# Lexicase Selection of Specialists

## TL;DR

This paper investigates how lexicase selection's ability to select specialists influences its effectiveness, showing that specialists contribute significantly to its performance and diversity in evolving solutions.

## Contribution

It reveals that selecting specialists is crucial for lexicase selection's success, providing insights into its advantages over error-aggregating methods.

## Key findings

- Lexicase selection's performance drops when specialists are excluded.
- Specialists help maintain diversity and drive evolution toward global solutions.
- Lexicase selection favors specialists more than tournament selection.

## Abstract

Lexicase parent selection filters the population by considering one random training case at a time, eliminating any individuals with errors for the current case that are worse than the best error in the selection pool, until a single individual remains. This process often stops before considering all training cases, meaning that it will ignore the error values on any cases that were not yet considered. Lexicase selection can therefore select specialist individuals that have poor errors on some training cases, if they have great errors on others and those errors come near the start of the random list of cases used for the parent selection event in question. We hypothesize here that selecting these specialists, which may have poor total error, plays an important role in lexicase selection's observed performance advantages over error-aggregating parent selection methods such as tournament selection, which select specialists much less frequently. We conduct experiments examining this hypothesis, and find that lexicase selection's performance and diversity maintenance degrade when we deprive it of the ability of selecting specialists. These findings help explain the improved performance of lexicase selection compared to tournament selection, and suggest that specialists help drive evolution under lexicase selection toward global solutions.