Evolution by Natural Selection: Mechanisms, Evidence, and Case Studies

Chapter 22: Evolution by Natural Selection

Introduction to Evolution by Natural Selection

Evolution by natural selection is a foundational theory in modern biology, explaining how species change over time through differential survival and reproduction. This chapter explores the mechanisms, evidence, and real-world examples of natural selection.

Major Influences on Darwin's Thinking

Darwin's Inspirations

• Artificial Selection: Darwin observed selective breeding in animals such as dogs and pigeons, noting how breeders could enhance specific traits over generations.

• Malthus' Principle of Population: Thomas Malthus proposed that populations grow faster than resources, leading to a "struggle for existence" where only some individuals survive and reproduce.

• Variation in Natural Populations: Darwin recognized that natural populations exhibit variation, which is essential for selection to act upon.

Example: All dog breeds descend from a common wolf ancestor, with selective breeding producing the diversity seen today.

Artificial Selection and Genetic Variation

Artificial Selection Reveals Genetic Variation

• Selective breeding demonstrates that variation exists and is heritable.

• Artificial selection can shift trait distributions in populations, as shown in experiments with fruit flies and bristle number.

Population	Trait Selected	Result
Original	Abdominal bristles	Wide range of bristle numbers
Selected for low bristles	Low bristle number	Shift toward fewer bristles
Selected for high bristles	High bristle number	Shift toward more bristles

Darwin's Four Postulates of Natural Selection

Criteria for Natural Selection

1. Variation: Individuals in a population vary in traits.

2. Heritability: Some trait differences are heritable.

3. Variable Survival and Reproduction: Not all individuals survive and reproduce equally.

4. Non-Random Survival: Individuals with advantageous traits are more likely to survive and reproduce.

Condensed Principle: Evolution by natural selection occurs when heritable variation leads to differential reproductive success.

Key Evolutionary Definitions

Fitness, Adaptation, and Selection

• Fitness: The ability of an individual to produce surviving, fertile offspring relative to others in the population.

• Adaptation: (n) A heritable trait that increases fitness in a particular environment; (v) The process by which such traits become more common.

• Selection: Differential reproduction resulting from heritable variation.

Measuring Natural Selection: Case Studies

Case Study 1: Antibiotic Resistance in Mycobacterium tuberculosis

• Background: Mycobacterium tuberculosis causes tuberculosis (TB), a deadly infectious disease.

• Antibiotic Resistance: TB rates surged due to the emergence of antibiotic-resistant strains, especially after the introduction of rifampin.

• Mechanism: Resistance arose from a point mutation in the rpoB gene, altering RNA polymerase and preventing rifampin binding.

• Natural Selection: Drug-resistant bacteria survived antibiotic treatment and reproduced, increasing the frequency of resistance alleles.

Key Points:

• Variation existed (resistant and non-resistant strains).

• Variation was heritable (genetic mutation).

• Reproductive success varied (only resistant bacteria survived).

• Selection occurred (resistant bacteria reproduced more).

Case Study 2: Beak Size Variation in Galápagos Finches

• Background: Peter and Rosemary Grant studied medium ground finches on Daphne Major Island.

• Heritability: Beak size and body form are heritable traits in these birds.

• Drought Experiment: A severe drought led to 84% mortality; birds with deeper beaks survived better due to food availability.

• Adaptive Tracking: Environmental changes (e.g., rainfall) shifted selection, favoring different beak shapes and sizes over time.

Year	Environmental Change	Selected Trait
Drought	Food scarcity	Deep beaks
Heavy Rainfall	Abundant small seeds	Small, pointed beaks

Genetic and Environmental Constraints on Evolution

Misconceptions and Constraints

• Natural Selection Does Not Change Individuals: It sorts existing variants; populations evolve, not individuals.

• Acclimatization vs. Adaptation: Acclimatization is a non-heritable change in phenotype; adaptation involves heritable genetic change.

• Not Goal-Directed: Evolution is not purposeful or progressive; mutations occur randomly, and adaptations do not arise because organisms "need" them.

• No Higher or Lower Organisms: All organisms are adapted to their environments; complexity is not a measure of evolutionary success.

• Fitness Trade-Offs: Adaptations often involve compromises (e.g., egg size vs. number, growth rate vs. lifespan).

• Genetic Constraints: Pleiotropy and genetic correlations can limit the direction and extent of evolutionary change.

• Environmental Constraints: Abiotic and biotic factors influence which traits are favored by selection.

Other Processes of Evolution

Beyond Natural Selection

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Gene Flow: Movement of alleles between populations.

• Mutation: Source of new genetic variation.

Equation for Evolution by Natural Selection:

$\Delta p = \frac{p(1 - p)s}{1 - s p}$ Where $\Delta p$ is the change in allele frequency, $p$ is the initial allele frequency, and $s$ is the selection coefficient.

Summary Table: Key Terms and Concepts

Term	Definition	Example
Fitness	Relative reproductive success	Number of surviving offspring
Adaptation	Heritable trait increasing fitness	Deep beak in finches during drought
Selection	Differential reproduction	Antibiotic resistance in bacteria
Genetic Drift	Random allele frequency change	Bottleneck effect
Gene Flow	Allele movement between populations	Migration
Mutation	New genetic variation	Point mutation in rpoB gene

Conclusion

Evolution by natural selection is a powerful mechanism explaining the diversity and adaptation of life. Understanding its principles, evidence, and limitations is essential for studying biology and addressing challenges such as antibiotic resistance and environmental change.