Straight line evolution, scientifically known as orthogenesis, is a historical concept describing evolution that proceeds along a predetermined, linear path, driven by internal forces within an organism.
Understanding Straight Line Evolution (Orthogenesis)
As defined by Bowler, orthogenesis literally means evolution in a straight line. It is generally understood to be "evolution that is held to a regular course by forces internal to the organism." This concept posits that evolutionary change is not random but is instead directed towards specific, predetermined goals or endpoints.
Key Characteristics of Straight Line Evolution
The defining features of straight line evolution, or orthogenesis, include:
- Internal Direction: Unlike modern evolutionary theory that emphasizes external environmental pressures, orthogenesis suggests that the trajectory of evolution is dictated by inherent, internal mechanisms or forces within the organism itself.
- Non-Random Variation: Variation is not considered random or undirected. Instead, new traits or changes are assumed to emerge in a way that guides the lineage towards a fixed, ultimate goal.
- Fixed Goals: Evolution is seen as having a predetermined end point or series of goals, with all changes contributing to the achievement of these objectives.
- Predictable Path: The evolutionary progression is expected to be linear and predictable, following a consistent course from one form to the next.
Why "Straight Line"?
The term "straight line" emphasizes the idea of a fixed, non-deviating trajectory. Imagine a species continually developing a larger size, more complex eyes, or a more specialized limb structure, not in response to varying environmental pressures, but as if following an internal blueprint towards an inevitable conclusion. This contrasts sharply with the branching, adaptive, and often unpredictable nature of evolution as understood today.
Core Concepts of Straight Line Evolution
| Concept | Description |
|---|---|
| Orthogenesis | The scientific term for straight line evolution, emphasizing a directed, non-random course. |
| Internal Forces | The presumed drivers of evolution, originating from within the organism, ensuring evolution proceeds along a "regular course." |
| Fixed Goals | The concept that variation is directed towards specific, pre-determined outcomes, making the evolutionary path predictable. |
| Non-Random | A fundamental tenet stating that genetic variations and subsequent evolutionary changes are not arbitrary but follow a predetermined direction. |
Important Distinction: Straight Line Evolution vs. Modern Evolutionary Theory
It's crucial to understand that straight line evolution (orthogenesis) is largely a historical concept and is generally not supported by modern evolutionary biology. Contemporary evolutionary theory, primarily based on natural selection and genetic drift, emphasizes random genetic variation and the role of environmental pressures in shaping the direction of evolution, leading to a branching, adaptive, and often opportunistic pattern rather than a linear, predetermined one.
