进化关系

A cladogram is a tree-shaped chart depicting hypothetical genealogical relationships between species. The tips represent specific species, while branch lengths indicate the degree of change between them. Nodes mark common ancestors, and species sharing a node are called sister groups. Cladograms help visualize how organisms diverged from shared ancestors over evolutionary time.

Examine the fossil's physical characteristics, or morphology, and compare them to known species on your cladogram. Based on these structural similarities and differences, hypothesize where the fossil fits on the tree. Add a new line or mark to indicate your proposed position, then refine this placement after comparing genetic data from related living species.

BLAST displays a graphic summary where your query sequence appears as a blue line, with matching database sequences shown as colored bars below. Bar length and color indicate alignment quality and identity score. The Sequences producing significant alignments list ranks matches by similarity, with statistics like Max, Total, Query Coverage, and Identity scores showing how closely related sequences are.

E values indicate the statistical significance of a sequence match. Lower E values mean the match is less likely to occur by random chance and more likely represents a true evolutionary relationship. An E value of 0 indicates a highly significant match, while higher E values suggest weaker, less reliable alignments between your query and database sequences.

After collecting multiple species from your BLAST alignments, return to the Sequences producing significant alignments list and select all boxes. Click Distance of tree results to generate a phylogram based on sequence similarity to your query sequence. Your fossil relative sequence appears highlighted in yellow, showing its position relative to other taxon groups in the evolutionary tree.

Morphology provides initial placement based on physical traits, but genetic sequences offer independent molecular evidence of evolutionary relationships. Comparing both approaches strengthens your hypothesis about fossil placement. If morphological and genetic placements differ, discussing these differences with classmates reveals how multiple data sources can support or challenge evolutionary hypotheses.

For each significant alignment, record the organism's scientific and common names listed under Source, and the protein the gene codes for under Definition. Click the Sequence ID to access detailed information. Collecting this data from multiple top matches allows you to construct an accurate phylogram and refine your hypothesis about where the fossil species belongs in the evolutionary tree.