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VARIATION Diversity within the same species

Paul Smart - The illustrated encyclopedia of the Butterfly world

 

Part 1: THE LEPIDOPTERA - Their origin and classification

 

Part 2: THE BUTTERFLY BODY - Its structure and function

Part 6: GENETICS –The mechanism of inheritance

Part 3: THE BUTTERFLY LIFE CYCLE - From egg to adult

Part 7: COLORATION - For concealment and advertisement

Part 4: ECOLOGY – Butterflies and their environment

Part 8: VARIATION – Diversity within the same species

Part 5: BUTTERFLIES MOBILITY - Distribution and migration

Part 9: MIMICRY - The art of impersonation

Variation is a fundamental characteristic of all animals. It is frequently overlooked that members of a species are not all  stereotyped replicas of their parents. They are not like mass-produced objects stamped out on a machine; they are living creatures, each individual being the embodiment and expression of a unique combination of genetic material.
Seen in this light, variation between individuals becomes less unexpected and it is certainly true that butterflies exhibit this trait more clearly than most other animals. Indeed, the range of variation in the patterns on the wings of some butterflies is so great that it may be very difficult to find two identical examples in the same species. Some species, notably those which include butterflies with pronounced patterns or markings, have a much greater tendency towards variability than others. Variations are also more common
in some families than others; and they are particularly prevalent in certain areas, especially where isolated populations occur. Indeed, French entomologists in the 19th century referred to Britain as le ays de varieties because of the comparative abundance of unusual forms which were to be found there. The abnormal forms may bear exaggerated or enhanced markings which make them even more attractive than the typical form  of the butterfly. This has led to entomologists specializing in the study of variations and devoting a great deal of effort to accumulating examples in collections of specimens, after the fashion of philatelists who collect different  watermarks on stamps.
There has been a tendency in the past for entomologists to justify these activities by creating a vast number of names, one for every distinguishable form of a species, when there are only tiny differences in detail. This is no more valid than inventing special scientific names for all the different minor variations among humans, as though blondes were fundamentally different from brunettes. Nevertheless many variations in both the colour pattern and the wing shape and size of butterflies are of interest and are produced by a number of well-known biological phenomena. The factors that may effect such variations can be internal or genetic, or they may be related to external influences in the butterfly's environment.
Geographical variation.
The variability of a species is most easily appreciated by examining a series of specimens of the butterfly taken from all over its geographical range, and from a variety of different habitats which it occupies within that range. In the case of a variable species it may be found that the wing size varies depending on where it was collected and that it may be darker coloured in some localities than in others. Sometimes it is possible to correlate these visible differences in the butterfly's appearance with features in the environment such as the type of food plant available, the soil type or geology or the altitude or latitude. The differences between butterflies at the extremes of the' range become more pronounced when the species is very widespread. Where there is a tendency for geographical isolation - that is, where physical barriers such as the sea, desert and mountain ranges prevent the meeting of butterflies of the same species living in different areas - local varieties often become very distinctive and common. When the isolation is total and when the physical separation has existed for a long period of time then each isolated group of a species tends to develop its own peculiar features, becoming a distinct and recognizable race or sub-species. When this process continues for a much longer period of time a separate species
may be formed.
The example shown on page 66 illustrates three sub-species of the birdwing butterfly Ornithoptera priamus, which occur in different regions of the Australasian archipelago. These widely different forms were selected to make the point dramatically obvious, but O. priamus  is even more variable than this, for at least a dozen separate green forms alone have been distinguished.
The apollo butterfly Parnassius apollo (Papilionidae) is probably the bestknown example of a variable species in Europe. This very attractive butterfly inhabits hillsides and mountain meadows throughout Europe and parts of Asia. It has tended to become isolated on different mountains or mountain ranges and a great many different local races have been described as a result of the great variability in the intensity of the black markings and the size and number of red spots.
In many instances the varieties of a species are not distinctly separate and the species shows a continuous gradation of forms over its geographical range. A progressive gradation from one extreme form to another from a distant area is described as a cline. An example of this clinal variation may be found in the large health Coenonympha tullia in the British Isles. Southern forms of this butterfly have more spots and a deeper ground colour than the northern examples but there is no clear demarcation between the
forms, and many subtle blends of intermediate coloration are found in localities lying between the geographical extremes.
In places where a number of closely related species of the same genus occur in different geographical areas the inter-relationship can be indicated by the use of a group name. Members of a species-group are not connected by intermediate forms but they are very closely related and are presumably derived from a common ancestral stock. These groups may be referred to by the name of the 'focal' species; for example, the various members of the Indo-Australian genus Troides (Papilionidae) are subdivided into the amphrysus species group, the helenaspecies group and so on. Similarly there are sub-species groups, which are more recently evolved, where the
individuals in the group are usually connected by intermediate forms.

 
 
 
The Red Pierrot Talicada nyseus - Picture: Phung My Trung
 

Variability, particularly among closely related species, makes it very difficult indeed to decide which animals belong to which species or sub-species, and indeed the whole demarcation of groups becomes very blurred. The Nymphalid butterflies of the genus Agrias are a case in point. A selection are shown on page 69' treated as one particular form (f. narcissus) of Agrias aedon. However, they also show some affinity with a form of a quite different species, A.claudina. In cases such as this, classification and naming of butterflies gets very confusing and becomes more a matter of opinion than an exact science.
Seasonal variation
Butterflies which produce more than one brood during a year frequently exhibit seasonal variation, especially if the different broods occur in widely different climatic conditions. In temperate regions there may be differences between the spring (vernal) and summer (aestival) forms of the same species. In the tropics there is not such a pronounced difference in temperature throughout the year as in higher latitudes but there are still distinct seasons associated with periods of rainfall or drought. The pattern of wet and dry seasons may be very pronounced and also very regular.
There are many examples of butterflies that show a variation in their colour pattern during the different seasons. It would appear that the critical factor that determines the colour pattern is the average temperature during the developmental stages of each brood of the butterfly, though humidity may also play a part. The effect of temperature is not constant for all families, for while many double-brooded Nymphalids produce darker form s at lower temperatures the reverse is true for many Pierids.
Seasonal dimorphism may also include variation in the size and shape of the wings. In the most extreme examples the seasonal forms may appear so dissimilar that it is difficult to believe that they belong to the same species.
The European map butterfly Araschnia levana (Nymphalidae)  has strikingly different seasonal forms. The spring brood (f. levana)resembles a fritillary on its upper side while the second generation (f. prorsa)resembles a small white admiral with blackish wings marked with white bars. The under sides are similar in the two broods. In this species environmental temperature  appears to affect the pigmentation indirectly by influencing the rate of development.
Another very striking example of seasonal dimorphism (see page 68) is found in an African Nymphalid, the gaudy commodore Precis octavia. This species has a wet season form  which is predominantly orange with black markings and a dry season form which is predominantly black and blue with virtually no orange at all. In parts of Africa which have pronounced wet and dry seasons the wet season form is abundant during the wettest months but declines and is replaced by the dry season form as the climate becomes more arid. It seems possible that it is the rate of seasonal change which stimulates the production of seasonal forms rather than absolute temperature. The reasons for the different colour forms of this butterfly are not immediately apparent yet the colour patterns must serve some function that improves the survival of the butterfly, or at least does not hinder it. Perhaps the answer lies in the way in which the butterfly behaves during the spells of drought. Dry seasons in the tropics correspond to the temperate winter in so far as they present severe environmental conditions for many plants and animals. Species dependent on moisture may become inactive and aestivate during months of drought conditions. Precis octavia commonly shelters and aestivates amongst rocks, and in such places the blue dry season form would appear to be better camouflaged than the rather conspicuous wet season form. A similar case is found in the closely related species Precis pelarga, in which the dry season forms of the butterfly have more elaborate wavy margins to the wings, and cryptically coloured under sides which presumably offer them greater protection while they are aestivating among dead leaves.
It is quite possible to induce artificially the different seasonal forms when the above species are reared in captivity, by experimentally altering the temperature or humidity regimes during development. It is also possible to induce abnormal colour forms by subjecting the larval and pupal stages to extremes of temperature. The dark 'melanic' forms of Nymphalidae (see page 74) have been artificially produced in this way, by using extremely low
temperatures. When the same experiment is tried using high temperatures, varieties with reduced dark areas are produced. In all such experiments the mortality rate is very high and such extreme examples are rare in the wild.
Another way in which the final colour pattern of a butterfly may vary as a result of external factors is if the insect is accidentally injured at a critical stage in its development. Such an injury is likely to result in a lack of some or all pigment from the affected area. These 'bleached' forms are particularly common in the meadow brown Maniola jurtma.
Genetic factors
Many variations in the colour patterns and appearance of butterflies are caused by genetic factors. Some genetic variants may be abnormalities or aberrant varieties while others are quite normal. The genetic process of inheritance has been dealt with in Chapter 6. This section deals with the types of variants produced by the genetic composition of butterflies.
Sexual dimorphism, or differences in the appearance of the male and female, is the most familiar example of genetically produced variation. In some species there may be very little difference between the sexes, but females are usually larger than their corresponding males. However, there are many instances of species in which the

 
 
 
Common Windmill Byasa polyeuctes - Picture: Phung My Trung
 

 

proportion, however small of the total population. Aberrations are variations of irregular or random appearance, and where special names have been assigned to them these are preceded by the abbreviation 'ab'.
Melanism
Some variations are due to spasmodically occurring disturbances in the normal pigmentation mechanism of butterflies. Melanic variants are an example. Melanins are pigments which are responsible for the black or brown colours in butterflies. They are produced during development by the action of an oxidizing agent, tyrosinase, on the colourless amino acid tytosin. The distribution of tyrosin may be influenced by genetic factors to produce butterflies in which the dark markings are much extended to give an overall dark appearance. One of these dark, or melanic, forms is shown opposite. Melanic forms of a number of the machaon group of swallowtails (Papilionidae) occur as rare varieties and are probably genetic recessives. In
part of the range of Papilio polyxenes a melanic form is recurrent and it has been suggested that this may be a reversion to an ancestral type. This is supported by the theory that the drab brown females of many butterfly species have retained the 'primitive' colour form for protection, while the males have evolved bright conspicuous patterns.
Albinism
Aberrant varieties in which a red or yellow ground colour is replaced by white are not strictly albinos. Albinism is the absence of the pigment melanin alone. Such a condition is rare in butterflies but has been isolated in breeding experiments and is therefore an inheritable factor. An albino form of the meadow brown Maniola jurtina is shown opposite.
Homeosis
Homeosis is another type of variation in which the normal developmental processes have become muddled. Homeotic specimens have part of the fore wing appearance exhibited on the hind wing or vice versa. This assumption of the form and character proper to another part of the body structure may be due to injury at an early stage of development. Extreme examples are rare. A specimen of the clouded yellow Colias crocea (Pieridae), where the dark central spot, appropriate to the fore wing, is reproduced on the hind wings, is
shown opposite. Strictly, this is also an example of heteromorphosis, since in the right fore wing this dark colour has also appeared in the 'wrong' area but on the same wing. Homeosis is, however, the general term used to describe both these phenomena.

 

 
 

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