Biological Evolution: Unity and Diversity

The theory of evolution explains how species change over time through a process called natural selection. According to this scientific theory, organis

Giant tortoises and Darwin’s finches are two famous examples that helped shape the theory of evolution. Charles Darwin observed these animals on the G

Charles Darwin’s theory of evolution was shaped by the ideas, observations, and discoveries of others. While traveling on the HMS Beagle, Darwin read

Why do some animals blend into their surroundings while others stand out? Why do certain traits, like a cheetah's speed or a polar bear’s thick fur, h

Have you ever wondered why some animals have spots, stripes, or specific colors? These visible traits, called phenotypes, are shaped by natural select

Fossils are preserved remains or traces of organisms that lived in the past. They provide important evidence for understanding the history of life on

Chordates are a large group of animals that share important features at some stage of development such as a notochord, dorsal nerve cord, and gill sli

Vertebrates are animals with backbones, and they come in many forms—from swimming fish and hopping frogs to flying birds and running mammals. The five

Fish are important for understanding the early evolution of vertebrates and how life adapted to aquatic environments over time. They demonstrate a wid

Jawless fish are some of the oldest and most interesting animals on Earth. They show us what early vertebrates (animals with backbones) may have looke

Cartilaginous fish are a special animal group that includes sharks, rays, and skates. They differ from most other fish because their skeletons are mad

Bony fish are the largest group of fish in the world. They are important because they show how fish evolved strong skeletons, new ways to stay afloat,

Viruses are microscopic infectious agents that can replicate only inside the living cells of organisms. Unlike bacteria or eukaryotic cells, viruses a

Bacteria are among the simplest and most ancient life forms, yet they are incredibly diverse and essential to life on Earth. These unicellular organis

Bacteria display a wide range of nutritional strategies that enable them to survive in nearly every environment on Earth, from deep-sea vents to the h

Bacteria are tiny, single-celled organisms that play a surprisingly big role in daily life. While some bacteria cause diseases, many others are helpfu

Archaea are ancient microorganisms that offer important insights into the early evolution of life on Earth. Although they resemble bacteria in appeara

Archaea are microscopic, single-celled organisms that form one of the three domains of life, alongside Bacteria and Eukarya. Although they resemble ba