Taxonomy & Evolution
1 - Taxonomy
2 - Butterfly Families and subfamilies
3 - What is a species ?
4 - Evolution and Speciation
5 - Lepidoptera and the Evolutionary table
- Butterfly World Census
The ancestors of
The strange creature below is a longhorn
caddis fly Mystacides azurea from
England. The 'pincers' are highly modified palpi which are used to
grip and manipulate the female prior to mating. Caddis flies are the
ancient ancestors of butterflies and moths. Their larvae are aquatic
and live in portable cases constructed from sand grains or fragments
of stems bonded to a silk tube surrounding their body. Modern day
bagworm moths ( Psychidae ) still have larvae which live inside cases.
Mystacides azurea Hungerford, England
The fossil record
The first insects appeared
on Earth about 300 million years ago. The earliest Lepidoptera evolved
from the Trichoptera ( caddis flies ) about 140-200 mya, coinciding
with the appearance of the first flowering plants. The fossilised
( Nymphalidae )
was found in the
Florissant beds, Colorado, USA and dates from the Oligocene period,
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.
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 may have 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
Charles Darwin went further when in
1859 he published 'The Origin of Species' which theorised that all
animals evolved by a process of natural selection from more basic
life forms. After considerable initial opposition the scientific
world accepted and expanded 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 that adaptations or traits
occur naturally in a percentage of a population due to mutation.
Evolution occurs as the more successful adaptations become dominant,
and former traits are bred out by the natural selection process.
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
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
( 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.
The greatest opportunities for
speciation occur in mountainous regions of the tropics. During an
ice age a particular species might be widespread in the valleys, but
during an interglacial period when global temperatures increase it
would be forced to retreat to cooler habitats at higher elevations.
After a long period of isolation subtle differences in the geology,
topography and climate of different mountains would cause each
population to gradually evolve changes in pattern that increased its
chances of survival, and thus several new species would evolve.
Later, during another ice age all of these species would descend to
warmer habitats, thus increasing the overall number of species.
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
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
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.
'subspecies' a valid taxon?
Subspecies are really nothing more
than convenience names given to geographically isolated races which
have adapted to various environments and evolved new anatomical /
behavioural traits that improve their survival rate. These races are
capable of interbreeding and producing fertile offspring if brought
together in a laboratory. They do not normally interbreed in the
wild due to geographical separation, but there are many cases
particularly in Amazonia where subspecies have been able to expand
their range to the point where they overlap and interbreed with
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 Plebejus
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.
Most biologists regard the species
as the terminal taxon, with the subspecies being a stepping stone, a
transitional taxon which is part of the process of speciation. They
consider that subspecies in time will theoretically evolve into new
species, replacing, or in some cases existing alongside the ancestor