Chapter 12: An Evolving Enemy – Natural Selection and Tuberculosis

Natural Selection and Tuberculosis

Introduction

This chapter explores the history and biology of tuberculosis (TB), the mechanisms of natural selection, and how evolutionary principles explain the emergence of drug-resistant pathogens. It also covers the foundational concepts of Darwinian evolution and the modern synthesis of genetics and evolution.

12.1 Return of a Killer

History of Tuberculosis

• Tuberculosis (TB) has affected humans for thousands of years.

• Evidence from Ancient Egypt (3000 B.C.): tubercular decay found in mummies.

• Hippocrates (406 B.C.) described a TB-like condition.

• In 1906, TB caused 2 out of 1000 deaths in the U.S.; currently, 1.5 TB deaths per 100,000.

Biology of Tuberculosis

• Cause: Bacterium Mycobacterium tuberculosis

• 2 billion people carry TB globally.

• New infection rate: 1 per second.

• Causes 2 million deaths per year.

• 90% of infections are symptomless (latent TB).

Symptoms of TB

• Cough producing blood

• Fever

• Fatigue

• Progressive wasting (weight loss, weakness)

• Formation of tubercles in lungs to isolate bacterial colonies

Consumption

• Antiquated name for TB, referring to the wasting symptoms due to lung tissue damage.

Transmission

• Spread through the air from infected individuals (e.g., sneezing releases 40,000 droplets).

• Highest risk: young children, elderly, immunocompromised individuals (poor nutrition, illness, drug abuse, AIDS).

Treatments

• 19th/early 20th century: long stays in rural sanatoriums, isolation, fresh air.

• 1940s: antibiotics revolutionized TB treatment.

• After 2 weeks of antibiotics, patients are no longer contagious.

• Treatment must continue for 6–12 months to fully eliminate the organism.

Antibiotic Resistance

• Since the 1980s, antibiotic-resistant TB cases have increased.

• Since 1993: 1% of cases are multidrug-resistant (MDR-TB); some are extensively drug-resistant (XDR-TB).

• Only 30–50% of XDR-TB infections are cured; high mortality in outbreaks.

12.2 Natural Selection Causes Evolution

Darwin's Theories

• Theory of Common Descent: All species descended from a single ancestor.

• Theory of Natural Selection: Explains how organisms evolved from a common ancestor through differential survival and reproduction.

Definition of Natural Selection

• Process by which traits that increase survival or reproduction become more common in a population.

• May result in new species due to accumulated changes.

• Driven by competition for survival and environmental variation.

Darwin's Four Observations

1. Variation: Individuals within populations vary (appearance, physiology, behavior).

2. Heritability: Some variation is passed to offspring.

3. Overproduction: More offspring are produced than can survive.

4. Non-random Survival: Survival and reproduction are not random; individuals with advantageous traits are more likely to survive and reproduce.

Key Terms

• Fitness: Relative survival and reproduction of one variant compared to others.

• Adaptation: Trait that increases individual fitness in a specific environment.

Examples of Adaptation

• Birds with larger bills survive better during droughts (can crack tough seeds).

• Flowers with bright colors and more nectar attract more pollinators, increasing reproductive success.

Testing Natural Selection

• Artificial Selection: Human-imposed selection (e.g., breeding dogs, pigeons).

• Natural Selection in the Lab: Manipulating environmental conditions (e.g., fruit flies in alcohol).

• Natural Selection in Wild Populations: Observing changes in traits (e.g., antibiotic resistance, bill size in finches, shell thickness in mussels).

12.3 Natural Selection Since Darwin

The Modern Synthesis

• Union of genetics and evolution.

• Genes: Segments of DNA with information about protein structure.

• Alleles: Different versions of the same gene; variation in alleles leads to variation in traits.

• Alleles arise through mutation.

• Half of a parent's alleles are passed to offspring via egg or sperm.

Mutation and Natural Selection

• Mutations introduce new alleles into populations.

• Natural selection acts on existing variation; beneficial mutations may increase in frequency.

Common Misunderstandings

• Natural selection cannot create new traits; it acts on existing variation.

• Selection does not produce "perfect" organisms, only those best fit for the current environment.

• Selection acts on populations, not individuals.

Table: Misunderstandings about Natural Selection

A Misunderstanding of Natural Selection	How Natural Selection Really Works
Natural selection cannot cause new traits to arise.	Only traits present in the population can be selected for. Example: The Dodo was not "stupid"; its population lacked hunter-avoiding traits.

Patterns of Selection

• Directional Selection: Traits shift in one direction (e.g., increased bill size in finches).

• Stabilizing Selection: Selection for average traits; extremes are selected against.

• Diversifying (Disruptive) Selection: Selection for extremes; average traits are selected against, leading to two or more variants.

12.4 Natural Selection and Human Health

TB and Darwin's Observations

• Variation: Some M. tuberculosis bacteria are antibiotic-resistant.

• Heritability: Resistance genes are passed to offspring.

• Overproduction: Antibiotics kill most bacteria, but some survive.

• Non-random Survival: Resistant bacteria survive and reproduce.

Drug Resistance and Selection

• Antibiotic treatment can result in directional selection for resistant bacteria.

• Maintaining drug therapy for the full prescribed duration is essential to prevent resistance.

• Combination drug therapy (drug cocktails) is effective against drug resistance by targeting multiple pathways.

Superbugs

• Pathogens like MRSA and XDR-TB have evolved resistance to most antibiotics.

• Human populations are unlikely to evolve resistance quickly enough to counter superbugs due to low exposure and genetic variation.

Review Questions

• Describe the history and biology of tuberculosis.

• List and explain Darwin's four observations about natural selection.

• Explain how natural selection causes evolutionary change.

• Provide examples of evidence supporting natural selection.

• Discuss how natural selection affects allele frequencies.

• Explain why natural selection does not produce perfectly adapted organisms.

• List and describe the three patterns of selection.

• Explain the effectiveness of combination drug therapy in combating resistance.

Key Equations

• Allele Frequency Change (Hardy-Weinberg Principle): $p^2 + 2pq + q^2 = 1$ where $p$ and $q$ are the frequencies of two alleles in a population.

Summary Table: Patterns of Selection

Pattern	Description	Example
Directional	Trait shifts in one direction	Finch bill size increases during drought
Stabilizing	Average trait favored	Human birth weight
Diversifying	Extremes favored	Shell thickness in mussels

Additional info: The notes expand on brief slide points to provide definitions, examples, and context for exam preparation.