Evolution is a feature of populations, not individuals. If the proto-chickens had 76 chromosomes, and some had a tendency to have offspring with 78 chromosomes

Evolution is a feature of populations, not individuals. If the proto-chickens had 76 chromosomes, and some had a tendency to have offspring with 78 chromosomes, there would be some 78 chromosome chickens born within the population. As long as those chickens bred with each other it would work. Soon they would become a distinct population of individuals breeding with each other.