In this article, efforts will be made to explain genetic erosion, its causes, and reasons for occurrence in both plant and animal species.
The Concept of Genetic Erosion in Agricultural Biodiversity
From the beginning of agriculture, farmers have domesticated hundreds of plant species, and within them, genetic variability has increased due to migration, natural mutations and crosses, and unconscious or conscious selection.
This gradual and continuous expansion of genetic diversity within crops continued for several millennia until scientific principles and techniques began to influence the development of agriculture.
The impact of humans on biodiversity has gradually increased with growing technology, population, production, and consumption rates.
Maxted and Guarino (2006) define genetic erosion as follows: “Genetic erosion is the permanent reduction in richness (or evenness) of common local alleles, or the loss of combinations of alleles over time in a defined area.”
This draws attention to the aspect of local adaptation. However, it is not clear why a definition should specify reductions in either richness or evenness.
The problem with taking too broad a definition to construct an indicator comes in the summing up, the aggregation. Neutral or trivial changes could mask critical changes when summed over loci, genotypes, populations, or species.
Most species that were originally diverse in Nigeria are becoming rare. It is clear that Nigeria’s plant diversity is being seriously eroded as a result of multiple environmental, political, and socioeconomic factors.
These conditions are also reported in other African countries, including those that are signatories to the Convention on Biological Diversity (CBD) of the United Nations Environment Programme (UNEP) (1994) and the Global Plan of Action (GPA) on plant genetic resources of the Food and Agricultural Organisation (FAO) (FAO, 1998).
A singular cause of genetic erosion in crops is often identified as the replacement of local varieties by improved or exotic varieties and species. This is due to the ever-increasing human population, greater competition for natural resources, and some interplay of natural factors.
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Indicators for Estimating Genetic Erosion in Agriculture
Brown et al. (1997) provided a useful list of features or indicators that could be measured singly or in combinations on individuals and populations of a given species in a defined area as part of a systematic effort to monitor changes in genetic diversity in the species.
1. The number of sub-specific entities: Formal taxonomic categories, such as subspecies, and also entities such as ecotypes, chromosome races, and landrace groups, are a useful first approximation of genetic diversity within a species.
2. Population size, numbers, and isolation: Small populations are at greater risk of loss of alleles, increased inbreeding, and extinction due to stochastic events. The number and isolation of populations in an area reflect both overall genetic diversity and how it is structured.
3. Environmental amplitude: The number of distinct habitats or environments in which a species is found in a study area (e.g., based on ecological and climatic classifications) reflects highly adaptive variation.
4. Genetic diversity at marker loci: Advances in molecular biology have produced powerful techniques used as diagnostic tools for investigating genetic variation in plants and animals. Commonly used techniques include RFLP, AFLP, and microsatellite analysis. These methods support conservation and sustainable use of plant and animal genetic resources by measuring diversity structure and temporal changes.
5. Quantitative genetic variation: Additive genetic variance of metric characters within populations reflects variation at multiple loci and measures the ability of a lineage to adapt to changing conditions.
6. Inter-population genetic structure: Markers and quantitative measures can assess not only diversity within populations but also genetic differentiation among populations, a critical component of overall genetic diversity in an area.
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Major Causes of Genetic Erosion in Agricultural Systems
Genetic diversity is dynamic and changes over time. Several issues contribute to genetic erosion in agriculture, including natural selection, dependence on improved crop varieties, poor management practices, habitat loss, and institutional limitations.
1. Natural Selection and Genetic Loss
Natural selection removes some genetic diversity, particularly at the population level. Too rapid a loss, or losses unrelated to natural processes, can create problems for conservation and restoration.
Besides habitat loss and fragmentation, other less obvious influences, such as inbreeding in small populations, cause cumulative genetic erosion. Improper maintenance of ex situ collections can also contribute.
Genetic erosion can occur during seed preparation, sub-sampling, exchange, storage, and regeneration (Sackville Hamilton and Chorlton, 1997).
2. Dependence on Improved Crop Varieties
The spread of modern varieties from crop improvement programs is a major cause of genetic erosion. Evidence from the 1970/71 FAO survey shows significant diffusion of modern cultivars. Landraces adapted to local conditions are most at risk of being lost through habitat destruction or replacement by elite germplasm.
Scientific plant breeding led to the wide distribution of high-yield, homogeneous varieties, suppressing landraces. Traits like yield potential drive farmers’ variety choices.
The Green Revolution continues to reduce genetic diversity. Urbanisation, land-use changes, deforestation, and disasters have caused extensive fragmentation and destruction of crop habitats.
For instance, improved okra (Abelmoscus esculentus) has replaced tall okra (A. caillei), while native varieties of sword bean (Canavalia ensiformis), African yam bean (Sphenostylis stenocarpa), and Lima bean (Phaseolus lunatus) are now becoming rare.
3. Poor Management Practices Affecting Biodiversity
Grazing pressure, fire, and excessive herbicide use negatively affect biodiversity. Fire destroys large forest areas annually, eliminating sensitive species such as Afromosia laxiflora, Ceiba pentandra, Entada abyssinica, Hildegardia barteri, and Holarrhena wulfbergia.
Though fire is common in the savanna, it is increasingly affecting rainforests. Bush burning and hunting for subsistence farming reduce biodiversity, deplete ecosystems, and destroy wildlife. Illegal grazing in game reserves further threatens biodiversity.
External forces promote the use of high-yield varieties, mechanisation, and chemical inputs to increase production and returns.
These change the decision-making process, encouraging monoculture and chemical use.
In such systems, ecosystem functions lost due to low agrobiodiversity are substituted by external inputs, putting agrobiodiversity components at risk, especially where cheaper alternatives exist.
4. Habitat Loss and Its Impact on Genetic Resources
Urbanisation and population growth lead to habitat loss, removing homes of plants and animals. If entire habitats are lost without regeneration or seedbank recovery, immediate genetic diversity loss occurs.
Even in cases where some diversity remains, it often declines in smaller populations. The 1980s famine threatened Ethiopia’s biological resources.
A study by Stephen et al. (2002) showed significant rice diversity reduction in the northeastern Philippines from 1996 to 1998 due to drought and typhoon-related flooding.
Erskine and Muehlbauer (1990) noted that a single drought season may lead to consumption of seed stocks, while prolonged drought alters cropping patterns and crop distribution. War and social disruptions also pose threats of genetic wipeout.
5. Other Institutional Causes of Genetic Erosion
Genetic erosion is also linked to limited support for gene banks and inconsistent institutional focus. In Eastern Europe, gene bank activities were reduced significantly towards the end of the last century, weakening conservation efforts and risking further loss of agricultural biodiversity.
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