Darwin’s Evolution: Genetics, Speciation, and Evidence
Evolution After Darwin
Mendel’s Discoveries and the Synthetic Theory of Evolution
Gregor Mendel discovered some of the basic laws of inheritance. Darwin’s proposals were enriched and updated with new knowledge of genetics, leading to the Synthetic Theory of Evolution. Key contributions include:
- The evolutionary unit is not the individual, but the population. Natural selection acts on the genetic wealth of the population.
- The origin of variability is mutation: a sudden change in DNA. Organisms carrying mutations show different characteristics that are heritable, occur randomly, and are often detrimental. Neutral mutations do not produce advantages or disadvantages.
Challenges to Gradualism: Punctuated Equilibrium
Darwin’s theory and the synthetic theory are gradualist, arguing that evolutionary changes are caused by a slow and gradual accumulation of small changes. However, Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibrium (or saltationism). They argued that if gradual evolution were true, the fossil record would be full of ever-changing species and intermediate forms. Instead, the fossil record shows that species experience long periods of stability interrupted by short periods of rapid change. Evolution, therefore, occurs at an irregular rhythm, with strong accelerations and sudden stops.
Evo-Devo: Evolutionary Developmental Biology
Evo-devo recognizes a hierarchy among genes. While some genes regulate small details, others control major decisions affecting other genes and determining an individual’s body plan. This explains major evolutionary leaps. Both punctuated equilibrium and evo-devo involve variability between individuals and natural selection (and are thus Darwinist), but do not propose a gradualist model.
The Origin of New Species
All living things come from a common ancestor. Evolution explains how, from a single species, two or more can form, leading to current biodiversity (speciation). This typically occurs as follows:
- Geographic Isolation: Two populations of the same species are separated by a geographical barrier, such as a sea or a mountain range.
- Independent Evolution: The two populations evolve independently. Mutations occur randomly, and because the number of genes is very high, different mutations will arise in each population.
- Divergence: The accumulation of differences between populations, combined with natural selection, leads to the emergence of two distinct species.
- Reproductive Isolation: Two populations belong to different species if they are not inter-fertile; crosses between individuals of both populations have no descendants or the offspring are sterile.
Geographical isolation is a factor that can cause speciation. Reproductive isolation is the criterion used to determine if two current populations belong to the same species.
Evolution: Fact and Theory
As Theodosius Dobzhansky famously stated, “Nothing in biology makes sense except in the light of evolution.” Evolution is a fact, but there are also evolutionary theories. In science, a fact is something confirmed. Facts must be explained by theories. If a theory fails to explain a fact, it does not negate that fact. Evolution is considered a historical fact, supported by data and arguments.