Evolution of Life & The Advent of Animals

Flashcards covering animal classification, taxonomy, evolutionary transitions, endosymbiosis, and the five key differentiating body plan traits of major animal phyla.

What are the four core components of taxonomy?

Taxonomy comprises Identification (determining distinct traits or genetic differences), Description (documenting morphology and distinguishing characteristics), Nomenclature (assigning scientific names according to formal codes), and Classification (placing organisms within a hierarchical system).

How was the Ratchetting toadlet (Uperoleia stridera) identified and classified as a distinct species?

Genetic samples first showed differences from individuals in another area. Subsequent fieldwork revealed that they sound different and lack red spots. It was then described in a published paper, named under the International Code of Zoological Nomenclature, and placed in the genus Uperoleia within the 'mop-attracted frog' group (an Old Gondwanan lineage related to South American forms).

What is the hierarchical sequence of taxonomic ranks in the Linnaean classification system from highest to lowest?

Domain $\rightarrow$ Kingdom $\rightarrow$ Phylum (plural: Phyla) $\rightarrow$ Class $\rightarrow$ Order $\rightarrow$ (Suborder) $\rightarrow$ Family $\rightarrow$ Genus (plural: Genera) $\rightarrow$ Species.

How are species and genus defined in the lecture notes, and what formatting rule applies to their scientific names?

A species is a group of individuals that can or do interbreed and produce offspring (with about $50$ definitions in biology). A genus is a group of similar species sharing characteristics and a common ancestor that do not interbreed. Scientific names must be written in italics, with the genus capitalized and the species in lowercase.

Why was the traditional taxonomic family Pongidae revised in modern primate classification?

Traditionally, Pongidae grouped chimpanzees, bonobos, gorillas, and orangutans, while Hominidae contained humans alone. However, humans share a more recent common ancestor with chimpanzees and bonobos than either does with gorillas. Grouping non-human great apes together into Pongidae created a paraphyletic group that excluded a descendant of their common ancestor (humans). In the revised classification, Pongidae contains orangutans only, and Hominidae includes humans, chimpanzees, bonobos, and gorillas.

What is convergent evolution, and what mammalian example is highlighted in the notes?

Convergent evolution is the independent evolution of similar morphological forms in unrelated lineages. For example, mole-like forms have evolved independently three times: marsupial moles in Australia, 'Western' moles in the Americas, and golden moles in Africa.

What do the tips, internal nodes, and root represent on a phylogenetic tree?

The tip is a terminal branch representing a taxon or individual; the internal node represents the inferred common ancestor of all lineages descended from it; and the root represents the shared common ancestor of all tips in the tree.

What are the six recognized kingdoms of life, and which are prokaryotic versus eukaryotic?

The six kingdoms are Bacteria, Archaea, Protista, Fungi, Plantae, and Animalia. Bacteria and Archaea are prokaryotes (simple cells lacking a membrane-bound nucleus). Protista, Fungi, Plantae, and Animalia are eukaryotes (possessing a membrane-bound nucleus, linear DNA, and membrane-bound organelles).

How does cell wall composition differ across the kingdoms of life?

Bacteria and Archaea possess cell walls made of peptidoglycan; Plantae and many Protista have cell walls made of cellulose; Fungi have cell walls made of chitin; and Animalia have no cell walls.

What does genomic sequencing reveal regarding the deepest split in the tree of life and the origin of eukaryotes?

The deepest evolutionary split is between Bacteria and Archaea. Eukaryotes are descended directly from an Archaeal common ancestor rather than forming an entirely independent third branch from the base. The universal common ancestor was likely a heterotrophic, unicellular prokaryote with a peptidoglycan cell wall.

Why has natural selection permitted Bacteria and Archaea to remain relatively unchanged in cellular complexity?

Natural selection does not require an increase in size or complexity to ensure survival. Increases in complexity are likely a product of time, during which evolution has explored more combinations, some of which proved viable.

When did eukaryotic cells originate, and what primary structural features characterize them?

Eukaryotes evolved approximately $1$ to $2\text{ billion}$ years ago. They feature a distinct nucleus containing linear DNA, complex internal communication networks, and membrane-bound organelles such as mitochondria and chloroplasts.

What does the endosymbiotic theory proposed by Lynn Margulis in 1970 state regarding mitochondria and chloroplasts?

It proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by a larger host cell. The engulfed cell supplied energy and food while the host provided protection. Over time, the relationship became obligate, and the endosymbionts evolved into organelles.

Which specific bacterial lineages gave rise to mitochondria and chloroplasts, and in which eukaryotic kingdoms are they present?

Mitochondria arose from an engulfed proteobacterium and are present in Plants, Animals, and Fungi. Chloroplasts arose from an engulfed cyanobacterium, evolved after mitochondria, and are present in Plants only.

What three lines of evidence support the endosymbiotic origin of mitochondria and chloroplasts?

1. Both are bounded by a double membrane.
2. Both possess their own circular genomes (characteristic of bacteria) rather than linear nuclear DNA.
3. Chloroplast DNA is most similar to cyanobacteria, while mitochondrial DNA is most similar to proteobacteria.

What are the two evolutionary models for the origin of multicellularity, and how do they work?

1. Staying together: Clonal daughter cells fail to separate after division (often due to a mutation in the final stage of mitosis), producing genetically identical cells without genetic conflict. This is the primary basis for plants and animals.
2. Coming together: Free-living independent cells aggregate (e.g., slime moulds in soil), in which only some cells in the aggregate reproduce.

What biological characteristics define an animal (Kingdom Animalia)?

Animals are eukaryotic, multicellular organisms with division of labour among specialized cells. Except for sponges (Porifera), their cells organize into tissues and organs. Embryos develop from a single-celled zygote, and they are chemoheterotrophs (obtaining energy by ingesting building blocks/intermediates they cannot produce themselves).

What broad patterns describe the diversity and habitat distribution of animal phyla?

There are $30$ to $36+$ recognized animal phyla (with no universal agreement among experts). At least $21$ phyla are fully aquatic, none are fully terrestrial, and over $90\%$ of all animal species are concentrated into nine main phyla.

What are the nine main animal phyla, their common examples, and approximate species counts?

1. Porifera: Sponges ($8{,}500$ species)
2. Cnidaria: Hydroids, jellyfish, corals ($10{,}000$ species)
3. Platyhelminthes: Flatworms ($10{,}000$ species)
4. Nematoda: Roundworms ($12{,}000$ species)
5. Mollusca: Snails, mussels, octopuses ($100{,}000$ species)
6. Annelida: Segmented worms ($13{,}000$ species)
7. Arthropoda: Shrimp, insects, spiders ($3{,}000{,}000$ species)
8. Echinodermata: Sea urchins, sea stars ($6{,}800$ species)
9. Chordata: Sea squirts, fish, frogs, birds, reptiles, mammals ($52{,}000$ species)

What are the key differences between radial symmetry and bilateral symmetry?

Radial symmetry allows the body to be divided into similar halves along any plane passing through the central axis; radially symmetrical animals are often sessile and attached to a substrate. Bilateral symmetry features a single plane dividing the body into mirror-image halves with anterior and posterior ends, promoting cephalization, concentrated sensory organs, and directed locomotion.

What are the three embryonic germ layers, and what does their presence determine about tissue level organization?

The germ layers are endoderm (inner), mesoderm (middle), and ectoderm (outer). Animals may have layers of cells without true tissues (e.g., Porifera), be diploblastic with two cell layers and tissues but no organs or mesoderm (e.g., Cnidaria, having a protein matrix mesoglea), or be triploblastic with three germ layers developing into complex organs.

How do protostomes and deuterostomes differ in blastopore fate and embryonic cleavage patterns?

In protostomes ('first mouth'), the blastopore becomes the mouth, and cleavage is spiral (e.g., flatworms, annelids, molluscs). In deuterostomes ('second mouth'), the blastopore becomes the anus, and cleavage is radial (e.g., echinoderms, chordates). Nematodes and arthropods exhibit protostome-like intermediate patterns.

How are the three types of body cavities in triploblastic animals defined?

Acoelomate: No body cavity between the gut and body wall (e.g., Platyhelminthes). Pseudocoelomate: A body cavity located between the endoderm and mesoderm (e.g., Nematoda). Coelomate: A true lined body cavity completely enclosed within the mesoderm (e.g., Annelida, Mollusca, Arthropoda, Echinodermata, Chordata). Note that the gut is not a body cavity.

How does the presentation of segmentation differ between annelids/arthropods and chordates?

Segmentation evolved multiple times independently. Annelids and arthropods retain obvious external segmentation, whereas in chordates external segmentation is usually lost in adulthood but remains evident internally in the arrangement of muscles and vertebrae.

According to the comparative phyla summary table, what are the symmetry, tissue layers, development pattern, and body plan of Echinodermata?

Symmetry: Radial (penta-radial in adults); Tissue layers: Three (triploblastic); Development: Deuterostome; Body plan: Coelomate.