The Evolutionary Tree of Life

All organisms on Earth today are the descendants of a single kind of unicellular organism that lived almost 4 billion years ago. But if that were the whole story, only one kind of organism might exist on Earth today. Instead, Earth is populated by many millions of different kinds of organisms that do not interbreed with one another. We call these genetically independent kinds species.

Why are there so many species? As long as individuals within a population mate at random and reproduce, structural and functional changes may evolve within that population, but only one species will exist. However, if a population becomes separated and isolated into two or more groups, individuals within each group will mate only with one another. When this happens, structural and functional differences between the groups may accumulate over time, and the groups may evolve into different species. The splitting of groups of organisms into separate species has resulted in the great diversity of life found on Earth today. The ways in which species form are explained in Chapter 24.

Sometimes humans refer to a species as "primitive" or "advanced." These and similar terms, such as "lower" and "higher," are best avoided in biology because they imply that some organisms function better than others. In fact, all living organisms are successfully adapted to their environments. The shape and strength of a bird's beak, or the form and dispersal mechanisms of a plant's seeds are examples of the rich array of adaptations found among living organisms (Figure 1.7). The abundance and success of prokaryotes—all of which are relatively simple organisms—read-ily demonstrates that they are highly functional. In this book, we use the terms simple and complex to refer to the level of complexity of a particular organism. We use the terms ancestral and derived to distinguish characteristics that appeared earlier from those that appeared later in evolution.

As many as 30 million species of organisms may live on Earth today. Many times that number lived in the past, but are now ex-

1.6 From Molecules to the Biosphere: The Hierarchy of Life

Bird Adaptations Environment

1.7 Adaptations to the Environment (a) Bird beaks are adapted to specific types of food items. (b) Plants cannot move, but their seeds have adaptations that allow them to travel varying distances from the parent plant.

1.7 Adaptations to the Environment (a) Bird beaks are adapted to specific types of food items. (b) Plants cannot move, but their seeds have adaptations that allow them to travel varying distances from the parent plant.

tinct. This diversity is the result of millions of splits in populations, known as speciation events. The unfolding of these events can be expressed as an evolutionary "tree" showing the order in which populations split and eventually evolved into new species (see Figure 1.8). An evolutionary tree, with its "trunk" and its increasingly finer "branches," traces the descendants coming from ancestors that lived at different times in the past. That is, a tree shows the evolutionary relationships among species and groups of species. The organisms on any one branch share a common ancestor at the base of that branch. The most closely related groups are together on the same branch. More distantly related organisms are on

different branches. In this book, we adopt the convention that time flows from left to right, so the tree in Figure 1.8 (and other trees in this book) lies on its side, with its root—the ancestor of all life—at the left.

The U.S. National Science Foundation is sponsoring a major initiative, called Assembling the Tree of Life (ATOL). Its goal is to determine the evolutionary relationships among all species on Earth. Achieving this goal is possible today because, for the first time, biologists have the technology to assemble the complete tree of life, from microbes to mammals. Data for ATOL come from a variety of sources. Fossils—the preserved remains of organisms that lived in the past—tell us where and when ancestral organisms lived and what they may have looked like. With modern molecular genetic techniques such as DNA sequencing, we can determine how many genes different species share, and information tech

Common ancestor of ^^m all organisms

Archaea and Eukarya share a common ancestor not shared by bacteria.

There are multiple protist lineages. Plants, fungi, and animals are descended from different protist ancestors.

Ancient

Common ancestor of ^^m all organisms

Archaea and Eukarya share a common ancestor not shared by bacteria.

There are multiple protist lineages. Plants, fungi, and animals are descended from different protist ancestors.

Tree Life Bacteria

Protists

Time

1.8 A Provisional Tree of Life The classification system used in this book divides Earth's organisms into three domains; Bacteria, Archaea, and Eukarya. Protists are descendants of multiple ancestors.

nology enables us to synthesize masses of genetic data. The ATOL initiative, one of the grandest projects of modern biology, is projected to take at least two decades and to involve hundreds of scientists working in a diverse array of fields. The reason it will take so long to complete is that most of Earth' species have not yet been described.

The Tree of Life will be an information framework for biology in much the same way that the periodic table of elements is an information framework for chemistry and physics. Evolution has conducted several billion years of free research and development. Every living thing carries a genetic "package" that has been tested by natural selection. Scientists can now unwrap and study these packages, learning much about the processes that produced them.

Although much remains to be accomplished, biologists know enough to have assembled a provisional tree of life, the broad outlines of which are shown in Figure 1.8. The branching patterns of this tree are based on a rich array of evidence, but no fossils are available to help us determine the earliest divisions in the lineages of life because those simple organisms had no parts that could be preserved as fossils. Therefore, molecular evidence has been used to separate all living organisms into three major domains. Organisms belonging to a particular domain have been evolving separately from organisms in the other domains for more than a billion years.

Organisms in the domains Archaea and Bacteria are prokaryotes. Archaea and Bacteria differ so fundamentally from one another in their metabolic processes that they are be

Protists

-*- Present lieved to have separated into distinct evolutionary lineages very early during the evolution of life. These two domains are described in Chapter 27.

Members of the other domain— Eukarya—have eukaryotic cells. The Eukarya are divided into four groups: Protista, Plantae, Fungi, and Animalia. The Protista (protists), the subject of Chapter 28, contains mostly single-celled organisms. The other three groups, referred to as kingdoms, are believed to have arisen from ancestral protists. All of their members are mul-ticellular.

Some bacteria, some protists, and most members of the kingdom Plantae (plants) convert light energy to chemical energy by photosynthesis. These organisms are called autotrophs ("self-feeders"). The biological molecules they produce are the primary food for nearly all other living organisms. The kingdom Plantae is covered in Chapters 29 and 30.

The kingdom Fungi, the subject of Chapter 31, includes molds, mushrooms, yeasts, and other similar organisms, all of which are heterotrophs ("other-feeders")—that is, they require a source of energy-rich molecules synthesized by other organisms. Fungi break down food molecules in their environment and then absorb the breakdown products into their cells. They are important as decomposers of the dead materials of other organisms.

Members of the kingdom Animalia (animals) are het-erotrophs that ingest their food source, digest the food outside their cells, and then absorb the breakdown products. Animals eat other forms of life to obtain their raw materials and energy. This kingdom is covered in Chapters 32, 33, and 34.

We will discuss the principal levels used in today's classification scheme for living organisms in Chapter 25. But to understand some of the terms we will use in the intervening chapters, you need to know that each species of organism is identified by two Latinized names (a binomial). The first name identifies the genus—a group of species that share a recent common ancestor—of which the species is a member. The second name is the species name. To avoid confusion, no combination of two names is assigned to more than one species. For example, the scientific name of the human species is Homo sapiens: Homo is our genus and sapiens is our species. The Pacific tree frogs Pieter Johnson studied are called, in scientific nomenclature, Hyla regilla.

Biology is the study of all of Earth's organisms, both those living today and those that lived in the past, so even extinct species are given binomials. These unique and exact names

Time

1.8 A Provisional Tree of Life The classification system used in this book divides Earth's organisms into three domains; Bacteria, Archaea, and Eukarya. Protists are descendants of multiple ancestors.

illuminate the tremendous diversity of life, and are important tools for biologists because, as in all the sciences, precise and unambiguous communication of research information is critical.

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  • Dahlak
    Where does protists belong on the phylogenetic tree?
    9 years ago
  • gundahar
    What is a plasma tree?
    7 years ago

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