Speciation & Evolutionary Evidence: Unpacking Biological Change

How New Species Originate: The Speciation Process

All living things descend from a common ancestor. Evolution explains how a single species can diverge into two or more, leading to the current biodiversity. This process, known as speciation, typically occurs as follows:

• Geographic Isolation: Two populations of the same species become separated by a geographical barrier, such as a sea or a high mountain range.
• Independent Evolution: Both populations then undergo independent evolutionary paths. Mutations occur randomly in each, and since the number of genes is very high, these mutations will differ between the two populations.
• Accumulation of Differences: The accumulation of these genetic differences, combined with the pressures of natural selection, eventually gives rise to two distinct species.

Two populations are considered different species if they cannot interbreed successfully. This means that crosses between individuals from both populations produce either no offspring or sterile offspring, a phenomenon known as reproductive isolation. While geographic isolation can initiate speciation, reproductive isolation is the definitive criterion used to determine if two current populations belong to the same species.

Is Evolution a Fact or a Theory?

As Theodosius Dobzhansky famously stated, "Nothing in biology makes sense except in the light of evolution." Evolution is widely considered a historical fact, but there are also evolutionary theories. In science, a fact is something confirmed through observation and experiment. Facts must be explained by theories. If a theory fails to explain a fact, it does not negate or question the fact itself. Evolution, as the change in heritable characteristics of biological populations over successive generations, is considered a historical fact.

Compelling Evidence for Evolution

Numerous lines of evidence support the reality of evolution:

• The Fossil Record: The discovery and study of fossils allow scientists to ascertain which species have appeared and become extinct over geological time, revealing transitional forms and evolutionary lineages.
• Comparative Anatomy: Examining the skeletal structure of forelimbs in diverse species—such as a human, a bat, a dolphin, and a bird—reveals a common underlying model and identical basic parts, indicating shared ancestry.
• Embryonic Development: The early stages of embryonic development in a fish, a reptile, a bird, and a mammal show remarkable similarities. These embryos undergo a process of progressive differentiation later on, but their initial resemblance points to common evolutionary origins.
• Molecular Biology: All living organisms are composed of the same fundamental chemical elements. They build their proteins using the same 20 amino acids, following identical procedures, all guided by instructions encoded in DNA. This universal molecular blueprint strongly supports a common ancestry for all life.