Evolution: Gradualism vs. Punctuated Equilibrium

Punctuated Equilibrium

The fossil record, while incomplete, reveals periods of stability punctuated by the sudden appearance of new species. These species often remain unchanged for extended periods before disappearing abruptly. In 1972, Gould and Eldredge proposed punctuated equilibrium to explain these sudden shifts, suggesting that not all evolutionary changes are gradual. Periods of stasis, where species remain unchanged, alternate with short bursts of rapid change (speciation), during which new species emerge from existing ones.

Gradualism vs. Punctuated Equilibrium

Gradualism

Species evolve along a single lineage, transforming slowly and continuously through small changes over long periods. The entire population transforms, not just isolated individuals.

Punctuated Equilibrium

Species do not follow a single lineage but branch out in bursts. Periods of stasis alternate with periods of speciation, where new species arise from small, isolated populations.

Species and Speciation

A species is a group of organisms sharing a common gene pool and capable of producing fertile offspring. Speciation, the formation of new species, requires reproductive isolation, interrupting gene flow between populations. Geographical barriers, like mountains, can cause isolation. Genetic material within isolated populations differentiates through mutation and recombination, with natural selection acting upon these changes.

Evidence of Evolution

Anatomical Evidence

• Homologous Structures: Similar internal structures with different external forms and functions, inherited from a common ancestor and adapted to different environments.
• Analogous Structures: Structures with the same function in different organisms but different origins.
• Vestigial Organs: Organs that were functional in ancestral species but are now disused.

Paleontological Evidence

Fossil remains reveal extinct organisms, showing that species have changed over time. The fossil record, though incomplete, supports the theory of evolution.

Embryological Evidence

Similarities in early embryonic development across different animal groups suggest common ancestry. These similarities diminish as development progresses.

Biogeographical Evidence

The geographical distribution of species supports evolution. Isolated populations in similar environments tend to evolve differently.

Biochemical Evidence

Molecular comparisons, such as DNA and protein sequences, reveal evolutionary relationships. Greater molecular similarity indicates a closer relationship.

Neo-Darwinism (Synthetic Theory of Evolution)

Around 1930, scientists integrated genetics with evolutionary theory, forming neo-Darwinism. This theory emphasizes mutations, genetic recombination, and natural selection as drivers of evolution. Key features include:

• Rejection of Lamarckism
• Genetic variability arises from mutation and recombination
• Natural selection acts on genetic variability
• Natural selection changes allele frequencies in populations
• Populations evolve, not individuals
• Evolution occurs gradually