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.