Final answer:
The systemic family development model explains family changes over time by recognizing that families progress in a fluid and non-linear manner.
Step-by-step explanation:
The systemic family development model explains family changes over time by recognizing that family life does not follow a strict set of stages, but rather progresses in a fluid and non-linear manner. This model, known as the family life course, acknowledges that families experience different events and challenges at different times, and that the timing and sequence of these events can vary. For example, in today's society, childbearing may not always occur with marriage, and families may go through different transitions based on individual circumstances.