Evidence for Evolution 2: Anatomy, Embryos, Biogeography and DNA

The same bones, different jobs

Feel your own arm: one bone from your shoulder to your elbow, two from your elbow to your wrist, then a group of small wrist bones and five fingers. Now picture a bat's wing, a whale's flipper and a cat's front leg. They look completely different and do different jobs: flying, swimming, walking. Yet inside, they have the same set of bones in the same order.

An engineer designing a wing and a flipper from scratch would not use the same parts. The best explanation is that bats, whales, cats and humans inherited the pattern from a common ancestor and it was modified over time. In this lesson you will look at four more kinds of evidence for evolution.

Comparative anatomy

Homologous structures have the same basic structure because they come from a common ancestor, even though they may now do different jobs. Vertebrate forelimbs are the classic example. Homologous structures are evidence of divergent evolution: one ancestral pattern modified in different directions.

Analogous structures do similar jobs but are built differently and did not come from the same structure in a common ancestor. A bird's wing (made of arm bones covered in feathers) and a butterfly's wing (thin sheets of tough material, with no bones) both allow flight. Analogous structures show convergent evolution: unrelated species facing similar challenges evolve similar solutions.

Vestigial structures are reduced parts that were useful in ancestors but have lost most or all of their original function:

• Whales have small pelvic and leg bones inside their bodies, not attached to the spine, left over from four-legged ancestors.
• Humans have a tailbone (coccyx) made of a few fused vertebrae, and tiny muscles that raise goosebumps, which fluff up fur in other mammals.
• Some snakes, such as pythons and boas, have tiny hind-limb spurs.

Vestigial structures make sense only if organisms descended from ancestors in which these parts worked.

Embryology

Early embryos of very different vertebrates look surprisingly alike. At an early stage, embryos of fish, birds and mammals (including humans) all have:

• a tail that extends beyond the anus, and
• a series of pharyngeal arches (folds in the neck region).

In fish, the arches develop into gills. In mammals, they develop into parts of the jaw, middle ear and throat. Humans have a tail as early embryos, which normally shrinks and becomes the coccyx.

These shared early stages make sense if vertebrates inherited the same basic developmental instructions from a common ancestor, and then evolution modified later stages. (Older textbooks used drawings by Ernst Haeckel that exaggerated the similarities. The real similarities are still clear in modern photographs, but students should know the old drawings were not accurate.)

Biogeography

Where species live is also evidence. Darwin noticed that:

• Island species resemble those on the nearest mainland. Galápagos species resemble South American ones, not those of similar islands elsewhere. Islands were colonized from the nearest land, and the colonists then evolved.
• Similar environments on different continents (such as deserts) hold different species that are adapted in similar ways. The cacti of the Americas and the cactus-like spurges (euphorbias) of Africa are not closely related.
• Australia has many marsupials (pouched mammals like kangaroos) found nowhere else. Australia separated from other continents long ago, and its mammals evolved in isolation.

Today we also know that continents move (plate tectonics). The distribution of fossils of the same species on now-separate continents matches when those continents were joined.

Molecular evidence: DNA and proteins

All living things use DNA and the same basic genetic code, which suggests that all life shares a common ancestor. When scientists compare the DNA (or the protein sequences it codes for) of different species:

• Closely related species have more similar sequences, because they split from a common ancestor recently and have had less time to build up differences.
• Distantly related species have more differences.

For example, the protein cytochrome c, which every cell uses in cellular respiration, is identical in humans and chimpanzees but differs more and more in species that are less closely related. Family trees (phylogenies) built from DNA usually match those built from fossils and anatomy, which is strong confirmation. You built cladograms from shared traits in the Diversity of Living Things unit; DNA comparisons are another way to build them.

Direct observation and the strength of many lines of evidence

Evolution has also been observed directly: in bacteria becoming resistant to antibiotics, insects to pesticides, weeds to herbicides, and the peppered moth and Darwin's finches changing as their environments changed. (The next lesson looks at resistance in detail.)

The theory of evolution is strong because many independent kinds of evidence (fossils, anatomy, embryology, biogeography, molecular biology and direct observation) all point to the same conclusion and the same family trees. Each field was developed separately, using different methods, and they agree.