To match each word to its definition:
1. **a version of a gene**: b. allele
- An allele is a specific version of a gene that determines a particular trait or characteristic in an organism. For example, in humans, the gene for eye color may have alleles for blue eyes, brown eyes, or green eyes.
2. **reproductive success**: a. fitness
- Fitness in the context of evolution refers to an organism's ability to survive, reproduce, and pass on its genes to the next generation. Reproductive success is a key component of fitness because individuals with traits that lead to successful reproduction are more likely to pass on those traits to their offspring.
By understanding these definitions, you can match the words "allele" and "fitness" to their corresponding definitions in the question.
