CODEWITHSUNDEEP
neural-networkgenetic-algorithmevolutionary-algorithm
Scooter
Scooter

Reputation: 1079

NEAT Two Identical Genes with Different Innovation Numbers

In paper written about the NEAT algorithm, the author states

A possible problem is that the same structural innovation will receive different innovation numbers in the same generation if it occurs by chance more than once. However, by keeping a list of the innovations that occurred in the current generation, it is possible to ensure that when the same structure arises more than once through independent mutations in the same generation, each identical mutation is assigned the same innovation number.

This makes sense because you don't want identical genes to end up with different innovation numbers. If they did, there would be problems when crossing over two genomes with identical genes but different innovation numbers because you would end up with an offspring with a copy of each gene from each parent, creating the same connection twice.

What doesn't make sense to me though is what happens if a mutation occurs between two genes, and then in the next generation the same mutation occurs? In the paper, it's very clear that only a list of mutations in the current generation is kept to avoid an "explosion of innovation numbers" but doesn't specify anything about what happens if the same mutation occurs across different generations.

Do you keep a global list of gene pairs and their corresponding innovation numbers to prevent this problem? Is there a reason why the paper only specifies what happens if the same mutation happens in the same generation and doesn't consider the case of cross-generational mutations?

Upvotes: 1

Views: 394

Answers (1)

Thomas Wagenaar
Thomas Wagenaar

Reputation: 6759

No. You don't keep a global list of gene pairs. You can if you want to avoid the same mutation from happening. But something that I want to point out: it doesn't matter. The only effect of the same mutation happening is that you will do some unnecessary computation and your global innovation number will increase.

For future genomes however, there is no chance that they will have two innovation numbers that are the same innovation.

Matching genes are inherited randomly, whereas disjoint genes (those that do not match in the middle) and excess genes (those that do not match in the end) are inherited from the more fit parent

So when two identical innovations occur, they will be either a disjoint or excess gene. These will get inherited from the more fit parent and only one parent can be fitter, so óne offspring will never have the same innovation genes.

Upvotes: 1

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