Mental Model: Fitness Landscapes

Fitness Landscapes are used to visualize the relationship between genetic makeup (genotype) and evolutionary fitness (the ability to survive and reproduce). A fitness landscape is a vast landscape divided into a grid of billions of squares. Each square represents a genotype—some squares represent birds; some fish; some humans; with the majority being all the variations of genetic possibility that couldn’t survive in reality. Each square is very similar to its neighbors: two of the same species with a small variation, or two different but related species. The closer the squares, the more similar the genotype, and the further the squares, the more different. The fitness of each genotype is represented by its height on the landscape. Valleys represent low fitness, mountain peaks high fitness.

Fitness Landscape

Over time, species tend to move up the landscape to the nearest peak (A), where all future paths of variation lead downward. The peak that a genotype “settles” on is most likely to be a local optimum, which is not necessarily the highest peak in the landscape (a global optimum). This is because selection pushes fitness towards nearby peaks (what is called a basis of attraction), but lacks the foresight to select the highest peak.

To get to a higher peak, a species may have to reduce its fitness in the near term (C) as it slowly traverses across a valley in order to improve fitness in the long term. In order to make this shift, there has to be sufficient instability or challenge; otherwise, an organism will not opt to leave the intermediate peak and suffer the unknown prospects of the valley. If the valley is too low or the higher peak too far away, it may be unreachable as the low fitness hurdle can’t be overcome. (An example is the lack of wheeled animals, which although beneficial is inaccessible due to the valley of low fitness genotypes around it.)

Evolution usually moves in small steps, but occasionally it takes wild leaps—a single mutation might give a creature an extra pair of legs or another radically different feature. Most of the time these leaps result in much lower fitness (B), and therefore don’t last. But other times it allows the genotype to jump to a higher peak without the slow process of going down before going up.

Every landscape has different terrain that can be on a scale from flat to rugged. A rugged or coarse landscape has many local peaks and deep valleys, while a flat landscape has only very small hills (all genotypes have about the same success rates).

Landscapes don’t remain static—they shift over time due to either environmental changes or adjustments as organisms move across it. The movement can vary from being stable (relatively flat and slow to change) to roiling (likely rugged and changing quickly). Given the likelihood of ever-shifting landscapes, the evolutionary mix of small steps and occasional wild leaps is the best possible way to adapt to the environment.

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