Describe the evidence supporting the theory of evolution by natural selection, including the fossil record, comparative anatomy, embryology, molecular evidence, and biogeography (Virginia 2018 Biology SOL BIO.7.a).

What this topic is asking

Virginia Biology SOL standard BIO.7 is about how populations change through time, and substandard BIO.7.a asks for the evidence supporting the theory of evolution by natural selection. The EOC expects you to name and explain the main lines of evidence (the fossil record, comparative anatomy, embryology, molecular evidence, and biogeography) and to understand why several independent lines that agree make a scientific theory strong. A common item gives you a diagram of limbs, a set of DNA sequences, or a description of fossils and asks which line of evidence it shows.

The fossil record

The fossil record shows that many organisms alive in the past no longer exist, and that today's organisms appeared in a sequence, not all at once. It is incomplete (not every organism fossilizes), but the order it does preserve is consistent with descent with modification.

Comparative anatomy

Comparing the body structures of different organisms is one of the clearest lines of evidence.

• Homologous structures have the same underlying structure (the same arrangement of bones) because the organisms inherited it from a common ancestor, even if the structures now perform different functions. The forelimbs of a human (grasping), a whale (swimming), and a bat (flying) share the same bone plan.
• Analogous structures have a similar function but a different underlying structure, because they evolved independently to meet a similar challenge. A bird's wing and an insect's wing both enable flight but are built differently. Analogous structures do not indicate close relationship.
• Vestigial structures are reduced parts that have little or no function in the organism now but were functional in an ancestor, such as the reduced pelvic bones in some whales.

The trap here is to confuse homologous with analogous: homologous means shared ancestry (same structure), analogous means shared function only.

Comparative embryology and molecular evidence

Comparative embryology shows that the early embryos of related vertebrates share features (for example, pharyngeal pouches), pointing to common developmental programs inherited from a shared ancestor.

Molecular evidence is among the most powerful. Because the genetic code is essentially universal, scientists can compare DNA base sequences and the amino-acid sequences of proteins across species. Closely related species have more similar sequences; the more differences there are, the longer ago the species diverged. Molecular evidence often confirms relationships first proposed from anatomy, and it can resolve cases where anatomy is ambiguous.

Biogeography

Island species that closely resemble mainland species, but with distinct adaptations, are a classic biogeographic pattern: they descend from mainland ancestors that reached the island and then evolved in isolation.

Try this

Q1. Explain the difference between homologous and analogous structures, and state which indicates common ancestry. [2]

• Cue. Homologous structures share the same underlying structure from a common ancestor (different functions allowed); analogous structures share only function but differ in structure. Homologous structures indicate common ancestry.

Q2. A scientist finds that two species have nearly identical sequences for a shared protein. What does this suggest, and which line of evidence is it? [2]

• Cue. It suggests the two species are closely related and share a recent common ancestor; this is molecular (biochemical) evidence.