Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 19

Many types of evidence enable researchers to construct phylogenies, which depict possible evolutionary relationships among species. Paleontology is the study of fossils. Other evidence comes from comparing anatomical and biochemical characteristics of modern species. Cladistics is the study of species diversification from shared ancestors.
A fossil may form when mineral replaces tissue, developing gradually or after a sudden catastrophe, or may be indirect evidence such as footprints. Ice or tar can preserve actual remains. Remains in amber dehydrate, which preserves DNA. The fossil record is incomplete. Few individuals are preserved and few of these escape erosion or are discovered.
Scientists estimate fossil age 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 provides an estimate of an absolute year date. This is radiometric dating. Potassium 40 can date fossils older than 300,000 years, and carbon 14 dating is used on fossils younger than 40,000 years; new techniques fill the gap.
Homologous structures are inherited from a shared ancestor but may differ in function. Analogous structures are similar in function but are not inherited from a common ancestor. They are similar responses to the same environmental challenge. Vestigial organs and similar embryonic structures in different species reflect genes retained from ancestors.
Molecular evolution considers similarities and differences between sequences of chromosome bands, a protein's amino acids, or a gene's DNA bases. Presumably, these sequences contain so many bits of information that it is unlikely similarities evolved by chance.
Similar chromosome band patterns may or may not reflect similarities at the gene level. DNA probes compare chromosomes of different species and identify syntenic, or corresponding, sections. Many genes and proteins are highly conserved among species.
A molecular tree diagram shows evolutionary relationships between species, based on a gene's mutation rate. A molecular clock estimates the time when two species diverged from a common ancestor. One set of data can yield many molecular trees. Parsimony analysis identifies the trees requiring the fewest mutations. Molecular clocks based on mitochondrial DNA are used to date recent events because mtDNA mutates 5 to 10 times faster than nuclear DNA.