OCR Specification focus:
‘Calculate genetic diversity in isolated populations, e.g., percentage of polymorphic loci; apply to zoos, rare breeds and pedigree animals.’
Genetic biodiversity refers to the variety of genes within a species, shaping its adaptability, resilience, and survival. Understanding and assessing this variation underpins effective conservation and species management.
Understanding Genetic Biodiversity
Genetic biodiversity describes the total number of different alleles present in a species or population. It represents variation in DNA sequences, leading to differences in allele combinations, phenotypes, and adaptability among individuals.
Genetic biodiversity: The range of genetic characteristics present within a species, population, or individual, contributing to variation and evolutionary potential.
A population with high genetic diversity possesses many alleles for each gene locus, whereas a population with low genetic diversity has few alleles and limited genetic combinations. High diversity increases the likelihood that some individuals will survive environmental changes, disease, or other pressures.
Importance of Assessing Genetic Biodiversity
Assessing genetic biodiversity allows scientists to:
Monitor the health of populations, particularly those that are isolated or endangered.
Guide breeding programmes in zoos, conservation projects, and for rare or pedigree animals.
Identify risks of inbreeding, genetic drift, or loss of adaptive potential.
Inform policies for species conservation and biodiversity management at both local and global levels.
Populations with reduced gene pools may struggle to adapt to environmental changes or resist new diseases, making assessment of genetic variation essential.
Methods of Assessing Genetic Diversity
Scientists assess genetic biodiversity by examining the variation at gene loci within a population. The most common method used in the OCR specification is the percentage of polymorphic loci.
Polymorphic locus: A gene locus that exists in more than one form (allele) within a population.
A monomorphic locus has only one allele, while a polymorphic locus has two or more alleles.
A labelled diagram showing a SNP (G/A) at a single base-pair position, illustrating how two alleles can exist at the same locus within a population. This directly underpins the concept of polymorphic loci used to quantify genetic diversity. The figure is minimal and free of extraneous detail. Source.
The greater the number of polymorphic loci, the higher the genetic diversity within that population.
Quantitative Assessment: Percentage of Polymorphic Loci
EQUATION
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Percentage of polymorphic loci = (Number of polymorphic loci ÷ Total number of loci) × 100
Polymorphic loci = Gene loci with two or more alleles present in the population
Total loci = Total number of loci examined in the study
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A population with a higher percentage of polymorphic loci demonstrates greater genetic diversity. Conversely, a low value indicates limited genetic variation and a higher risk of inbreeding.
Genetic Biodiversity in Isolated Populations
Isolated populations—those separated geographically, behaviourally, or reproductively—are particularly vulnerable to reduced genetic diversity. Over generations, limited gene flow leads to inbreeding, loss of alleles, and fixation of harmful mutations.
Causes of Isolation
Habitat fragmentation: Urbanisation, agriculture, or deforestation cutting off subpopulations.
Captive breeding: Zoos or rare breed programmes with small breeding groups.
Geographical barriers: Mountains, rivers, or oceans restricting gene exchange.
Consequences of Reduced Genetic Diversity
Inbreeding depression: Accumulation of deleterious alleles reducing fertility and survival.
Reduced adaptability: Lower potential for natural selection to act effectively.
Increased extinction risk: Small populations cannot respond to environmental pressures.
Applications: Zoos, Rare Breeds, and Pedigree Animals
Assessing genetic diversity is crucial in conservation and breeding programmes to maintain viable, healthy populations.
Zoos
In zoological collections, individuals are often from small founder populations. Breeding programmes such as studbooks track genetic lineages to:
Maximise heterozygosity (presence of different alleles at gene loci).
Avoid inbreeding by pairing genetically distinct individuals.
Maintain representative genetic variation of wild populations.
Studbooks enable coordinated breeding between institutions, preserving rare alleles and maintaining population viability.
A standard pedigree diagram showing generations, mating lines, and offspring, as used in conservation breeding programmes. Such charts help managers avoid close matings and maintain genetic diversity across small captive populations. The figure focuses on general pedigree conventions only, without disease-specific extras. Source.
Rare Breeds
In agricultural species, traditional or rare breeds may have unique genetic traits valuable for future breeding or resilience. Monitoring their genetic diversity ensures:
Conservation of distinctive traits (e.g., disease resistance, climate tolerance).
Prevention of allele loss due to overuse of commercial breeds.
Maintenance of a broad genetic base in domesticated species.
Pedigree Animals
Selective breeding of pedigree animals, such as dogs or horses, often reduces genetic diversity. Breeders must:
Avoid excessive inbreeding to prevent inherited disorders.
Maintain genetic variation through outcrossing when necessary.
Use genetic testing to identify carriers of harmful alleles and plan ethical breeding.
Molecular Techniques in Genetic Biodiversity Assessment
Modern molecular tools enable precise measurement of genetic variation across individuals and populations.
Common Techniques
DNA sequencing: Determines nucleotide differences between individuals.
Microsatellite analysis: Examines short, repetitive DNA regions that vary between individuals.
Single nucleotide polymorphism (SNP) analysis: Detects small genetic variations across the genome.
Gel electrophoresis: Separates DNA fragments by size, allowing comparison of allele patterns.
These methods provide accurate data on allele frequency, heterozygosity, and genetic distance between populations, complementing classical calculations like the percentage of polymorphic loci.
Stylised microsatellite genotyping results: left, gel electrophoresis lanes with major and minor (stutter) bands; right, an automated capillary electropherogram with allele peaks. These outputs are used to score alleles at loci and quantify population genetic diversity. Extra detail shown (stutter peaks and capillary traces) goes slightly beyond OCR depth but remains useful context. Source.
Maintaining Genetic Biodiversity
Once genetic diversity has been assessed, conservation strategies aim to preserve or enhance it through:
Habitat connectivity: Encouraging gene flow between isolated groups.
Managed breeding: Rotating breeding partners to maintain allele diversity.
Population monitoring: Regular genetic testing to detect losses of diversity.
Genetic rescue: Introducing individuals from other populations to increase variation.
Implications for Conservation
High genetic biodiversity supports species’ long-term survival by enabling adaptation, disease resistance, and evolutionary potential. Understanding and quantifying it through measures such as the percentage of polymorphic loci is therefore central to modern conservation biology and aligns directly with the OCR A-Level Biology specification for this subsubtopic.
FAQ
A population bottleneck occurs when a large population is drastically reduced in size due to an event such as disease, habitat loss, or natural disaster. The surviving individuals carry only a small fraction of the original genetic variation.
As a result:
Many alleles, especially rare ones, are lost.
Genetic diversity decreases significantly.
The new population is more vulnerable to environmental changes and inbreeding depression.
Even if the population size later increases, its gene pool remains limited, reducing long-term adaptability.
Both measure aspects of genetic diversity but describe different levels of variation.
Heterozygosity refers to the proportion of individuals in a population that are heterozygous for a given gene locus.
Percentage of polymorphic loci describes how many gene loci in total are polymorphic (have more than one allele).
Heterozygosity focuses on the genotype of individuals, while polymorphic loci assess diversity across the entire genome. Together, they give a fuller picture of genetic variation.
In captivity, effective population size is often smaller than the actual number of individuals due to:
Unequal breeding success (some individuals contribute more genes).
Selective pairing for desired traits.
Limited founder population.
These factors cause genetic drift, where alleles are lost by chance over generations, reducing diversity despite apparent population stability. Regular genetic monitoring and exchange of individuals between institutions can mitigate this loss.
Inbreeding increases the likelihood that offspring inherit identical alleles from both parents. This raises the chance of homozygosity for recessive harmful mutations.
Consequences include:
Reduced fertility and growth rates.
Higher rates of genetic disorders.
Lower resistance to disease and environmental stress.
The overall result is decreased population fitness and survival, particularly in small or isolated groups where genetic diversity is already low.
In small, isolated populations, random fluctuations in allele frequency have a stronger effect. Genetic drift can quickly lead to loss of alleles, even beneficial ones.
In contrast, large populations buffer these effects because random changes are averaged out across many individuals.
Over time, genetic drift in small populations reduces variation, increasing the chance of fixation of harmful alleles and making the population less adaptable to environmental changes.
Practice Questions
Question 1 (2 marks)
Explain what is meant by the term polymorphic locus and state how the proportion of polymorphic loci can be used to assess genetic biodiversity within a population.
Mark Scheme
1 mark: Polymorphic locus correctly defined as a gene locus that has two or more alleles present within a population.
1 mark: Recognition that a higher proportion of polymorphic loci indicates greater genetic biodiversity or genetic variation within the population.
Question 2 (5 marks)
Conservation biologists compared the genetic diversity of two isolated populations of a rare mammal species.
Population A had 20% of its loci polymorphic, whereas Population B had 55%.
Discuss what these results suggest about the genetic health of each population and describe how conservationists might act to maintain or increase genetic biodiversity in Population A.
Mark Scheme
1 mark: Correct interpretation that Population B has higher genetic diversity than Population A (based on proportion of polymorphic loci).
1 mark: Identification that higher genetic diversity increases adaptability and reduces inbreeding risk.
1 mark: Recognition that Population A’s low genetic diversity could lead to inbreeding depression, accumulation of harmful alleles, or reduced survival.
1 mark: Description of conservation actions such as introducing individuals from other populations (genetic rescue) or managing breeding to maximise heterozygosity.
1 mark: Mention of monitoring through genetic testing or maintaining studbooks to track lineages and preserve allele diversity.
