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Taxonomy & Evolution
Section 1 - Taxonomy
Section 2 - Butterfly Families and
subfamilies
Section 3 - What is a species / subspecies ?
Section 4 - Evolution and Speciation
Section 5 - Lepidoptera and the Evolutionary
table
What is a species ?
Morphological species
In past times any two butterflies that looked identical in structure
and markings were assumed to be the same species, and named
accordingly. However we now know that there are many species
involved in "mimicry rings" whereby several different species can
produce visually identical adults.
Conversely we
also know that
it is quite common for a
single species to produce several different forms or morphs, which
previously might have been classified as different species.
A good example of this phenomenon is the Mocker
Swallowtail Papilio dardanus -
the male is cream in colour, and possesses tails on the hindwings,
but the female has an entirely different wing shape, does not
possess tails, and produces several forms, each differing in colour and
pattern, but all mimicking various toxic species from the Danaine
genera Danaus and
Amauris, and the Acraeine genus
Bematistes.
Another problem with morphological species is that apparently
identical adult butterflies can have early stages ( eggs,
caterpillars, chrysalises ) that differ in structure. Scientists
therefore began wherever possible to examine the full lifecycle
rather than just the adult butterflies, and as a result several new
species were discovered, e.g. the
apparently identical Clouded Yellows
Colias alfacariensis
and
C. hyale were thought to be the same species until Berger
discovered distinct differences in the larvae and pupae.
Closely related
morphological species are often so similar that it is not possible
from visual examination to determine whether they represent
varieties / subspecies of one butterfly, or whether they are
biologically different ( but visually identical ) species.
Biological species
These are defined as creatures which breed with other biologically
identical creatures, and produce fertile offspring.
To
prevent interbreeding, each species of butterfly and moth has uniquely shaped genitalia -
the male "key" only fitting the correct female "lock".
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Hybrids
- Despite Nature's
mechanisms for the prevention of interbreeding, hybridisation
between species does occasionally occur, but hybrids
are by definition
infertile, so they cannot pass on their characteristics to another generation.
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Often the male and female of a particular butterfly are totally
different in appearance, and may not initially be recognised as
belonging to the same taxon. By examining the "lock and key"
genitalia, taxonomists have often discovered that 2 insects
previously classified under entirely different genera are actually
the same biological species !
Cladistics
Cladistics is a system whereby
comparative data is analysed quantitatively, and used
to construct cladograms. These represent the evolutionary tree of
life, and are used to illustrate the assumed phylogenetic relations and
evolutionary history of groups of organisms.
In simple terms, if
two sample
specimens were analysed and
found to share only a low number of common characteristics,
this would imply that they were only distantly related, i.e. that
they split from their common ancestor a very long time ago. They
would therefore be placed in separate families. At
the other extreme, if the number of shared characteristics was
extremely high, it would imply that the specimens were very closely
related, having split from a common ancestor relatively recently.
Hence they would be placed in the same family, and possibly in the
same genus.
For both sample insects to be
recognised as belonging to the same species, ALL characteristics
( of the same sex ) would of course be identical.
The
usual method favoured
by entomologists involves
microscopic examination and measurement of the constituent parts
of the genitalia of both sexes. Dozens, or sometimes hundreds of these characteristics
need to be analysed and quantified to enable the relationships
between species to be established.
Genitalia analysis is the primary tool used in determining
relationships and "evolutionary trees", but only tells part of the
story. Other methods of determination include study of wing-scale
structure, leg structure,
egg, caterpillar and chrysalis morphology, larval host-plant usage,
behavioural analysis and DNA sequencing.
W hat
is a "subspecies" ?
Butterflies can loosely be divided into 3 camps regarding genetic
interchange :
The first group
comprises species
such as the Painted Lady Vanessa cardui,
the Small White Pieris rapae and the
Pea Blue / Long-tailed Blue Lampides
boeticus which are migratory in behaviour and cosmopolitan
in distribution. Their nomadic nature brings them into fairly
regular contact with distant "cousins" of their own species, so
genetic interchange occurs frequently. A Painted Lady in Australia
therefore is genetically almost identical to a Painted Lady in
Europe, Africa, Asia or North America. Accordingly all races are
designated as Vanessa cardui, and
there are no recognised subspecies.
The second group
comprises of endemics - species which are native to and confined
to a limited area such as a particular island or mountain range.
Examples include Calisto confusa
which is found only in Haiti; Baronia
brevicornis, found only in deciduous scrub forests of s.w.
Mexico; Eresia sticta which is
restricted to Costa Rica; and Henotesia
comorensis - a butterfly confined to the Comoro Islands
north-west of Madagascar. By definition endemics have no other
races of their own species with which they can interbreed, so
there are no subspecies.
The vast majority
of butterfly species fall into the third group, which comprises
those species which are quite widespread in distribution, but
whose populations, due to changes in climate or vegetation have
become isolated from each other for hundreds or thousands of
years. Such populations have no opportunity to interbreed, so
genetic interchange is virtually non-existent, and accordingly
each isolated population develops its own characteristics. The
Scottish race of the Large Heath e.g. lacks ocelli on the wings
and is known as Coenonympha tullia scotica,
while the larger and much darker race from Bosnia-Herzegovina,
with very prominent ocelli, is recognised as a different
subspecies Coenonympha tullia lorkovici.
The subspecies
phenomenon is particularly prevalent in South America, where areas
of rainforest have during several periods of the Earth's climatic
history become isolated as a result of glaciation or
desertification. The result of this long term separation of
populations is particularly noticeable in the genus
Heliconius - in the case of the
Common Longwing Heliconius erato for
example there are no less than 29 recognised subspecies, each with
a different pattern.
If
populations have been isolated for several millennia the
differences between them can be very pronounced - patterns,
colours, size, and even the shape of the wings can be different,
particularly if the races evolve in different habitats and
climates.
Sometimes, due to
the periodic contraction and expansion of forested areas,
subspecies which have been isolated for many thousands of years
can be reunited, thus it is possible, and in fact not uncommon, to
find more than one subspecies occurring in the same area of
forest.
By definition,
all subspecies of any given species are physically capable of
interbreeding and can produce fertile offspring. Although such
interbreeding may not necessarily occur naturally, it can be
induced in captivity.
Some
evolutionists argue that if a subspecies becomes isolated for a
long enough period of time, it will ultimately evolve structural
differences ( e.g. in the genitalia ) that make it physically
impossible for it to interbreed with other subspecies, and that if
and when this occurs the 2 forms have become sufficiently
different for them to be reclassified as separate species, i.e.
the evolution of a new species will have occurred.
Others argue that
DNA
analysis of various subspecies on remote islands in Melanesia
indicates that when populations become isolated, inbreeding and
genetic entropy occurs, which is contradictory to most theories of
evolution, which state that as species evolve they advance -
thereby increasing rather than decreasing the number of chemical
base pairs in their genome.
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