Populations—groups of individuals of the same species—are defined by a suite of emergent characteristics, such as average age of first reproduction, average life span, and average number of offspring. These characteristics may vary from population to population; for example, the average age of first reproduction in humans varies from 19 in Mozambique to 31 in South Korea. However, these characteristics tend to vary more between species than they do within species. As an example, human birth rates are nothing like that of the Pacific salmon, which can produce over 10,000 offspring per individual.
In this chapter, we’ll focus on another, often-puzzling aspect of populations—sex ratios. We’ll talk about organisms in which the sex-ratios are typically 1:1, with relatively equal numbers of males and females (e.g., humans), and populations that are either consistently skewed (e.g., a 1:5 male:female sex ratio is typical of the American alligator; meaning for every male there are 5 females, notice that this means that the proportion of males is 1 male out of 6 total alligators or 1/6 or 16.7%), or characterized by variability (e.g., the sex-ratio of peafowl populations varies quite a bit). Before we move on, let’s make sure you are interpreting ratios correctly.