Grovolution OverviewNatural selection on artificial life

Grovolution simulates plants growing, reproducing and dying in an environment with sunlight that shines from above. Plants acquire energy by absorbing light with their leaves and expend their energy on growth, maintenance and generating spores. When a plant runs out of energy, it dies. Each plant has a genome that determines its growth and shape, and this genome is inherited, from parent to offspring, with the chance of mutations.

Evolutionary progression over time towards taller plants

These ingredients are sufficient for natural selection to occur. Plants with forms that increase their chance of survival will be more likely to have offspring, and plants with less advantageous forms tend to leave fewer offspring. Over time, small advantageous changes can accumulate and lead to large evolutionary change. It should be emphasized that while the plants and environment are simulated, the evolution they undertake is very much real – this program does not simulate evolution, it demonstrates it.

The primary selection pressure on the plants is light competition—shorter plants are more likely to be shades by their neighbors and receive less light, and taller plants are likely to receive more light. Plants with better access to sunlight will increase their distribution of seedpods allowing them increased speed in reaching their full growth potential.

Plants in the simulation have three parts: leaves, seedpods and branches. Leaves are the part of a plant that collects light and provides energy to the plant. Without these, a plant would have no way to feed itself and would starve. Seedpods are the parts that produce seeds that can become plants of the next generation. Without these, a plant cannot reproduce. Branches provide the physical structure for the plant. They support the weight of the plant’s parts, so heavier plants need thicker branches. Each branch can have multiple parts growing from its end – either leaves, seedpods or more branches.

The currency of a plant’s life is energy. A plant can gain energy in only one way – absorbing light with leaves. A number of things sap the plant’s energy. Growing takes energy, as well as maintaining existing parts. When plants produce seeds, they put some of their own energy into the seeds which, if the seed is destined to become a new plant, will serve as that new plant’s initial energy reserve. When a plant runs out of energy entirely, it dies of starvation and disappears from the program. Plants can also die of ‘bad luck’ as there is a small chance of death for every tick of the clock. This simulates all of the other reasons a plant might die: disease, fire, herbivores, etc.

A plant’s seedpods produce seeds at a constant rate which go into the seed pool. This seed pool is not visible but exists in the program’s memory. Each seed contains a copy of the genome of the plant that created it. Pairs of seeds in the seed pool combine to form new plants (a bit different from how real plants reproduce). When two seeds combine, they shuffle their genomes together to create a new genome that is a combination of the two. In this manner, the plant reproduction is sexual – each plant has two parents and inherits half its genome from each. Mutations can also occur at this stage, introducing new, random variation into the genomes.

Each plant has a genome, which like ours, is made up of a four-letter alphabet. It specifies all of the details about how the plant will grow. A change to the genome can change how the plant grows. Genomes change in two ways: they are shuffled together when two parent plants sexually reproduce to make a child plant; and they acquire mutations, when one letter in the genome randomly changes to another.

The forms into which the plants evolve is a result of the process of natural selection. Plants that happen to be better suited to live in the current environment are more likely to survive, reproduce and spread the genes that made them successful. Conversely, plants that are ill-suited to their environment are more likely to die without leaving offspring. While the mutations introduced into genomes are random, which organisms survive (the process of natural selection) is not. It is through this continuous process of ‘survival of the fittest’ that advantageous mutations accumulate in the gene pool and harmful mutations are discarded. Over time, the accumulation of advantageous mutations can lead to large changes, resulting in plants that are larger and more complex than their ancestors.