Phylogeny is the historical development of groups of organisms. It shows how organ systems became more complex and how major groups arose from one another. The exam asks about the main aromorphoses in plant and animal evolution, and especially the "ladder" of animal organ systems — in which group each feature first appeared.
§1. Phylogeny of the plant kingdom
Algae live in water; the body is a thallus, with no true tissues or organs. Land plants evolved from ancient green algae.
Psilophytes were the first land plants (in the Palaeozoic). Covering and conducting tissues appeared, but there were no true roots or leaves (only rhizoids). Two branches arose from them.
Mosses — the gametophyte dominates the life cycle; there are no roots and conducting tissue is poorly developed. They are an evolutionary dead end — no other groups arose from them.
Ferns and their allies (horsetails, club mosses, ferns) — the sporophyte dominates; true roots, stems and leaves are present. Fertilisation needs water (sperm swim in water).
Gymnosperms — the seed appeared (embryo, food store, seed coat); thanks to the pollen tube, fertilisation no longer depends on water. Ovules lie exposed on cone scales.
Angiosperms (flowering plants) — the flower, the fruit (protects and disperses seeds), double fertilisation, vessels in the xylem. Today they are in a state of biological progress.
The main trend in plant evolution: the gametophyte shrinks and the sporophyte becomes dominant, while reproduction becomes independent of water.
In flowering plants the gametophyte is greatly reduced: the male gametophyte is the pollen grain (two or three cells), the female gametophyte is the embryo sac (seven cells, eight nuclei).
Coelenterates (cnidarians) — two cell layers (ectoderm and endoderm), radial symmetry, a diffuse nervous system, a single body opening.
Flatworms — three layers (mesoderm appears), bilateral symmetry, organ systems. No body cavity (the space is filled with parenchyma).
Roundworms — a primary body cavity; the gut ends in an anus.
Annelids — a secondary body cavity (coelom), a body divided into segments, the first closed circulatory system, parapodia.
From ancestors close to annelids came arthropods (chitinous exoskeleton, jointed limbs) and molluscs (shell, mantle).
Chordates — a notochord, a neural tube above it, gill slits in the pharynx, and a heart on the ventral side. The lancelet is a primitive chordate.
The vertebrate sequence: fish → (from lobe-finned fish) amphibians → reptiles (reproduction on land, embryonic membranes) → birds and mammals (independently, from different reptile groups).
Worked example
Which group does an animal belong to if it is three-layered and bilaterally symmetrical, has a primary body cavity and a gut ending in an anus, but no circulatory system?
Three layers and bilateral symmetry — more advanced than flatworms.
A primary body cavity and an anus are features of roundworms.
There is no circulatory system yet (it appears in annelids).
Answer: roundworms (for example, the human roundworm Ascaris).
§3. Evolution of animal organ systems
Nervous system:
diffuse (net-like) — in hydra;
ganglionic: in flatworms a cerebral ganglion and longitudinal nerve cords; in annelids a ventral nerve cord; in arthropods the ganglia fuse into a complex "brain";
tubular — in chordates, on the dorsal side; in vertebrates it divides into brain and spinal cord, and in mammals the cerebral cortex is highly developed, with sulci and gyri.
Respiratory system: the whole body surface (cnidarians, worms) → gills (fish, crustaceans, many molluscs) → tracheae (insects) → lungs: simple sac-like in amphibians (the skin also takes part in breathing), honeycombed in reptiles, spongy lungs with air sacs in birds ("double breathing"), alveoli and a diaphragm in mammals.
Circulatory system:
absent — in cnidarians, flatworms and roundworms;
closed — in annelids (no heart; ring vessels contract);
open — in molluscs and arthropods (blood flows into the body cavity);
closed with a heart — in vertebrates.
Fish — a two-chambered heart (atrium + ventricle), one circulation, only venous blood in the heart.
Amphibians — a three-chambered heart (2 atria + 1 ventricle), two circulations, blood mixes in the ventricle.
Reptiles — three-chambered, with an incomplete septum in the ventricle, so blood mixes less (crocodiles have a four-chambered heart).
Birds and mammals — a four-chambered heart, arterial and venous blood do not mix → warm-bloodedness. Birds keep the right aortic arch, mammals the left.
Digestive system: intracellular digestion (unicellular organisms, partly cnidarians) → a gut cavity with one opening (cnidarians, flatworms) → a digestive tube with an anus (from roundworms onward) → a tube divided into sections, with digestive glands (liver, pancreas).
Support and movement: cilia and flagella → skin-muscle sac (worms) → exoskeleton (the chitinous cover of arthropods) → endoskeleton (the notochord in chordates, the backbone in vertebrates).
Worked example
Arrange these animals in order of decreasing mixing of blood in the heart: lizard, frog, pigeon.
Frog — three-chambered heart, no septum in the ventricle — a lot of mixing.
Lizard — three-chambered, incomplete septum — less mixing.
Pigeon — four-chambered, no mixing.
Answer: frog → lizard → pigeon.
§4. Evidence for phylogeny: comparative anatomy and embryology
Relationships between groups cannot be observed directly, so they are reconstructed from several independent lines of evidence.
Homologous organs share the same origin and structural plan but may have different functions: the human arm, the bat’s wing and the whale’s flipper. They are evidence of a common ancestor (the result of divergence).
Analogous organs have the same function but different origins: the wing of a butterfly and of a bird, the gills of a fish and of a crayfish. They result from convergence and do not show relationship.
Vestigial organs have lost their function and become reduced: the coccyx, the appendix and the remnant of a third eyelid in humans; remnants of pelvic bones in whales.
Atavisms are ancestral traits that reappear in individual organisms: a tail in a human, extra nipples, a thick coat of body hair.
Embryological evidence. Early embryos of all vertebrate classes are similar: they have a notochord, pharyngeal (gill) pouches and a tail. F. Müller and E. Haeckel expressed this as the biogenetic law.
Biogenetic law: ontogeny is a short and rapid recapitulation of phylogeny.
The law is not exact: an embryo repeats the embryonic stages of its ancestors, not their adult forms, and some stages are lost or altered. A. N. Severtsov extended it with his theory of phylembryogenesis.
Molecular evidence: the more closely the amino-acid sequences of proteins and the nucleotide sequences of DNA match, the more closely the species are related. For example, human and chimpanzee DNA are extremely similar.
Worked example
Which pairs are homologous and which are analogous: a) a bird’s wing and a bee’s wing; b) a horse’s foreleg and a seal’s flipper; c) a cactus spine and a pea tendril (both modified leaves); d) a fish’s fin and a dolphin’s flipper?
a) the bird’s wing is a forelimb skeleton, the bee’s wing is a fold of the chitinous cuticle: different origins → analogous.
b) both are mammalian forelimbs with the same bone plan → homologous.
c) both derive from leaves → homologous.
d) they look alike, but the dolphin’s flipper is built from a limb skeleton; the similarity is an adaptation to life in water → analogous.
Answer: homologous — b, c; analogous — a, d.
§5. Transitional forms and the phylogenetic tree
Transitional forms combine features of two major groups and document the transition between them:
Archaeopteryx (Jurassic) — bird features (feathers, wings) and reptile features (teeth in the jaws, a long bony tail, clawed fingers on the wing);
Ichthyostega — an early tetrapod between lobe-finned fishes and amphibians;
theriodonts (mammal-like reptiles) — a group between reptiles and mammals;
seed ferns — fossil plants between ferns and gymnosperms.
Living primitive groups also help to understand phylogeny: the lancelet shows the chordate body plan in its simplest form, the coelacanth is a living lobe-finned fish, and the platypus and echidna are egg-laying mammals.
A phylogenetic tree is a diagram of the origin of groups. A branch point stands for a common ancestor; the more recent the split between two groups, the more closely they are related. Modern groups on the tree do not descend from one another — they are sister branches from a shared ancestor.
Check yourself
Which plants were the first on land?
In which plants did fertilisation first become independent of water?
In which animals did a closed circulatory system first appear?
Malpighian tubules are the excretory organs of which animals?
What kind of blood flows through a fish’s heart?
In which group did the mesoderm first appear?
Remnants of pelvic bones in whales are an example of what kind of evidence?
What kind of organs are a bird’s wing and a butterfly’s wing?
Common mistakes
Saying the circulatory system first appeared in molluscs — it appeared in annelids.
Reversing the trend — in evolution it is the sporophyte that became dominant.
Writing that ferns evolved from mosses — both are separate branches from psilophytes.
Giving flatworms an anus — it first appeared in roundworms.
Calling the circulatory system of molluscs and arthropods closed — it is open.
Thinking birds evolved from mammals — both arose from different groups of reptiles.