Construct an argument using valid and reliable sources to support the claim that evidence from comparative morphology (analogous vs. homologous structures), embryology, biochemistry, and genetics supports common descent (GSE SB6.c).

What this topic is asking

Standard SB6.c asks you to construct an argument that several independent lines of evidence support common descent, the claim that all living organisms are related through shared ancestors. For the Georgia Milestones Biology EOC you must know the main lines (the fossil record, comparative anatomy, embryology, and biochemistry and genetics) and be able to interpret data, especially a table of DNA or protein differences, to say which organisms are most closely related. These are reasoning items: the strength of the case comes from many lines agreeing.

The fossil record

The fossil record shows that life on Earth has changed over time: older (deeper) rock layers contain different, often simpler organisms than younger layers. It also preserves transitional forms, fossils with features of two groups (such as fossils linking fish to early four-limbed land animals, or dinosaurs to birds), which capture change between forms. The EOC uses the fossil record to establish that species are not fixed but have a history of change.

Homologous, analogous, and vestigial structures

Comparing anatomy across species gives three terms the EOC tests closely:

• Homologous structures share the same underlying structure because of common ancestry, even if they now do different jobs. The classic example is the forelimb of a human (arm), whale (flipper), bat (wing), and cat (leg): all share the same bone arrangement inherited from a shared ancestor.
• Analogous structures share a function but not structure or ancestry, because they evolved separately for the same purpose. A bird's wing and an insect's wing both enable flight but are built differently, so they are not evidence of close ancestry.
• Vestigial structures are reduced remnants of features that were functional in an ancestor (the human tailbone, the hip bones of a whale). Their presence is evidence of descent from ancestors that used those features.

Embryological similarities

Closely related species often look strikingly similar as early embryos, sharing features (such as a tail and pharyngeal pouches in vertebrate embryos) that may not appear in the adult. These developmental similarities are evidence that the species inherited a shared developmental pattern from a common ancestor.

Molecular evidence: the strongest line

The most powerful modern evidence comes from comparing DNA and protein sequences. Because all life uses the same genetic code and similar molecules, the degree of similarity in DNA or proteins reflects how recently two species shared a common ancestor:

• The more similar the sequences, the more recently the species diverged from a common ancestor.
• The more differences accumulated, the longer ago they shared an ancestor.

So a table of DNA or amino-acid differences lets you rank relatedness: the species with the fewest differences from a given organism is its closest relative. This molecular evidence agrees with the anatomical and fossil evidence, which is why common descent is so strongly supported, and it is the basis for building the cladograms in SB4.

Try this

Q1. Explain the difference between homologous and analogous structures, giving an example of each. [3 points]

• Cue. Homologous structures share the same underlying structure from a common ancestor (the forelimb bones of a human and a whale); analogous structures share a function but not structure or ancestry (a bird's wing and an insect's wing).

Q2. State what the fossil record shows about life on Earth. [2 points]

• Cue. That life has changed over time (older rock layers contain different, often simpler organisms than younger layers) and that transitional forms link groups.