Describe the lines of evidence for evolution and common ancestry, including the fossil record, comparative anatomy, embryology, biogeography, and molecular (DNA and protein) evidence (Ohio's Learning Standards for Science, Biology, B.E).

What this topic is asking

The Ohio Evolution strand (B.E) treats evolution as a well-supported explanation backed by many independent lines of evidence, and B.E.2 explicitly expands biological classification to molecular evidence. The Ohio Biology EOC turns this into items where you identify a line of evidence and explain what it suggests about common ancestry. The crosscutting idea is patterns: the same patterns appear across fossils, anatomy, development, geography, and molecules, and they all point to descent from shared ancestors. Because the standards ask you to argue from evidence, you should be able to say not just what the evidence is but why it supports evolution.

The fossil record

Fossils are the preserved remains or traces of organisms from the past, usually found in layers (strata) of sedimentary rock. Two features make them evidence for evolution.

• Change over time. Deeper, older layers contain different (often simpler or extinct) organisms than shallower, younger layers, showing that life on Earth has changed rather than stayed fixed.
• Transitional forms. Some fossils show features of two groups, linking them. Fossils with both reptile and bird features, or with both fish and amphibian features, are what you would expect if one group descended from another.

The fossil record is incomplete (fossilisation is rare), but the order in which forms appear is consistent with evolution.

Comparative anatomy

Comparing the body structures of living species reveals two telling patterns.

• Homologous structures. Body parts with the same basic structure but different functions, such as the forelimb bones of a human (grasping), a whale (swimming), a bat (flying), and a cat (walking). The shared plan is best explained by a common ancestor whose limb was later modified by selection for each lifestyle.
• Vestigial structures. Reduced or apparently functionless features that were useful in an ancestor, such as the tiny hip bones in some whales. They make sense as leftovers from ancestors that used them.

Be careful with analogous structures (a bird's wing and an insect's wing): they do the same job but have different underlying structures, so they show similar selection pressures, not common ancestry. Homology, not analogy, is the evidence for shared ancestry.

Embryology

Closely related species often pass through similar stages of early development. Vertebrate embryos (fish, chicken, human) look alike early on and share features such as a tail and pharyngeal (throat) structures, even where the adults differ greatly. Similar development reflects shared genes inherited from a common ancestor.

Biogeography

Biogeography is the study of where species live. Species on an island or continent often resemble nearby mainland species more than they resemble similar species elsewhere in the world, because they descended from ancestors that reached or lived in that region and then diverged. Island groups such as the Galapagos finches, each suited to a local food source but clearly related, are a classic example.

Molecular evidence

The strongest modern evidence is molecular. All living things use DNA as the genetic material and share essentially the same genetic code (the same codons specify the same amino acids), which points to a single origin of life. Beyond that:

• The more similar two species' DNA or protein sequences are, the more recently they shared a common ancestor.
• Differences accumulate through mutation over time, so sequence differences act like a clock: closely related species have few differences, distantly related species have many.

This links directly to building phylogenetic trees and cladograms from molecular data, which B.E.2 highlights.

Try this

Q1. State the difference between a homologous and an analogous structure. [2]

• Cue. Homologous structures share the same underlying structure but have different functions and indicate common ancestry; analogous structures have the same function but different underlying structures and do not indicate close common ancestry.

Q2. Explain why the fact that nearly all organisms use the same DNA code is evidence for common ancestry. [2]

• Cue. A shared genetic code across all life is most simply explained if all organisms inherited it from a single common ancestor, rather than the same code arising independently many times.