Eighth Grade Science: Embryology & Constructing Cladograms
Understanding the evolutionary relationships linking diverse biological organisms requires looking beyond adult anatomy to explore developmental biology and phylogenetic classification. Embryology—the study of organisms during their earliest stages of development—reveals shared ancestral pathways hidden in adult forms. Cladistics organizes this anatomical and genetic evidence into branching family trees known as cladograms, mapping evolutionary divergence across deep time.
Comparative Embryology: Shared Developmental Blueprints
Comparative embryology is the scientific study of the similarities and differences in the embryos of different species during their developmental stages. Long before a vertebrate embryo develops the distinctive features of a human, a chicken, a tortoise, or a fish, it passes through embryonic stages that display shared anatomical landmarks.
Vertebrate Embryonic Commonalities
Fish Embryo Bird Embryo Human Embryo
(Early) (Early) (Early)
+-----+ +-----+ +-----+
/ (O) \ / (O) \ / (O) \
| ===== | | ===== | | ===== | <-- Pharyngeal Pouches
| | | | | | (Gill Slits)
\ / \ / \ /
\~~~~~/ \~~~~~/ \~~~~~/ <-- Post-Anal Tail
+-------------------------------------------------------------+
| Four Universal Chordate Embryonic Features |
+-------------------------------------------------------------+
| 1. Notochord: A flexible internal rod of cartilage that |
| develops along the dorsal side (becomes the spine). |
| |
| 2. Dorsal Hollow Nerve Cord: Tube of nervous tissue running |
| above notochord (develops into spinal cord and brain). |
| |
| 3. Pharyngeal Slits / Pouches: Paired throat openings; |
| become gills in fish, and ears/throat/jaw in humans. |
| |
| 4. Post-Anal Tail: An extension of the skeletal body plan |
| past the anus (retained in animals; regresses in humans).|
+-------------------------------------------------------------+
Why Embryos Reveal Ancestry
Why does a developing human embryo temporarily possess pharyngeal gill slits and a post-anal tail complete with tail vertebrae during its fifth week of gestation? These structures do not develop because a human embryo was once a fish; they develop because vertebrates inherit their basic developmental genetic master switches (Hox genes) from a common aquatic chordate ancestor. In fish, these pharyngeal arches develop into functional gills. In terrestrial mammals, natural selection repurposed these ancient embryonic arches to form the bones of the middle ear (malleus and incus), the hyoid bone, and the parathyroid glands.
Cladistics: Reconstructing Evolutionary Trees
Cladistics is the biological classification system that groups organisms into evolutionary branches based on shared derived characteristics inherited from common ancestors. A branching evolutionary diagram constructed through cladistic analysis is called a cladogram (or phylogenetic tree).
Anatomy of a Basic Cladogram
(Taxon D)
(Taxon C) /
(Taxon B) / /
(Taxon A) / / /
/ / / /
/ / / /
+----------------+----------------+----------------+
/ / /
(Outgroup) / /
| [ Node 1 ] [ Node 2 ] <-- Common Ancestor
| | |
v v v
Root Ancestor Trait 1 Trait 2 <-- Shared Derived Traits
Essential Cladogram Components
To read and construct cladograms, you must understand four structural elements:
- Root: The base of the cladogram representing the ancestral lineage from which all downstream organisms evolved.
- Node: A branching intersection point where a single ancestral lineage divides into two or more descendant lineages. Each node represents the most recent common ancestor shared by the diverging branches.
- Clade: An evolutionary group consisting of an ancestral species and all of its evolutionary descendants (a monophyletic group).
- Outgroup: A distantly related species that lacks the derived characteristics of the primary group being studied, serving as an evolutionary baseline for comparison.
Shared Ancestral versus Shared Derived Characters
When constructing cladograms, evolutionary biologists differentiate between ancestral traits and derived traits:
- Shared Ancestral Character (Plesiomorphy): A primitive anatomical feature that originated in an ancient ancestor and is shared by all members of the clade (e.g., all mammals possess a vertebral column, but because reptiles and fish also have backbones, a backbone cannot be used to divide mammalian orders).
- Shared Derived Character (Synapomorphy): A novel evolutionary feature that arose within an evolutionary lineage and is unique to a specific clade (e.g., mammary glands and hair are shared derived traits unique to mammals, distinguishing them from all other vertebrates).
Step-by-Step Construction of a Cladogram
Constructing a cladogram involves building a character matrix to track which traits are present ($1$) or absent ($0$) across candidate species, and applying the Principle of Parsimony.
Sample Vertebrate Character Matrix
+---------------+-----------+-----------+-----------+-----------+
| Organism | Backbone | Amniotic | Four Limbs| Hair / |
| | (Vertebra)| Egg | (Tetrapod)| Milk |
+---------------+-----------+-----------+-----------+-----------+
| Lancelet (Out)| 0 | 0 | 0 | 0 |
| Trout (Fish) | 1 | 0 | 0 | 0 |
| Frog (Amphib) | 1 | 0 | 1 | 0 |
| Lizard (Rept) | 1 | 1 | 1 | 0 |
| Tiger (Mammal)| 1 | 1 | 1 | 1 |
+---------------+-----------+-----------+-----------+-----------+
Resulting Phylogenetic Cladogram
Lancelet Trout Frog Lizard Tiger
\ / / / /
\ / / / /
\ / / / / <-- Hair / Milk
\ / / / +
\ / / / <-- Amniotic Egg
\/ / +
\ / <-- Four Limbs (Tetrapod)
\ +
\ / <-- Vertebral Column (Backbone)
\ +
\ /
+ <-- Ancestral Root
The Principle of Maximum Parsimony
When multiple evolutionary branching trees are mathematically possible, biologists apply the Principle of Maximum Parsimony (Occam's Razor). The most parsimonious cladogram is the simplest scientific hypothesis that requires the fewest evolutionary trait transitions or character state changes. Evolution is assumed to follow the simplest, most direct pathway unless evidence proves otherwise.
Chapter Exercises
Exercise 1: Describe two anatomical structures present in early human embryos that are not present in adult humans. Explain why these structures develop during gestation and identify what happens to them as development proceeds.
Exercise 2: Examine the character matrix provided in this chapter. Identify the organism that serves as the outgroup and justify its selection. Explain why the frog branches off after the development of four limbs, but before the appearance of the amniotic egg.
Exercise 3: On a cladogram, Species X and Species Y share a common ancestor at Node 3, while Species X and Species Z share a common ancestor at Node 1 (located further down the tree closer to the root). State which pair of species is more closely related and justify your answer using cladistic principles.
Solutions and Explanations
Solution 1: Two embryonic anatomical structures present in early human embryos that are absent in adult humans are pharyngeal arches (pouches) and a post-anal tail. During the fifth week of human embryonic development, prominent tissue folds form in the throat region that are anatomically identical to the pharyngeal pouches of fish and amphibian embryos, alongside an extended post-anal tail containing several tail vertebrae. These structures appear because humans share deep genetic developmental pathways inherited from ancient common chordate ancestors. In fish, the pharyngeal pouches develop into gill slits; in humans, they are repurposed during development to form the Eustachian tubes, middle ear bones, and parathyroid glands. The human post-anal tail regresses through programmed cell death (apoptosis), leaving the fused vertebrae of the coccyx (tailbone) beneath the skin.
Solution 2: In the character matrix, the lancelet serves as the outgroup because it scores zero across all four analyzed derived traits (it lacks a vertebral column, four limbs, an amniotic egg, and hair/milk). The outgroup provides an ancestral baseline against which all derived evolutionary innovations in the ingroup (trout, frog, lizard, tiger) are measured. The frog branches off after the evolution of four limbs because amphibians are tetrapods that possess two pairs of walking limbs, distinguishing them from limbless bony fish like the trout. However, the frog branches off before the evolution of the amniotic egg because amphibians lay soft, unshelled, gelatinous eggs that must remain submerged in aquatic environments to prevent desiccation; only reptiles, birds, and mammals possess the waterproof, self-contained amniotic egg.
Solution 3: Species X and Species Y are more closely related to each other than Species X is to Species Z. In cladistics, evolutionary relatedness is determined by the recency of a shared common ancestor. Node 3 is situated higher up on the cladogram, closer to the tips of the branches, meaning that the common ancestor represented by Node 3 lived more recently in geological time than the ancestor at Node 1 near the base of the tree. Because Species X and Species Y share a more recent common ancestor, they have had less time to diverge and share more derived genetic and anatomical characteristics with each other than with Species Z.