Human Adaptation: Genetic Variation and Natural Selection

Introduction to Human Adaptation and Genetic Diversity

  • Origins of Genetic Diversity: Contemporary human populations exhibit patterns of genetic diversity that are the result of several foundational processes:

    • Migration Patterns: Movement out of Africa and subsequent movement within and across different global regions.
    • Admixture: Interbreeding with archaic hominids, specifically the Denisovans and the Neanderthals.
    • Gene Flow: Later genetic exchange between various human populations.
    • Natural Selection: The process of adaptation to specific environmental pressures which alters the frequency of certain genetic variations.

  • Environmental Exposure: As populations migrate into diverse environments, they encounter unique environmental pressures. These pressures act as selective forces, leading to adaptations that change the frequency of specific alleles in one population relative to others.

The Mechanism of Natural Selection

  • Evolutionary Forces: While there are four primary evolutionary forces—mutation, gene flow, random genetic drift, and natural selection—natural selection is the only force that is explicitly adaptive.

  • The Process of Selection:

    • Selection occurs when specific genotypes (the sets of alleles inherited from parents) allow certain individuals to leave more offspring than others.
    • Over time, this process leads to a shift in the frequency of alleles within the population.

  • Measuring Fitness: Fitness is the metric used to quantify natural selection and consists of two primary components:

    • Viability: The ability to survive, particularly surviving through reproductive age.
    • Fertility: The capacity to produce offspring.
    • Competition: Interactions with others for resources also play a critical role in determining fitness.

  • Selection on Alleles:

    • If a specific allele (in either homozygous or heterozygous form) provides higher fitness than the alternative homozygote, its frequency will increase over time.
    • Conversely, if an allele imposes a fitness cost, its frequency will decrease.
    • The rate of change depends on whether the cost is expressed only in the homozygote or also in the heterozygote form.
    • Natural selection can also maintain deleterious alleles in a population through balancing selection.

Global Patterns of Adaptation

  • Selective Pressures and Associated Genes: Research has identified several regions of the genome under selective pressure from various environmental exposures:

    • Diet: Variations associated with a marine diet and lactose tolerance ($MCM6$ gene).
    • Infectious Disease: Variations providing resistance to malaria ($HBB$, $G6PD$) and cholera.
    • Altitude: Genes such as $EPAS1$ are linked to survival at high elevations.

  • Genetic Signatures: By examining the genomes of different populations, scientists can identify "signatures" or regions within the DNA that demonstrate they have been under selective pressure.

Case Study: Malaria as a Powerful Selective Pressure

  • Causative Agent: Plasmodium falciparum is the parasite primarily responsible for malaria.

  • Historical Impact: Malaria represents one of the strongest selective pressures on the human genome in recent history. The incidence and prevalence of the disease are thought to have increased significantly following the initiation of agriculture globally.

  • Resistance Adaptations: The human genome has co-evolved with the parasite, leading to an increase in variations associated with better outcomes, including:

    • Haemoglobin Gene ($HBB$): Includes the sickle cell variant ($HBS$).
    • Glucose-6 Phosphate Dehydrogenase ($G6PD$): Variants associated with enzyme function.
    • Thalassemia: Other genetic variants affecting haemoglobin structure.

  • Fitness Implications: Malaria is highly destructive, causing thousands of deaths annually. Because it has a higher fatality rate in children than in adults, it has a massive negative impact on fitness, making resistance variations highly advantageous.

Balancing Selection and Sickle Cell Disease

  • Directional Selection (Absence of Malaria):

    • Wild type ($AA$): Fitness = $1$ ($100\,\%$).
    • Heterozygote ($AS$): Fitness = $1$.
    • Homozygote Sickle Cell ($SS$): Fitness decreases dramatically due to the fatal nature of sickle cell disease.
    • In this scenario, the $S$ allele would eventually disappear from the population.

  • Heterozygote Advantage (Malaria Endemic Regions):

    • Heterozygote ($AS$): This is the most fit genotype (Fitness = $1$) as it provides protection against malaria.
    • Wild type ($AA$): Fitness drops to $0.9$ (a $10\,\%$ decrease relative to the heterozygote) due to susceptibility to malaria.
    • Homozygote ($SS$): Remains detrimental.
    • Result: The $S$ allele is maintained in the population at a low frequency, sometimes as high as $10\,\%$.

G6PD and the Selective Sweep

  • The Mahidol Variant: A specific variation of the $G6PD$ gene (the $487 A$ variant) studied in Southeast Asian populations.

  • Evidence of Advantage:

    • In children aged 1 to 91 \text{ to } 9, individuals with the Mahidol variant show a significantly lower density of the Plasmodium vivax parasite in their blood compared to those with the wild-type ($487 G$) variant.
    • While the effect is less obvious in older age groups (e.g., 10 to 2410 \text{ to } 24 and above $25$), mortality early in life has the strongest impact on fitness.

  • Selective Sweep Signature:

    • In individuals carrying the Mahidol variant, the genetic region around the mutation exhibits a "selective sweep."
    • This is characterized by long, shared stretches of DNA (orange bars) that are roughly $2,000,000$ bases (2 Mb) in length.
    • In contrast, the wild-type DNA shows much smaller shared regions (white space indicating lack of sharing), which is the standard genetic background without selective pressure.

Disease Interaction and Maladaptation

  • Co-infection: In regions such as parts of Africa, populations deal with multiple infectious diseases simultaneously (HIV, Tuberculosis, Malaria).

  • Maladaptation Example: An adaptation for one disease can be detrimental for another.

    • Gene: Chemokine receptor number $5$ ($CCR5$).
    • Variation: A specific deletion in the gene.
    • Effect: Two copies of this deletion are thought to be protective against HIV. However, if infected with West Nile virus, this same variation is associated with a worse outcome and faster disease progression.

Dietary Adaptation: Lactase Persistence

  • Hunter-Gatherer Antecedents: Before agriculture, humans were likely adapted to a lifestyle of "feast and famine," characterized by a "thrifty genotype."

  • The Agricultural Shift: The onset of agriculture changed population mobility and diet, specifically through the domestication of livestock and the utilization of milk.

  • Lactase Persistence Phenotype: The ability to digest lactose into adulthood via the persistence of the lactase enzyme. This phenotype is common in European populations and specific African populations.

  • Convergent Evolution:

    • The phenotype is the result of mutations in the $MCM6$ gene region (upstream of the $LCT$ gene).
    • The specific genetic variations differ: The $-13910$ base pair SNP is primarily seen in Europeans, while different SNPs are found in African populations.
    • Timeline: Selective sweeps occurred independently approximately $10,000$ to $7,000$ years ago in African populations and 2,000 to 3,0002,000 \text{ to } 3,000 years ago in European populations.

High Altitude Adaptation and Convergent Selection

  • Environmental Pressure: Sustained hypoxia (critically low oxygen levels).

  • Tibetan Adaptation (e.g., Sherpas):

    • Gene: $EPAS1$ (part of the Hypoxia Inducible Factor pathway, or $HIF$).
    • Variation: A non-coding variant that leads to decreased haemoglobin concentration in the blood.
    • History: This variation likely appeared $50,000$ years ago and was introduced to the population through introgression from Denisovans.

  • Andean Adaptation (e.g., Peru):

    • Gene: $EPAS1$.
    • Variation: A missense variant at position $194$.
    • Mutation: A change from Histidine to Arginine (denoted as HIS→ARGHIS \rightarrow ARG).
    • Protein Impact: This version of the $EPAS1$ gene affects the HIF−2αHIF-2 \alpha protein structure.
    • History: This variation arose independently approximately $10,000$ years ago and similarly results in lower haemoglobin concentration.

  • Significance of Convergent Evolution: When different genetic pathways lead to the same phenotype (the ability to perform in high altitudes) in different populations, it is a definitive indicator of extremely strong selective pressure.