Variation in taxonomic and systematic characters

Variation in Taxonomic and Systematic Characters
A Geographic Variation in one or more characteristics over space.
B Sexual Variation in one or more traits between or within a sex.
C Individual Variation in one or more characteristics within the lifetime of an
individual organism.
Noor Zada, M.Sc Zoology, KUST, Kohat
Variation:
Variation is a deviation in structural, functional or developmental characters of an organism
from its parents, from others in the same population.
Genotypic variation is the difference in genotype within population or species as a result of
mutation, recombination or gene interaction with some evolutionary significance.
Phenotypic variation is variation in structural and functional characters resulting from
environmental factors on one or more genotypes; this may not be of evolutionary significance.

1. Geographic Variation
As implied by the name this type of variation occurs over geographic space. This includes,
but is not limited to, latitudinal, longitudinal, and altitudinal variation of characters.
Usually, researchers examine variation in demes, populations, and species and look for any
"geographic" correlation with any of this variation. Within logical boundaries, if a
"geographic" component to the variation can be identified then one may hypothesize that the
observed variation within the taxon may be clinal and may be correlated with an
environmental component. If so, such a character may have a selective advantage in some
geographic areas or, developmentally, characters may exist in some geographic areas because
of temperature, light, or other physical influences early in ontogeny.

2. Sexual Variation
As implied by the name of this type of variation males and females frequently vary from
one another for characteristics. This is most often known as sexual dimorphism and the
characteristics that do differ are often referred to as primary or secondary sexual
features. These latter terms will be discussed below.
Sexually dimorphic characters may not vary in larval and juvenile individuals; variation
may be obtained only later in life. Adults may display these characters throughout the
year or may only possess them during the breeding season. In the former case, usually
the dimorphic characters are most extreme during the breeding season. Some species do
not vary in secondary sexual features and sex is determined only by examination of
gonadal tissue.

3. Individual Variation
1. Morphological Variation (Non Genetic Variation)
I. Age variations
Common in many groups of organisms to have different looking juveniles or larvae from adults.
Many synonyms have resulted from this phenomenon.
For example, Linnaeus described the immature striped goshawk as a different species from adult.
This was obviously the result of molting pattern differences.
Allometric Variations
This type of variation is typically thought of as being under genetic control. Allometric growth or
variation results when the size of some particular structure or number of structures is
disproportionate relative to other structures or the rest of the body. Thus, individuals of different
sizes may have different sized traits. For body measurements this is solved with multivariate
statistics or use of ratios. In fishes sometimes the number and sizes of breeding tubercules of adult
males may be correlated with their body size.

Seasonal Variation in Individuals.
In species that survive for more than one year or more than one
reproductive season, characteristics may vary depending upon the
season. For example, the plumage of birds, antlers of some mammals,
and general breeding coloration.
II. Social Variation.
In some social insects (bees and wasps, termites) certain castes are
developed (reproductive, workers, soldiers). The individuals may be
males, females or both. The different structural types that are observed
may be the result of different larval food or may be due to hormonal or
other controls. Obviously these different forms should not be considered
different species.

III. Ecological Variation
Habitat Variation.
Populations of a single species may occur in different habitats in the same region and are often
visibly different depending upon the habitat that they are found in. Taxonomic treatment of local
variants of this nature have fluctuated between two extremes. Some researchers have considered
them to be different species while others have considered them to be non-genetic variants.
Obviously, one can only determine the status of such entities with additional information derived
from controlled growth studies or genetic analyses.
Temporary Climatic Conditions
Some species have tremendous phenotypic plasticity and for some traits a different phenotype is
produced in years of extreme conditions (drought, cold, warm weather) relative to those from other
year classes under normal conditions. Fishes are commonly dwarfed in bad years. Stunted growth or
periods of exceptionally rapid growth can be displayed in different proportional traits.

Host-Determined Variation
Parasitic species may display different traits dependent upon the host on which they
feed. Cocoons can vary in color depending upon wasp host. Some wasps may be
winged or wingless, depending upon host. Some hosts may display different traits
when parasitized. Color patterns may vary with fishes sometimes if they are
parasitized (usually this is obvious).
Density-Dependent Variation
Crowding can influence morphological variations. This can be a result of reduced food
supply or not. Under crowded conditions the phenotypes may vary from those reared
under less crowded conditions: This phenomenon is particularly common with locusts.

Neurogenic or Neurohumoral Variation
Color change in individuals due to regions in environment. Accomplished through
the concentration or dispersal of color bearing bodies known as chromatophores.
This has been observed in chameleons, some lower vertebrates, crustaceans,
cephalopods, and flat fishes.
A common example of this phenomenon is that of colored fishes begin placed
into a white bucket. After being in the bucked for a few minutes the coloration is
"washed out" and the organism looks different to you than it did 5 minutes
previously.

IV. Traumatic Variation
This type of variation occurs with varying frequency depending on the group. It is
usually obvious, but in some cases may be subtle and misleading.
Parasite individual variation
Typical patterns discovered in a host individual will include swelling, distortion, and
perhaps mechanical injury. With insects parasites can alter head size, wing venation,
and other structural features. Parasitized fishes may appear pale and soft, have darks
spots on the body, have weak fin rays, pop-eyed and pot-bellied appearance, have
small mouths, nostrils that are joined, no lateral lines, an increase in scale numbers
and other abnormalities.

Teratological or accidental
Alterations in development. Usually these are externally induced but can be developmental
and may be from hormonal control. External stimuli for aberrant morphologies may be
mechanical, physical, or chemical. Usually obvious because individuals appear as freaks!
Post-mortem Changes
Common in some museum specimens that have been fixed or preserved or pinned. Colors are
often lost or fade, preserved bodies may appear odd in some cases. In some instances some
color patterns do not appear until after specimens are fixed. Some insects that are yellow turn
red in cyanide.
Thus, in taxonomic and systematic studies there is no substitute for observing your taxa in life
from various locations within its range. Also, be sure to take careful notes of life colors.

2. Genetic Variation
Before, the same individual is actually or potentially subject to change
in appearance. In addition to this non-inherited variation, there is much
interpopulational variation which is primarily due to differences in
genetic constitution. This variation can be more or less arbitrarily
divided into two such classes.

I. Sex-Associated Variation
Among the genetically determined variants within a population, there may be some
that are sexually associated. They may be sex-linked (expressed in one sex only) or
be otherwise associated with one or the other sex.
Primary sex differences - Those that involve primary sex organs used in
reproduction (gonads, genitalia). Where the sexes are otherwise quite similar, these
will rarely be a source of taxonomic confusion.

Secondary sex differences - Many groups display pronounced sexual dimorphism.
These differences can be quite striking. Different sexes have frequently been described as
different species until more work has been done on a group.
Alteration of Generations - In some groups there may be an agamic stage that looks
quite different from a reproducing stage. In aphids the parthenogenetic females are wingless
whereas the sexual females have wings.
Gonadromorphs and Intersexes- Gonadromorphs display male characters on one
part of the body and female on the other. Due to unequal somatic distribution of sex
chromosomes. Spiders
Intersexes - exhibit a blending of male and female traits. Thought to result from upset in balance
of male tendency and female tendency genes. Can be from irregularities in fertilization on
mitosis or physiological disturbance due to parasitism. Occur most frequently in areas of
interspecific hybrids.

II. Non-Sex Associated Individual Variation
Continuous Variation - Most common type of variation due to slight genetic
differences which exist between individuals. No two individuals are exactly alike in
a population genetically or morphologically. This is one of the foremost tasks of the
taxonomist. No single individual is "typical" of the characters of a population. Only
with statistics of the whole population can we arrive at the true picture of the whole
population.
Each character is likely to show different degrees of variation in a population.
Likewise there will be differing degrees of variation between species for a character.

Discontinuous Variation - Differences between individuals in a population are, in
general, slight and intergrading. In some species, however, can be grouped into different
classes determined by some characters. This discontinuous variation is frequently termed
polymorphism. Frequency such polymorphisms may be controlled by a single gene.
· Cichlasoma minckleyi - This is a cichlid species found only in some small bodies of
water in northern Mexico. In these populations biologists have identified two different
morphologies in tooth structure. After careful study, this appears to be an instance of a
polymorphism in tooth structure.
Peppered moth - industrial melanism.
Many bird species have been proposed to demonstrate this type of polymorphic variation
within populations for morphological characteristics.
Some butterflies which mimic poisonous species may have more than one morphotype in a
population. By possessing multiple mimics to poisonous butterfly species the polymorphic
species has an advantage when it comes to predation. This is special form of Batesian
mimicry.

Variation in taxonomic and systematic characters

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