Unifying themes connect concepts from many fields of biology. 

Systems - All levels of life have systems of related parts

A system is an organized group of interacting parts. 

• A cell is a system of chemicals a processes
• A body system includes organs that interact
• An ecosystem includes living and nonliving things that interact

Structure and function -  are related in biology

Structure determines function

• Proteins with different structures have different functions
• Intestinal muscle cells have a different structure and function than heart muscle cells
• Different species have different structures with different functions

Homeostasis - organisms must maintain homeostasis to survive in diverse environments

Homeostasis is the maintenance of constant internal conditions

• Homeostasis is usually maintained through negative feedback
• Negative feedback systems return a condition to its normal point

Evolution - explains the unity and diversity of life

Evolution is the change in living things over time

• The genetic makeup of a population of a species changes
• Can occur through natural selection of adaptations
• Adaptations are inherited traits that benefit a species that are passed to future generations

Watch the video below for more insight into the four unifying themes of biology.   https://youtu.be/ibhnP5suqK8