabidopsis thaliana (plant), Saccharomyces cerevisiae (budding yeast), and C. elegans (worm). In addition, mouse, rat, and zebrafish genomes have been sequenced. Eukaryotic organisms are also useful to the research community. The genome of Plasmodium (the organism that causes malaria) has also been sequenced. The goals of these sequencing projects are to prepare gene linkage maps and physical maps. A gene linkage map pinpoints the location of genes based on their connection to certain marker gene sequences. A physical map, in comparison, gives the actual number of bases between genes on a chromosome; therefore, it locates the gene of interest more precisely.

 

Ultimately, scientists hope to learn the actual names and sequences of all 3 billion nitrogenous base pairs in the human genome. Automation and computerization are essential tools in the sequencing, and the development of the specific technology is underway.

Chapter 12: Principles of Evolution

  History of the Theory of Evolution

 

Evolution implies a change in one or more characteristics in a population of organisms over a period of time. The concept of evolution is as ancient as Greek writings, where philosophers speculated that all living things are related to one another, although remotely. The Greek philosopher Aristotle perceived a “ladder of life,” where simple organisms gradually change to more elaborate forms. Opponents of this concept were led by several theologians who pointed to the biblical account of creation as set forth in the Book of Genesis. One prelate, James Ussher, calculated that creation had taken place on October 26, 4004 B.C., at 9 a.m.

 

Opponents of the creationist argument were encouraged by geologists who postulated that Earth is far older than 4,004 years. In 1785, James Hutton postulated that Earth was formed by an ancient progression of natural events, including erosion, disruption, and uplift. In the early 1800s, Georges Cuvier suggested that Earth was 6,000 years old, based on his calculations. In 1830, Charles Lyell published evidence pushing the age of Earth back several million years.

 

Amid the controversy over geology and Earth’s age, French zoologist Jean-Baptiste Lamarck suggested a theory for evolution based on the development of new traits in response to a changing environment. For example, the neck of the giraffe stretched as it reached for food. Lamarck’s theory of “use and disuse” gained favor, and his concept of “acquired characteristics” was accepted until the time of Charles Darwin, many years later.

 

Charles Darwin was the son of an English physician. As a naturalist on the ship H.M.S. Beagle, Darwin traveled to remote regions of South America and other destinations. His observations on this trip led him to develop his own theory of evolution. Darwin was particularly interested in the finches and tortoises of the Galapagos Islands. He pondered how different species of animals could have developed on this remote set of islands 200 miles west of Ecuador.

 

Darwin returned to England from South America in 1838 and continued to ponder the theory of evolution. He was influenced by Thomas Malthus’s Essay on the Principle of Population. In his book, Malthus pointed out the human population’s continual struggle for survival and that a population’s natural tendency is to produce more offspring than can possibly survive. Darwin applied this principle to animals and plants, and his theory of evolution began to develop.

 

In 1858, another English naturalist, Alfred Russel Wallace, developed a concept of evolution similar to Darwin’s. Wallace wrote a paper on the subject and corresponded with Darwin. The two men decided to simultaneously present papers on evolution to London’s scientific community in 1858. The next year, 1859, Darwin published his famous book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The book has become known simply as The Origin of Species.

 

Evidence for Evolution

 

In his book, Darwin offered several pieces of evidence that supported evolution. He attempted to convince the scientific community of the validity of his theory.

 

Paleontology

One piece of evidence offered by Darwin is found in the science of paleontology. Paleontology deals with locating, cataloging, and interpreting the life forms that existed in past millennia. It is the study of fossils—the bones, shells, teeth, and other remains of organisms, or evidence of ancient organisms, that have survived over eons of time.

Paleontology supports the theory of evolution because it shows a descent of modern organisms from common ancestors. Paleontology indicates that fewer kinds of organisms existed in past eras, and the organisms were probably less complex. As paleontologists descend deeper and deeper into layers of rock, the variety and complexity of fossils decrease. The fossils from the uppermost rock layers are most like current forms. Fossils from the deeper layers are the ancestors of modern forms.

Comparative anatomy

More evidence for evolution is offered by comparative anatomy (see Figure 12-1). As Darwin pointed out, the forelimbs of such animals as humans, porpoises, bats, and other creatures are strikingly similar, even though the forelimbs are used for different purposes (that is, lifting, swimming, and flying, respectively). Darwin proposed that similar forelimbs have similar origins, and he used this evidence to point to a common ancestor for modern forms. He suggested that various modifications are nothing more than adaptations to the special needs of modern organisms.

 

Figure 12-1   The forelimbs of a human and four animals showing the similarity in construction. This similarity was offered by Darwin as evidence that evolution has occurred.

Darwin also observed that animals have structures they do not use. Often these structures degenerate and become undersized compared with similar organs in other organisms. The useless organs or body parts are called vestigial organs. In humans, they include the appendix, the fused tail vertebrae, the wisdom teeth, and muscles that move the ears and nose. Darwin maintained that vestigial organs may represent structures that have not quite disappeared. Perhaps an environmental change made the organ unnecessary for survival, and the organ gradually became nonfunctional and reduced in size. For example, the appendix in human ancestors may have been an organ for digesting certain foods, and the coccyx at the tip of the vertebral column may be the remnants of a tail possessed by an ancient ancestor.

 

Embryology

 

Darwin noted the striking similarity among embryos of complex animals such as humans, chickens, frogs, reptiles, and fish. He wrote that the uniformity is evidence for evolution. He pointed out that human embryos pass through a number of embryonic stages inherited from their ancestors because they have inherited the developmental mechanisms from a common ancestor. These mechanisms are modified in a way that is unique to an organism’s way of life.

The similarities in comparative embryology are also evident in the early stages of development. For example, fish, bird, rabbit, and human embryos are similar in appearance in the early stages. They all have gill slits, a two-chambered heart, and a tail with muscles to move it. Later on, as the embryos grow and develop, they become less and less similar. The branch of biology that focuses on embryos and their development is called embryology.

 

Comparative biochemistry

 

Although the biochemistry of organisms was not well known in Darwin’s time, modern biochemistry indicates there is a biochemical similarity in all living things. This comparison of biochemical processes with ancient species is called comparative biochemistry. For example, the same mechanisms for trapping and transforming energy and for building proteins from amino acids are nearly identical in almost all living systems. DNA and RNA are the mechanisms for inheritance and gene activity in all living organisms. The structure of the genetic code is almost identical in all living things. This uniformity in biochemical organization underlies the diversity of living things and points to evolutionary relationships.

 

Domestic breeding

 

From observing the domestic breeding experiments of animal and plant scientists, Darwin developed an idea about how evolution takes place. Domestic breeding brings about new forms that differ from ancestral stock. For example, pigeon fanciers have developed many varieties of pigeons through domestic breeding experiments. In effect, evolution has taken place under the guidance of human hands. The development of new agricultural crops by farmers and botanists provides more evidence for directed evolution.

 

Geographic distribution

 

Darwin was particularly interested in the life forms of the Galapagos Islands. He noticed how many of the birds and other animals on the islands were found only there. The finches were particularly puzzling; Darwin found 13 species of finches not found anywhere else in the world, as far as he knew. He concluded that the finches had evolved from a common ancestral group that probably reached the island many generations earlier. In the isolation of the Galapagos Islands, the original finches had probably evolved into the 13 species.

 

The geographic distribution of species in geographic areas can help to explain evolution. For instance, alligators are located only in certain regions of the world, presumably because they have evolved in those regions. The islands of Australia and New Zealand have populations of animals found nowhere else in the world because of their isolated environments.

 

Mechanisms of Evolution

 

Populations evolve, but individual organisms do not. A population is an interbreeding group of individuals of one species in a given geographic area at the same time. A population evolves because the population contains the collection of genes called the gene pool. As changes in the gene pool occur, a population evolves.

 

Mutation

 

Mutation, a driving force of evolution, is a random change in an organism’s genetic makeup, which influences the population’s gene pool. It is a change in the nature of the DNA in one or more chromosomes. Mutations give rise to new alleles; therefore, they are a source of genetic variation in a population.

 

Mutations may be harmful or benign, but they may also be beneficial. For example, a mutation may permit organisms in a population to produce enzymes that will allow them to use certain food materials. Over time, these types of individuals survive, while those that don’t have the mutations are more likely to perish. Therefore, natural selection tends to remove the less-fit individuals, allowing more-fit individuals to survive and form a population.

 

Gene flow

 

Another mechanism of evolution may occur during the migration of individuals from one group or location to another. When the migrating individuals interbreed with the new population, they contribute their genes to the gene pool of the local population. This establishes gene flow in the population.

Gene flow occurs, for example, when wind carries seeds far beyond the bounds of the parent plant population. As another example, animals may be driven off from a herd. This forces them to migrate to a new population, thereby bringing new genes to a gene pool. Gene flow tends to increase the similarity between remaining populations of the same species because it makes gene pools more similar to one another.

 

Genetic drift

 

Another mechanism for evolution is genetic drift, which can occur when a small group of individuals