Biology Notes

Natural Selection

Darwin's Impact

• Development of the Theory: Charles Darwin formulated the theory of evolution by natural selection, co-published with Alfred Russel Wallace in 1858 and expanded in On the Origin of Species in 1859.

• Challenging Prevailing Beliefs:

  • Fixity of Species and Special Creation: Replaced the long-held view that species were divinely created, unchanging, and immutable.

  • Lamarckism: Disproved the Lamarckian idea of the inheritance of acquired characteristics (that physiological traits developed during an individual's lifetime are transmitted to offspring).

  • Common Descent: Established that all living organisms share common ancestors and have diversified through descent with modification.

Process of Natural Selection

• Variation: Individuals within a population exhibit natural phenotypic variations arising from genetic differences (mutations, sexual reproduction, and crossing over during meiosis).

• Competition: Organisms produce more offspring than the environment can sustain, leading to intense competition for limiting resources (food, territory, mates, and water).

• Survival of the Fittest: Individuals possessing adaptations best aligned with their environment have a higher probability of survival and reproductive success.

• Environmental Pressures: Selective agents—both biotic (predators, disease-causing pathogens, competitors) and abiotic (temperature, rainfall, soil composition)—exert selection pressure, favoring individuals with advantageous phenotypes.

Stages of Natural Selection

$\text{Variation} \rightarrow \text{Overproduction} \rightarrow \text{Competition} \rightarrow \text{Survival} \rightarrow \text{Reproduction} \rightarrow \text{Change in Population}$

1. Variation

   • Heritable genetic and phenotypic differences pre-exist within a population.

2. Overproduction

   • Organisms produce more progeny than the ecosystem's carrying capacity can support.

3. Competition

   • Resource scarcity leads to a struggle for survival among individuals.

4. Survival

   • Organisms possessing advantageous traits are better equipped to withstand environmental pressures.

5. Reproduction

   • Surviving individuals reproduce, passing favorable alleles to subsequent generations.

6. Change in Population Over Time

   • The frequency of adaptive alleles increases in the gene pool across generations, driving evolutionary adaptation.

Evolution

Definition of Evolution

• Evolution is the cumulative change in the heritable characteristics and allele frequencies of biological populations across successive generations.

Mechanisms of Evolution

• Mutation:

  • Spontaneous changes in the nucleotide sequence of DNA.

  • The primary and ultimate source of new genetic variation, introducing novel alleles into a gene pool.

• Natural Selection:

  • Differential reproductive success based on phenotypic fitness.

  • Non-randomly increases the frequency of beneficial alleles while decreasing deleterious ones.

• Genetic Drift:

  • Unpredictable, random shifts in allele frequencies from generation to generation due to chance events.

  • Exerts the greatest impact on small populations, notably through:

  • Bottleneck Effect: A catastrophic event drastically cuts population size, randomly reducing genetic diversity.

  • Founder Effect: A small group isolates from a larger population, establishing a colony with an unrepresentative gene pool.

• Gene Flow:

  • The transfer of alleles into or out of a population resulting from the migration of fertile individuals or gametes.

  • Introduces new genetic diversity to populations and counteracts divergence between geographically separated populations.

Convergent vs. Divergent Evolution

• Convergent Evolution:

  • Characteristics: Distantly related or unrelated species independently evolve similar phenotypic characteristics.

  • Selection Pressures: Occurs when different lineages encounter comparable environmental challenges and ecological niches.

  • Resulting Structures: Produces analogous structures—features sharing similar functions and appearances but possessing distinct internal anatomy and separate evolutionary origins (e.g., streamlined bodies in dolphins and sharks; wings of insects and birds).

• Divergent Evolution:

  • Characteristics: Two or more related populations or species sharing a recent common ancestor accumulate differences and diverge phenotypically.

  • Selection Pressures: Driven by exposure to differing ecological pressures, geographical separation, or adaptive radiation into open niches.

  • Resulting Structures: Produces homologous structures—features sharing an anatomical layout derived from a common ancestor, often modified for disparate functions (e.g., the pentadactyl limb in humans, bats, whales, and cats).

Speciation

Definition of a Species

• A species is a group of natural populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, and are reproductively isolated from other such groups.

Process of Speciation

1. Variation Within a Population

   • An ancestral population possesses existing genetic and phenotypic diversity.

2. Isolation

   • A barrier disrupts gene flow between subpopulations:

   • Geographical Isolation (Allopatric): Physical separation by bodies of water, mountain ranges, or habitat fragmentation.

   • Reproductive and Behavioural Isolation (Sympatric): Speciation within the same geographic area due to behavioral differences (courtship rituals), temporal differences (breeding seasons), or mechanical/gametic incompatibility.

3. Selection Pressures

   • Isolated groups encounter distinct biotic and abiotic conditions, creating differential selection pressures that favor different adaptations.

4. Genetic Divergence

   • Mutations, natural selection, and genetic drift operate independently in each gene pool, causing genetic differences to accumulate over time.

5. Reproductive Isolation

   • Over time, accumulated genetic differences prevent the separated populations from interbreeding successfully if reunited, marking the completion of speciation.

Evidence for Evolution

Fossil Record

• Provides a chronological sequence of organisms preserved across sedimentary rock strata.

• Documents transitional forms (intermediate fossils) demonstrating evolutionary connections between major taxonomic groups (e.g., Archaeopteryx displaying non-avian reptilian and avian characteristics; Tiktaalik bridging lobe-finned fish and early tetrapods).

Comparative Anatomy

• Homologous Structures: Anatomical features sharing a common structural framework inherited from a common ancestor, adapted for different functional roles, demonstrating divergent evolution.

• Analogous Structures: Structures with similar functions that evolved independently due to similar ecological niches, demonstrating convergent evolution.

• Vestigial Structures: Functionless or rudimentary remnants of structures that had important functions in ancestral organisms (e.g., pelvic bones in cetaceans, human appendix).

Comparative Genetics and Biochemistry

• All terrestrial life shares a universal genetic code (DNA/RNA) and identical fundamental biochemical pathways (such as glycolysis and transcription/translation).

• The degree of nucleotide sequence similarity in DNA or amino acid sequence similarity in shared proteins (e.g., Cytochrome c, hemoglobin) reflects evolutionary kinship; more closely related taxa show fewer sequence differences.

Biogeography

• The geographical distribution of extant and fossil species mirrors geological history and plate tectonics.

• Isolated landmasses feature distinct endemic lineages that evolved in place (e.g., the evolutionary radiation of marsupials in Australia and Darwin's finches on the Galápagos Islands).

Comparative Embryology

• Embryos of distinct vertebrate classes display remarkable structural commonalities during early development (e.g., pharyngeal arches/pouches, post-anal tails).

• These shared developmental patterns reflect conserved regulatory genes (such as Hox genes) passed down from a common ancestor.