THE LIVING WORLD
Unit One. The Study of Life
1. The Science of Biology
1.9. Four Theories Unify Biology as a Science
The Cell Theory: Organization of Life
As was stated at the beginning of this chapter, all organisms are composed of cells, life’s basic units. Cells were discovered by Robert Hooke in England in 1665. Hooke was using one of the first microscopes, one that magnified 30 times. Looking through a thin slice of cork, he observed many tiny chambers that reminded him of monks’ cells in a monastery. Not long after that, the Dutch scientist Anton van Leeuwenhoek used microscopes capable of magnifying 300 times, and discovered an amazing world of single-celled life in a drop of pond water like you see in figure 1.10. He called the bacterial and protist cells he saw “wee animalcules.” However, it took almost two centuries before biologists fully understood their significance. In 1839, the German biologists Matthias Schleiden and Theodor Schwann, summarizing a large number of observations by themselves and others, concluded that all living organisms consist of cells. Their conclusion forms the basis of what has come to be known as the cell theory. Later, biologists added the idea that all cells come from other cells. The cell theory, one of the basic ideas in biology, is the foundation for understanding the reproduction and growth of all organisms. The nature of cells and how they function is discussed in detail in chapter 4.
Figure 1.10. Life in a drop of pond water.
All organisms are composed of cells. Some organisms, including these protists, are single-celled, while others, such as plants, animals, and fungi, consist of many cells.
The Gene Theory: Molecular Basis of Inheritance
Even the simplest cell is incredibly complex, more intricate than a computer. The information that specifies what a cell is like—its detailed plan—is encoded in a long cablelike molecule called DNA (deoxyribonucleic acid). Researchers James Watson and Francis Crick discovered in 1953 that each DNA molecule is formed from two long chains of building blocks, called nucleotides, wound around each other. You can see in figure 1.11 that the two chains face each other, like two lines of people holding hands. The chains contain information in the same way this sentence does—as a sequence of letters. There are four different nucleotides in DNA (symbolized as A, T, C, and G in the figure), and the sequence in which they occur encodes the information. Specific sequences of several hundred to many thousand nucleotides make up a gene, a discrete unit of hereditary information. A gene might encode a particular protein, or a different kind of unique molecule called RNA, or a gene might act to regulate other genes. All organisms on earth encode their genes in strands of DNA. This prevalence of DNA led to the development of the gene theory. Illustrated in figure 1.12, the gene theory states that the proteins and RNA molecules encoded by an organism’s genes determine what it will be like. The entire set of DNA instructions that specifies a cell is called its genome. The sequence of the human genome, 3 billion nucleotides long, was decoded in 2001, a triumph of scientific investigation. How genes function is the subject of chapter 12. In chapter 13 we explore how detailed knowledge of genes is revolutionizing biology and having an impact on the lives of all of us.
Figure 1.11. Genes are made of DNA.
Winding around each other like the rails of a spiral staircase, the two strands of a DNA molecule make a double helix. Because of its size and shape, the nucleotide represented by the letter A can only pair with the nucleotide represented by the letter T, and likewise G can only pair with C.
Figure 1.12. The gene theory.
The gene theory states that what an organism is like is determined in large measure by its genes. Here you see how the many kinds of cells in the body of each of us are determined by which genes are used in making each particular kind of cell.
The Theory of Heredity: Unity of Life
The storage of hereditary information in genes composed of DNA is common to all living things. The theory of heredity, first advanced by Gregor Mendel in 1865, states that the genes of an organism are inherited as discrete units. A triumph of experimental science developed long before genes and DNA were understood, Mendel’s theory of heredity is the subject of chapter 10. Soon after Mendel’s theory gave rise to the field of genetics, other biologists proposed what has come to be called the chromosomal theory of inheritance, which in its simplest form states that the genes of Mendel’s theory are physically located on chromosomes, and that it is because chromosomes are parceled out in a regular manner during reproduction that Mendel’s regular patterns of inheritance are seen. In modern terms, the two theories state that genes are a component of a cell’s chromosomes (like the 23 pairs of human chromosomes you see in figure 1.13), and that the regular duplication of these chromosomes during sexual reproduction is responsible for the pattern of inheritance we call Mendelian segregation. Sometimes a character is conserved essentially unchanged in a long line of descent, reflecting a fundamental role in the biology of the organism, one not easily changed once adopted. Other characters might be modified due to changes in DNA.
Figure 1.13. Human chromosomes.
The chromosomal theory of inheritance states that genes are located on chromosomes. This human karyotype (an ordering of chromosomes) shows banding patterns on chromosomes that represent clusters of genes.
The Theory of Evolution: Diversity of Life
The unity of life, which we see in the retention of certain key characteristics among many related life-forms, contrasts with the incredible diversity of living things that have evolved to fill the varied environments of earth. These diverse organisms are sorted by biologists into six kingdoms, as you learned in section 1.1. Organisms placed in the same kingdom have in common some general characteristics. In recent years, biologists have added a classification level above kingdoms, based on fundamental differences in cell structure. The six kingdoms are each now assigned into one of three great groups called domains: Bacteria, Archaea, and Eukarya (figure 1.14).
Figure 1.14. The three domains of life.
Biologists categorize all living things into three overarching groups called domains: Bacteria, Archaea, and Eukarya. Domain Bacteria contains the kingdom Bacteria, and domain Archaea contains the kingdom Archaea. Domain Eukarya is composed of four more kingdoms: Protista, Plantae, Fungi, and Animalia.
The theory of evolution, advanced by Charles Darwin in 1859, attributes the diversity of the living world to natural selection. Those organisms best able to respond to the challenges of living will leave more offspring, he argued, and thus their traits become more common in the population. It is because the world offers diverse opportunities that it contains so many different life-forms.
Today scientists can decipher many of the thousands of genes (the genome) of an organism. One of the great triumphs of science in the century and a half since Darwin is the detailed understanding of how Darwin’s theory of evolution is related to the gene theory—of how changes in life’s diversity can result from changes in individual genes (figure 1.15).
Figure 1.15. The theory of evolution.
Darwin's theory of evolution proposes that many forms of a gene may exist among members of a population, and that those members with a form better suited to their particular habitat will tend to reproduce more successfully, and so their traits become more common in the population, a process Darwin dubbed "natural selection." Here you see how this process is thought to have worked on two pivotal genes that helped generate the diversity of finches on the Galapagos Islands, visited by Darwin in 1831 on his round-the-world voyage on HMS Beagle.
Key Learning Outcome 1.9. The theories uniting biology state that cellular organisms store hereditary information in DNA. Sometimes DNA alterations occur, which when preserved result in evolutionary change. Today's biological diversity is the product of a long evolutionary journey.
Inquiry & Analysis
Does the Presence of One Species Limit the Population Size of Others?
Implicit in Darwin's theory of evolution is the idea that species in nature compete for limiting resources. Does this really happen? Some of the best evidence of competition between species comes from experimental field studies, studies conducted not in the laboratory but out in natural populations. By setting up experiments in which two species occur either alone or together, scientists can determine whether the presence of one species has a negative impact on the size of the population of the other species. The experiment discussed here concerns a variety of seed-eating rodents that occur in North American deserts. In 1988, researchers set up a series of 50-meter x 50-meter enclosures to investigate the effect of kangaroo rats on smaller seed-eating rodents. Kangaroo rats were removed from half of the enclosures, but not from the other enclosures. The walls of all the enclosures had holes that allowed rodents to come and go, but in plots without kangaroo rats the holes were too small to allow the kangaroo rats to enter.
The graph to the right displays data collected over the course of the next three years as researchers monitored the number of the smaller rodents present in the enclosures. To estimate the population sizes, researchers determined how many small rodents could be captured in a fixed interval. Data were collected for each enclosure immediately after the kangaroo rats were removed in 1988, and at three-month intervals thereafter. The graph presents the relative population size—that is, the total number of captures averaged over the number of enclosures (an average is the numerical mean value, calculated by adding a list of values and then dividing this sum by the number of items in the list. For example, if a total of 30 rats were captured from 3 enclosures, the average would be 10 rats). As you can see, the two kinds of enclosures do not contain the same number of small rodents.
1. Applying Concepts
a. Variable. In the graph, what is the dependent variable?
b. Relative Magnitude. Which of the two kinds of enclosures maintains the highest population of small rodents? Does it have kangaroo rats or have they been removed?
2. Interpreting Data
a. What is the average number of small rodents in each of the two plots immediately after kangaroo rats were removed? After one year? After two?
b. At what point is the difference between the two kinds of enclosures the greatest?
3. Making Inferences
a. What precisely is the observed impact of kangaroo rats on the population size of small rodents?
b. Examine the magnitude of the difference between the number of small rodents in the two plots. Is there a trend?
4. Drawing Conclusions
Do these results support the hypothesis that kangaroo rats compete with other small rodents to limit their population sizes?
5. Further Analysis
a. Can you think of any cause other than competition that would explain these results? Suggest an experiment that could potentially eliminate or confirm this alternative.
b. Do the populations of the two kinds of enclosures change in synchrony (that is, grow and shrink at the same times) over the course of a year? If so, why might this happen? How would you test this hypothesis?
Test Your Understanding
1. Biologists categorize all living things based on related characteristics into large groups, called
2. Living things can be distinguished from nonliving things because they have
c. cellular organization.
d. response to a stimulus.
3. Living things are organized. Choose the answer that illustrates this organization and that is arranged from smallest to largest.
a. cell, atom, molecule, tissue, organelle, organ, organ system, organism, population, species, community, ecosystem
b. atom, molecule, organelle, cell, tissue, organ, organ system, organism, population, species, community, ecosystem
c. atom, molecule, organelle, cell, tissue, organ, organ system, organism, community, population, species, ecosystem
d. atom, molecule, cell wall, cell, organ, organelle, organism, species, population, community, ecosystem
4. At each level in the hierarchy of living things, properties occur that were not present at the simpler levels. These properties are referred to as
a. novelistic properties.
b. complex properties.
c. incremental properties.
d. emergent properties.
5. The five general biological themes include
a. evolution, energy flow, competition, structure determines function, and homeostasis.
b. evolution, energy flow, cooperation, structure determines function, and homeostasis.
c. evolution, growth, competition, structure determines function, and homeostasis.
d. evolution, growth, cooperation, structure determines function, and homeostasis.
6. When you are trying to understand something new, you begin by observation, and then put the observations together in a logical fashion to form a general principle. This method is called
a. inductive reasoning.
b. rule enhancement.
c. theory production.
d. deductive reasoning.
7. When trying to figure out explanations for observations, you usually construct a series of possible hypotheses. Then you make predictions of what will happen if each hypothesis is true, and
a. test each hypothesis, using appropriate controls, to determine which hypothesis is true.
b. test each hypothesis, using appropriate controls, to rule out as many as possible.
c. use logic to determine which hypothesis is most likely true.
d. reject those that seem unlikely.
8. Which of the following statements is correct regarding a hypothesis?
a. After sufficient testing, you can conclude that it is true.
b. If it explains the observations, it doesn’t need to be tested.
c. After sufficient testing, you can accept it as probable, being aware that it may be revised or rejected in the future.
d. You never have any degree of certainty that it is true; there are too many variables.
9. Cell theory states that
a. all organisms have cell walls and all cell walls come from other cells.
b. all cellular organisms undergo sexual reproduction.
c. all living organisms use cells for energy, either their own or they ingest cells of other organisms.
d. all living organisms consist of cells, and all cells come from other cells.
10. The gene theory states that all the information that specifies what a cell is and what it does
a. is different for each cell type in the organism.
b. is passed down, unchanged, from parents to offspring.
c. is contained in a long molecule called DNA.
d. All of the above.