21.2 Major Features of Plant and Animal Evolution
Key Takeaways
Land plants came from a green algal ancestor, and the major transitions are cuticle and embryos, vascular tissue, seeds, and flowers.
Mosses and other bryophytes lack vascular tissue and need moisture for swimming sperm, while ferns have vascular tissue and do not make seeds.
Gymnosperms have pollen and seeds but do not make flowers or fruit. Angiosperms have flowers and fruit.
The animal survey contrasts radial and bilateral symmetry, a coelom, and protostome versus deuterostome development.
Vertebrate features run from jawless fishes through jaws, bony skeletons, lobe-fins, amphibians, the amniotic egg, reptiles, birds within dinosaurs, and mammals with hair and milk. Humans are not the goal of that sequence.
21.2 Major Features of Plant and Animal Evolution
Land plants and animals gained major features as branching innovations. Learn the transitions, not every phylum. Do not read the vertebrate list as a path whose purpose is a human.
From a green alga to a flowering plant
Land plants arose from a freshwater green algal ancestor among the charophyte algae. Life on land favored traits that slow drying and that protect the young plant. Name four transitions in order: cuticle and embryos, vascular tissue, seeds, and flowers.
A cuticle is a waxy covering that slows water loss. An embryo is the multicellular young sporophyte, protected in parental tissue. Those features mark life on land. They are not veins, seeds, or flowers.
Bryophytes, including mosses, liverworts, and hornworts, keep that early condition. The leafy moss is the gametophyte. It produces eggs and swimming sperm, and the sperm need a film of water, so mosses stay moist. Bryophytes have no xylem or phloem and make no seeds, flowers, or fruit. A moss is not a small fern.
Seedless vascular plants added xylem and phloem. Ferns are the familiar example. Vascular tissue moves water, minerals, and sugars and supports a taller body. The fern you see is the sporophyte. It releases spores, its gametophyte is small and separate, and sperm still swim. Ferns do not make seeds. Horsetails and club mosses belong in this group. A club moss has vascular tissue and is not a true moss.
Gymnosperms include conifers, cycads, ginkgo, and gnetophytes. They added pollen and seeds. Pollen delivers sperm without a film of external water. A seed packages an embryo, food, and a coat, so the embryo can wait through harsh conditions. Conifer seeds often sit on cone scales. Gymnosperms make neither flowers nor fruit. A fleshy juniper cone is still a cone, not fruit from a flower.
Angiosperms added flowers and fruit. A flower often attracts a pollinator. The ovary wall becomes fruit and helps disperse enclosed seeds. Double fertilization is another angiosperm feature: one sperm fertilizes the egg, and a second sperm forms endosperm, the food in the seed. Do not assign flowers or fruit to a pine, or seeds to a fern or a moss.
| Plant group | Feature added in the land-plant story | Still missing |
|---|---|---|
| Bryophytes, such as mosses | Cuticle and a protected embryo, on land | Vascular tissue, seeds, flowers, and fruit |
| Seedless vascular plants, such as ferns | Xylem and phloem | Seeds, flowers, and fruit |
| Gymnosperms, such as pines | Pollen and seeds | Flowers and fruit |
| Angiosperms, the flowering plants | Flowers and fruit | None of those four transitions |
Animal body plans, in a short survey
Sponges lack true tissues and lack the symmetry of the groups below. Radial symmetry arranges parts around a central axis, as in a jellyfish or a sea anemone. Adult sea stars look radial, often in five parts, but their larvae are bilateral and sea stars are deuterostomes. Shape alone does not place a sea star with a jellyfish.
Bilateral symmetry produces left and right sides, a head and a tail, and a back and a belly. Cephalization concentrates sense organs at the head, which suits an animal that moves in one direction. Insects, fishes, and people are bilateral.
A coelom is a fluid-filled body cavity lined by mesoderm, so organs can grow and move inside it. Flatworms are acoelomate and have no such cavity. Roundworms are pseudocoelomate: a cavity is present, but mesoderm does not line every side. Annelids, mollusks, arthropods, echinoderms, and chordates have a true coelom.
In a protostome the blastopore becomes the mouth. Mollusks, annelids, and arthropods are the examples. In a deuterostome the blastopore becomes the anus and the mouth forms later. Echinoderms and chordates, including vertebrates, are deuterostomes. Surveys also teach spiral, determinate cleavage for protostomes and radial, indeterminate cleavage for deuterostomes. The contrast to apply first is the fate of the blastopore.
Vertebrate innovations, not a ladder
These features appeared on branching lineages. Humans are one twig, not the goal, and earlier vertebrates are not unfinished people.
The earliest vertebrates were jawless fishes. Lampreys and hagfish still lack jaws. Jaws came from skeletal supports of the pharyngeal arches. Sharks and rays have jaws and skeletons of cartilage. Bony skeletons distinguish the bony fishes, including ray-finned fishes such as tuna. Lobe-finned fishes, such as lungfish and coelacanths, have fleshy fins with internal bone. Tetrapods arose among lobe-finned relatives, not from a ray-finned fish.
Amphibians are tetrapods that use land, but most still lay eggs in water or very damp places. The amniotic egg changed that limit. Its amnion, chorion, allantois, and yolk sac enclose the embryo, manage waste and gases, and store food, so reproduction need not occur in a pond. Reptiles are amniotes, typically scaled, with a shelled egg.
Birds are living dinosaurs. Feathers, and flight in most living species, sit inside that lineage. Mammals are a separate amniote branch, marked by hair and milk. Mammals did not evolve from birds, and birds did not evolve from mammals. Monotremes lay eggs. Marsupials and placental mammals keep the embryo in the mother, and amniotic membranes remain part of development.
Warning
Mosses are not vascular. Ferns do not make seeds. Gymnosperms do not make flowers or fruit. A club moss is not a true moss, and a fleshy juniper cone is not fruit from a flower.
How to place an unfamiliar organism
- No vascular tissue, and sperm that must swim: a bryophyte such as a moss.
- Vascular tissue, spores, and no seeds: a fern or another seedless vascular plant.
- Pollen and seeds, but no flowers or fruit: a gymnosperm.
- Flowers and fruit: an angiosperm.
- For animals, use symmetry, the presence of a true coelom, and whether the blastopore becomes the mouth or the anus.
- Place a vertebrate by the feature it shows: jaws, bone, lobe-fins, an amphibian tie to water, an amniotic egg, feathers within dinosaurs, or hair and milk.
Plant order is cuticle and embryos, then vascular tissue, then seeds, then flowers. Nothing in the animal list exists in order to end at a human.
Which sequence names major plant features in the order they appear in the land-plant story?
Seeds in mosses, then flowers in ferns, then vascular tissue only in gymnosperms.
Flowers first, then vascular tissue, then a cuticle, with swimming sperm only in flowering plants.
Fruit in gymnosperms, then cones that count as flowers, then vascular tissue in mosses.
Cuticle and embryos, then vascular tissue, then seeds, then flowers.
A plant has xylem and phloem, releases spores, and has swimming sperm. It makes no seeds, flowers, or fruit. Which description fits it?
It is a fern, a seedless vascular plant.
It is a moss, because every land plant without flowers is a nonvascular bryophyte.
It is an angiosperm, because flowers are optional and a frond is a fruit.
It is a pine, because a pine has no vascular tissue and its cones are spores rather than seeds.
Which statement about animal body plans and vertebrates is correct?
Radial symmetry is the body plan of insects, and a coelom is a kind of flower.
Humans are the goal of vertebrate evolution, and jawless fishes are unfinished versions of people.
Bilateral animals have a head-to-tail axis, a coelom is a body cavity lined by mesoderm, and birds are living members of the dinosaur lineage.
In deuterostomes the mouth forms from the blastopore, and birds stand outside the dinosaur lineage.
Sections you finish are checked off in the contents.