INTERSPECIFIC ADAPTATIONS

Interspecific interactions have shaped many other characteristics of animals. Camouflage occurs when an animal’s color patterns help hide the animal, or a developmental stage, from another animal. Cryptic coloration (L. crypticus, hidden) is a type of camouflage that occurs when an animal takes on color patterns in its environment to prevent the animal from being seen by other animals. Countershading is a kind of camouflage common in frog and toad eggs. These eggs are darkly pigmented on top and lightly pigmented on the bottom. When a bird or other predator views the eggs from above, the dark of the top side hides the eggs from detection against the darkness below. On the other hand, when fish view the eggs from below, the light undersurface blends with the bright air-water interface.
Some animals that protect themselves by being dangerous or distasteful to predators advertise their condition by conspicuous coloration. The sharply contrasting white stripe(s) of a skunk and bright colors of poisonous snakes give similar messages. These color patterns are examples of warning or aposematic coloration (Gr. apo, away from sematic, sign).
Resembling conspicuous animals may also be advantageous. Mimicry (L. mimus, to imitate) occurs when a species resembles one, or sometimes more than one, other species and gains protection by the resemblance.

SYMBIOSIS

Some of the best examples of adaptations arising through coevolution come from two different species living in continuing, intimate associations, called symbiosis (Gr. sym, together bio, life). Such interspecific interactions influence the species involved in dramatically different ways. In some instances, one member of the association benefits, and the other is harmed. In other cases, life without the partner would be impossible for both.
Parasitism is a common form of symbiosis in which one organism lives in or on a second organism, called a host. The host usually survives at least long enough for the parasite to complete one or more life cycles. The relationships between a parasite and its host(s) are often complex. Some parasites have life histories involving multiple hosts. The definitive or final host is the host that harbors the sexual stages of the parasite. A fertile female in a definitive host may produce and release hundreds of thousands of eggs in its lifetime. Each egg gives rise to an immature stage that may be a parasite of a second host. This second host is called an intermediate host, and asexual reproduction may occur in this host. Some life cycles may have more than one intermediate host and more than one immature stage. For the life cycle to be completed, the final immature stage must have access to a definitive host. Many examples of coevolutionary interactions between host and parasite are cited in Part Two of this text.

Commensalism is a symbiotic relationship in which one member of the relationship benefits, and the second is neither helped nor harmed. The distinction between parasitism and commensalism is somewhat difficult to apply in natural situations. Whether or not the host is harmed often depends on such factors as the host’s nutritional state. Thus, symbiotic relationships may be commensalistic in some situations and parasitic in others.

Mutualism is a symbiotic relationship that benefits both members. Examples of mutualism abound in the animal kingdom, and many examples are described elsewhere in this text.

COEVOLUTION

The evolution of ecologically related species is sometimes coordinated such that each species exerts a strong selective influence on the other. This is coevolution.
Coevolution may occur when species are competing for the same resource or during predator–prey interactions. In the evolution of predator–prey relationships, for example, natural selection favors the development of protective characteristics in prey species. Similarly, selection favors characteristics in predators that allow them to become better at catching and immobilizing prey. Predator–prey relationships coevolve when a change toward greater predator efficiency is countered by increased elusiveness of prey. Coevolution is obvious in the relationships between some flowering plants and their animal pollinators. Flowers attract pollinators with a variety of elaborate olfactory and visual adaptations. Insect-pollinated flowers are usually yellow or blue because insects see these wavelengths of light best. In addition, petal arrangements often provide perches for pollinating insects. Flowers pollinated by hummingbirds, on the other hand, are often tubular and red. Hummingbirds have a poor sense of smell but see red very well. The long beak of hummingbirds is an adaptation that allows them to reach far into tubular flowers. Their hovering ability means that they have no need for a perch.

INTERSPECIFIC COMPETITION

When members of different species compete for resources, one species may be forced to move or become extinct, or the two species may share the resource and coexist. While the first two options (moving or extinction) have been documented in a few instances, most studies have shown that competing species can coexist. Coexistence can occur when species utilize resources in slightly different ways and when the effects of interspecific competition are less severe than the effects of intraspecific competition. Robert MacArthur studied five species of warblers that all used the same caterpillar prey. Warblers partitioned their spruce tree habitats by dividing a tree into preferred regions for foraging. Although foraging regions overlapped, competition was limited, and the five species coexisted

INTRASPECIFIC COMPETITION

Competition occurs when animals utilize similar resources and in some way interfere with each other’s procurement of those resources. Competition among members of the same species, called intraspecific competition, is often intense because the resource requirements of individuals of a species are nearly identical. Intraspecific competition may occur without individuals coming into direct contact. (The “early bird that gets the worm” may not actually see later arrivals.) In other instances, the actions of one individual directly affect another. Territorial behavior and the actions of socially dominant individuals are examples of direct interference.

Population Density

Density-independent factors influence the number of animals in a population without regard to the number of individuals per unit space (density). For example, weather conditions often limit populations. An extremely cold winter with little snow cover may devastate a population of lizards sequestered beneath the litter of the forest floor. Regardless of the size of the population, a certain percentage of individuals will freeze to death. Human activities, such as construction and deforestation, often affect animal populations in a similar fashion.
Density-dependent factors are more severe when population density is high (or sometimes very low) than they are at other densities. Animals often use territorial behavior, song, and scent marking to tell others to look elsewhere for reproductive space. These actions become more pronounced as population density increases and are thus density dependent. Other density-dependent factors include competition for resources, disease, predation, and parasitism.

POPULATION REGULATION

The conditions that an animal must meet to survive are unique for every species. What many species have in common, however, is that population density and competition affect populations in predictable ways.

POPULATION GROWTH

Animal populations change over time as a result of birth, death, and dispersal. One way to characterize a population with regard to the death of individuals is with survivorship curves. There are three kinds of survivorship curves. Individuals in type 1 (convex) populations survive to an old age, then die rapidly. Environmental factors are relatively unimportant in influencing mortality, and most individuals live their potential life span. Some human populations approach type I survivorship. Individuals in type II (diagonal) populations have a constant probability of death throughout their lives. The environment has an important influence on death and is no harsher on the young than on the old. Populations of birds and rodents often have type II survivorship curves. Individuals in type III (concave) populations experience very high juvenile mortality. Those reaching adulthood, however, have a much lower mortality rate. Fishes and many invertebrates display type III survivorship curves.
A second attribute of populations concerns population growth. The potential for a population to increase in numbers of individuals is remarkable. Rather than increasing by adding a constant number of individuals to the population in every generation, the population increases by the same ratio per unit time. In other words, populations experience exponential growth. Not all populations display the same capacity for growth. Such factors as the number of offspring produced, the likelihood of survival to reproductive age, the duration of the reproductive period, and the length of time it takes to reach maturity all influence reproductive potential.
Exponential growth cannot occur indefinitely. The constraints that climate, food, space, and other environmental factors place on a population are called environmental resistance. The population size that a particular environment can support is the environment’s carrying capacity and is symbolized by K. In these situations, growth curves assume a sigmoid, or flattened S, shape, and the population growth is referred to as logistic population growth.

ANIMALS AND THEIR ABIOTIC ENVIRONMENT

An animal’s habitat (environment) includes all living (biotic) and nonliving (abiotic) characteristics of the area in which the animal lives. Abiotic characteristics of a habitat include the availability of oxygen and inorganic ions, light, temperature, and current or wind velocity. Physiological ecologists who study abiotic influences have found that animals live within a certain range of values, called the tolerance range, for any environmental factor. At either limit of the tolerance range, one or more essential functions cease. A certain range of values within the tolerance range, called the range of optimum, defines the conditions under which an animal is most successful.
Combinations of abiotic factors are necessary for an animal to survive and reproduce. When one of these is out of an animal’s tolerance range, it becomes a limiting factor. For example, even though a stream insect may have the proper substrate for shelter, adequate current to bring in food and aid in dispersal, and the proper ions to ensure growth and development, inadequate supplies of oxygen make life impossible. Often, an animal’s response to an abiotic factor is to orient itself with respect to it; such orientation is called taxis. For example, a response to light is called phototaxis. If an animal favors well-lighted environments and moves toward a light source, it is displaying positive phototaxis. If it prefers low light intensities and moves away from a light source, it displays negative phototaxis.

MOSAIC EVOLUTION

Rates of evolution can vary both in populations
and in molecules and structures. A species is a mosaic of different
molecules and structures that have evolved at different
rates. Some molecules or structures are conserved in evolution;
others change more rapidly. The basic design of a bird provides a
simple example. All birds are easily recognizable as birds because
of highly conserved structures, such as feathers, bills, and a certain
body form. Particular parts of birds, however, are less conservative
and have a higher rate of change. Wings have been modified for
hovering, soaring, and swimming. Similarly, legs have been modified
for wading, swimming, and perching. These are examples of
mosaic evolution.

GENE DUPLICATION

Recall that most mutations are selected against. Sometimes, however,
an extra copy of a gene is present. One copy may be modified,
but as long as the second copy furnishes the essential protein,
the organism is likely to survive. Gene duplication, the accidental
duplication of a gene on a chromosome, is one way that extra genetic
material can arise.
Vertebrate hemoglobin and myoglobin are believed to have
arisen from a common ancestral molecule. Hemoglobin carries
oxygen in red blood cells, and myoglobin is an oxygen storage
molecule in muscle. The ancestral molecule probably carried out
both functions. However, about 1 billion years ago, gene duplication
followed by mutation of one gene resulted in the formation of
two polypeptides: myoglobin and hemoglobin. Further gene duplications
over the last 500 million years probably explain why most
vertebrates, other than primitive fishes, have hemoglobin molecules
consisting of four polypeptides.

MOLECULAR EVOLUTION

Many evolutionists study changes in animal structure and function
that are observable on a large scale—for example, changes in
the shape of a bird’s bill or in the length of an animal’s neck. All
evolutionary change, however, results from changes in the base sequences
in DNA and amino acids in proteins. Molecular evolutionists
investigate evolutionary relationships among organisms
by studying DNA and proteins. For example, cytochrome c is a
protein present in the cellular respiration pathways in all eukaryotic
organisms. Organisms that other research has
shown to be closely related have similar cytochrome c molecules.
That cytochrome c has changed so little during hundreds of millions
of years suggests that mutations of the cytochrome c gene are
nearly always detrimental, and are selected against. Because it has
changed so little, cytochrome c is said to have been conserved
evolutionarily.
Not all proteins are conserved as rigorously as cytochrome c.
Although variations in highly conserved proteins can help establish
evolutionary relationships among distantly related organisms,
less conserved proteins are useful for looking at relationships
among more closely related animals. Because some proteins are
conserved and others are not, the best information regarding evolutionary
relationships requires comparing as many proteins as
possible in any two species.

SYMPATRIC SPECIATION

A third kind of speciation, called sympatric (Gr. sym, together)
speciation, occurs within a single population. Even though organisms
are sympatric, they still may be reproductively isolated from
one another. Many plant species are capable of producing viable
forms with multiple sets of chromosomes. Such events could lead
to sympatric speciation among groups in the same habitat. While
sympatric speciation in animals is uncommon, it has been documented
in two species of bats and several species of insects and fish.

PARAPATRIC SPECIATION

Another form of speciation, called parapatric (Gr. para, beside)
speciation, occurs in small, local populations, called demes. For
example, all of the frogs in a particular pond or all of the sea
urchins in a particular tidepool make up a deme. Individuals of a
deme are more likely to breed with one another than with other
individuals in the larger population, and because they experience
the same environment, they are subject to similar selection pressures.
Demes are not completely isolated from each other because
individuals, developmental stages, or gametes can move among
demes of a population. On the other hand, the relative isolation of
a deme may mean that its members experience different selection
pressures than other members of the population. If so, speciation
can occur. Although most evolutionists theoretically agree that
parapatric speciation is possible, no certain cases are known. Parapatric
speciation is therefore considered of less importance in the
evolution of animal groups than allopatric speciation.

ALLOPATRIC SPECIATION

Allopatric (Gr. allos, other patria, fatherland) speciation occurs
when subpopulations become geographically isolated from one another.
For example, a mountain range or river may permanently separate
members of a population. Adaptations to different environments
or neutral selection in these separate populations may result in
members not being able to mate successfully with each other, even if
experimentally reunited. Many biologists believe that allopatric speciation
is the most common kind of speciation.
The finches that Darwin saw on the Galápagos Islands are a
classic example of allopatric speciation, as well as adaptive radiation
Adaptive radiation occurs when a number of
new forms diverge from an ancestral form, usually in response to
the opening of major new habitats.
Fourteen species of finches evolved from the original finches
that colonized the Galápagos Islands. Ancestral finches, having emigrated
from the mainland, probably were distributed among a few
of the islands of the Galápagos. Populations became isolated on various
islands over time, and though the original population probably
displayed some genetic variation, even greater variation arose. The
original finches were seed eaters, and after their arrival, they probably
filled their preferred habitats rapidly. Variations within the original
finch population may have allowed some birds to exploit new islands
and habitats where no finches had been. Mutations changed the
genetic composition of the isolated finch populations, introducing
further variations. Natural selection favored the retention of the
variations that promoted successful reproduction.
The combined forces of isolation, mutation, and natural selection
allowed the finches to diverge into a number of species
with specialized feeding habits. Of the 14 species of
finches, six have beaks specialized for crushing seeds of different
sizes. Others feed on flowers of the prickly pear cactus or in the
forests on insects and fruit.

POSTMATING ISOLATION

Postmating isolation prevents successful fertilization and
development, even though mating may have occurred. For example,
conditions in the reproductive tract of a female may not support
the sperm of another individual, which prevents successful
fertilization. Postmating isolation also occurs because hybrids are
usually sterile (e.g., the mule produced from a mating of a male
donkey and a mare is a sterile hybrid). Mismatched chromosomes
cannot synapse properly during meiosis, and any gametes produced
are not viable. Other kinds of postmating isolation include
developmental failures of the fertilized egg or embryo.

PREMATING ISOLATION

Premating isolation prevents mating from taking place. For
example, impenetrable barriers, such as rivers or mountain ranges,
may separate subpopulations. Other forms of premating isolation
are more subtle. If courtship behavior patterns of two animals are
not mutually appropriate, mating does not occur. Similarly, individuals
with different breeding periods or that occupy different
habitats are unable to breed with each other.

SPECIATION

Speciation is the formation of new species. A requirement
of speciation is that subpopulations are prevented from interbreeding.
This is called reproductive isolation. When subpopulations
are reproductively isolated, natural selection and genetic
drift can result in evolution taking a different course in each subpopulation.
Reproductive isolation can occur in different ways.

SPECIES

According to a biological definition, a species is a group of populations in which genes are actually, or potentially, exchanged through interbreeding.

BALANCED POLYMORPHISM

Polymorphism occurs in a population when two or more distinct
forms exist without a range of phenotypes between them. Balanced
polymorphism (Gr. poly, many morphe, form) occurs
when different phenotypes are maintained at relatively stable frequencies
in the population and may resemble a population in
which disruptive selection operates.
Sickle-cell anemia results from a change in the structure of
the hemoglobin molecule. Some of the red blood cells of persons
with the disease are misshapen, reducing their ability to carry oxygen.
In the heterozygous state, the quantities of normal and sickled
cells are roughly equal. Sickle-cell heterozygotes occur in some
African populations with a frequency as high as 0.4. The maintenance
of the sickle-cell heterozygotes and both homozygous genotypes
at relatively unchanging frequencies makes this trait an example
of a balanced polymorphism.
Why hasn’t natural selection eliminated such a seemingly
deleterious gene? The sickle-cell gene is most common in regions
of Africa that are heavily infected with the malarial parasite,
Plasmodium falciparum. Sickle-cell heterozygotes are less susceptible
to malarial infections; if infected, they experience less severe
symptoms than do homozygotes without sickled cells. Individuals
homozygous for the normal allele are at a disadvantage because
they experience more severe malarial infections, and individuals
homozygous for the sickle-cell allele are at a disadvantage because
they suffer from the severe anemia that the sickle cells cause. The
heterozygotes, who usually experience no symptoms of anemia,
are more likely to survive than either homozygote. This system is
also an example of heterozygote superiority—when the heterozygote
is more fit than either homozygote. Heterozygote superiority
can lead to balanced polymorphism because perpetuation of the
alleles in the heterozygous condition maintains both alleles at a
higher frequency than would be expected if natural selection
acted only on the homozygous phenotypes.