Ch14

Evolution and Human Health

Historical Context

  • Louis Pasteur (1858): Proposed that contagious diseases, such as cholera, are caused by germs.

  • Charles Darwin (1859): Published "On the Origin of Species," marking the birth of evolutionary biology and modern medicine, which have developed in parallel.

Pathogen Evolution and Host Interaction

  • Fundamental Events in Evolution:

    • Evolution involves changes in genotype frequencies within a population.

    • There exists a conflict between pathogens and their hosts:

    • Pathogens consume the host's tissues and replicate as more pathogens.

    • Hosts attempt to mitigate damage by slowing or killing pathogens.

  • Immunity:

    • The human immune system can recognize foreign proteins.

    • Pathogens often exhibit large population sizes, short generation times, and high mutation rates.

COVID-19 Context

  • SARS-CoV-2 and MERS-CoV: Outline of genetic structures of viruses and their respective open reading frames (ORFs) is important for understanding viral evolution and their impacts on human health.

Pandemic Influences on Human Health

  • 1918 Flu Pandemic:

    • Infected approximately 20% of the global population.

    • Resulted in 50 to 100 million deaths.

  • Antigenic Sites: Defined as specific parts of a foreign protein recognized by the immune system.

Genetic Divergence and Survival of Pathogens

  • Antigenic Sites and Nucleotide Substitutions:

    • Surviving flu lineages had a higher fraction of amino acid replacements in antigenic sites, allowing them to persist while others failed.

    • Evolutionary Analysis: Scientists analyzed hemagglutinin genes to forecast viral evolution, leading to correct predictions of dominant strains in 9 out of 11 flu seasons. This analysis supports annual vaccine design.

Gene Swapping among Influenza Strains

  • Nucleoprotein (NP): Identified as crucial for host specificity.

  • Flu strains demonstrate the ability to exchange genes.

Evolving Pathogens and Antibiotic Resistance

  • Antibiotics: Chemicals that eliminate bacteria by disrupting biochemical processes.

    • Evidence shows that antibiotics can select for resistant bacteria, leading to rapid evolutionary responses in bacterial populations.

  • Costs of Resistance:

    • While antibiotic resistance costs are assumed, natural selection can eliminate such costs over time. The evolution of resistance considers trade-offs in bacterial populations.

Transmission and Virulence Hypotheses

  • Coincidental Evolution Hypothesis: The virulence of pathogens may not be selected directly but instead may be a by-product of selection on other traits.

  • Shortsighted Evolution Hypothesis: Pathogens can evolve within a host before transmission to new hosts.

  • Trade-off Hypothesis: Suggestions that strains can maintain virulence while increasing transmission probability despite host mortality.

Vertical and Horizontal Transmission

  • Viral Selection: Viral strains favor traits that promote reproduction and transmission within and between hosts:

    • During vertical transmission, viral strains may facilitate faster bacterial division of host bacteria.

    • During horizontal transmission, strains that induce host bacteria to secrete more viral copies are favored.

Virulence in Human Pathogens

  • Vector Transmission: Parasites transmitted by vectors tend to exhibit greater virulence than direct contact-transmitted parasites due to differing survival strategies.

Implications of Genetic Traits in Human Health

  • Genetics and Stroke: An allele L68Q linked to early stroke risk in Icelandic carriers, revealing life span and selection patterns.

  • Nearsightedness: Evidence suggesting myopia is partially heritable, potentially caused by environmental influences.

  • Breast Cancer Incidence: Approximately 1 in 8 North American women may develop breast cancer. Potential causes include:

    • Pathogen-related causes.

    • Diseases associated with modern civilization.

Breast Cancer and Biological Factors

  • MMTV Influence: Mouse Mammary Tumor Virus (MMTV) is correlated with breast cancer development in laboratory studies.

    • Analysis of 321 cancer tissue samples showed a significant correlation between MMTV presence and cancer occurrence.

  • Menstrual Cycling: Links between menstrual cycle regularity and breast cancer risk:

    • Earlier menstruation start, later childbirth, and less time spent nursing correlate with higher cancer risk.

Obesity and Environmental Factors

  • Variations in Obesity: Much of the variability in human weight is attributed to differences in environmental exposures.

  • Microbial Influence: Changes in microbiota can lead to obesity, as demonstrated when germ-free mice gained weight after being inoculated with microbes.

Fever as an Adaptive Response

  • Fever Mechanisms: Two hypotheses explain fever in relation to infections:

    1. Fever manipulates hosts to facilitate pathogen growth (if this hypothesis is true, reducing fever would aid the host).

    2. Fever acts as an adaptive defense, impeding pathogen reproduction or enhancing immune response at higher temperatures (if true, antipyretic drugs may hinder recovery).

Conclusion

  • Environmental Responses and Physiology: By analyzing adaptations in organisms, such as desert iguanas and their preferred temperature ranges, researchers can assess the intricate relationships between climate, health, and evolutionary biology.