B.7ABG Evidence of Common Ancestry

Student Expectation

The student is expected to analyze and evaluate how evidence of common ancestry among groups is provided by the fossil record, biogeography, and homologies, including anatomical, molecular, and developmental; AND analyze and evaluate scientific explanations concerning any data of sudden appearance, stasis, and sequential nature of groups in the fossil record; AND analyze and evaluate scientific explanations concerning the complexity of the cell.

Key Concepts

• The study of the similarities between species in the fossil record shows that organisms have a common ancestry. Variations in the physical features, such as shell shape or bone structure, of specific species reflect changes over time. The fossil record has also revealed links between large groups of organisms such as fish and reptiles, and reptiles and birds.
• Some species evolution is explained by the study of biogeography (geographic isolation and continental drift). Geographic isolation occurs when members of a single species are split into separate populations and can no longer mate because of geographic barriers; the result is two isolated gene pools that diverge over time, eventually becoming two separate species. Continental drift caused the ancient supercontinent Pangaea to split into separate continents; when this happened, some members of the same species were separated by water and became separate species over time.
• Homologies are seen in anatomical structures, genetics, and the developmental stages of different organisms. Species with common origins (e.g., birds, reptiles, and mammals) all show similar patterns of anatomy, such as those in the bones of the forelimbs. Genetic studies of the similarities between DNA sequences are revealing additional information about the similarities between species. Embryological studies also show that different species have similar developmental stages, suggesting evolution from a common ancestor.
• The fossil record reflects that the rates and patterns of evolution are not constant over geologic time. Species can change very little for long periods of time (evolutionary stasis); they can change very little for a long time, then undergo dramatic change over a relatively short period of time, followed by another long period of little change (punctuated equilibrium); or they can change gradually and sequentially (gradualism).
• The Endosymbiotic Theory states that mitochondria and chloroplasts were once free-living prokaryotes that adapted to living inside other cells and eventually formed a symbiotic relationship with their host cells.

Fundamental Questions

• How do scientific observations and evidence of common ancestry help to answer questions about the history of life?
• How does the fossil record reflect changes in groups of organisms over time?
• What are the different types of evolution?
• What are some factors that influence evolution?