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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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