The species problem is a mixture of difficult related questions that often come up when biologists define the word "species". There are a wide range of approaches to defining how we identify species and how species function in nature; each approach is known as a species concept. The number and types of species concepts which exist are constantly changing, but there are at least 26 recognized species concepts. Biological reality means that a definition that works well for some organisms (e.g., birds) will be useless for others (e.g., bacteria) particularly due to concerns regarding if the species reproduces asexually or sexually. The scientific study of the species problem has been called microtaxonomy.