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Taxonomy & Evolution
1 - Taxonomy
2 - Butterfly Families and
subfamilies
3 - What is a species / subspecies ?
4 - Evolution and Speciation
5 - Lepidoptera and the Evolutionary
table
Evolution
The fossil
record
Prodryas persephone (
Nymphalidae ) is one of several fossilised butterflies found in the
Florissant beds, Colorado, USA. It dates from
the Oligocene period, 30mya.
( image supplied )
Estimates of the age of the earliest
insect fossils date back to at least 300 million years ago. The earliest Lepidoptera are generally supposed to have evolved from the Trichoptera (
caddis flies ) about 140-200 mya, coinciding with the appearance of the first flowering plants.
Where & when
did the first butterflies appear ?
Long ago the land masses of the Earth were divided into 4
continents - Laurentia, Baltica, Siberia and Gondwanaland. They
gradually converged, and about 350 mya became linked, forming
the super-continent Pangaea. It seems probable that the earliest butterflies
originated on Pangaea, which then began to
break up about 130 mya, ultimately forming the present day
continents. This may partly explain why all the butterfly families are represented on more than one continent.
A few subfamilies, and many tribes, are limited in their global distribution
-
the Ithomiini e.g. are found only in the neotropics, and the
Tellervini only in the Australian
( Papuan ) region. It seems likely that the latter evolved from their alleged parent
subfamily Danainae, AFTER the break-up of Pangaea. However, it is
important to realise that the break-up
was very gradual, and it is quite possible that all subfamilies
and genera may have come into
existence at a time when the new continents were still partially
linked.
Evolutionary Theory
The philosopher Aristotle ( 384-322
BC ) believed in spontaneous generation - the notion that all living
things arose from non-living sources. This probably reflected
unexplained phenomena such as swarms of flies apparently being
generated spontaneously from dead carcasses. This in turn probably
led to religious concepts of reincarnation, resurrection and the
"soul". St Thomas Aquinas even concluded that the spontaneous
generation of insects was the work of the devil!
The notion of evolution was first
mooted by the French biologist Jean-Baptiste Lamarck in 1800. He
believed that
animal anatomy and behaviour were not fixed, but adapted to
circumstances. Lamarck considered that these adaptations could be transmitted to
new generations. He believed evolution was driven by 2 forces - one
which drove animals from simple to complex forms, and another that
adapted animals to local environments and differentiated them from
each other.
Darwin's Theory of Evolution
Darwin went further when in 1838
he published "The Origin of Species" which theorised that all
animals evolved by a
process of natural selection from more basic life forms. The
scientific world gradually accepted and later expanded upon these
theories, concluding that life began billions of years ago when
viruses, bacteria or similar "primitive" entities somehow
arose in
the "primordial soup" of early oceans.
Put simply, the theory states
adaptations or traits occur naturally in a percentage of a
population, due to mutation. Evolution occurs when the more
successful adaptations become dominant, and former traits are "bred
out" of the population by a process known as "natural
selection".
Speciation
Environmental conditions change over a period of time,
during which the less
adaptable
life forms become extinct. Others acquire, via the birth of mutant
offspring, new properties that make them more capable of surviving.
These traits can be transmitted genetically to future generations,
as demonstrated by Mendel's experiments with hybridisation in the
late 19th century.
Most theorists believe that new
species probably evolve when the habitat of the ancestral species
undergoes major changes. Only those mutations that are able to
survive these changes can pass on their genes to future generations.
The ancestral form often becomes extinct, and the new form evolves
over a period of time, eventually becoming so different from its
ancestor that it is regarded as a new species.
Evolution does not however
necessarily require extinction of the ancestral species. Natural
barriers can emerge such as wide rivers, deserts or mountain ranges,
which isolate
populations of a given species from each other. In isolation the
natural variation which exists in any population will ensue
that new traits arise in both populations. One type of
biological or behavioural trait may work best in population A,
while a very different adaptation may work better in the
environment occupied by population B. Thus, provided that their
ranges do not overlap, two "subspecies" will
evolve.
The figure below illustrates how an
emerging mountain range can split a butterfly population. The
environment of one population remains largely unchanged, but the
other population is subjected to a much drier climate in the
rain-shadow of the mountain. In the latter population, only the
variants ( mutations) that are best suited to the new
environment will survive and pass on their genes to the next
generation. A process of natural selection "survival of the fittest"
will slowly bring about the evolution of a new species.
Two or more subspecies can still
theoretically interbreed and produce viable offspring. However if they remain
isolated for several thousand generations they slowly evolve
along different pathways, and eventually reach the stage when they
are so different that they can no longer interbreed. When this stage is
reached, they become "new" species.
By
way of example, a warm period in the Earth's history might cause a
particular species of Erebia Mountain
Ringlet to contract its range altitudinally - retreating to higher
ground where the climate was still cool enough to sustain the
vegetation on which its caterpillars depend.
It is quite likely
that at such a time, a formerly widespread mid-elevation species
would find itself reduced to a dozen or so small populations, each
occupying its own isolated mountain top, and unable to cross
intervening hostile habitats to interbreed with other populations.
Subtle differences in the geology,
topography and rainfall of different mountains would prompt each
population to evolve ( via natural selection ) changes in pattern
and colour that increased its chances of survival - thus several
new Erebia species would evolve.
Sub-species
Some taxonomists regard the species
as the terminal taxon, but the majority accept the existence of
"sub-species", i.e. geographically isolated races which have adapted
to different environments and evolved new anatomical or behavioural
characteristics that improve their survival rate. These races are
capable of interbreeding if brought together in a laboratory, but do
not interbreed in the wild due to geographical separation.
If and when these races become
significantly different from the ancestral race, they are regarded
as a subspecies, although few agree on what exactly defines a
subspecies!
Examples of subspecies include
Papilio machaon brittanicus which is
widely distributed throughout the Holarctic region but in Britain is
restricted to the Norfolk Broads; and
Hipparchia semele thyone and Plejebus
argus caernensis, both of which occur across much of the
Holarctic including Britain, although these particular subspecies
are restricted to a tiny area in north Wales.
The subspecies named above all have
certain things in common - compared to the forms elsewhere in the
Holarctic they have slightly different markings, and are
significantly smaller in wingspan. One convincing explanation for
the latter phenomenon is that they may be subject to a form of
natural selection against mobility, associated with reduced areas of
suitable breeding habitat.
Evolution in action ?
Most evolutionists consider that
'sub-species' are examples of evolution in action, speculating
that they are geographically isolated transitional taxa, evolving
from one species into another, as new forms mutate which are more
capable of surviving in different environments.
If this were true, sub-species
would by definition be genetically more complex than the ancestral
species from which they evolved. In fact the opposite is the case -
DNA analysis of Ornithoptera
populations on Indonesian islands shows that when isolated from
the ancestral populations they have become genetically impoverished.
Sub-species therefore, as proposed by d'Abrera,
are species undergoing entropy - in isolation they are losing
genetic material through in-breeding, and as a result are
less
capable of coping with environmental change.
This
is demonstrated in the case of the isolated British sub-species of Papilio machaon,
which has become dependent upon, and restricted to, an extremely limited range of
habitats & foodplants, whereas the ancestral populations in
mainland Eurasia and North America are far more adaptable and
capable of utilising a much wider range of foodplants.
Modern evolution synthesis accepts
that genetic entropy occurs, but points out that evolution has two
driving forces - natural selection and genetic drift. The latter is
an independent process that produces random changes due to chance,
rather than by selection. Genetic drift is more prevalent in small
isolated populations, and probably therefore speeds up the
evolutionary process in such populations. Theoretically, they will
also ultimately become subject to natural selection, and will evolve
in the more widely accepted sense of the word.
Speciation Frequency
There are several factors affecting
the rate at which speciation occurs. It is likely to happen more
frequently at habitat boundaries, where an existing species has
to adapt it's biology and behaviour in order to survive on
different foodplants or in different habitats.
It will also be more frequent in
tropical habitats than in temperate regions, because there will
be more generations during any given time period. In subarctic
regions many species take 2 years to complete their lifecycle.
The moth
Gynaephora groenlandica
spends up to 14 years as a
larva!
Most butterflies in temperate North
America and Europe produce one or two generations each year. In
the tropics however there can be 10 or more generations in the
same period. The entire lifecycle of one common tropical
butterfly Hypolimnas misippus takes
only 23 days to complete.
The more generations there are
in a year, the more chances there are for random mutations to
occur, and the more opportunities there are for speciation.
It has been mathematically
calculated that complete substitution of a gene under a natural
selection rate of only 1% takes less than 4000 generations. In
the case of a species like Heliconius
erato, which produces 10 generations a year it it quite
feasible then for speciation to occur at intervals of just 400
years.
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