How Species Evolve: Neo-Darwinism vs Punctuated Equilibrium

Objections to Darwin's Evolutionary Theory

Following Darwin's evolutionary theories, two major objections were filed. On one hand, there was a lack of proof of why variations occurred in these organisms, to which he had no response. On the other hand, there was a lack of intermediate fossil records between groups of organisms, often called missing links.

The Rise of Neo-Darwinism

At the beginning of the 20th century, with the rediscovery of Mendel's laws and the discovery of the phenomenon of genetic mutation, Darwinism regained strength. The increased fossil record and the introduction of statistics enabled the formulation in the 1940s and 1950s of the synthetic theory of evolution or Neo-Darwinism.

Sources of Genetic Variation

There are two sources of variation in organisms:

• Mutations
• The combination of genes through the process of sexual reproduction

Natural selection acts on combinations of genes so that those best adapted to a particular environment are passed to the next generation, while others are eliminated. Natural selection acts on populations, not individuals. The accumulation of small variations changes gene frequencies almost imperceptibly within populations, so that over time we can speak of a new species. This process of slow and total transformation of one species into another is called anagenesis.

Punctuated Equilibrium vs. Gradualism

Against the idea of gradual and continuous change that the synthetic theory proposed, another theory emerged that advocates a different idea. Facing the accumulation of small changes within a species (a process called microevolution), there is another process capable of producing new species suddenly (macroevolution).

The Punctuated Equilibrium Theory

In the geological field, and taking into account the lifetime of species on Earth, the authors of the punctuated equilibrium theory estimate that the process of species formation is faster because it occurs over a period of time between 5,000 and 50,000 years.

The fossil record shows that after the sudden appearance of a species, it does not change substantially during the remainder of its existence. This phenomenon is known as stasis. Therefore, the punctuated equilibrium theory advocates an evolutionary mechanism that is fast and branching (cladogenesis).

A Common Ancestry

In any case, if both theories propose a process of species formation from an ancestral species, tracing this process backwards in time brings us to an organism that can be considered the ancestor of all species.