   Among philosophers of biology there has been a wide consensus that the solution to problem of defining individual ‘fitness’ is given by treating it as a probabilistic disposition. As such it causally intervenes between the relationship of environments to organisms that cause it, and the actual rates of reproduction that are its effects. Thus, fitness turns out to be a “garden-variety” dispositional concept like ‘magnetic’ or ‘fragile’. These properties, and all dispositions are distinguished both from the actual behaviour to which they give rise—e.g., attracting iron filing or breaking in the case of magnetism or fragility; some items are magnetic and others fragile without ever actually attracting iron filings or breaking. Similarly, an organism can have a probabilistic disposition to have \(n\) offspring and yet “unluckily” never actually reproduce.