Biology is frequently described as the scientific study of living things, but it is often taught as a collection of facts about organisms. Science is a dynamic process, constantly evolving as new technologies and new information become available. In this chapter, and throughout the book, we will emphasize the process of science that leads to our current understanding of the structure, function, interrelationships, and evolution of living things.
Biology, which began as a descriptive science, is based on testable hypotheses and predictions, as are the other sciences such as physics and chemistry.
The Nature of Science
Science is a process, a way of knowing about the natural world. Natural phenomena have natural causes that can be discovered by observation and explained by forming and testing hypotheses. Scientific understanding of the natural world is dynamic. It continually changes, becoming richer, as we gain new insights or invent new tools to study natural processes.
Two Approaches to Scientific Study
Inductive reasoning (induction) is a discovery process that uses specific observations to construct general principles. If every living thing we examine microscopically is composed of cells, we can induce the general principle that all living things are composed of cells even though we haven’t looked at all living things. If we accept the general principle that living things are composed of cells and observe a fossil with cellular structure, we can deduce (deduction) that the fossil was once alive.
Hypothesis Testing
A hypothesis is a tentative explanation for an observation or a prediction of what will occur. Both inductive and deductive reasoning can give rise to testable hypotheses. Scientific experiments are carefully designed to test specific hypotheses. Hypotheses are accepted if they are supported by testing, but rejected if the data are inconsistent with the predicted results.
Scientific Theories
In science an explanation that is broad in scope and well supported by a large body of evidence is called a theory. Scientific theories generate new hypotheses that drive discovery, and there is always the possibility that new discoveries will force us to modify or even reject our current theories. Note that this definition is virtually the opposite of the common usage of the term theory.
Life, which seems like such an obvious concept, is impossible to define precisely because of the great variety of living things.
The Properties of Life
Life defies simple definition, but there are a number of properties that, if taken together, describe the characteristics of living things. Among these are cellular organization, ability to grow (increase in size; increase in cell number; and/or develop specialized cells, tissues, and organs), reproductive capability, responsiveness to stimuli, utilization of chemical energy, homeostasis (ability to self-maintain the living condition), and ability to evolve.
Hierarchical Organization
Cells are composed of subcellular units, various organelles within a protoplasmic matrix, that themselves are not alive. These, in turn, are constructed from atoms and molecules. Many organisms are unicellular, but multicellular organisms are composed of specialized cells organized into tissues, organs, and organ systems. Different organisms interact with each other at different levels in their environment: with other organisms of the same species in populations; with other species in communities; and with the physical environment in ecosystems.
Emergent Properties
At each higher level in the hierarchy of life, novel properties emerge that could not be predicted based on the properties of the lower levels. These emergent properties are the result of the arrangement of parts and interactions between them as complexity increases. For example, chlorophyll molecules can absorb light energy but it is simply released as fluorescent light of lower energy. If the same molecules are embedded in chloroplast membranes, the same light energy can be converted into the chemical energy of ATP and NADPH that is used to synthesize sugar from CO2.
A basic theory of biology is that structures called cells are the fundamental unit of living things.
Living Things Are Composed of Cells
Cells consist of one or more membrane-bound compartments that are the smallest units to exhibit the properties of life. Organisms are composed of one or more cells.
Prokaryotic
Cells of the smallest organisms (<10 μm diameter) consist of a single membrane-bound compartment without a nucleus or any other special membrane-bound organelles. These prokaryotic (that is, “before a nucleus”) organisms are typically unicellular.
Eukaryotic
Larger cells (>10 μm diameter) contain multiple compartments called organelles. Each organelle has a specific function and is bound by one or two membranes. The presence of a membrane-bound nucleus defines these cells as eukaryotic (that is, “true nucleus”). Eukaryotic organisms may be unicellular or multicellular.
Cells Come from Preexisting Cells
Under the conditions on earth today, all cells arise from either the asexual division of existing cells or the sexual fusion of specialized reproductive cells, called gametes. In asexual cell division, a special process of nuclear division, called mitosis, ensures that each daughter cell receives exactly the same genetic information as the mother cell. In sexual reproduction, a related process, called meiosis, ensures that the offspring cell will receive half of its genetic information from each parent.
The condition that cells come from preexisting cells implies a hereditary relationship in which traits of one generation are passed on to the next.
Many Traits Are Inherited
The structural and functional characteristics of organisms (phenotypes) are frequently inherited from their parents. The unit of inheritance for any particular trait is a gene.
Chromosomal Basis of Inheritance
Genes are located on chromosomes in the nucleus of eukaryotic cells or on the bacterial chromosome of prokaryotic cells. There are hundreds of different genes on each chromosome.
Molecular Basis of Inheritance
Genes are coded in DNA molecules that are part of the structure of chromosomes. Each chromosome contains duplicate copies of the DNA on identical chromatids held together by a centromere so that the information can be accurately passed from one generation of cells to the next every time a cell divides. The genetic information coded by the DNA is translated into polypeptides (enzymes and other proteins) in the cytoplasm of the cell, thus giving rise to the phenotypic characteristics of that cell.
Long before Mendel’s discovery of a mechanism to explain inheritance, plant and animal breeders recognized that certain traits could be passed unerringly from one generation to the next, resulting in purebreds and true-breeding lines. Yet other traits could be changed over time, allowing breeders to develop new breeds of animals and new plant varieties.
The Unity of Life
Cell Theory and Genetic Theory provide powerful evidence that organisms share a succession of common ancestors. The human gene for insulin can be inserted into a bacterium and the bacterium will begin to produce human insulin. Clearly, the machinery of life in the prokaryotic bacterium follows the same commands as the eukaryotic cells in the human pancreas.
The Diversity of Life
While all organisms share a kinship through their common ancestry, species diverged as modifications evolved to better adapt species to their environmental conditions. Darwin called the evolutionary history implied by the unity and diversity of life “descent with modification.” Today we call it the theory of biological evolution. Evolution is the basic underlying foundation for all of biology.
Natural Selection
Darwin was not the first scientist to propose a theory of evolution, but he was the first to provide a massive amount of evidence from a wide variety of scientific disciplines. More importantly, he proposed a mechanism to explain how evolution occurs——the theory of natural selection. Natural selection remains the most broadly supported theory to explain the unity and diversity of life. It is based on three observations and two inferences.
Individual Variation
Choose any phenotypic character in a population of individuals, and the expression of that trait will vary ...
