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Genetic Diversity and Adaptation

AQA spec ref: 3.4.5 - Genetic diversity and natural selection

Genetic diversity is the variation in alleles within and between populations. It is the raw material for natural selection - without variation, selection has nothing to act on, and populations cannot adapt. This note covers the sources of genetic diversity, how selective pressures act on it, and how populations and species respond over time.

What Is Genetic Diversity?

Genetic diversity is the number of different alleles of genes present within a population. It is quantified by measures such as:

  • Number of different alleles at a given locus
  • Proportion of loci that are polymorphic (have more than one allele)
  • Average heterozygosity across loci

A population with high genetic diversity has many different alleles at many loci - individuals differ substantially at the genetic level. A population with low genetic diversity is more genetically uniform.

Genetic diversity matters because:

  • It provides the variation needed for natural selection to produce adaptation
  • It reduces the risk of extinction: a diverse population is more likely to contain some individuals that can survive a novel threat (new pathogen, climate change, etc.)
  • Low diversity is associated with inbreeding depression - reduced fitness due to expression of harmful recessive alleles

Sources of Genetic Diversity

1. Mutation - the ultimate source of all new alleles. Random, unpredictable changes to DNA base sequences produce new variants. Without mutation, no new alleles could ever arise, and all diversity would gradually be eliminated by selection or drift. See Mutations for types.

2. Meiosis - reshuffles existing alleles into new combinations:

  • Crossing over in prophase I - non-sister chromatids exchange segments, creating recombinant chromosomes with new combinations of alleles (see Meiosis)
  • Independent assortment in metaphase I - each bivalent aligns randomly, so any combination of maternal and paternal chromosomes can be distributed to gametes. With 23 chromosome pairs in humans, this gives 2²³ ≈ 8 million combinations from assortment alone.

3. Random fertilisation - any sperm can fertilise any egg (from the ~8 million possible combinations each), giving 8 million × 8 million ≈ 64 trillion possible genotypes for any offspring.

4. Gene flow (migration) - individuals moving between populations bring their alleles with them. Immigration increases genetic diversity in the receiving population; emigration may reduce diversity. Gene flow also counteracts genetic drift, tending to homogenise allele frequencies between populations.

Factors That Reduce Genetic Diversity

Selective breeding (artificial selection): breeders choose individuals with specific desired traits to reproduce. Only a subset of alleles is passed on. Over generations, many alleles are lost. Modern crop varieties and livestock breeds have dramatically reduced genetic diversity compared to their wild relatives - this is why monocultures are vulnerable to disease (e.g. Irish potato famine, caused by Phytophthora infestans, which swept through the genetically uniform potato crop).

Founder effect: when a small group of individuals establishes a new population (colonises a new island, is geographically isolated), they carry only a small, random sample of the original population's alleles. The new population has reduced genetic diversity. Harmful recessive alleles that happened to be common in the founders become common in the new population. Example: Amish populations in Pennsylvania, which were founded by a small group from Europe, have higher frequencies of certain recessive genetic disorders (e.g. Ellis-van Creveld syndrome).

Population bottleneck: a sudden drastic reduction in population size - due to disaster, hunting, disease, or habitat loss - causes most individuals (and their alleles) to be lost. The surviving small population has reduced diversity. Even if the population later recovers in size, it retains the low diversity of the bottleneck. Example: cheetahs are so genetically similar that skin grafts can be exchanged between unrelated individuals without rejection - the result of a severe bottleneck ~12,000 years ago.

Genetic drift: in small populations, allele frequencies can change randomly from generation to generation simply by chance (which individuals survive and reproduce, which gametes fuse). Unlike selection, genetic drift is non-directional - it doesn't favour better-adapted alleles. Over time, drift can eliminate alleles or fix them at 100% entirely by chance. Drift is strongest in small populations.

Natural Selection

Natural selection is the mechanism by which populations become better adapted to their environment. It acts on the variation that already exists (due to mutations and genetic recombination) and causes changes in allele frequency over generations.

The logic of natural selection:

  1. Variation - individuals in a population vary in their phenotype (due to different alleles)
  2. Heredity - phenotypic differences are (at least partly) heritable - passed from parents to offspring
  3. Differential survival/reproduction - in any environment, some phenotypes (and therefore genotypes) confer better survival or reproductive success (fitness)
  4. Selection - individuals with fitter phenotypes survive longer and reproduce more, passing their alleles to more offspring
  5. Allele frequency change - over generations, the alleles associated with higher fitness become more common; those associated with lower fitness become less common

Selection acts on phenotypes but what is inherited are genotypes (alleles). The environment determines which phenotypes are fitter.

Directional Selection

One extreme phenotype is more fit than the others. The population mean shifts in the direction of the favoured extreme over successive generations.

Examples:

  • Antibiotic resistance in bacteria: the "resistant" extreme is strongly favoured when antibiotics are present. Bacteria with resistance alleles survive and reproduce; those without are killed. The population mean shifts rapidly toward resistance.
  • Industrial melanism in Biston betularia: before industrialisation, pale moths were camouflaged on lichen-covered trees; dark moths were predated more. After industrialisation (trees blackened with soot), dark moths were better camouflaged; pale moths were predated. The population frequency shifted from mostly pale to mostly dark in areas near industrial centres.
  • Selective breeding: breeders apply directional selection artificially, choosing individuals at one extreme of the distribution.

Stabilising Selection

Intermediate phenotypes are more fit; both extremes are selected against. The variation in the population decreases over time, and the mean stays approximately the same.

Example - human birth weight: very low birth weight babies have higher neonatal mortality (insufficient reserves, organ development); very high birth weight babies also have higher mortality (difficult delivery, birth complications). The optimal birth weight is ~3.5 kg, and there is strong selection against both extremes. This explains why human birth weight has remained clustered around 3.5 kg for thousands of years despite variation in nutrition.

Disruptive Selection

Both extremes are favoured over the intermediate phenotype. The distribution becomes bimodal (two peaks). This is relatively rare but can be a driver of speciation if the two extreme morphs become reproductively isolated.

Example - beak size in African seed-cracker finches: small beaks crack soft seeds efficiently; large beaks crack hard seeds efficiently; intermediate beaks are inefficient at both. Both small-beaked and large-beaked birds are more fit than intermediate-beaked birds. The population becomes bimodal for beak size.

Adaptations

An adaptation is an inherited characteristic that increases an organism's fitness - its ability to survive and reproduce - in a particular environment. Adaptations arise through natural selection acting repeatedly on heritable variation over many generations.

Three types of adaptation:

Anatomical/morphological - structural features of the body:

  • Thick fur and white coat in Arctic mammals (insulation + camouflage)
  • Streamlined body shape in fish and cetaceans (hydrodynamic efficiency)
  • Deep root systems in desert plants (access to deep water)
  • Hollow bones in birds (reduces mass for flight)

Physiological - internal biochemical or functional processes:

  • Production of antifreeze proteins in Antarctic fish (lowers freezing point of blood)
  • Counter-current heat exchange in Arctic mammals' limbs (conserves body heat)
  • Enzymes with different optimal temperatures in organisms from different environments
  • Thick, waterproof cuticle in xerophytes (reduces water loss)
  • Ability to concentrate urine in desert mammals (conserves water)

Behavioural - actions and responses:

  • Migration to warmer climates in winter (avoids food shortage)
  • Hibernation in cold-adapted mammals (reduces energy expenditure)
  • Nocturnal activity in desert animals (avoids daytime heat)
  • Courtship displays and mate choice (maximise reproductive success)

Summary

  • Genetic diversity = number of different alleles in a population; provides variation for selection
  • Sources: mutation (new alleles), meiosis (crossing over + independent assortment), random fertilisation, gene flow
  • Reduced by: selective breeding, founder effect, bottleneck, genetic drift
  • Natural selection: variation→heredity→differential reproduction→allele frequency change
  • Directional: one extreme favoured → mean shifts (e.g. antibiotic resistance)
  • Stabilising: intermediate favoured → variance decreases (e.g. birth weight)
  • Disruptive: both extremes favoured→bimodal distribution
  • Adaptations: anatomical, physiological, behavioural - all inherited, increase fitness

AQA Exam Tips

  • Define genetic diversity precisely: "the number of different alleles of genes within a population" - not "the amount of variation" (too vague).
  • Natural selection mechanism: when asked to explain natural selection, use all five steps - variation exists, variation is heritable, some phenotypes are more fit, these individuals reproduce more, their alleles increase in frequency. AQA mark schemes expect all steps.
  • Founder effect vs bottleneck: founder effect = small group establishes new population; bottleneck = existing population is drastically reduced. Both result in reduced genetic diversity. AQA distinguishes them.
  • Directional selection - allele frequency: state that the favoured allele increases in frequency over generations, and unfavourable alleles decrease. Selection doesn't create new alleles - it only changes their frequencies.
  • Adaptation definition: must include "inherited" and "increases fitness/survival and reproduction." Do not just say "a feature that helps an organism survive" - state that it is inherited and increases reproductive success.
  • Antibiotic resistance: mutations conferring resistance occur spontaneously before antibiotics are introduced; antibiotics select for them (they don't cause the mutations). AQA commonly tests this misconception.