Question

Darwin's theory of natural selection

Hint:

Remember the logical sequence: variation → competition → differential survival → inheritance → cumulative change.

Answer 1

Statement: Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring; over generations this leads to evolution of populations. 

Key points: 

• Variation: Individuals in a population show heritable variation in traits.
• Overproduction: Organisms produce more offspring than can survive, creating competition for limited resources.
• Struggle for existence: Limited resources result in a struggle in which not all individuals survive to reproduce.
• Differential survival and reproduction: Individuals with favourable heritable traits have higher survival or reproductive success.
• Inheritance: Advantageous traits are passed to offspring, changing the frequency of alleles in the population.
• Cumulative change: Small, accumulated changes across generations can produce adaptation and, over long periods, speciation.

Mechanism: 
Natural selection acts on phenotypic variation already present in a population; environmental pressures (predators, climate, resources, disease) favour individuals with traits that confer higher fitness; these individuals contribute proportionally more genes to the next generation, shifting population characteristics. 

Types of selection: \[\begin{array}{rl} \bullet & \text{Stabilizing selection: Favors intermediate phenotypes, reducing variation.} \\ \bullet & \text{Directional selection: Favors one extreme phenotype, shifting the population mean.} \\ \bullet & \text{Disruptive selection: Favors both extremes, which can promote divergence and speciation.} \\ \end{array}\] 

Evidence supporting the theory: \[\begin{array}{rl} \bullet & \text{Fossil record: Transitional forms and sequences of change over time.} \\ \bullet & \text{Comparative anatomy: Homologous structures implying common ancestry.} \\ \bullet & \text{Embryology: Similar early developmental stages among related groups.} \\ \bullet & \text{Biogeography: Geographic distribution patterns consistent with divergence and adaptation.} \\ \bullet & \text{Direct observation: Examples such as antibiotic resistance and industrial melanism.} \\ \bullet & \text{Molecular biology: DNA and protein similarities that reflect evolutionary relationships.} \\ \end{array}\] 

Limitations and modern extension: 
Darwin did not know the genetic mechanism of inheritance; the modern synthesis integrated Mendelian genetics and population genetics with natural selection, explaining how variation is produced and transmitted and how allele frequencies change.