4. A fossil may form when mineral replaces tissue gradually or after a sudden catastrophe, or may be indirect evidence such as footprints.
5. In petrifaction, minerals replace living tissue. Phosphatization is a form of petrifaction that replaces very small structures with calcium phosphate.
6. Fossil age is estimated in relative and absolute terms. The rock layer a fossil is in provides a relative date. The ratio of a stable radioactive isotope to its breakdown product gives an absolute date, which is a more precise range of time when an organism lived. This is a type of radiometric dating.
17.3 Comparing Structures
7. Homologous structures are inherited from a shared ancestor, but may differ in function. Analogous structures are similar in function due to convergent evolution.
8. Vestigial structures and similar embryonic structures in different species reflect actions of genes retained from ancestors.
9. Molecular evolution considers similarities and differences among sequences of chromosome bands, a protein's amino acids, a gene's DNA bases, or genomes. These sequences contain so many bits of information that it is unlikely that similarities happened by chance. More likely is descent from a shared ancestor.
10. Similar chromosome band patterns may not reflect similarity at the gene level. DNA probes can reveal synteny, or corresponding sections. Many genes and proteins are highly conserved.
11. A molecular clock estimates the time when two species diverged from a common ancestor by comparing DNA or protein sequences. Molecular clocks based on mitochondrial DNA are used to date recent events because this DNA mutates faster than nuclear DNA.