Evolution: Natural Selection, Evidence and Common Ancestry

1. Natural selection: how populations change

In 1858, Charles Darwin and Alfred Russel Wallace proposed natural selection. Darwin's book On the Origin of Species followed in 1859. The idea rests on four observations, remembered as VIST:

1. Variation: individuals in a population differ.
2. Inheritance: many differences are passed from parents to offspring in their genes.
3. Selection: more offspring are born than can survive. Individuals whose traits suit the environment are more likely to survive and reproduce.
4. Time: over many generations, helpful traits become more common.

Where does variation come from? Two sources. First, mutations — random changes in DNA. Second, the shuffling of genes when egg and sperm form (meiosis) and join (fertilization). The environment does not create the helpful trait; it selects among traits that already exist.

Think of a sports team. The coach does not make new players. The coach picks the best ones already trying out. Nature works the same way.

Adaptations can be:

• structural: a cactus's spines, a duck's webbed feet;
• behavioral: birds migrating, opossums "playing dead";
• physiological: a snake's venom, an animal hibernating.

Evolution we can observe:

• Antibiotic resistance: when an antibiotic is used, the few bacteria with resistance genes survive and multiply. Soon most of the population is resistant. This is why doctors warn against overusing antibiotics.
• Peppered moths in England: during the Industrial Revolution, soot from factories darkened tree bark. Birds saw dark moths less easily on dark bark. So dark moths became more common. After clean-air laws, light moths became common again.

Key point: individuals do not evolve; populations do. A single bacterium does not "become" resistant because it needs to. Resistant bacteria were already present and out-reproduced the others.

2. Evidence for evolution

In plain words: many different kinds of clues all point to the same answer.

A detective trusts a story more when fingerprints, video and a witness all agree. Evolution is like that. It is a scientific theory — a broad explanation backed by many separate kinds of evidence.

Reading rock layers (law of superposition): in undisturbed layers of sedimentary rock, lower layers are older. A fossil found in a deep layer is older than one in a layer above it. Radiometric dating of certain rocks gives ages in years.

Homologous vs. analogous: homologous structures share an origin but may do different jobs (a bat wing and a human hand). Analogous structures do the same job but evolved separately (a bird wing and a butterfly wing). Only homologous structures are evidence of a close common ancestor.

3. Common ancestry and cladograms

All living things are related through common ancestors, like branches of one huge family tree. Think of cousins. You did not come from your cousin. You both came from the same grandparents. Humans did not evolve from chimpanzees. Humans and chimpanzees both came from a shared ancestor. That ancestor lived millions of years ago. Since then, each line has kept evolving.

A cladogram shows these relationships:

• The tips are the organisms being compared (usually living species).
• Each node (branch point) is a common ancestor.
• Shared derived traits are marked along the main line. A trait is shared by every organism that branches off after (above) its mark.
• Two organisms are more closely related when their branches meet at a more recent node (higher on the tree).

A table like this is called a character table. You can turn it into a cladogram. The trait shared by the most organisms goes at the bottom. The trait shared by the fewest goes at the top.

Molecular evidence — clues from DNA and proteins — often agrees with cladograms. Species that split apart more recently have fewer differences in their DNA and proteins.

4. Speciation: how new species form

A species is a group of organisms that can interbreed and produce fertile offspring. Horses and donkeys can mate. But their young (mules) almost never can have young of their own. So horses and donkeys are separate species.

How speciation often happens:

1. Isolation: a barrier splits a population in two. The barrier might be a river that changes course, a mountain range, or seas that cover a land bridge. This is geographic isolation.
2. Different pressures: each group faces different food, climate and predators. Natural selection, and random chance, change each group differently. Mutations arise separately in each group.
3. Reproductive isolation: after many generations, the groups are very different. Even if they meet again, they cannot breed, or their young cannot have young. They are now separate species.

Classic example: Darwin's finches on the Galápagos Islands. Finches on different islands evolved different beaks to fit the local food. Thick beaks crack seeds. Thin beaks catch insects. Over time, this produced more than a dozen species from one ancestral group.