Chapter 12: An Evolving Enemy – Natural Selection and Tuberculosis

Natural Selection and Tuberculosis

Learning Outcomes

• Describe the history of tuberculosis in human populations and explain why current treatments are ineffective against some strains.

• List the four observations that led to the inference of natural selection.

• Explain how natural selection causes evolutionary change.

• Provide examples of evidence supporting the hypothesis that natural selection leads to the evolution of populations.

• Describe how natural selection works on allele frequencies in a population.

• Discuss why natural selection does not result in “perfectly adapted” organisms or drive organisms toward some ideal state.

• List the three patterns of selection, provide examples, and explain their outcomes.

• Explain why combination drug therapy is effective against drug resistance.

12.1 Return of a Killer

History of Tuberculosis

Tuberculosis (TB) has affected humans for thousands of years:

• Ancient Egypt (3000 B.C.): Tubercular decay found in mummies.

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

• 1906: Caused 2 out of 1000 deaths in the U.S.

• Currently: 1.5 TB deaths per 100,000 people.

Characteristics of Tuberculosis

• Cause: Bacterium Mycobacterium tuberculosis

• 2 billion people carry TB worldwide.

• New infection rate: 1 per second.

• Causes 2 million deaths per year.

• 90% of infections are symptomless.

Symptoms of TB

• Cough producing blood

• Fever

• Fatigue

• Long, relentless wasting (weight loss, weakness)

• Tubercles (nodules) form in lungs to isolate bacterial colonies

Consumption

• Antiquated name for TB

• "Wasting" symptoms due to lung tissue damage

Transmission of Tuberculosis

• Spread through the air from infected individuals

• A single sneeze releases 40,000 droplets

• Highest risk: young children, elderly, individuals with poor health (malnutrition, illness, drug abuse, AIDS)

Tuberculosis Treatments

• 19th/early 20th century: long stays in rural sanatoriums with fresh air; isolation to reduce spread

• 1940s: Antibiotics revolutionized TB treatment

• After 2 weeks on antibiotics, individuals are no longer contagious

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

Antibiotic Resistance

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

• Since 1993: 1% of cases did not respond to standard treatment (multidrug-resistant TB, MDR-TB)

• 76 of 2000 cases: resistant to second-line drugs (XDR-TB)

• Only 30–50% of active XDR-TB infections are cured

• Example: 52 of 53 patients died in an XDR-TB outbreak in South Africa

12.2 Natural Selection Causes Evolution

Darwin's Theories from the Origin of Species

• Theory of Common Descent: All species descended from a single ancestor; widely accepted within 20 years.

• Theory of Natural Selection: Explains how organisms evolved from a common ancestor; took 60 more years to be accepted.

Definition of Natural Selection

Natural selection is the process by which physical or behavioral traits that increase survival or reproduction become more common in a population, while less favorable traits are lost.

• May result in new species due to accumulated changes

• Driven by competition for survival

• Most important cause of evolution

• Passive process determined by variation and environment

Darwin's Four Observations

1. Individuals within populations vary.

2. Some variation among individuals can be passed to offspring.

3. Populations produce more offspring than will survive.

4. Survival and reproduction are not random.

Observation 1: Variation Within Populations

• Variation in appearance (e.g., coat color in wolves)

• Blooming time in flowers

• Amount of caffeine in coffee plant seeds

• Variant: Each different type of individual in a population

Observation 2: Heritability of Variation

• Resemblance between parents and offspring

• Pigeon breeders produced offspring with desired traits

• Mendel’s work supported inheritance of traits

Observation 3: Overproduction of Offspring

• Even slow-breeding animals can produce large populations quickly

Observation 4: Non-Random Survival and Reproduction

• Subset of individuals who survive and reproduce is not arbitrary

• Fitness: Relative survival and reproduction of one variant

• Adaptation: Traits that increase individual fitness in an environment

• Individuals with adaptations are more likely to survive and reproduce

Adaptations

• Birds with larger bills can crack open tough seeds and are more fit during droughts

• Traits may increase number of offspring (e.g., bright flower color, greater nectar production to attract pollinators)

Darwin's Inference: Natural Selection Causes Evolution

• Favorable inherited variations increase in frequency

• Unfavorable variations tend to be lost

• Evolution: Change in traits of individuals in a population over generations

Testing Natural Selection

• Artificial selection: Selection imposed by human choice (e.g., breeding of horses, fancy pigeons, domestic dog breeds)

• Natural selection in the lab: Manipulating environmental conditions (e.g., fruit flies in alcohol; fast-metabolizing flies increase over generations)

• Natural selection in wild populations: Examples include antibiotic resistance in M. tuberculosis, bill size in Galapagos finches, thicker shells in blue mussels

12.3 Natural Selection Since Darwin

The Modern Synthesis

• Union between genetics and evolution

• Genes are segments of genetic material with information about protein structure

• Actions of proteins determine physical traits

• Different versions of the same gene are alleles; variation in alleles leads to variation in traits

• Different alleles arise through mutation

• Half of the alleles carried by a parent are passed to offspring via egg or sperm

Mutation and Natural Selection

• Mutations introduce new alleles into a population

• Natural selection acts on these alleles, increasing or decreasing their frequency

Common Misunderstandings of Natural Selection

• Natural selection cannot cause new traits to arise; it only acts on existing variation

• Selection acts on populations, not individuals

• Adaptations may be beneficial in most, but not all, conditions

• Natural selection fits organisms to the current environment, not future ones

A Misunderstanding of Natural Selection	How Natural Selection Really Works
Natural selection cannot cause new traits to arise. Example: The Dodo was too stupid to adapt to human hunters, so it had to go extinct.	Only traits present in the population can be selected for. The Dodo was not "stupid"; the population simply did not contain variants with hunter-avoiding traits.

Patterns of Selection

• Directional selection: Population traits move in a particular direction (e.g., increase in bill size in finches)

• Stabilizing selection: Extreme variants are selected against; population stabilizes around the average trait

• Diversifying (disruptive) selection: Changes result in two or more variants (e.g., selection for both small and large beak sizes)

12.4 Natural Selection and Human Health

TB and Darwin's Observations

• Organisms in populations vary: Bacterial variants of M. tuberculosis that resist antibiotics exist

• Variation can be passed on: Genes for antibiotic resistance are inherited by other bacteria

• More organisms are produced than survive: Antibiotics eliminate most bacteria in an infected individual

• Survival is not random: Bacteria with resistance are more likely to survive and reproduce

Drug Resistance and Selection

• Simple antibiotic treatment can result in directional selection in bacteria

• Some bacteria are more resistant to antibiotics

• Maintaining drug therapy for months is necessary to prevent resistance

• Combination drug therapy (drug cocktail) is effective against drug resistance

Superbugs

• MRSA (Methicillin-resistant Staphylococcus aureus): Evolved from common bacteria into a dangerous pathogen; untreatable by most antibiotics

• Humans vary in immune capacity, but most are never exposed to TB; human evolution alone will not solve the superbug problem

Key Terms and Concepts

• Natural Selection: Process by which traits that increase survival/reproduction become more common

• Adaptation: Trait that increases fitness in a particular environment

• Fitness: Relative survival and reproduction of a variant

• Allele: Different version of a gene

• Mutation: Change in DNA sequence, source of new alleles

• Directional, Stabilizing, Diversifying Selection: Patterns of evolutionary change in populations

Relevant 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	Traits shift in one direction	Increase in bill size in finches
Stabilizing	Average traits favored; extremes selected against	Human birth weight
Diversifying	Both extremes favored; average selected against	Beak size in birds with two food sources

Applications and Examples

• Antibiotic resistance in M. tuberculosis and MRSA

• Artificial selection in domestic animals

• Natural selection in laboratory fruit flies

• Evolution of shell thickness in blue mussels