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Taxonomy & Evolution
1 - Taxonomy
2 - Butterfly Families and subfamilies
3 - What is a species ?
4 - Evolution and Speciation
5 - Lepidoptera and the Evolutionary table
6
- Butterfly World Census
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 African
Mocker Swallowtail Papilio dardanus -
the male is cream in colour and possesses tails on the hindwings.
The female however lacks tails, and produces several forms, each
differing in colour and pattern, but all mimicking various 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. Taxonomists
therefore began to examine the full lifecycle rather than just the
adult butterflies, and consequently several new species were
discovered, e.g. the
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 can sometimes be visually indistinguishable,
making it almost impossible 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 has uniquely shaped
genitalia - the male "key" only fitting the correct female "lock".
The males and females of
some butterflies are totally different in appearance, and may not
initially be recognised as belonging to the same taxon. By examining
the genitalia, taxonomists have often discovered that 2 insects
previously classified under entirely different genera are actually
the same biological species !
Hybrids
Despite Nature's mechanisms for the prevention of interbreeding,
hybridisation between species does occasionally occur. Hybrids
however are always infertile, so they are unable to pass on their
characteristics to another generation.
Cladistics and
phylogenetics
Cladistics is a system whereby comparative data is analysed
quantitatively and is used
to construct cladograms, dendrograms and matrices. These are visual
representations of the evolutionary tree of life and are used to
depict the assumed evolutionary relationships between 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. At the other extreme, if
the number of shared characteristics was high it would imply that
the specimens were closely related, having split from a common
ancestor relatively recently. Hence they would probably be placed in
the same genus. For both insects to be recognised as the same
species, ALL of the characteristics would of course have to be
identical.
Since the late 20th century phylogenetic analysis has increasingly
been used to differentiate species and to establish evolutionary
relationships between various subfamilies and genera. A biologist
will typically begin by analysing 100 or so different
characteristics. These could include anything from the spikes on a
butterfly's leg to the structure of the eyes, the presence or
absence of a branching wing vein, or the chemical content of the
larval foodplants.
From this initial list of 100 characteristics the biologist will
then select about 40-50 that experience indicates should be the most
reliable for the final analysis. These are each weighted according
to their perceived relevance to the study. Computers are then used
to perform complex algorithms which determine the "distance" between
closely related genera, tribes or species. The selection of
characteristics used has a dramatic impact on the outcome of an
analysis. It is virtually unknown for two or more studies to produce
identical results. Consequently while there is general agreement
about higher level ( superfamily : family ) classification, there
are wide differences of opinion about the relationships between
various genera, tribes and species.
Genetic interchange
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 little opportunity to interbreed, so genetic
interchange is minimal, 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
race from Bosnia-Herzegovina has very prominent ocelli, and 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
Heliconius erato there are no less than
29 subspecies, each with a different pattern. Each subspecies of
erato corresponds in colour and pattern with one of the 29
subspecies of another Heliconius
species - H.melpomene.
Heliconius erato emma, Rio Pindayo, Peru ©
Adrian Hoskins
Heliconius erato petiverana, Costa Rica ©
Maris Pukitis
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, so that their ranges of distribution overlap. 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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