tag:blogger.com,1999:blog-26871088815213511612024-02-20T05:38:32.847-08:00What is ZoologyZoology (Gr. zoon, 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.Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.comBlogger82125tag:blogger.com,1999:blog-2687108881521351161.post-72768594842216936832015-01-17T10:19:00.002-08:002015-01-17T10:19:54.918-08:00INTERSPECIFIC ADAPTATIONS<div dir="ltr" style="text-align: left;" trbidi="on">
Interspecific interactions have shaped many other characteristics
of animals. <b>Camouflage</b> occurs when an animal’s color patterns
help hide the animal, or a developmental stage, from another animal. <b>Cryptic coloration</b> (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. <b>Countershading</b> 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.<br />
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 <b>aposematic coloration</b>
(Gr. apo, away from sematic, sign).<br />
Resembling conspicuous animals may also be advantageous.
<b>Mimicry</b> (L. mimus, to imitate) occurs when a species resembles
one, or sometimes more than one, other species and gains protection
by the resemblance.</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com1tag:blogger.com,1999:blog-2687108881521351161.post-19996052488757609172015-01-17T10:16:00.000-08:002015-01-17T10:16:39.245-08:00SYMBIOSIS<div dir="ltr" style="text-align: left;" trbidi="on">
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.<br />
<b>Parasitism</b> 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.<br />
<br />
<b>Commensalism</b> 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.<br />
<br />
<b>Mutualism</b> 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.</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com1tag:blogger.com,1999:blog-2687108881521351161.post-20637801738998514062015-01-17T10:12:00.000-08:002015-01-17T10:12:04.449-08:00COEVOLUTION<div dir="ltr" style="text-align: left;" trbidi="on">
The evolution of ecologically related species is sometimes coordinated
such that each species exerts a strong selective influence
on the other. This is <b>coevolution</b>.<br />
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.</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-27656353204819894162015-01-17T10:08:00.000-08:002015-01-17T10:08:50.536-08:00INTERSPECIFIC COMPETITION<div dir="ltr" style="text-align: left;" trbidi="on">
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 <b>interspecific competition</b> 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</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-84692103530851419902014-09-10T11:28:00.001-07:002014-09-10T11:28:27.984-07:00INTRASPECIFIC COMPETITION<div dir="ltr" style="text-align: left;" trbidi="on">
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
<b>intraspecific competition</b>, 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.</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-43277228514186984012014-09-10T11:27:00.000-07:002014-09-10T11:27:17.446-07:00Population Density<div dir="ltr" style="text-align: left;" trbidi="on">
<b>Density-independent factors</b> 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.<br />
<b>Density-dependent factors</b> 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.</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-30199959388844013132014-09-10T11:25:00.001-07:002014-09-10T11:25:45.953-07:00POPULATION REGULATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
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.</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-10044902963213733012014-09-10T05:50:00.000-07:002014-09-10T05:50:08.693-07:00POPULATION GROWTH<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
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.<br />
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 <b>exponential growth</b>. 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.<br />
Exponential growth cannot occur indefinitely. The constraints
that climate, food, space, and other environmental factors
place on a population are called <b>environmental resistance</b>. The population size that a particular environment can support is the
<b>environment’s carrying capacity</b> 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 <b>logistic population
growth.</b></div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-74784010891054983242014-09-10T05:45:00.000-07:002014-09-10T05:45:10.265-07:00ANIMALS AND THEIR ABIOTIC ENVIRONMENT<div dir="ltr" style="text-align: left;" trbidi="on">
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 <b>tolerance range</b>, 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 <b>range of optimum</b>, defines the conditions under which
an animal is most successful.<br />
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 <b>limiting factor.</b> 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 <b>phototaxis</b>. 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.
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-67846985749967846342012-09-24T05:00:00.004-07:002012-09-24T05:00:25.436-07:00MOSAIC EVOLUTION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
Rates of evolution can vary both in populations<br />
and in molecules and structures. A species is a mosaic of different<br />
molecules and structures that have evolved at different<br />
rates. Some molecules or structures are conserved in evolution;<br />
others change more rapidly. The basic design of a bird provides a<br />
simple example. All birds are easily recognizable as birds because<br />
of highly conserved structures, such as feathers, bills, and a certain<br />
body form. Particular parts of birds, however, are less conservative<br />
and have a higher rate of change. Wings have been modified for<br />
hovering, soaring, and swimming. Similarly, legs have been modified<br />
for wading, swimming, and perching. These are examples of<br />
<b><span style="font-size: large;">mosaic evolution</span></b>.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-78064478685775537782012-09-24T04:59:00.000-07:002012-09-24T04:59:13.272-07:00GENE DUPLICATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
Recall that most mutations are selected against. Sometimes, however,<br />
an extra copy of a gene is present. One copy may be modified,<br />
but as long as the second copy furnishes the essential protein,<br />
the organism is likely to survive. Gene duplication, the accidental<br />
duplication of a gene on a chromosome, is one way that extra genetic<br />
material can arise.<br />
Vertebrate hemoglobin and myoglobin are believed to have<br />
arisen from a common ancestral molecule. Hemoglobin carries<br />
oxygen in red blood cells, and myoglobin is an oxygen storage<br />
molecule in muscle. The ancestral molecule probably carried out<br />
both functions. However, about 1 billion years ago, gene duplication<br />
followed by mutation of one gene resulted in the formation of<br />
two polypeptides: myoglobin and hemoglobin. Further gene duplications<br />
over the last 500 million years probably explain why most<br />
vertebrates, other than primitive fishes, have hemoglobin molecules<br />
consisting of four polypeptides.<br />
<br /></div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-17181567231971221702012-09-24T04:58:00.000-07:002012-09-24T04:58:18.385-07:00MOLECULAR EVOLUTION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
Many evolutionists study changes in animal structure and function<br />
that are observable on a large scale—for example, changes in<br />
the shape of a bird’s bill or in the length of an animal’s neck. All<br />
evolutionary change, however, results from changes in the base sequences<br />
in DNA and amino acids in proteins. Molecular evolutionists<br />
investigate evolutionary relationships among organisms<br />
by studying DNA and proteins. For example, cytochrome c is a<br />
protein present in the cellular respiration pathways in all eukaryotic<br />
organisms. Organisms that other research has<br />
shown to be closely related have similar cytochrome c molecules.<br />
That cytochrome c has changed so little during hundreds of millions<br />
of years suggests that mutations of the cytochrome c gene are<br />
nearly always detrimental, and are selected against. Because it has<br />
changed so little, cytochrome c is said to have been conserved<br />
evolutionarily.<br />
Not all proteins are conserved as rigorously as cytochrome c.<br />
Although variations in highly conserved proteins can help establish<br />
evolutionary relationships among distantly related organisms,<br />
less conserved proteins are useful for looking at relationships<br />
among more closely related animals. Because some proteins are<br />
conserved and others are not, the best information regarding evolutionary<br />
relationships requires comparing as many proteins as<br />
possible in any two species.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-13807454930097056552012-09-23T09:48:00.004-07:002012-09-23T09:48:58.583-07:00SYMPATRIC SPECIATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
A third kind of speciation, called <b><span style="font-size: large;">sympatric</span></b> (Gr. sym, together)<br />
<b><span style="font-size: large;">speciation</span></b>, occurs within a single population. Even though organisms<br />
are sympatric, they still may be reproductively isolated from<br />
one another. Many plant species are capable of producing viable<br />
forms with multiple sets of chromosomes. Such events could lead<br />
to sympatric speciation among groups in the same habitat. While<br />
sympatric speciation in animals is uncommon, it has been documented<br />
in two species of bats and several species of insects and fish.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-70083962683138576692012-09-23T09:48:00.000-07:002012-09-23T09:48:09.709-07:00PARAPATRIC SPECIATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
Another form of speciation, called <b style="font-size: x-large;">parapatric</b> (Gr. para, beside)<br />
<b><span style="font-size: large;">speciation</span></b>, occurs in small, local populations, called <b><span style="font-size: large;">demes</span></b>. For<br />
example, all of the frogs in a particular pond or all of the sea<br />
urchins in a particular tidepool make up a deme. Individuals of a<br />
deme are more likely to breed with one another than with other<br />
individuals in the larger population, and because they experience<br />
the same environment, they are subject to similar selection pressures.<br />
Demes are not completely isolated from each other because<br />
individuals, developmental stages, or gametes can move among<br />
demes of a population. On the other hand, the relative isolation of<br />
a deme may mean that its members experience different selection<br />
pressures than other members of the population. If so, speciation<br />
can occur. Although most evolutionists theoretically agree that<br />
parapatric speciation is possible, no certain cases are known. Parapatric<br />
speciation is therefore considered of less importance in the<br />
evolution of animal groups than allopatric speciation.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-28474769160154432092012-09-23T09:45:00.003-07:002012-09-23T09:46:45.703-07:00ALLOPATRIC SPECIATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
<b><span style="font-size: large;">Allopatric</span></b> (Gr. allos, other patria, fatherland) <b><span style="font-size: large;">speciation</span></b> occurs<br />
when subpopulations become geographically isolated from one another.<br />
For example, a mountain range or river may permanently separate<br />
members of a population. Adaptations to different environments<br />
or neutral selection in these separate populations may result in<br />
members not being able to mate successfully with each other, even if<br />
experimentally reunited. Many biologists believe that allopatric speciation<br />
is the most common kind of speciation.<br />
The finches that Darwin saw on the Galápagos Islands are a<br />
classic example of allopatric speciation, as well as adaptive radiation<br />
<b><span style="font-size: large;">Adaptive radiation</span></b> occurs when a number of<br />
new forms diverge from an ancestral form, usually in response to<br />
the opening of major new habitats.<br />
Fourteen species of finches evolved from the original finches<br />
that colonized the Galápagos Islands. Ancestral finches, having emigrated<br />
from the mainland, probably were distributed among a few<br />
of the islands of the Galápagos. Populations became isolated on various<br />
islands over time, and though the original population probably<br />
displayed some genetic variation, even greater variation arose. The<br />
original finches were seed eaters, and after their arrival, they probably<br />
filled their preferred habitats rapidly. Variations within the original<br />
finch population may have allowed some birds to exploit new islands<br />
and habitats where no finches had been. Mutations changed the<br />
genetic composition of the isolated finch populations, introducing<br />
further variations. Natural selection favored the retention of the<br />
variations that promoted successful reproduction.<br />
The combined forces of isolation, mutation, and natural selection<br />
allowed the finches to diverge into a number of species<br />
with specialized feeding habits. Of the 14 species of<br />
finches, six have beaks specialized for crushing seeds of different<br />
sizes. Others feed on flowers of the prickly pear cactus or in the<br />
forests on insects and fruit.<br />
<br />
<br /></div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-23352970159785732742012-09-23T09:42:00.004-07:002012-09-23T09:42:58.168-07:00POSTMATING ISOLATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
<b><span style="font-size: large;">Postmating isolation</span></b> prevents successful fertilization and<br />
development, even though mating may have occurred. For example,<br />
conditions in the reproductive tract of a female may not support<br />
the sperm of another individual, which prevents successful<br />
fertilization. Postmating isolation also occurs because hybrids are<br />
usually sterile (e.g., the mule produced from a mating of a male<br />
donkey and a mare is a sterile hybrid). Mismatched chromosomes<br />
cannot synapse properly during meiosis, and any gametes produced<br />
are not viable. Other kinds of postmating isolation include<br />
developmental failures of the fertilized egg or embryo.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-24239780221969458332012-09-23T09:41:00.004-07:002012-09-23T09:41:47.985-07:00PREMATING ISOLATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
<b><span style="font-size: large;">Premating isolation</span></b> prevents mating from taking place. For<br />
example, impenetrable barriers, such as rivers or mountain ranges,<br />
may separate subpopulations. Other forms of premating isolation<br />
are more subtle. If courtship behavior patterns of two animals are<br />
not mutually appropriate, mating does not occur. Similarly, individuals<br />
with different breeding periods or that occupy different<br />
habitats are unable to breed with each other.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-56405187914220193302012-09-23T09:40:00.003-07:002012-09-23T09:40:34.587-07:00SPECIATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
<b><span style="font-size: large;">Speciation</span></b> is the formation of new species. A requirement<br />
of speciation is that subpopulations are prevented from interbreeding.<br />
This is called <b><span style="font-size: large;">reproductive isolation</span></b>. When subpopulations<br />
are reproductively isolated, natural selection and genetic<br />
drift can result in evolution taking a different course in each subpopulation.<br />
Reproductive isolation can occur in different ways.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com1tag:blogger.com,1999:blog-2687108881521351161.post-81901834432937610692012-09-23T09:39:00.000-07:002012-09-23T09:39:25.436-07:00SPECIES<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
According to a biological definition, a species is a group of populations in which genes are actually, or potentially, exchanged through interbreeding.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-78127431633417296212012-09-23T09:36:00.002-07:002012-09-23T09:37:22.242-07:00BALANCED POLYMORPHISM<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
Polymorphism occurs in a population when two or more distinct<br />
forms exist without a range of phenotypes between them. <b><span style="font-size: large;">Balanced</span></b><br />
<b><span style="font-size: large;">polymorphism</span></b> (Gr. poly, many morphe, form) occurs<br />
when different phenotypes are maintained at relatively stable frequencies<br />
in the population and may resemble a population in<br />
which disruptive selection operates.<br />
Sickle-cell anemia results from a change in the structure of<br />
the hemoglobin molecule. Some of the red blood cells of persons<br />
with the disease are misshapen, reducing their ability to carry oxygen.<br />
In the heterozygous state, the quantities of normal and sickled<br />
cells are roughly equal. Sickle-cell heterozygotes occur in some<br />
African populations with a frequency as high as 0.4. The maintenance<br />
of the sickle-cell heterozygotes and both homozygous genotypes<br />
at relatively unchanging frequencies makes this trait an example<br />
of a balanced polymorphism.<br />
Why hasn’t natural selection eliminated such a seemingly<br />
deleterious gene? The sickle-cell gene is most common in regions<br />
of Africa that are heavily infected with the malarial parasite,<br />
Plasmodium falciparum. Sickle-cell heterozygotes are less susceptible<br />
to malarial infections; if infected, they experience less severe<br />
symptoms than do homozygotes without sickled cells. Individuals<br />
homozygous for the normal allele are at a disadvantage because<br />
they experience more severe malarial infections, and individuals<br />
homozygous for the sickle-cell allele are at a disadvantage because<br />
they suffer from the severe anemia that the sickle cells cause. The<br />
heterozygotes, who usually experience no symptoms of anemia,<br />
are more likely to survive than either homozygote. This system is<br />
also an example of heterozygote superiority—when the heterozygote<br />
is more fit than either homozygote. Heterozygote superiority<br />
can lead to balanced polymorphism because perpetuation of the<br />
alleles in the heterozygous condition maintains both alleles at a<br />
higher frequency than would be expected if natural selection<br />
acted only on the homozygous phenotypes.<br />
<br /></div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-33203280943863619882012-09-23T09:30:00.001-07:002012-09-23T09:34:28.180-07:00MODES OF NATURAL SELECTION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
For certain traits, many populations have a range of phenotypes,<br />
characterized by a bell-shaped curve that shows that phenotypic<br />
extremes are less common than the intermediate phenotypes.<br />
Natural selection may affect a range of phenotypes in three ways.<br />
<b><span style="font-size: large;">Directional selection</span></b> occurs when individuals at one phenotypic<br />
extreme are at a disadvantage compared to all other individuals<br />
in the population. In response to this selection,<br />
the deleterious gene(s) decreases in frequency, and all other<br />
genes increase in frequency. Directional selection may occur when<br />
a mutation gives rise to a new gene, or when the environment<br />
changes to select against an existing phenotype.<br />
Industrial melanism, a classic example of directional selection,<br />
occurred in England during the Industrial Revolution. Museum<br />
records and experiments document how environmental<br />
changes affected selection against one phenotype of the peppered<br />
moth, Biston betularia.<br />
In the early 1800s, a gray form made up about 99% of the<br />
peppered moth population. That form still predominates in nonindustrial<br />
northern England and Scotland. In industrial areas of<br />
England, a black form replaced the gray form over a period of about<br />
50 years. In these areas, the gray form made up only about 5% of<br />
the population, and 95% of the population was black. The gray<br />
phenotype, previously advantageous, had become deleterious.<br />
<br />
The nature of the selection pressure was understood when<br />
investigators discovered that birds prey more effectively on moths<br />
resting on a contrasting background. Prior to the Industrial Revolution,<br />
gray moths were favored because they blended with the<br />
bark of trees on which they rested. The black moth contrasted<br />
with the lighter, lichen-covered bark and was easily spotted by<br />
birds. Early in the Industrial Revolution, however,<br />
factories used soft coal, and spewed soot and other pollutants into<br />
the air. Soot covered the tree trunks and killed the lichens where<br />
the moths rested. Bird predators now could easily pick out gray<br />
moths against the black background of the tree trunk, while the<br />
black form was effectively camouflaged.<br />
In the 1950s, the British Parliament enacted air pollution<br />
standards that have reduced soot in the atmosphere. As expected,<br />
the gray form of the moth has experienced a small but significant<br />
increase in frequency.<br />
Another form of natural selection involves circumstances<br />
selecting against individuals of an intermediate phenotype.<br />
<b><span style="font-size: large;">Disruptive selection</span></b> produces distinct subpopulations.<br />
Consider, for example, what could happen in a population of<br />
snails with a range of shell colors between white and dark brown<br />
and living in a marine tidepool habitat with two background colors.<br />
The sand, made up of pulverized mollusc shells, is white, and<br />
rock outcroppings are brown. In the face of shorebird predation,<br />
what phenotypes are going to be most common? Although white<br />
snails may not actively select a white background, those present<br />
on the sand are less likely to be preyed on than intermediate phenotypes<br />
on either sand or rocks. Similarly, brown snails on rocks<br />
are less likely to be preyed on than intermediate phenotypes on either<br />
substrate. Thus, disruptive selection could produce two distinct<br />
subpopulations, one white and one brown.<br />
When both phenotypic extremes are deleterious, a third<br />
form of natural selection—<b><span style="font-size: large;">stabilizing selection</span></b>—narrows the<br />
phenotypic range. During long periods of environmental<br />
constancy, new variations that arise, or new combinations<br />
of genes that occur, are unlikely to result in more fit phenotypes<br />
than the genes that have allowed a population to survive for thousands<br />
of years, especially when the new variations are at the extremes<br />
of the phenotypic range.<br />
A good example of stabilizing selection is the horseshoe<br />
crab (Limulus), which lives along the Atlantic coast of the United<br />
States. Comparison of the fossil record with living<br />
forms indicates that this body form has changed little over 200<br />
million years. Apparently, the combination of characteristics present<br />
in this group of animals is adaptive for the horseshoe crab’s<br />
environment.<br />
<br /></div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com1tag:blogger.com,1999:blog-2687108881521351161.post-89602137340337179702012-09-23T09:18:00.001-07:002012-09-23T09:28:00.735-07:00NATURAL SELECTION REEXAMINED<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
The theory of natural selection remains preeminent in modern<br />
biology. Natural selection occurs whenever some phenotypes<br />
are more successful at leaving offspring than other phenotypes.<br />
The tendency for natural selection to occur—and upset<br />
<br />
Hardy-Weinberg equilibrium—is selection pressure. Although<br />
natural selection is simple in principle, it is diverse in operation.<br />
</div>
Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-24709474313216730962012-03-24T02:13:00.000-07:002012-03-24T02:13:25.722-07:00MUTATION<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
Mutations are changes in the structure of genes and chromosomes. The Hardy-Weinberg theorem assumes that no mutations occur or that mutational equilibrium exists. Mutations, however, are a fact of life. Most importantly, mutations are the origin of all new genes and a source of variation that may prove adaptive for an animal. Mutation counters the loss of genetic material from natural selection and genetic drift, and it increases the likelihood that variations will be present that allow a group to survive future environmental shocks.<br />
<br />
Mutations are random events, and the likelihood of a mutation is not affected by the mutation’s usefulness. Organisms cannot filter harmful genetic changes from advantageous changes before they occur. The effects of mutations vary enormously. Most are deleterious. Some may be neutral or harmful in one environment and help an organism survive in another environment.<br />
<br />
Mutational equilibrium exists when a mutation from the wild-type allele to a mutant form is balanced by mutation from the mutant back to the wild type. This has the same effect on allelic frequency as if no mutation occurred. Mutational equilibrium rarely exists, however. Mutation pressure is a measure of the tendency for gene frequencies to change through mutation.<br />
</div>Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-80551923917553109612012-03-24T02:11:00.000-07:002012-03-24T02:11:11.549-07:00GENE FLOW<div dir="ltr" style="text-align: left;" trbidi="on">
The Hardy-Weinberg theorem assumes that no individuals enter a population from the outside (immigrate) and that no individuals leave a population (emigrate). Immigration or emigration upsets the Hardy-Weinberg equilibrium, resulting in changes in gene frequency (evolution). Changes in gene frequency from migration of individuals are gene flow. Although some natural populations do not have significant gene flow, most populations do.</div>Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0tag:blogger.com,1999:blog-2687108881521351161.post-51282735631093033232012-03-24T02:10:00.000-07:002012-03-24T02:10:19.216-07:00HARDY-WEINBERG THEOREM<div dir="ltr" style="text-align: left;" trbidi="on">
<br />
In 1908, English mathematician Godfrey H. Hardy and German physician Wilhelm Weinberg independently derived a mathematical model describing what happens to the frequency of alleles in a population over time. Their combined ideas became known as the Hardy-Weinberg theorem. It states that the mixing of alleles at meiosis and their subsequent recombination do not alter the frequencies of the alleles in future generations, if certain assumptions are met. Stated another way, if certain assumptions are met, evolution will not occur because the allelic frequencies will not change from generation to generation, even though the specific mixes of alleles in individuals may vary.<br />
The assumptions of the Hardy-Weinberg theorem are as follows:<br />
<br />
1. The population size must be large. Large size ensures that gene frequency will not change by chance alone.<br />
<br />
2. Mating within the population must be random. Every individual must have an equal chance of mating with any other individual in the population. If this condition is not fulfilled, then some individuals are more likely to reproduce than others, and natural selection may occur.<br />
<br />
3. Individuals cannot migrate into, or out of, the population. Migration may introduce new genes into the gene pool or add or delete copies of existing genes.<br />
<br />
4. Mutations must not occur. If they do, mutational equilibrium must exist. Mutational equilibrium exists when mutation from the wild-type allele to a mutant form is balanced by mutation from the mutant form back to the wild type. In either case, no new genes are introduced into the population from this source.<br />
<br />
These assumptions must be met if allelic frequencies are not changing—that is, if evolution is not occurring. Clearly, these assumptions are restrictive, and few, if any, real populations meet them. This means that most populations are evolving. The Hardy-Weinberg theorem, however, does provide a useful theoretical framework for examining changes in gene frequencies in populations.<br />
</div>Anonymoushttp://www.blogger.com/profile/13977651999505398642noreply@blogger.com0