Chapter 1. Zoology: An Evolutionary And Ecological Perspective

1
Zoology:
An Evolutionary and
Ecological Perspective

Chapter Outline

1. 1.1 Zoology: An Evolutionary Perspective
   • Evolutionary Processes
   • Animal Classification and Evolutionary Relationships
2. 1.2 Zoology: An Ecological Perspective
   • World Resources and Endangered Animals

Zoology (Gr. zoon, animal + logos, to study) is the study of animals. It is one of the broadest fields in all of science because of the immense variety of animals and the complexity of the processes occurring within animals. There are, for example, more than 20,000 described species of bony fishes and more than 400,000 described (and many more undescribed) species of beetles! It is no wonder that zoologists usually specialize in one or more of the subdisciplines of zoology. They may study particular functional, structural, or ecological aspects of one or more animal groups (table 1.1), or they may choose to specialize in a particular group of animals (table 1.2).

Ichthyology, for example, is the study of fishes, and ichthyologists work to understand the structure, function, ecology, and evolution of fishes. These studies have uncovered an amazing diversity of fishes. One large group, the cichlids, is found in Africa (more than 1,000 species), Central and South America (300 species), India (3 species), and North America (1 species). Members of this group have an enormous variety of color patterns (figure 1.1), habitats, and body forms. Ichthyologists have described a wide variety of feeding habits in cichlids. These fish include algae scrapers, like Eretmodus, that nip algae with chisel-like teeth; insect pickers, like Tanganicodus;and scale eaters, like Perissodus. All cichlids have two pairs of jaws. The mouth jaws are used for scraping or nipping food, and the throat jaws are used for crushing or macerating food before it is swallowed.

(a)

(b)

FIGURE 1.1

Cichlids.

 Cichlids of Africa exist in an amazing variety of color patterns, habitats, and body forms. (a) This dogtooth cichlid (Cynotilapia afra) is native to Lake Malawi in Africa. The female of the species broods developing eggs in her mouth to protect them from predators. (b) The fontosa (Cyphontilapia fontosa) is native to Lake Tanganyika in Africa.

Many cichlids mouth brood their young. A female takes eggs into her mouth after the eggs are spawned. She then inhales sperm released by the male, and fertilization and development take place within the female's mouth! Even after the eggs hatch, young are taken back into the mouth of the female if danger threatens (figure 1.2). Hundreds of variations in color pattern, body form, and behavior in this family of fishes illustrate the remarkable diversity present in one relatively small branch of the animal kingdom. Zoologists are working around the world to understand and preserve this enormous diversity.

FIGURE 1.2

A Scale-Eating Cichlid.

Scale-eaters (Perissodus microlepis) attack from behind as they feed on scales of prey fish. Two body forms are maintained in the population. In one form, the mouth is asymmetrically curved to the right and attacks the prey's left side. The second form has the mouth curved to the left and attacks the prey's right side. Both right- and left-jawed forms are maintained in the population so that prey will not eventually become wary of being attacked from one side. Perissodus microlepis is endemic (found only in) to Lake Tanganyika. A male with its brood of young is shown here.

1.1 ZOOLOGY: AN EVOLUTIONARYPERSPECTIVE

LEARNING OUTCOMES

1. Formulate a hypothesis regarding the evolutionary origin of contrasting color patterns in two closely related species of fish.
2. Explain how our taxonomic system is hierarchical.

Animals share a common evolutionary past and evolutionary forces that influenced their history. Evolutionary processes are remarkable for their relative simplicity, yet they have had awesome effects on life-forms. These processes have resulted in an estimated 4 to 100 million species of animals living today. (Over 1 million animal species have been described.) Many more, about 90%, existed in the past and have become extinct. Zoologists must understand evolutionary processes if they are to understand what an animal is and how it originated.

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TABLE 1.1
EXAMPLES OF SPECIALIZATIONS IN ZOOLOGY

SUBDISCIPLINE	DESCRIPTION
Anatomy	Study of the structure of entire organisms and their parts
Cytology	Study of the structure and function of cells
Ecology	Study of the interaction of organisms with their environment
Embryology	Study of the development of an animal from the fertilized egg to birth or hatching
Genetics	Study of the mechanisms of transmission of traits from parents to offspring
Histology	Study of tissues
Molecular biology	Study of subcellular details of structure and function
Parasitology	Study of animals that live in or on other organisms at the expense of the host
Physiology	Study of the function of organisms and their parts
Systematics	Study of the classification of, and the evolutionary interrelationships among, animal groups

TABLE 1.2
EXAMPLES OF SPECIALIZATIONS IN ZOOLOGY BY TAXONOMIC CATEGORIES

SUBDISCIPLINE	DESCRIPTION
Entomology	Study of insects
Herpetology	Study of amphibians and reptiles
Ichthyology	Study of fishes
Mammalogy	Study of mammals
Ornithology	Study of birds
Protozoology	Study of protozoa

Evolutionary Processes

Organic evolution (L. evolutus, unroll) is change in the genetic makeup of populations of organisms over time. It is the source of animal diversity, and it explains family relationships within animal groups. Charles Darwin published convincing evidence of evolution in 1859 and proposed a mechanism that could explain evolutionary change. Since that time, biologists have become convinced that evolution occurs. The mechanism proposed by Darwin has been confirmed and now serves as the nucleus of our broader understanding of evolutionary change (chapters 4 and 5).

Understanding how the diversity of animal structure and function arose is one of the many challenges faced by zoologists. For example, the cichlid scale eaters of Africa feed on the scales of other cichlids. They approach a prey cichlid from behind and bite a mouthful of scales from the body. The scales are then stacked and crushed by the second set of jaws and sent to the stomach and intestine for protein digestion. Michio Hori of Kyoto University found that there were two body forms within the speciesPerissodus microlepis. One form had a mouth that was asymmetrically curved to the right, and the other form had a mouth that was asymmetrically curved to the left. The asymmetry allowed right-jawed fish to approach and bite scales from the left side of their prey and the left-jawed fish to approach and bite scales from the right side of their prey. Both right- and left-jawed fish have been maintained in the population; otherwise, the prey would eventually become wary of being attacked from one side. The variety of color patterns within the species Topheus duboisi has also been explained in an evolutionary context. Different color patterns arose as a result of the isolation of populations among sheltering rock piles separated by expanses of sandy ­bottom. Breeding is more likely to occur within their isolated populations because fish that venture over the sand are ­exposed to predators.

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Animal Classification and Evolutionary Relationships

 

Evolution not only explains why animals appear and function as they do, but it also explains family relationships within the animal kingdom. Zoologists have worked for many years to understand the evolutionary relationships among the hundreds of cichlid species. Groups of individuals are more closely related if they share more of their genetic material (DNA) with each other than with individuals in other groups. (You are more closely related to your brother or sister than to your cousin for the same reason. Because DNA determines most of your physical traits, you will also more closely resemble your brother or sister.) Genetic studies suggest that the oldest populations of African cichlids are found in Lakes Tanganyika and Kivu, and from these the fish invaded African rivers and Lakes Malawi, Victoria, and other smaller lakes (figure 1.3). The history of these events is beginning to be ­understood and represents the most rapid known origin of species of any animal group. For example, the origin of Lake Victoria's more than 500 cichlid species has been traced to an invasion of ancestral cichlids, probably from Lake Kivu approximately 100,000 years ago. This invasion continued up to about 40,000 years ago when volcanic eruptions isolated the fauna of Lakes Kivu and Victoria. That time period is long from the perspective of a human lifetime, but it is a blink of the eye from the perspective of evolutionary time. There is firm geological evidence that Lake Victoria nearly dried out and then refilled 14,700 years ago. This event probably did not result in the ­extinction of all cichlids in the lake because the lake basin may have retained smaller bodies of water, and thus refuges for some cichlid species. After Lake Victoria refilled, these refuge populations could have provided the stock for recolonizing the lake. Exactly how many of the more than 500 species of Lake Victoria's cichlids arose in the last 14,700 years is an unanswered question.

FIGURE 1.3

Lakes Victoria, Tanganyika, and Malawi.

These lakes have cichlid populations that have been traced by zoologists to an ancestry that is approximately 200,000 years old. Cichlid populations originated in Lake Kivu and Lake Tanganyika and then spread to the other lakes.

Like all organisms, animals are named and classified into a hierarchy of relatedness. Although Karl von Linne (1707–1778) is primarily remembered for collecting and classifying plants, his system of naming—binomial nomenclature—has also been adopted for animals. A two-part name describes each kind of organism. The first part indicates the genus, and the second part indicates the species to which the organism belongs. Each kind of organism—for example, the cichlid scale-eater Perissodus microlepis—is recognized throughout the world by its two-part name. Above the species and genus levels, organisms are grouped into families, orders, classes, phyla, kingdoms, and domains, based on a hierarchy of relatedness (figure 1.4). Organisms in the same species are more closely related than organisms in the same genus, and organisms in the same genus are more closely related than organisms in the same family, and so on. When zoologists classify animals into taxonomic groupings they are making hypotheses about the extent to which groups of animals share DNA, even when they study variations in traits like jaw structure, color patterns, and behavior, because these kinds of traits ultimately are based on the genetic material.

FIGURE 1.4

Hierarchy of Relatedness.

The classification of a housefly, horsefly, cichlid fish, and human illustrates how the classification system depicts degrees of relatedness.

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How Do We Know about Genetic Relationships among Animals?

A

s shown by the example of Lake Victorian cichlids, zoologists often ask questions about genetic relationships among groups of animals. These family relationships are depicted in tree diagrams throughout this book. Early studies of genetic relationships involved the analysis of inherited morphological characteristics like jaw and fin structure that can be readily measured. With the advent of molecular biological techniques, zoologists have added to their repertoire of tools the analysis of variation in a series of enzymes, called allozymes, and DNA structure. These techniques allow zoologists to directly observe genetic relationships because the more DNA that two individuals, or groups of individuals, share, the more closely they are related. Because proteins, like enzymes, are encoded by DNA, variations in the structure of a protein also reflect genetic relationships. These topics are discussed in more detail in chapters 3, 4, and 5.

Evolutionary theory has affected zoology like no other single theory. It has impressed scientists with the fundamental unity of all of life. As the cichlids of Africa illustrate, evolutionary concepts hold the key to understanding why animals look and act in their unique ways, live in their particular geographical regions and habitats, and share characteristics with other ­related animals.

SECTION REVIEW 1.1

Our knowledge of evolutionary processes helps zoologists under­stand the great diversity of structure and function present in animals. Evolution also helps zoologists understand relationships among animals. These evolutionary relationships are ultimately based on shared DNA, they are reflected in inherited morphological characteristics, and they are represented by groupings in the classification system. The hierarchical nature of the naming system is reflected in groups becoming more inclusive as one moves from species to domain.

Why can taxonomists use similarities in DNA, similarities in morphological characteristics, or both when investigating taxonomic (evolutionary) relationships among animals?

1.2 ZOOLOGY: AN ECOLOGICALPERSPECTIVE

LEARNING OUTCOMES

1. Explain how the failure to understand ecological relationships among animals and their environment has resulted in detrimental environmental consequences.
2. Analyze the relationships between human population growth and threats to world resources.

Just as important to zoology as an evolutionary perspective is an ecological perspective.Ecology (Gr. okios, house + logos, to study) is the study of the relationships between organisms and their environment (chapter 6). Throughout our history, humans have depended on animals, and that dependence too often has led to exploitation. We depend on animals for food, medicines, and clothing. We also depend on animals in other, more subtle ways. This dependence may not be noticed until human activities upset the delicate ecological balances that have evolved over hundreds of thousands of years.

In the 1950s, the giant Nile perch (Lates niloticus) was introduced into Lake Victoria in an attempt to increase the lake's fishery (figure 1.5). This voracious predator reduced the cichlid population from 99% to less than 1% of the total fish population and has resulted in the extinction of many cichlid species. Because many of the cichlids fed on algae, the algae in the lake grew uncontrolled. When algae died and decayed, much of the lake became depleted of its oxygen. The introduction of nonnative water hyacinth, which has overgrown portions of the lake, has resulted in further habitat loss. To make matters worse, when Nile perch are caught, their excessively oily flesh must be dried. Fishermen cut local forests for the wood needed to smoke the fish. This practice has resulted in severe deforestation around Lake Victoria. The resulting runoff of soil into the lake has caused further degradation. Decreased water quality not only presented problems for the survival of individual cichlids, but increased turbidity also interfered with critical behavioral functions. Many of these species rely on their bright colors as visual cues during mating. Mouth-brooding species rely on vision to pick up developing eggs. The loss of Lake Victorian cichlids may be the largest extinction event of vertebrate species in modern human history.

FIGURE 1.5

Introduction of the Nile perch (Lates niloticus) in an attempt to improve Lake Victoria's fishery has resulted in the extinction of many cichlid species and has indirectly contributed to decreased water quality and deforestation.

There are some hopeful signs in this story. Although many Lake Victorian species have been lost forever, some cichlids are recovering. Heavy fishing pressure on the Nile perch has reduced its population density. (It still comprises more than 50% of catch weight—down from about 90% in the 1980s.) This decline has promoted the recovery of some cichlids that feed on small animals in the upper portions of open-water areas. (The Nile perch is predominately a bottom-dwelling predator.) Clearly, thinking ahead about the ecological ramifications of the seemingly simple act of introducing a foreign species could have averted this disaster.

Ecological problems also threaten Lake Tanganyika's cichlid populations. The area to the north of the lake has experienced nearly 100% deforestation. One-half of the forests on the Tanzania side of the lake are deforested to maintain a meager agricultural subsistence for human populations. Overfishing, agricultural runoff, and wastes from growing urban populations have led to some cichlid extinctions in the lake.

World Resources and Endangered Animals

There is grave concern for the ecology of the entire world, not just Africa's greatest lakes. The problems, however, are most acute in developing countries, which are striving to attain the same wealth as industrialized nations. Two problems, global overpopulation and the exploitation of world resources, are the focus of our ecological concerns.

Population

Global overpopulation is at the root of virtually all other environmental problems. Human population growth is expected to continue in the twenty-first century. Virtually all of this growth is in less developed countries, where 5.4 billion out of a total of 7 billion humans now live. Since a high proportion of the population is of childbearing age, the growth rate will increase in the twenty-first century. By the year 2050, the total population of India (1.6 billion) is expected to surpass that of China (1.4 billion) and the total world population will reach 9.3 billion. As the human population grows, the disparity between the wealthiest and poorest nations is likely to increase.

World Resources

Human overpopulation is stressing world resources. Although new technologies continue to increase food production, most food is produced in industrialized countries that already have a high per-capita food consumption. Maximum oil production is expected to continue in this millennium. Continued use of fossil fuels adds more carbon dioxide to the atmosphere, contributing to the greenhouse effect and climate change. Deforestation of large areas of the world results from continued demand for forest products, fuel, and agricultural land. This trend contributes to climate change by increasing atmospheric carbon dioxide from burning forests and impairing the ability of the earth to return carbon to organic matter through photosynthesis. Deforestation also causes severe regional water shortages and results in the extinction of many plant and animal species, especially in tropical forests. Forest preservation would result in the identification of new species of plants and animals that could be important human resources: new foods, drugs, building materials, and predators of pests (figure 1.6). Nature also has intrinsic value that is just as important as its provision of resources for humans. Recognition of this intrinsic worth provides important aesthetic and moral impetus for preservation.

(a)

(b)

FIGURE 1.6

Tropical Rain Forests: A Threatened World Resource.

(a) A Brazilian tropical rain forest. (b) A bulldozer clear-cutting a rain forest in the Solomon Islands. Clear-cutting for agriculture causes rain forest soils to quickly become depleted, and then the land is often abandoned for richer soils. Cutting for roads breaks continuous forest coverage and allows for easy access to remote areas for exploitation. Loss of tropical forests results in the extinction of many valuable forest species.

Solutions

An understanding of basic ecological principles can help prevent ecological disasters like those we have described. Understanding how matter is cycled and recycled in nature, how populations grow, and how organisms in our lakes and forests use energy is fundamental to preserving the environment. There are no easy solutions to our ecological problems. Unless we deal with the problem of human overpopulation, however, solving the other problems will be impossible. We must work as a world community to prevent the spread of disease, famine, and other forms of suffering that accompany overpopulation. Bold and imaginative steps toward improved social and economic conditions and better resource management are needed.

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“Wildlife Alerts” that appear at the end of selected chapters in the first two parts of this text remind us of the peril that an unprecedented number of species face around the world. Endangered or threatened species from a diverse group of animal phyla are highlighted.

SECTION REVIEW 1.2

As with the introduction of the Nile perch into Lake Victoria, our failure to understand complex ecological relationships among animals often results in detrimental consequences that require many decades, or even evolutionary time frames, to heal. Many of these detrimental consequences are direct or indirect results of the overpopulation of our planet by our own species.

What is another example of how the careless disregard of ecological relationships has resulted in detrimental environmental consequences? (If you cannot think of an example on your own, see the “Wildlife Alert” boxes in subsequent chapters.)

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WILDLIFE ALERT
An Overview Of The Problems

Extinction has been the fate of most plant and animal species. It is a natural process that will continue. In recent years, however, the threat to the welfare of wild plants and animals has increased dramatically—mostly as a result of habitat destruction. Tropical rain forests are one of the most threatened areas on the earth. In certain areas, such as Ecuador, forest coverage has been reduced by 95%. United Nations scientists estimate that 24 hectares (60 acres) of tropical forests are cleared every minute. Thirteen million hectares (32 million acres) of forests are lost each year. This is about twice the land area of Ireland or the size of the state of Arkansas. This decrease in habitat has resulted in tens of thousands of extinctions. Accurately estimating the number of extinctions is impossible in areas like rain forests, where taxonomists have not even described most species. We are losing species that we do not know exist, and we are losing resources that could lead to new medicines, foods, and textiles. Other causes of extinction include climate change, pollution, and invasions from foreign species. Habitats other than rain forests—grasslands, marshes, deserts, and coral reefs—are also being seriously threatened.

No one knows how many species living today are close to extinction. As of 2011, the U.S. Fish and Wildlife Service and the International Union for Conservation of Nature (IUCN) lists between 1,100 and 1,400 species in the United States as endangered or threatened. The IUCN lists 19,500 species as endangered or threatened world-wide. An endangered species is in imminent danger of extinction throughout its range (where it lives). A threatened species is likely to become endangered in the near future. Box figure 1.1 shows the number of endangered and threatened species in different regions of the United States. Clearly, much work is needed to improve these alarming statistics.

In the chapters that follow, you will learn that saving species requires more than preserving a few remnant individuals. It requires a large diversity of genes within species groups to promote species survival in changing environments. This genetic diversity requires large populations of plants and animals.

Preservation of endangered species depends on a multifaceted conservation plan that includes the following components:

1. A global system of national parks to protect large tracts of land and wildlife corridors that allow movement between natural areas
2. Protected landscapes and multiple-use areas that allow controlled private activity but also retain value as a wildlife habitat
3. Zoos and botanical gardens to save species whose extinction is imminent

BOX FIGURE 1.1

Map Showing Approximate Numbers of Endangered and Threatened Species in the United States.

Because the ranges of some organisms overlap two or more states, the sum of all numbers is greater than the sum of all endangered and threatened species. The total number of endangered and threatened species in the United States is between 1,100 and 1,300.

SUMMARY

1. 1.1   Zoology: An Evolutionary Perspective
   • Zoology is the study of animals. It is a broad field that requires ­zoologists to specialize in one or more subdisciplines.
     Animals share a common evolutionary past and evolutionary forces that influenced their history.
     Evolution explains how the diversity of animals arose.
     Evolutionary relationships are the basis for the classification of animals into a hierarchical system. This classification system uses a two-part name for every kind of animal. Higher levels of classification denote more distant evolutionary relationships.
2. 1.2   Zoology: An Ecological Perspective
   • Animals share common environments, and ecological principles help us to understand how animals interact within those environments.
     Human overpopulation is at the root of virtually all other environmental problems. It stresses world resources and results in pollution, climate change, deforestation, and the extinction of many plant and animal species.

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CONCEPT REVIEW QUESTIONS

1. All of the following are examples of specializations in zoology except one. Select the exception.
   1. Ichthyology
   2. Mammalogy
   3. Ornithology
   4. Histology
2. A change in the genetic makeup of populations of organisms over time is a definition of
   1. binomial nomenclature.
   2. organic evolution.
   3. evolution.
   4. ecology.
3. Which of the following do zoologists use to study the genetic ­relationships among animals?
   1. Inherited morphological characteristics
   2. Enzyme structure
   3. DNA structure
   4. All of the above are used by zoologists to study genetic relationships.
4. Which one of the following statements is true?
   1. Members of the same class are always more closely related to each other than members of the same order.
   2. Members of different orders may be more closely related to each other than members of the same family.
   3. Members of the same family are more closely related to each other than members of different orders.
   4. Members of the same order are always more closely related to each other than members of the same class.
5. All of the following may result from deforestation except one. ­Select the exception.
   1. Climate change is promoted.
   2. Extinction of many plant and animal species occurs.
   3. Regional water shortages occur.
   4. Long-term improvement in the standard of living in less ­developed countries occurs.
   5. Loss of important human resources such as new drugs and food occurs.
6. By the year 2050, most human population growth will occur in  and result in a world population of about .
   1. less developed countries; 7 billion
   2. less developed countries; 9.3 billion
   3. less developed countries; 20.5 billion
   4. developed countries; 5.5 billion
   5. developed countries; 10.2 billion

ANALYSIS AND APPLICATION QUESTIONS

1. How is zoology related to biology? What major biological concepts, in addition to evolution and ecology, are unifying principles shared between the two disciplines?
2. What are some current issues that involve both zoology and ­questions of ethics or public policy? What should be the role of ­zoologists in helping to resolve these issues?
3. Many of the ecological problems facing our world concern events and practices that occur in less developed countries. Many of these practices are the result of centuries of cultural ­evolution. What ­approach should people and institutions of ­developed countries take in helping to encourage ecologically minded resource use?
4. Why should people in all parts of the world be concerned with the extinction of cichlids in Lake Victoria?

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