Tag: fitness landscape

Lay of the Landscape

The more complex the world gets, the more we need models to simplify it. One of the models I return to often is fitness landscapes, which can help solve problems, design better experiences, and explain the world around us.

Imagine you and a group of friends are on a team playing a game.

The game takes place on a huge playing field with rough, mountainous terrain, like the Himalayas or Alps. The only goal is to increase your team’s average altitude. This seems easy enough, but there are a few catches: (1) any player can only see a few feet ahead of them, (2) the terrain slowly changes over time, and (3) if a player drops below a certain altitude for long enough, they’re eliminated. Given these rules, what strategies would your team use to find the highest peaks?

This is a metaphor for the “game” that species must play to survive in an ecosystem.

The terrain is a fitness landscape representing a library or design space of every possible variation of organism, spread out over a nearly infinite surface. The closer together on the surface, the more similar the genotype. This means single species would be clustered together. Dogs would be near wolves, far from fish, and even farther from fungi.

Altitude indicates the fitness of the organism — or how likely it is to survive in a particular environment. The higher it is on the landscape, the better the design and more fit the organism. Below a certain threshold, organisms can’t survive and species go extinct.

As a model, landscapes can help show us visually and mathematically how to find the best designs. The original concept was developed by evolutionary theorist Sewell Wright in 1931, and focused only on biological entities. But a design space could represent almost any set of possibilities — as long as it has building blocks or variables that combine into many variations, each with a value (or fitness level) that can be assigned. This means it could apply to design spaces of problems, equations, technologies, strategies, memes, or even sets of LEGOs.

Features of landscapes

A vast majority of the variations on a typical landscape are bad designs. These are oceans of low fitness, below the surface of which organisms are incapable of survival or reproduction.

But certain regions — springing out of the oceans like islands or continents — are full of a range of potential variations, all with some usable level of fitness. The basic features of these regions of terrain are:

  • Local peaks or plateaus — A point or area of high fitness where all surrounding paths go down.
  • Global peak — The highest peak in the region. The fittest entity in the area. The best design of all similar variations.
  • Valleys — Flatter areas of low fitness adjacent to hills and mountain ranges.
  • Pits or Crevasses — Deep holes of low fitness below the “sea-level” of survival.

Peaks are good. Pits are bad. And crossing valleys is very risky: you could find higher fitness, but likely not.

The unconscious process of evolution drives genotypes uphill over time, finding and settling on peaks of fitness until the landscape shifts or some other factor forces a move. More on this later.

Continue reading “Lay of the Landscape” →

Mental Model: Fitness Landscapes

UPDATE (September 2020): I wrote a more in-depth, detailed explanation of fitness landscapes and how they can be applied across disciplines. The original (shorter) version is still below but I’d recommend the latest one for a better understanding of the model.

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.