Evolution doesn’t just tell us why species are different; it also reveals why they are connected. The unifying theme of life is that all organisms, no matter how diverse, share common ancestors. Understanding this concept gives us a framework for making sense of biology as a whole.
Unit 1 THE CHEMISTRY OF LIFE
Concept 1.2 The Core Theme: Evolution accounts for the unity and diversity of life
The Dual Role of Evolution: Unity and Diversity
If you compare a hummingbird, a sea horse, and a giraffe, they seem wildly different. But if you take a closer look—beneath the feathers, fins, and fur—you’ll find the same underlying skeletal structure. How is that possible?
This is where evolution comes in. Descent with modification explains that while species change over time to adapt to new environments (diversity), they also retain traits inherited from a common ancestor (unity).
The fossil record is a time capsule of this process, showing how organisms have transformed over billions of years. DNA comparisons reinforce these connections, proving that all life on Earth is deeply related.
The Three Domains of Life
To make sense of life’s incredible diversity, scientists classify all living things into three major domains:
- Bacteria – The most widespread and diverse prokaryotes.
- Archaea – Another group of prokaryotes, often found in extreme environments.
- Eukarya – All organisms with complex cells, including:
- Plants (make their own food through photosynthesis)
- Fungi (absorb nutrients from surroundings)
- Animals (consume other organisms for energy)
- Protists (mostly unicellular, but incredibly diverse)
This classification system helps scientists understand how life evolved and how different organisms are related.
Darwin’s Big Idea: Natural Selection
The theory of natural selection, first proposed by Charles Darwin in 1859, explains how evolution happens. His observations were simple yet profound:
- Variation exists – Individuals in a population have different traits, many of which are heritable.
- Not everyone survives – Organisms produce more offspring than the environment can support, leading to competition.
- Survival isn’t random – Individuals with traits that help them survive and reproduce pass those traits to the next generation.
Over generations, these small changes add up, leading to adaptations that shape the form and function of species.
A classic example? The Galápagos finches Darwin studied. Each island had finches with different beak shapes, perfectly suited to their food sources. Over time, these adaptations created new species—all descended from a common ancestor.
The Tree of Life: Mapping Evolution
Scientists often represent evolution as a tree, with branches showing how species are connected. Imagine tracing your family tree back 3.5 billion years—you’d eventually land on the first life forms, tiny prokaryotic cells that started it all.
This branching pattern explains why bat wings, human arms, and whale flippers share the same bone structure. Evolution doesn’t create entirely new designs from scratch; it modifies existing traits to fit different environments.
Evolution in Action
Natural selection isn’t just something that happened in the past—it’s happening right now. Some examples:
- Antibiotic resistance – Bacteria evolve rapidly to survive modern medicine.
- Climate change adaptation – Animals are changing migration patterns and behaviors in response to global warming.
- Human evolution – Our genes are still evolving, influenced by diet, disease, and culture.
Wherever life exists, evolution never stops.
Why Evolution Matters
Understanding evolution isn’t just about science—it’s about seeing how everything is connected. It explains the past, shapes the present, and helps predict the future of life on Earth. From medicine to conservation, evolution is the key to unlocking biology’s greatest mysteries.