How do we represent the evolutionary relationships between species, and what does common ancestry mean?
Explain how species are related through common ancestry and how an evolutionary tree (phylogenetic diagram) represents these relationships, interpreting branching to infer relatedness (NYSSLS LS4, patterns; systems and system models).
A NYSSLS-level answer on common ancestry for the New York Life Science: Biology Regents: what common ancestry means, how an evolutionary tree represents relationships, and how to read branching points to judge how closely species are related.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this topic is asking
NYSSLS LS4 asks you to understand common ancestry and to read an evolutionary tree (phylogenetic diagram) that represents how species are related. On the Life Science: Biology Regents this is tested almost entirely as a model-reading task: given a tree, decide which species are most closely related and justify it. The crosscutting concepts are patterns and systems and system models.
Common ancestry
All the evidence for evolution (fossils, anatomy, embryology, molecules, see evidence for evolution) points to species being linked by common ancestors, ultimately tracing back to shared ancestry for all life. Common ancestry is why related species share so many features: they inherited them from the same source.
Evolutionary trees
An evolutionary tree (also called a phylogenetic tree or cladogram) is a branching diagram showing how species are related. Reading one correctly is the key skill:
- The tips (ends of the branches) are the species being compared.
- Each branch point (node) represents a common ancestor from which the lineages split.
- Lineages that branch off more recently (closer to the tips) share a more recent common ancestor and are more closely related.
Relatedness and similarity
Because closely related species shared an ancestor recently, less time has passed for their lineages to change separately, so they have more inherited features in common, including more similar DNA and proteins. This connects the tree to the molecular evidence: the tree predicts that more closely related species (joining more recently) should have more similar DNA, and they generally do. The exam often pairs a tree with a question about which species should have the most similar DNA.
Reading carefully
A common trap is to read a tree by the left-to-right order of the tips, or by how near two tips are along the top. Neither is reliable. The only correct way is to trace each branch back to where two lineages join (their common ancestor) and compare how recent those junctions are. Rotating a branch at a node does not change the relationships, only the drawing.
Try this
Q1. Explain what is meant by common ancestry. [2]
- Cue. Two or more species descended from the same ancestral species; they share an ancestor at some point in the past.
Q2. On an evolutionary tree, how do you decide which two species are most closely related? [2]
- Cue. Find which two share the most recent common ancestor (where their branches join closest to the present); those are the most closely related.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Life Science sample, 2024)3 marksAn evolutionary tree shows that species X and Y share a recent common ancestor, while species Z branches off much earlier. (a) State which two species are most closely related. (b) Explain how you can tell from the tree. (c) Predict which two species would be expected to have the most similar DNA.Show worked answer →
A 3-point constructed-response item assessing patterns and reading a model.
(a) 1 point: species X and Y.
(b) 1 point: X and Y share a more recent common ancestor (their branches meet closer to the present), while Z branched off earlier, so X and Y are more closely related.
(c) 1 point: X and Y, because more closely related species (sharing a more recent common ancestor) are expected to have more similar DNA.
Markers reward reading the recent shared branch point and linking closer relatedness to more similar DNA.
Regents (Life Science CR, 2025)2 marksBiologists state that all life shares common ancestry. (a) Explain what is meant by common ancestry. (b) Explain why two species that share a more recent common ancestor are considered more closely related.Show worked answer →
A 2-point item on common ancestry.
(a) 1 point: common ancestry means that different species descended from the same ancestral species (share an ancestor) at some point in the past.
(b) 1 point: a more recent common ancestor means less time has passed for the two lines to change separately, so they share more inherited features and are more closely related.
Markers reward "descended from a shared ancestor" and "more recent ancestor means more shared features / more closely related".
Related dot points
- Describe the lines of evidence for evolution (fossils, comparative anatomy, embryology and molecular/DNA evidence) and explain how each supports common ancestry (NYSSLS LS4, patterns; structure and function).
A NYSSLS-level answer on the evidence for evolution for the New York Life Science: Biology Regents: the fossil record, comparative anatomy and homologous structures, embryology, and molecular evidence such as DNA, and how each supports common ancestry.
- Explain how variation, overproduction, competition and differential survival lead to natural selection, and how this changes the proportion of traits in a population over time (NYSSLS LS4, cause and effect; patterns).
A NYSSLS-level answer on natural selection for the New York Life Science: Biology Regents: how variation, overproduction, competition and differential survival drive evolution, with the Beaks of Finches investigation and worked examples.
- Explain how new species form when populations become reproductively isolated and diverge, and how environmental change can lead to extinction (NYSSLS LS4, cause and effect; stability and change).
A NYSSLS-level answer on speciation and extinction for the New York Life Science: Biology Regents: how reproductive isolation and divergence form new species, and how environmental change and a poor match of traits lead to extinction.
- Explain what biodiversity is, why genetic and species diversity matter for the resilience of populations and ecosystems, and how human activity threatens it (NYSSLS LS4, stability and change; cause and effect).
A NYSSLS-level answer on biodiversity for the New York Life Science: Biology Regents: what biodiversity is, why genetic and species diversity make populations and ecosystems more resilient, and how human activity threatens it.
- Describe the structure of DNA (the antiparallel double helix and base pairing) and explain how complementary base pairing allows DNA to be copied accurately during replication (NYSSLS LS3, structure and function; patterns).
A NYSSLS-level answer on DNA for the New York Life Science: Biology Regents: the double-helix structure, base pairing, why DNA is a stable store of information, and how complementary base pairing allows accurate replication.
Sources & how we know this
- New York State P-12 Science Learning Standards (Life Science) — New York State Education Department (2016)
- Educator Guide to the Regents Examination in Life Science: Biology — New York State Education Department (2025)