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Q1:
Explain antibiotic resistance observed in bacteria in light of Darwinian selection theory.
Solution :
Given Variables & Theoretical Foundation
In the context of Darwinian selection, the development of antibiotic resistance in bacterial populations is not an induced change in individuals, but a shift in the genetic composition of the population over successive generations due to differential survival [Darwinian Theory of Natural Selection].
- Variation: Naturally occurring genetic variations exist within a bacterial population (arising from mutations or horizontal gene transfer).
- Selection Pressure: The introduction of an antibiotic serves as an environmental filter.
- Differential Reproduction: Only bacteria possessing resistance genes survive to reproduce, passing those traits to progeny.
Step 1: The Pre-Selection State (Stochastic Variation)
In any large population of bacteria, stochastic genetic mutations occur. Occasionally, a mutation confers a biochemical advantage, such as the synthesis of an enzyme (e.g., $\beta$-lactamase) capable of degrading a specific antibiotic, or an alteration in the cell wall protein structure that prevents antibiotic binding [Mutation Theory]. At this stage, the resistant strain exists in a negligible frequency.
Step 2: The Introduction of Selective Pressure
When an antibiotic is administered, it acts as a selective agent. Bacteria lacking the resistance gene are susceptible and undergo lysis or inhibition of metabolic pathways.
Mathematically, if $N_s$ is the number of susceptible bacteria and $N_r$ is the number of resistant bacteria, the fitness function $W$ for susceptible bacteria drops to $W_s \approx 0$ upon exposure to the antibiotic, while $W_r \approx 1$ for the resistant strain.
Step 3: Visual Representation of Selection
Step 4: Inheritance and Population Shift
The few resistant bacteria survive the selection event. Because bacteria possess high reproductive rates ($R_0 \gg 1$), the resistant survivors undergo rapid binary fission. Through vertical gene transfer (reproduction) and horizontal gene transfer (conjugation, transformation, transduction), the resistance gene spreads throughout the remaining population [Principles of Bacterial Genetics].
| Mechanism | Impact on Evolution |
|---|---|
| Natural Selection | Eliminates unfit phenotypes (susceptible bacteria). |
| Reproductive Success | Resistant bacteria pass alleles to next generation. |
| Directional Selection | The population mean phenotype shifts toward resistance. |
Analytical Summary
Darwinian selection dictates that the environment does not create the mutation; rather, the environment selects for the pre-existing advantageous variation. Antibiotic resistance is a classic example of Directional Selection, where the allele frequency of the resistant gene increases over time within the bacterial gene pool, rendering the antibiotic ineffective against the evolved population.
Final Solution: Antibiotic resistance is an evolutionary phenomenon driven by natural selection, wherein environmental pressure (antibiotics) eliminates susceptible individuals, allowing survivors with pre-existing resistance mutations to propagate their genotype, ultimately leading to a resistant bacterial population.
More Questions from Class 12 Biology Evolution EXERCISES
- Q1: Explain antibiotic resistance observed in bacteria in light of Darwinian selection theory.
- Q2: Find out from newspapers and popular science articles any new fossil discoveries or controversies about evolution.
- Q2: Find out from newspapers and popular science articles any new fossil discoveries or controversies about evolution.
- Q3: Attempt giving a clear definition of the term species.
- Q3: Attempt giving a clear definition of the term species.
- Q4: Try to trace the various components of human evolution (hint: brain size and function, skeletal structure, dietary preference, etc.)
- Q4: Try to trace the various components of human evolution (hint: brain size and function, skeletal structure, dietary preference, etc.)
- Q5: Find out through internet and popular science articles whether animals other than man has self-consciousness.
- Q5: Find out through internet and popular science articles whether animals other than man has self-consciousness.
- Q6: List 10 modern-day animals and using the internet resources link it to a corresponding ancient fossil. Name both.
- Q6: List 10 modern-day animals and using the internet resources link it to a corresponding ancient fossil. Name both.
- Q7: Practise drawing various animals and plants.
- Q7: Practise drawing various animals and plants.
- Q8: Describe one example of adaptive radiation.
- Q8: Describe one example of adaptive radiation.
- Q9: Can we call human evolution as adaptive radiation?
- Q9: Can we call human evolution as adaptive radiation?
- Q10: Using various resources such as your school Library or the internet and discussions with your teacher, trace the evolutionary stages of any one animal, say horse.
- Q10: Using various resources such as your school Library or the internet and discussions with your teacher, trace the evolutionary stages of any one animal, say horse.
CBSE Solutions for Class 12 Biology Evolution
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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