What evolution means and where the idea came from

Chapter 1 defined evolution as a heritable change in a population from one generation to the next. This chapter takes that idea further: how biologists came to accept it, what evidence supports it, and what changes in DNA lie underneath it.

Small changes and big ones

You can look at evolution at two scales. Up close, you watch a single gene: how common each of its alleles (the gene’s alternative versions, from Chapter 17) is in a population, and how that share rises or falls over the generations. Evolution at this scale, a change in a single gene or in how common its alleles are in a population over time, is microevolution.

Step back across thousands or millions of generations and you see new species appear, or whole groups of related species branching from one ancestor. That large-scale view is macroevolution. Whales arising from four-legged land mammals is an example.

Both scales run on the same processes, such as mutation and natural selection. The difference is only how much time and change you’re looking at: macroevolution is microevolution added up over long stretches of time.

What makes a group a species

Chapter 1 described a species as organisms that share a distinctive form and are close genetic relatives. For organisms that reproduce sexually, such as most plants and animals, biologists add a practical test: members of the same species can interbreed in nature and produce offspring that are viable (able to survive) and fertile (able to reproduce themselves).

A horse and a donkey can mate, but their offspring, a mule, is almost always sterile. That’s why horses and donkeys count as two species. The members of one species living in the same area at the same time, with the chance to interbreed, form a population, the unit that evolves.

Check yourselfA Great Dane and a Chihuahua look nothing alike, yet they belong to the same species. How can that be?

Species membership depends on whether organisms can interbreed and produce viable, fertile offspring, not on how they look. Dogs of different breeds can produce fertile puppies, so all dogs are one species.

An idea that didn’t hold up

For centuries, many people believed every kind of living thing had stayed the same since it first appeared. By the late 1700s, a few naturalists were arguing that species change over time. The best known of them, Lamarck, proposed a mechanism: an organism changes its behavior in response to its environment, that behavior modifies its body during its life, and the modified traits are passed to its offspring.

In his view, giraffes that stretched for high leaves lengthened their necks, and their young were born with longer necks. This idea is the inheritance of acquired characteristics. Research has rejected it as a general explanation. Muscles built at the gym don’t change the genes in a person’s eggs or sperm, so a trait acquired during life generally isn’t passed on. The rare exceptions are certain epigenetic changes carried in sperm or eggs, the epigenetic inheritance of Chapter 18.

What led to the theory of evolution

Two naturalists, Charles Darwin and Alfred Wallace, independently reached the same explanation for how species change. Three lines of thinking fed into it:

• An old Earth. Geologists argued that Earth was shaped mainly by slow, ongoing processes such as erosion, working at the same gradual pace in the past as today (an idea called uniformitarianism), rather than mostly by sudden catastrophes. Slow processes can make big changes only over enormous spans of time, so Earth must be very old. That leaves time for living things to change slowly too.
• Too many offspring. Populations can grow far faster than their food supply. So not every individual that is born can survive and reproduce.
• Island species. On long collecting voyages, both men noticed that island species resemble species on the nearest mainland, yet differ from them. On the Galápagos Islands off South America, several finch species have beaks of different sizes and shapes, each suited to a different food: crushing seeds, picking insects off trees, or probing cactus fruits. All of them descend from a single mainland species that reached the islands.

Neither man knew how traits are inherited, because genes hadn’t been discovered yet. They could only observe that offspring resemble their parents. Genetics later filled in that missing piece: heritable variation comes from differences in genes.

Check yourselfWhy did the idea of a very old Earth matter to the theory of evolution?

Evolution by natural selection changes species slowly, over many generations. An Earth only a few thousand years old wouldn’t leave enough time for that; an Earth shaped by slow processes over vast spans of time would.