Animal adaptations

Many of the features that are most interesting and beautiful in biology are adaptations. Adaptations are the result of long evolutionary processes in which succeeding generations of organisms become better able to live in their environments. Specialized structures, physiological processes, and behaviors are all adaptations when they allow organisms to cope successfully with the special features of their environments. Adaptations ensure that individuals in populations will reproduce and leave well-adapted offspring, thus ensuring the survival of the species.

Mutation and Natural Selection

Adaptations arise through mutations—inheritable changes in an organism’s genetic material. These rare events are usually harmful, but occasionally, they give specific survival advantages to the mutated organism and its offspring. When certain individuals in a population possess advantageous mutations, they are better able to cope with their specific environmental conditions. As a result, they will contribute more offspring to future generations compared with those individuals in the population that lack the mutation. Over time, the number of individuals that have the advantageous mutation will increase in the population at the expense of those that do not have it. Individuals with an advantageous mutation are said to have a higher “fitness” than those without it because they tend to have comparatively higher survival and reproductive rates. This is natural selection.

Over very long periods of time, evolution by natural selection results in increasingly better adaptations to environmental circumstances. Natural selection is the primary mechanism of evolutionary change, and it is the force that either favors or selects against mutations. Although natural selection acts on individuals, a population gradually changes as those with adaptations become better represented in the total population. Predaceous fish, for example, which rely on speed to pursue and overtake prey, would benefit from specific adaptations that would increase their swimming speed. Therefore, mutations causing a more sleek and hydrodynamically efficient form would be beneficial to the fish predator. Such changes would be adaptations if they resulted in improved predation success, improved diet, and subsequently greater reproductive success, compared with slower members of the population. Natural selection would favor the mutations because they confer specific survival advantages to those that carry the mutations and impose limitations on those lacking these advantages. Thus, those individuals with special adaptations for speed would have a competitive advantage over individuals that can only swim more slowly. These attributes would be passed to their more numerous offspring and, in evolutionary time, speed and hydrodynamic efficiency would increase in the population.

Another important contributor to how species pass genes and traits through successive generations was Gregor Johann Mendel (1822-1884). In the early nineteenth century, Mendel was a monk who lived in Saint Thomas' Abbey in what is now the Czech Republic. Having already attained several advanced degrees, including philosophy and physics, Mendel turned his attention to the subject of heredity and how characteristics are passed down through generations. He began to catalog and study variations in plants. From 1856-1863, Mendel conducted a regimen of experiments involving over 10,000 pea plants that he grew in the monastery’s garden. Mendel studied traits in pea plants that included color, shape, height, and the position of the flower.

Mendel cross-fertilized the pea plants by intermixing pollen and noted the way in which characteristics appeared to be transmitted through subsequent generations. Among Mendel's important findings were that genes are passed to offspring in pairs, and that they contain the traits inherited between generations. Offspring will obtain a gene from each parent. When the parents contain different traits, the genes and characteristics that appear in the offspring are called the dominant trait. This will supersede the same characteristic, or recessive trait, that exists in the other parent. Unlike Darwin, Mendel worked in obscurity. His work never achieved mass recognition until decades after his death. Although the works of both Darwin and Mendel are now accepted as scientific bedrock, there has remained a divergence of opinion on whose theories better explain the process of continuous change in species.

Environment and Survival

Although natural selection serves as the instrument of change in shaping organisms to very specific environmental features, highly specific adaptations may ultimately be a disadvantage. Adaptations that are specialized may not allow sufficient flexibility (generalization) for survival in changing environmental conditions. The nature of the environment ultimately controls the degree of adaptative specialization. Environments, such as the tropics with predictable, uniform climates and long, uninterrupted periods of climatic stability, are biologically complex and have high species diversity. Scientists generally believe that this diversity results, in part, from complex competition for resources and from intense predator-prey interactions. Because of these factors, many narrowly specialized adaptations have evolved when environmental stability and predictability prevail. By contrast, harsh physical environments with unpredictable or erratic climates favor organisms with general adaptations or adaptations that allow flexibility. Regardless of the environment type, organisms with both general and specific adaptations exist because both types of adaptation enhance survivorship under different environmental circumstances.

Structural adaptations are parts of organisms that enhance survival ability. For example, scientists believe opossums originated in South America but now live globally in various climates. Some have adapted to eat poisonous snakes and amphibians, while the world’s only semiaquatic marsupial, the water opossum, eats fish and crustaceans. Other examples of structural adaptations include camouflage, which enables organisms to hide from predators or prey. Others are specialized mouth parts that allow organisms to feed on specific food sources. Forms of appendages, such as legs, fins, or webbed toes, allow efficient movement; protective spines that make it difficult for the organism to be eaten are all structural adaptations. These adaptations enhance survival by assisting individuals in dealing with the rigors of the physical environment, obtaining nourishment, competing with others, hiding, or confusing predators.

Physiological Versus Behavioral Adaptation

Metabolism is the sum of all chemical reactions taking place in an organism, whereas physiology consists of the processes involved in an organism carrying out its function. Physiological adaptations are changes in the metabolism or physiology of organisms, giving them specific advantages for a given set of environmental circumstances. Because organisms must cope with the rigors of their physical environments, physiological adaptations for temperature regulation, water conservation, varying metabolic rate, and dormancy or hibernation allow organisms to adjust to the physical environment or respond to changing environmental conditions.

Desert environments, for example, pose a special set of problems for organisms. Hot, dry environments require physiological mechanisms that enable organisms to conserve water and resist prolonged periods of high temperature. Highly efficient kidneys and other excretory organs that assist organisms in retaining water are physiological adaptations related to the metabolisms of desert organisms. The kangaroo rat is a desert rodent that is extremely well adapted to its habitat. Kangaroo rats do not drink but can obtain all their water from the seeds they eat. They produce highly concentrated urine and feces with very low water content. Camels have also adapted to the desert environment with eyelashes and closable nostrils to avoid irritations from sand, hooves that prevent them from sinking in the sand, and a hump that stores fat for future water or food needs.

Adaptation to a specific temperature range is also an important physiological adaptation. Organisms cannot live in environments with temperatures beyond their range of thermal tolerance, but some organisms are adapted to warmer and others to colder environments. Metabolic response to temperature is quite variable among animals, but most animals are either homeothermic (warm-blooded) or poikilothermic (cold-blooded). Homeotherms maintain constant body temperatures at specific temperature ranges. Although a homeotherm’s metabolic heat production is constant when the organism is at rest and when the environmental temperature is constant, strenuous exercise produces excess heat that must be dissipated into the environment, or overheating and death will result. Physiological adaptations that enable homeotherms to rid their bodies of heat are the ability to increase blood flow to the skin’s surface, sweating, and panting, all of which promote heat loss to the atmosphere.

Behavioral adaptations allow organisms to respond appropriately to various environmental stimuli. Actions taken in response to various stimuli are adaptive if they enhance survival. Migrations are behavioral adaptations because they ensure adequate food supplies or the avoidance of adverse environmental conditions. For example, gray whales migrate from the Arctic Ocean to Mexico's coast each year to give birth in the warmer water and then return to the Arctic's ample nutrient supply. Other examples of behavioral adaptations include courtship rituals that help in species recognition prior to mating, reflex and startle reactions allowing for quick retreats from danger, and social behavior that fosters specialization and cooperation for group survival are behavioral adaptations.

Because organisms must also respond and adapt to an environment filled with other organisms—including potential predators and competitors—adaptations that minimize the negative effects of biological interactions are favored by natural selection. The interaction between species is so close that each species often strongly influences the others in the interaction and serves as the selective force causing change. Under these circumstances, species evolve together in a process called coevolution. The adaptations resulting from coevolution have a common survival value to all the species involved in the interaction. The coevolution of flowers and their pollinators is a classic example of these tight associations and their resulting adaptations. Hummingbirds have adapted to have longer beaks to access the nectar is specific plants, which have also adapted to produce nectar that appeals to hummingbirds, ensuring pollination.

Adaptation in Theory and Practice

Charles Darwin and Alfred Russel Wallace, the mid-nineteenth-century biologists who formulated the theory of evolution by natural selection, found much of the evidence for their theory in the adaptations they observed in nature. They reasoned that organisms with similar body forms and structures were closely related evolutionarily and had common ancestors, which are now extinct. The modern study of relatedness among species and the evolutionary history of organisms is called systematics, and this discipline aids in understanding evolution and adaptations.

The methods used to study adaptations are largely the same as those used to examine the theory of evolution. Evolution, however, is a slow process, and, as a result, it is extremely difficult to test the theory. Instead, evidence must be collected from the past, and closely related organisms must be examined carefully to reconstruct how adaptations may have come into being. Scientists can then speculate on how adaptations occurred and how they helped organisms to survive.

Because fossils are a historical record of evolutionary change, scientists use fossils to reconstruct evolutionary histories. Comparative anatomists use similar structures in different living organisms with essentially the same function to show how adaptations for a specific mode of life arose. The fins of some ancient fish and the limbs of mammals, for example, have strikingly similar bones that have a common origin, but the appendages have been modified for locomotion in very different environments. Adaptations are also studied in relation to biogeography, the geographical distribution of organisms. On the Galápagos Islands, fourteen species of finches, now known as Darwin’s finches, are distributed geographically based on their adaptations. Although the species that gave rise to these fourteen species are extinct, the existing species and their distributions suggest how evolution proceeded and how the adaptations came about.

A classic example of recent evolutionary change and adaptation comes from England. The peppered moth, with a mottled gray color, is well adapted to resting quietly on pale tree bark, with which it blends nicely. This adaptive coloration (camouflage) enhanced the moth’s survival because the moths could remain largely undetected by predators during daylight hours. Between 1850 and 1950, however, industrialization near urban centers blackened tree trunks with soot, making the gray form disadvantageous, as it stood out on the contrasting background. During this period, the gray moths began to disappear from industrial areas, but a black-colored variant, previously rare, became increasingly common in the population. These circumstances made it possible for scientists to test whether the peppered moth’s camouflage was adaptive.

In a simple experiment, moths were raised in the laboratory, and equal numbers of gray and black moths were released in both industrial and unpolluted rural areas. Sometime later, only half of the gray-colored moths could be recovered from the industrial sites, while only half of the black forms could be recovered from the rural sites, compared with the total number released. These results enabled the scientists to conclude that increased predation on the gray moths in industrial areas led to a greater fitness of the black moths, so the frequency of black moths increased in the population. The reverse was true at the rural sites. This is the first well-documented case of natural selection causing evolutionary change, illustrating camouflage's adaptive significance.

The various ways of examining adaptations (by evolutionary history, comparative anatomy, and biogeography) demonstrate how adaptations are structurally and functionally important. These approaches also give scientists insight into the survival benefits of various adaptations.

The Function of Adaptation

Adaptations can be general or highly specific. General adaptations define broad groups of organisms whose general lifestyle is similar. For example, mammals are homeothermic, provide care for their young, and have many other adaptations in common. At the species level, however, adaptations are more specific and give a narrow definition to organisms that are more closely related. Slight variations in a single characteristic, such as bill size in the seed-eating Galápagos finches, are adaptive in that they enhance the survival of several closely related species. Understanding how adaptations function to make species distinct also furthers the knowledge of how species are related to one another.

Why so many species exist is one of the most intriguing questions of biology. The study of adaptations offers biologists an explanation. Because there are many ways to cope with the environment, and because natural selection has guided the course of evolutionary change for billions of years, the vast variety of species existing on the Earth today is simply an extremely complicated variation on the theme of survival.

Principal Terms

Coevolution: joint evolutionary change caused by the close interaction of two or more species; each species serves as the natural selection agent for the other(s)

Competition: striving for a limited resource

Evolution: a process, guided by natural selection, that changes a population’s genetic composition and results in adaptations

Fitness: the ability of an organism to produce offspring that, in turn, can reproduce successfully; the fitness of organisms increases as a result of natural selection

Natural selection: the elimination of individuals with hereditary characteristics that hinder the organism’s ability to survive and reproduce and the preservation of those with traits beneficial to survival

Population: a group of individuals of the same species in a particular location

Species: a group of organisms that can successfully interbreed to produce living, successfully reproducing offspring

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