Grouping Species Using DNA
Use shared derived traits, including anatomical and DNA evidence, to explain cladistics. Compare corresponding DNA sequences and introduce calibrated molecular clocks. Explain birds' placement within theropod dinosaurs using fossil and skeletal evidence. Compare convergent and parallel evolution with stated conventions. Describe horizontal gene transfer and revisions to classification as evidence changes.
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You have compared traits that help survival. To test ancestry, ask whether similar traits were inherited from a shared ancestor or evolved separately. Cladistics tests ancestry using inherited shared derived characters. A derived character changes an ancestral condition; losing a feature can count too. Evidence can come from body features, fossils and molecules, including DNA. A phylogenetic tree is a testable hypothesis about evolutionary relationships.
Crocodiles resemble lizards, but share a more recent common ancestor with birds than with lizards. Crocodiles and birds belong to archosaurs. Lizards belong to a different reptile branch. Anatomical evidence and DNA comparisons support this relationship.
Scientists align corresponding DNA stretches inherited from common ancestors. Compare the letters at matching positions and count differences. With three sequences, compare each pair separately. Equal-length stretches changing at comparable rates can suggest relative split times, but a few letters alone cannot establish a real species tree.
A molecular clock connects DNA changes to estimated time using change rates and dated evidence, such as fossils. Rates vary among genes and lineages. Under a comparable-rate model, 4 differences suggest less elapsed time than 7 in equal-length stretches. Counts alone do not give exact dates.
Under the stated comparable-rate model, Pair X is predicted to have the more recent shared ancestor.
Birds evolved within theropod dinosaurs. Fossils and inherited skeletal features support this placement without requiring DNA from Tyrannosaurus. A clade includes an ancestor and all its descendants, so birds remain dinosaurs. The same rule keeps whales within mammals.
Convergent evolution produces similar features independently in different lineages. Bird and bat flight wings evolved separately, although their forelimb bones share older ancestry. Cacti and some euphorbias independently evolved thick, water-storing stems. Independent evolution concerns the particular feature, not a lack of all common ancestry.
Parallel evolution often describes independent similar changes from similar inherited starting mechanisms. Here, two skink lineages independently reduce pigment through changes in the same inherited pigment pathway. We call this parallel evolution. Relatedness alone does not decide the label, and scientific usage varies.
Horizontal gene transfer moves DNA between organisms outside parent-to-offspring inheritance. It occurs within or between species, is common in bacteria and also occurs in other groups. A transferred gene can have a different history from other genes. Networks can show transfer without erasing ancestry. Matching DNA alone does not prove transfer.
Scientists revise classifications when tested evidence supports a different relationship. One matching gene does not settle every branch. When genes disagree, compare more genes and check other evidence before deciding. The newest claim is not automatically the best-supported explanation.
DNA, fossils and body features test ancestry. Birds remain dinosaurs. Independently evolved similarities, changing DNA rates and gene transfer can complicate comparisons.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked videos credit Khan Academy, the California Academy of Sciences and the American Museum of Natural History. The Museum video carries its stated rights notice.
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Ask how a shared feature arose
Two organisms can resemble each other for different reasons. A feature may have been inherited from a shared ancestor, or similar features may have evolved independently.
Scientists test these possibilities with several kinds of evidence. Fossils preserve anatomical features. DNA comparisons examine corresponding inherited sequences. Patterns across many features can help distinguish possible relationships.
A phylogenetic tree summarizes a hypothesis about ancestry. One matching feature cannot settle every branch, and a matching gene alone does not establish horizontal transfer.
A descendant remains in its ancestral clade
Classification gives organisms names and places them in groups such as genera and families. A phylogenetic tree proposes relationships through shared ancestry. A list of classification ranks and a branching tree are connected, but they are not the same representation.
A clade includes an ancestor and all descendants. A descendant does not leave the clade when its appearance changes. Whales remain mammals, and birds remain dinosaurs.
Fossils and inherited skeletal features support birds' placement within theropod dinosaurs. This does not require recovered DNA from extinct non-bird dinosaurs.
Read connections rather than spacing
On a rooted phylogenetic tree, follow two tips backward until their branches meet. That meeting represents their shared ancestor. Compare relationships by finding which pair shares the more recent common ancestor.
Tips drawn beside each other are not automatically the closest relatives. Rotating branches around a branch point can change tip order without changing ancestry.
Drawn branch length represents time or genetic change only when the diagram supplies the relevant scale. A long line by itself does not establish an older split.
Genes can have different histories
Parent-to-offspring inheritance connects generations. Horizontal transfer adds another route: DNA enters an organism from a donor that is not its parent. Donor and recipient can belong to the same species or different species.
A received gene may resemble the donor's gene while the recipient's other genes support a different relationship. Researchers investigate such conflicts by comparing more genes and other evidence.
A network can display transfer alongside ordinary descent. Adding transfer connections does not mean organisms lose their ancestors or that every gene follows the same history.
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