Yeasts secrete enzymes that break down carbohydrates (through fermentation) to yield carbon dioxide and alcohol. The source of carbohydrates are either living hosts or non-living hosts such as rotting vegetation, or the moist body cavities of animals. Yeasts are considered by some scientists to be closely related to the algae, lacking only in photosynthetic capability—perhaps as a result of an evolutionary trend toward a lifestyle dependent upon host nutrition. Ecologically yeasts are decomposers that secrete enzymes which dismantle the complex carbon compounds of plant cell walls and animal tissues, which they convert to sugars for their own growth and sustenance. Yeast reproduction may involve sexual spore production or asexual budding, dependent upon surrounding conditions. Though yeasts are highly tolerant of environmental variations in temperature and acidity, they thrive in warm and moist places high in oxygen and low in carbon dioxide. Whether or not they reproduce through asexual budding depends on the favorability of surrounding conditions: when times are good, yeast clones are produced by budding. In times of environmental stress, yeasts produce spores which are capable of withstanding periods of environmental hardship—perhaps even to lie dormant, until conditions improve and the mingling of genes can take place with the spore of another yeast. This rare version of yeast reproduction provides for genetic
variation when conditions demand it, though budding is the predominant mode of yeast reproduction.