|What are animal genetic resources?||| Print ||
Genetic diversity, species, breeds, strains and populations
Diversity in general usage refers to the range of differences among some set of entities. Biological diversity thus refers to variety within the living world-the biosphere. The term biodiversity is commonly used to describe the number, variety and variability of living organisms. Diversity can only be measured if some quantitative value can be ascribed to it and these values compared. To do this, biodiversity is divided into its constituent elements, i.e. genes, species and ecosystems which correspond to three fundamental and hierarchically related levels of biological organisation.
The most common usage of the word biodiversity is as a synonym of species diversity or species richness. This is perhaps because the living world is most widely considered in terms of species (see photo set 1). Thus, discussion of global biodiversity is typically presented in terms of global numbers of species in different taxonomic groups. Estimates for the total number of species currently existing on earth vary from 5 million to nearly 100 million. A conservative working estimate suggests there might be around 12.5 million. Of these, only an estimated 1.7 million have been described to date. In terms of species number alone, life on earth appears to consist essentially of insects and micro-organisms!
Using species as the level at which to consider diversity does have disadvantages. Species cannot be recognised and enumerated by systematists with total precision. Moreover, the concept of what a species is differs considerably between groups of organisms (see photo set 2). Worse still, enumeration of the number of species alone, if not supported by data on the diversity within species, may be misleading. That is, a count of the number of species only provides a partial picture of biological diversity. Implicit within the term species is the concept of degree or extent of variation; that is, organisms that differ widely from each other in some respect by definition contribute more to overall diversity than those which are very similar. The more different one species is from another, the greater its contribution to the overall measure of global biodiversity.
Genetic diversity represents the heritable variation within and between populations of organisms. The populations may be entire species or a specific collection of individuals within a species such as a breed, strain, line, herd/flock etc. The diversity ultimately resides in the variations in the sequence of the four base pairs which, as components of nucleic acids, constitute the genetic code. New genetic variation arises in individuals by gene and chromosome mutations and, in organisms with sexual reproduction, is spread through the population by recombination.
The genetic diversity-the pool of genetic variation-in an interbreeding population is acted upon by selection, be it natural or artificial. Differential survival results in changes of the frequency of genes within the population and this constitutes population evolution. The significance of genetic variation is thus clear: it enables both natural evolutionary change and artificial selective breeding to occur.
The term animal genetic resources (AnGR) is used to include all animal species, breeds and strains that are of economic, scientific and cultural interest to humankind in terms of food and agricultural production for the present or the future. Another equivalent term increasingly used is farm animal genetic resources. There are more than 40 species of animals (Table 1) that have been domesticated (or semi-domesticated) during the past 10 to 12 thousand years which contribute directly (through animal products used for food and fibre) and indirectly (through functions and products such as draft power, manure, transport, store of wealth etc.) [Mammalian species-the evolutionary relationships]. Common species include cattle, sheep, goats, pigs, chickens, horses, buffalo, but many other domesticated animals such as camels, donkeys, elephants, reindeer, rabbits etc. are important to different cultures and regions of the world (see Module 1, Section 4).
Table 1. List of animal species used for food and agriculture
a Not domesticated.
Domestic animal diversity (DAD) refers to the genetic variation or diversity existing among the species, breeds, strains and individuals for all animal species which have been domesticated to meet human needs for food and agricultural production, and their immediate wild relatives.
The concept of a breed, in which all members have a pedigree tracing their ancestry, was developed primarily in Western Europe during the 18th century. Today, in the developed world breeds are recognised as distinct intra-specific groups, the members of which share particular characteristics that distinguish them from other such groups, and formal organisations usually exist for each breed or breed group. In its strictest sense, a breed designates a closed population-mating pairs are drawn only from within the population and relationships among individuals are documented. Members of a breed have developed under the same selection pressures and share common ancestry. However, breeds are rather dynamic and, in many instances, are not completely closed populations. Changes in breeding objectives also affect breed characteristics over time (see photographs below).
Photograph 1: Differences possible in livestock breeds and species over time and space.
The term breed as a formal designation often has little meaning outside areas of Western influence, where pedigree recording is often non-existent. However, even under these circumstances, there exist strains or geographically separated interbreeding populations, or populations separated by cultural or community 'boundaries' or differential preferences for specific animal attributes. In any case, breed as a concept is rather complex in the context of developing countries. In developing countries, examples of breeds that could not fit the Western world definitions of the same include the East African shorthorn zebu, which is found in most parts of East Africa (see different breeds/strains of East African Zebu cattle). Others include the Yellow cattle found all the way from South China, Lao, Thailand to Malaysia, the Djallonké sheep of West Africa [Breed information]; and the Grass-cutter also of West Africa [CS 1.32 by Mensah and Okeyo] [Breed Information].
Livestock populations developed in different ecological or geographical areas will become genetically distinct as a result of genetic drift and differential selection pressures, provided they have also been reproductively isolated from other populations developed under different conditions. Thus the indigenous livestock from different regions of the world should probably be assumed a priori to represent different 'breeds'. It seems clear that populations with different adaptive characteristics or possessing unique physiological characteristics should be recognised as different breeds. This distinction should be drawn even if the populations are shown to be relatively closely related based upon measures of genetic distance [Chinese pig breeds].
Within a breed there may be differentiation between populations due to differences in selection objectives. The best example in temperate breeds is the Holstein-Friesian. The Holstein of the USA is a bigger animal producing much more milk, but with less butter fat and protein contents than Friesian strains previously found in Europe which, in the middle of the last century, were developed into smaller dual purpose (meat and milk) animals. The [Friesian strain in New Zealand] is also unique, as it has been selected to produce milk from pasture-based production systems. Examples in Africa include the Boran/Borana (see Kenya Boran bull and Ethiopian Borana cow) strains in Kenya and Ethiopia and the strains of Djallonké sheep in several countries in West and Central Africa [Djallonké sheep]. Increasingly, the move is to recognise strains found in different countries as distinct breeds (see Breed information); [DAD-IS]; [DAGRIS]. This is principally a response to the Convention on Biological Diversity which emphasises national ownership of genetic resources.
The World Watch List for Domestic Animal Diversity [WWL-DAD] prepared by the Food and Agriculture Organization of the United Nations (FAO) in 1993, and which has since been revised two times (1995 and 2000), has defined a breed as: either a homogenous, sub-specific group of domestic livestock with definable and identifiable external characteristics that enable it to be separated by visual appraisal from other similarly defined groups within the same species, or a homogenous group for which geographical separation from phenotypically similar groups has led to general acceptance of its separate identity.
Phenotypic and molecular characterisation contribute to breed classification
Clearly, the definition of a breed is, to say the least, a bit 'woolly'. More challenges creep in when degree and time of reproductive isolation between populations cannot be clearly determined. When breed identity is documented through pedigree records, one can presumably document the time of genetic isolation and thereby place some boundaries on likely distinctiveness between candidate breeds. However, a relatively small proportion of the world's livestock is listed in herd books. When potential 'breeds' are physiologically similar and have overlapping and, often, large ranges, we should probably then utilise measures of genetic relatedness to help sort out breed distinctions. Thus, if we have basically similar animals across a wide area (for example, fat-rumped sheep in Africa or the so called 'Small East African Zebu' [CS 1.10 by Okomo]; [Yak ILRI] or the South East Asian Yellow cattle with little phenotypic variation among populations and little reproductive isolation between adjacent populations, estimates of genetic distances among populations at the extremes of the range may be very helpful in assigning estimates of genetic uniqueness and, more importantly, in assigning conservation priorities relative to other populations. Where herd books exist that appear to document genetic uniqueness among breeds, measures of genetic distance can supplement this information in situations where breeds appear otherwise quite similar.
Thus, we work at two levels. First, there are the populations that are clearly distinct and unique based on adaptations and physiology. Here, there are degrees of 'distinctiveness', but either a group of knowledgeable breeders and scientists or a good discriminate analysis should be able to isolate the distinct genes and determine the extent of shared genes.
Next, are the populations that are not so easily discriminated and whose genetic uniqueness must be determined. It is at this level that 'breed' distinctions become a combination of genetic and cultural distinctions (Ankole-cattle strains) of the various strains of Ankole strains of cattle. Since, in these cases, we now have very imperfect information regarding genetic relationships and breed characteristics, including production and reproductive traits as well as adaptive attributes, we would do well to recognise the cultural definition as potentially valid until we have better information. This is basically the principle underlying ongoing AnGR research work in many laboratories/institutions worldwide, including ILRI. That is, to both recognise breeds when the owners claim that they are distinct and to proceed to attempt to acquire objective measures of genetic relatedness. In making these distinctions among breeds, and especially when several apparently similar breeds are found in the same area, a population can be accorded tentative breed identity when groups of farmers in the area can be identified who:
|Last Updated on Wednesday, 02 November 2011 13:23|