Evolution in Present Times – Study Notes

Evolution in Present Times: Overview

  • Evolution and natural selection are ongoing processes that normally take centuries and are difficult to observe directly, but can be seen in organisms that multiply rapidly.

DDT and Malaria Control: History and Resistance

  • Malaria is caused by Plasmodium and is carried by the female Anopheles mosquito, transferring the parasite from person to person.
  • Malaria is a dangerous disease that can cause deaths.
  • Eradication efforts historically targeted the mosquito host; in the past, the insecticide DDT was used to kill the mosquito.
  • By 1959 it was found that DDT was no longer effective in killing all of the mosquitoes; mosquitoes had developed resistance to DDT, increasing their survival chances.
  • Resistance means the mosquitoes evolved to withstand DDT, reducing the insecticide’s effectiveness.
  • This situation is explained by natural selection: variation exists in the mosquito population (some mosquitoes are resistant, others are not).
  • When DDT is sprayed, susceptible (non-resistant) mosquitoes are killed while resistant mosquitoes survive.
  • Resistant mosquitoes then have a higher chance to reproduce and their numbers increase due to less competition.
  • Repeated spraying with DDT leads to a larger population of DDT-resistant mosquitoes; when sprayed again, susceptible mosquitoes die and resistant ones persist, causing the insecticide to become ineffective over time.
  • The situation could have been mitigated by using a variety of insecticides (insecticide rotation or integrated pest management).

Galápagos Finches: Beak Size and Adaptation

  • There is a great deal of variation among Galápagos finches, including beak size and body size.
  • If the environment changes, the availability of food changes as well.
  • Finches with beaks suited to the new food supply will survive, while those with beaks adapted to foods in short supply may struggle due to insufficient food.
  • The finch with the appropriate beak has a higher chance to reproduce and pass on its genes to the next generation.
  • Over time, the finches with inappropriate beaks are replaced by those with appropriate beaks better suited to the environment.

Tuberculosis (TB) and Drug Resistance

  • TB is caused by the bacterium Mycobacterium tuberculosis.
  • TB can be treated with antibiotics, and full recovery is possible if the antibiotic course is completed as directed.
  • If patients do not complete their antibiotic course, drug-resistant TB can develop, making the antibiotics ineffective.
  • The immune system alone cannot fully cope with the bacteria because the bacteria reproduce rapidly.
  • Drug-resistant TB examples include MDR TB (multidrug-resistant) and XDR TB (extremely drug-resistant).
  • The development of drug resistance involves variation in the bacterial population: some bacteria are resistant to antibiotics while others are not.
  • When antibiotics are taken, susceptible bacteria are killed while resistant bacteria survive.
  • The resistant bacteria then reproduce rapidly, increasing their numbers and creating a new, more resistant population.
  • As a result, the disease becomes harder to cure with standard antibiotics.

HIV/AIDS and Antiretroviral Resistance; Treatment Strategies

  • HIV is a virus; there is no cure, but antiretroviral drugs (ARVs) slow the progression of HIV/AIDS.
  • HIV can become resistant to ARVs, meaning the virus can’t be killed by the drugs.
  • The virus reproduces quickly, forming new colonies of drug-resistant virus, leading to evolved drug resistance.
  • To prevent resistance, patients are treated with a cocktail (combination) of drugs.
  • A cocktail of drugs helps prevent the virus from easily developing resistance because multiple targets are attacked simultaneously.

Key Terms and Concepts ( glossary-style notes )

  • DDT: Insecticide used to kill Anopheles mosquitoes.
  • Mycobacterium tuberculosis: Bacteria that causes TB.
  • Drug resistant: A pathogen that cannot be killed by an antibiotic.
  • MDR TB: Multiple drug resistant TB.
  • XDR TB: Extreme drug resistant TB.
  • HIV: Human immunodeficiency virus; causes HIV/AIDS.
  • ARV: Antiretrovirals.
  • Cocktail of drugs: A combination of drugs used to treat HIV/AIDS.
  • Cocktail of drugs (HIV context): A treatment approach using multiple ARVs together.

How Natural Selection Is Used to Explain These Phenomena

  • Variation in populations provides raw material for selection (e.g., mosquitoes with differing levels of DDT resistance; finches with varying beak shapes; TB bacteria with varying antibiotic susceptibility; HIV strains with differing ARV susceptibility).
  • Selection pressure (DDT spraying, food availability, antibiotic treatment, ARV treatment) favors the survival and reproduction of the resistant or better-adapted variants.
  • Over generations, the frequency of resistance or advantageous traits increases in the population.
  • This leads to evolved populations that are better adapted to their changing environments or treatment regimens.

Simple Mathematical View of Selection (conceptual)

  • If two alleles exist (R for resistant, S for susceptible) with fitnesses wR and wS, the next generation allele frequency can be described by:
  • p' = \frac{p \cdot wR}{p \cdot wR + (1-p) \cdot w_S} where p is the current frequency of R.
  • The change in allele frequency is \Delta p = p' - p.
  • A basic population genetics expression that often applies in population studies is the Hardy–Weinberg principle: p^2 + 2pq + q^2 = 1 assuming no evolution, random mating, infinite population size, no migration, and no mutation.
  • In real-world scenarios described here, selection pressures violate HW assumptions, leading to changes in allele frequencies over time.

Connections to Foundational Principles and Real-World Relevance

  • Variation is essential for evolution; observed in DDT resistance among mosquitoes, beak/body size variation among Galápagos finches, antibiotic resistance in TB bacteria, and ARV resistance in HIV.
  • Selection pressures (pesticide use, food scarcity, antibiotics/ARVs, treatment regimens) shape which variants survive and reproduce.
  • Reproductive advantage of fit individuals leads to gene propagation of advantageous traits.
  • Practical implications: the need for rotation of insecticides, maintaining food supply strategies that reduce selective pressure, strict antibiotic stewardship, and use of ARV cocktails to prevent drug resistance.
  • Ethical and public health considerations: environmental impact of insecticides, equity in access to effective treatments, and the global challenge of antimicrobial resistance (AMR).

Summary of Quiz/Practice Questions (Pages 19–28 of Transcript)

  • Page 19: Infected with HIV virus are given a…
    • A. Cocktail of drugs
    • B. Combination of drugs
    • C. Both A and B
    • D. Neither A nor B
    • Answer (based on transcript): C
  • Page 20: Drugs used to treat HIV/AIDS…
    • A. Antibiotics
    • B. Anti-retrovirals
    • C. Immunization
    • D. None of the above
    • Answer: B
  • Page 21: If a TB patient does not complete the course of antibiotics, they may suffer from…
    • A. MDR TB
    • B. XDR TB
    • C. Both A and B
    • D. Neither A nor B
    • Answer: C
  • Page 22: TB is caused by…
    • A. Human immunodeficiency virus
    • B. Mycobacterium tuberculosis
    • C. A virus
    • D. A pathogen
    • Answer: B
  • Page 23: The pathogen that causes TB is a…
    • A. Bacteria
    • B. Virus
    • C. Both A and B
    • D. Neither A nor B
    • Answer: A
  • Page 24: The vector for malaria is the/a…
    • A. Female Anopheles mosquito
    • B. Plasmodium
    • C. Virus
    • D. Bacteria
    • Answer: A
  • Page 25: The pathogen of malaria is …
    • A. The Female Anopheles mosquito
    • B. The Plasmodium
    • C. A Virus
    • D. A Bacteria
    • Answer: B
  • Page 26: The malaria vector can be killed by…
    • A. Antibiotics
    • B. Antibodies
    • C. DDT
    • D. None of the above
    • Answer: C
  • Page 27: A resistant mosquito is one that is…
    • A. Not affected by DDT
    • B. Affected by DDT
    • C. Not affected by antibodies
    • D. None of the above
    • Answer: A
  • Page 28: DDT resistant mosquitoes were first detected in…
    • A. Columbia in 1959
    • B. South Africa in 1959
    • C. India in 1959
    • D. Turkey in 1959
    • Answer (from transcript listing): A

Practical Implications and Study Tips

  • Understand how variation and selection interact to drive resistance in real-world contexts (pesticides, antibiotics, ARVs).
  • Be able to explain the difference between MDR TB and XDR TB and why incomplete antibiotic courses contribute to resistance.
  • Remember the concept of a drug cocktail and why combination therapies are used for HIV/AIDS.
  • Review the glossary terms and the relationships among DDT, malaria transmission, and vector control.

Connections to Foundational Principles in Life Sciences

  • Variation provides raw material for evolution.
  • Selection pressures guide which variants persist.
  • Inheritance passes advantageous traits to future generations.
  • Evolution can be observed in short-generation organisms (mosquitoes, bacteria) more readily than in long-lived species.
  • Ethical considerations include responsible pesticide use, antibiotic stewardship, and equitable access to treatment.