Lec. 19 population variations
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Variations within populations can be either genetic or non-genetic. Non-genetic variations include individual variations over time (age or seasonal variations), social variations, and ecological variations caused by different habitats or environmental conditions. Genetic variations are inherited and include sexual dimorphism where males and females differ, gynandromorphs which have both male and female characteristics in different parts of the body, and intersexes that have mosaic sex characteristics. Understanding these natural variations is important for taxonomists to accurately identify and classify species.
Lec. 19 population variations
- 1. Population Variations DR. MUHAMMAD RASOOL
- 2. Variations Variations within a population is known as intrapopulational variations. Different individuals belonging the same species may be very different like some species contains various phena. Often a phenon of one species resemble a corresponding phenon of another species much more closely that it does with any other phenon of the same species. This is called intrapopulational variations or individual variations. For example, the females of some species birds and insects more closely resemble the females of other species than the males of its own species. These individual varaints is a source of confusion for most of taxonomists. Thus a thorough knowledge of individual variations is essential for taxonomist.
- 3. Types of Variations 1. Non Genetic 2. Genetic
- 4. Non Genetic Variations Those variations in an individual that belong to same interbreeding population and are non-inherited are called non genetic variations or extrinsic variations. In preserved museum specimen it is impossible to determine whether a given variant has a genetic basis or not. Nevertheless, it is important for the taxonomist to understand that many types of variation exist and it is usually possible to make valid conclusion about the status of a given variant on the basis of field observation and available experimental evidence.
- 5. Types of Non Genetic Variations Individual Variations in Time: individual variations in time may be either age variations or seasonal variations. Age Variations: There are many larval stages in the life of some animals during which they are quietly different, which cause confusion to taxonomist. In reptiles, birds and mammals there are no larval stages but still their young ones differ from the adult stages. In fishes the young forms are so different from the adult that they are often placed in different families. For example, young eel (Anguilla) and caterpillars are quite different from their adults. The free living larvae of sessile coelenterates, the larvae of molluscs, echinoderms and insects are quite different from their adults. The cercaria stage larva of liver fluke does not resemble the adult, similarly in young deer there are less antlers than old deer, all this cause confusion to taxonomists. Seasonal Variations in an individual: The animal appearance varies with the seasons during the year. Many birds like ducks, shore birds, warblers and tangers have bright nuptial plumage during breeding season which become dull after the breeding season. Aortic fox have grey colour in summer and white appearance during winter. Similarly polar bear have grey appearance in summer and white in winter.
- 6. Types of Non Genetic Variations Individual Variations in Time: Seasonal Variations in consecutive generations: Many species of short lived invertebrates, particularly, insects produce several generations in the course of year i.e. the new forms of spring are quite different from the young ones produced in summer. Similarly the individuals produced in the wet season differ from those produced in the dry season. In some tropical butterflies this phenomenon of dry and wet weather is more prominent. Cyclomorphosis: It is a special kind of seasonal variation where the population of a species undergo regular morphological changes throughout the season due to different kind of food, changes in temperature etc. This phenomenon is present in fresh water organisms like Daphnia, Rotifers and Cladocerans.
- 7. Non Genetic Variations Social Variations: In social insects such as bees, wasps and termites have different forms of individuals in their colonies (Queen, Worker, Drone and soldier). These different forms are due to different food or hormonal control. Ecological Variations: The different forms of a species produced due to different environmental conditions. It may be of the following types. Habitat Variations (Ecophenotypic): Population of a single species that occur in different habitats in the same region are often visibly different. In snails and mussels there are different forms in different habitats and often they are considered as different species.
- 8. Non Genetic Variations: Ecological Variations induced by temporary Climatic Conditions: Some animals like fish have highly unusual phenotypic conditions throughout the year due to unusual conditions like drought, cold and food supply. Host-Determined Variations: The same species of a parasite have different forms in different hosts. As in different hosts, the conditions are different so the parasite morphology is changed accordingly to adapt to the host body conditions. Ebeling in 1938 studied variations in the scale insect “Lecanum corni” grown on different hosts. Those of the apricot were having large bodies and short appendages as compared to those from Christmas berry which was having small bodies and long appendages.
- 9. Non Genetic Variations: Ecological Density Dependent Variations: The effect of crowding is some time reflected in morphological variation. Uvarov (1921) has shown that gregarious species of locusts exist in three unstable biological phases; solitary, gregarious and transitional. These phases differ in anatomy, colour and behaviour causing problems for the taxonomists. Allometric Variations: It is the disproportionate growth of a structure in relation to the rest of the body. This phenomenon is well known in insects which involves disproportionate growth of head in ants, mandibles in stag beetle etc.
- 10. Non Genetic Variations: Ecological Neurogenic or Neurohormonal Variations: It refers to colour change in individuals in response to environment. These changes are accomplished through the concentration or dispersal of colour bearing bodies called chromatophores. Such variations were first studied in chameleon. The same phenomenon of colour change called metachrosis is also found in crustaceans, cephalopods and cold-blooded vertebrates.
- 11. Non Genetic Variations Traumatic (Injury or Disease) Variations: Traumatic variations occur with varying frequency in different groups of animals. The abnormal nature of this type of variation is usually obvious, but in some cases it is subtle and may misleading. There are three types of this variation. Parasitic induced Variations: Apart from the familiar effects of parasitism as swelling, distortion and mechanical injury, parasites also produce conspicuous structural and morphological variations in the host due to parasitism. For example, when stylop (bee parasite) enters the bee, the head size of the bee is reduced abdomen enlarges and wing venation changes.
- 12. Non Genetic Variations Accidental/Terotological Variations: Accidental variations are usually extremely induced although it may work internally through some developmental or hormonal system. Such variations are extremely diverse and in most animals may be readily identified, because such individuals deviate markedly from the type on those forms which undergo metamorphosis. Injuries to an earlier stage may produce later abnormalities which are not easily recognized. For example, in beetles certain types of pupal injury may produce symmetrical abnormalities in punctation, surface sculpturing or segmentation of the appendages. In butterflies it may result in symmetrical modification of wing pattern. In most cases even with such clearly notable differences the abnormal nature of the variation may be detected by a specialist without much difficulty. Post-mortem changes: In many groups of animals, it is impossible to prevent post-mortem changes of preserved specimens. For example, orange yellow bird of paradise fade to white in collections. The Plumage of the chines jay white in life, but turns to blue in collection due to loss of volatile yellow component in the pigment. Similarly yellow wasp when exposed to cyanide for preservation it gets bright red.
- 13. Genetic Variations The variations in an individual that belong to the same interbreeding populations and which are inherited are called Genetic, inherited or intrinsic variations. There are three types of genetic variations. Musca Spp
- 14. Genetic Variations: Types Sexual Dimorphism The difference between male and female is known as sexual dimorphism. In this case the male and female are differentiated from one another. These are sex associated variations which may be Primary Sex difference: The difference between male and female at the time of birth is called primary sex difference like gonads and genitalia. Secondary Sex difference: The difference between male and female which occurs after puberty is called secondary sex differences. For example, the king parrot, Electus roratus, of the Paupan region in which the male is green with an orange bill and the female is red with black bill. Both of these male and female were considered as different species for nearly 100 years which was later on proved by naturalist that they bonged to the same species. Similarly the males of African ant Dorylus were so unlike than other ant species that they were not considered as ant for a long time ant was put in different family. Similarly in some wasps, the females have small wings and males have long and are so different that some taxonomists used a different nomenclature for the two sexes.
- 15. Genetic Variations: Types Sexual Dimorphism King Parrot: Male and Female
- 16. Genetic Variations: Types Gynandromorphs The individuals that show male characters in one part of the body and female characters in other part of the body. Thus the two halves of the body have different sexes. In these organisms the sex characters are scattered in mosaic form. They are produced by unequal somatic distribution of chromosomes, particularly sex chromosomes. For example, Gynandromorphs of alfa alfa butterfly, (Papilis dardanus) has the left wing of female and the right wing of male.
- 17. Genetic Variations: Types Intersexes: Intersexes are individuals with mosaic sex. These are likely to exhibit a blending of male and female characters. It is due to either hormonal imbalance or due to male and female tendency genes. Intersexes have been studied in Lymantria (genus of moth).
- 18. Genetic Variations: Types Reproductively Different Generations In many insects one generation alternates the other generation and each generation differs from one another, this is very confusing to taxonomists. For example, in genus Cynips (gall wasps) the agamic generation is so different from the bisexual one that it has given different scientific names. Similarly in the aphids (plant lice) the parthenogenetic wingless females ae very different from the winged females of sexual generation.
- 19. Genetic Variations: Types Ordinary Genetic Variations These variations do not involve sex characters. It is of two types.
- 20. Genetic Variations: Types Ordinary Genetic Variation These variations do not involve sex characters and is of Two types Discontinuous Variations or Poly Morphism and Continuous Variations Polymorphism: The same individual with different shapes (appearance) are polymorphic individuals and the phenomenon is called polymorphism. In many species of Hemiptera and Coleoptera, the same population contain flying and flightless individuals. The spotting in lady beetles (Coccinellidae) is well known example of genetic polymorphism, as industrial melanism in moths. Alfalfa butterfly, Colias eurytheme, have different female forms, on is white and the other resembling the orange-coloured male. The African Swallowtail butterflies of the genus Papilio have one male form and five female forms. Alfa alfa Butterfly Melania Snail
- 21. Genetic Variations: Types Ordinary Genetic Variation Continuous Variations The commonest type of individual variations is due to slight genetic differences among individuals. No two individuals (except monozygotic twins) in a population are exactly alike genetically or morphologically. These differences are very slight and only detected by special techniques. A model study of variability based on 2877 skins of the house sparrow (Passer domesticus) has been presented by Selander and Johnston (1967). Among their skin colours each character showed different degree of variability within a single population. The species of the snail genus Melania (fresh water and brackish water) have been described largely on the basis of shell characters, so some have spines and spiral ribs while absent in other members of the genus. Alfa alfa Butterfly Melania Snail