Why are homologous structures evidence of common ancestry but analogous structures are not?

Homologous structures share the same embryonic origin and bone pattern despite different functions — inherited from a common ancestor. Analogous structures evolved independently under similar selective pressures.

Two species have 5% DNA sequence divergence. If the mutation rate is 1% per 10 million years, when did they diverge?

T = d/(2μ) = 0.05/(2 × 0.001) = 25 million years ago (divide by 2 because both lineages accumulate mutations).

How does biogeography (geographic distribution of species) support evolution? Give an example.

Island species resemble nearby mainland species more than distant island species with similar environments — colonization + local adaptation, not independent creation.

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Evidence of Evolution

Phylogenetic trees, homologous structures, and fossil record analysis

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How it works

Evolution is supported by converging evidence from multiple fields. Comparative anatomy reveals homologous structures (same developmental origin, different function — e.g., human arm vs. whale flipper) indicating common ancestry, and analogous structures (different origin, similar function — e.g., bird wing vs. insect wing) showing convergent evolution. The fossil record documents transitional forms (e.g., Tiktaalik between fish and tetrapods) and patterns of appearance/extinction. Molecular biology provides the strongest evidence: DNA and protein sequence comparisons show that more closely related species share more similar sequences. The molecular clock hypothesis proposes that neutral mutations accumulate at a roughly constant rate, allowing estimation of divergence times. Phylogenetic trees (cladograms) organize this evidence into branching diagrams showing evolutionary relationships based on shared derived characters (synapomorphies).

Step-by-step

Switch between four evidence views using the Evidence Type slider.
In Tree view, explore the phylogenetic tree and click species to highlight branches.
In Anatomy view, compare homologous limb structures across vertebrates.
In Fossil view, scroll through a stratigraphic timeline.
In Molecular view, see DNA sequence alignment and similarity percentages.

Key formulas

$d = \frac{\text{mutations}}{\text{total sites}} \times 100\%$ Sequence divergence: percentage of differing nucleotides between two aligned sequences
$t = \frac{d}{2\mu}$ Molecular clock: divergence time estimated from sequence divergence (d) and mutation rate (μ)

Frequently asked questions

Why are homologous structures evidence of common ancestry but analogous structures are not?: Homologous structures share the same embryonic origin and bone pattern despite different functions — inherited from a common ancestor. Analogous structures evolved independently under similar selective pressures.
Two species have 5% DNA sequence divergence. If the mutation rate is 1% per 10 million years, when did they diverge?: T = d/(2μ) = 0.05/(2 × 0.001) = 25 million years ago (divide by 2 because both lineages accumulate mutations).
How does biogeography (geographic distribution of species) support evolution? Give an example.: Island species resemble nearby mainland species more than distant island species with similar environments — colonization + local adaptation, not independent creation.