Everything you need to know for C4.1 — Populations & Communities.

Any two of the following — pick the ones you can defend in one sentence:

• Hair / fur made of keratin covering the body (insulation and sensory function).
• Mammary glands that secrete milk to nourish young.
• Three middle-ear bones (malleus, incus, stapes) — evolved from reptilian jaw bones.
• A neocortex region of the brain (complex cognition).
• Heterodont dentition — incisors, canines, premolars and molars.
• A muscular diaphragm separating thoracic and abdominal cavities.
• Endothermy with a four-chambered heart (birds also evolved these independently).

Best two for an exam: hair/fur + mammary glands — uniquely mammalian (synapomorphies).

Yes. Genus is the second-narrowest taxonomic rank (only species is narrower), so two species in the same genus share a recent common ancestor and many derived characteristics. They belong to the same clade at that level and are usually morphologically and genetically very similar.

Examples: Panthera leo (lion) and Panthera tigris (tiger) share Panthera — both are big cats and can produce hybrids (ligers, tigons). Homo sapiens and the extinct Homo neanderthalensis shared the Homo genus and a common ancestor only ~600,000 years ago.

Caveat: two species in the same genus are more closely related to each other than to a species in a different genus — but less closely related than two populations of the same species.

Mutations have two broad sources:

• 1. Spontaneous (endogenous) causes — DNA replication errors; chemical instability of DNA (depurination, deamination); errors in DNA repair.
• 2. Induced causes (mutagens) — ionising radiation (X-rays, gamma rays); UV light (thymine dimers); chemicals (benzene, mustard gas); some viruses (e.g. HPV).

Best answer for "list two": (1) errors during DNA replication, and (2) exposure to mutagens such as UV light or ionising radiation.

A clade is built using shared derived characters (synapomorphies). Main evidence types:

• Molecular evidence (most reliable today) — DNA, RNA or amino-acid sequences of conserved proteins (cytochrome c, ribosomal RNA). Fewer differences = more recent common ancestor.
• Morphological / anatomical — homologous structures (pentadactyl limb), bone structure. Beware analogous structures from convergent evolution.
• Embryological — shared developmental stages (vertebrate embryos all have pharyngeal arches).
• Fossil evidence — transitional fossils (Tiktaalik linking fish to tetrapods).
• Behavioural and biochemical — shared metabolic pathways, courtship rituals.

Modern cladistics relies primarily on molecular data — it's quantitative and less subject to convergent evolution.

A gene pool is the total collection of all alleles in a population. Variation within it comes from:

• 1. Mutation — the original source of all new alleles.
• 2. Sexual reproduction / meiosis — shuffles existing alleles via crossing over (prophase I), independent assortment (metaphase I), and random fertilisation.

Other contributors: gene flow (migration) bringing new alleles in, and genetic drift changing frequencies randomly.

Best answer for "list two": (1) mutation (new alleles), and (2) meiosis + sexual reproduction (new allele combinations).

Natural selection requires variation — without it, every individual would have identical fitness. The logic flows in five steps:

• (1) Individuals show heritable variation in their traits (from mutation and sexual reproduction).
• (2) Populations overproduce offspring — more are born than the environment can support.
• (3) Some variants are better suited to the environment.
• (4) These individuals survive and reproduce more — higher fitness — passing alleles on.
• (5) Over generations, beneficial alleles increase in frequency. This is evolution.

Concrete example: In peppered moths, light (typica) and dark (carbonaria) forms existed — that was the variation. Industrial soot darkened bark; dark moths survived predation more often. Over ~50 years the dark allele's frequency rose from <2% to >95% in industrial cities.

Key: selection cannot create new traits — it can only act on the variation that already exists.

Four conditions must combine:

• 1. Genetic variation within the population — from mutation and sexual reproduction.
• 2. Isolation that prevents gene flow:
  • Geographic (allopatric) — mountain, river, ocean, ice age.
  • Reproductive (sympatric) — temporal, behavioural, mechanical, gametic, polyploidy.
• 3. Different selection pressures on the two isolated populations — drives divergence.
• 4. Time — enough generations for genetic differences to accumulate until interbreeding is no longer possible.

Bottom line: speciation = variation + isolation + divergent selection + time → reproductive isolation between two new species.

Carl Woese (1990) split all life into three domains based on ribosomal RNA sequences:

• Bacteria — prokaryotic; single circular chromosome; cell walls of peptidoglycan; binary fission. Examples: E. coli, Streptococcus, cyanobacteria.
• Archaea — also prokaryotic but biochemically distinct. No peptidoglycan; membrane lipids have ether-linked branched isoprene chains (Bacteria use ester-linked straight chains); RNA polymerase resembles eukaryotes. Many live in extreme environments. Examples: methanogens, halophiles, thermophiles.
• Eukarya — true membrane-bound nucleus and organelles. Includes Protista, Fungi, Plantae, Animalia.

Relationship: Archaea and Eukarya are more closely related to each other than either is to Bacteria — eukaryotes likely arose from an archaeal ancestor that engulfed a bacterium (mitochondrion).

Gene pool = the total collection of all alleles for every gene in a population. The genetic library.

Allele frequency = the proportion of one particular allele relative to all alleles for that gene — a number 0 to 1.

Analogy: gene pool is a bag of marbles; allele frequency is the percentage that are red.

Worked example: 100 pea plants, flower-colour gene with alleles P and p — 200 total alleles. If 140 are P, 60 are p:

• Gene pool contains P and p (plus alleles of every other gene).
• Allele frequency of P = 140/200 = 0.70 (70%).
• Allele frequency of p = 60/200 = 0.30 (30%).

Key: evolution is defined as a change in allele frequencies in a gene pool over generations.

Both produce new species but differ in the isolating mechanism.

Similarities:

• Both require reproductive isolation that prevents gene flow.
• Both rely on genetic variation and divergent selection (or drift).
• Both produce two or more species from one ancestor.

Differences:

Summary: Allopatric requires a geographic barrier; sympatric produces reproductive isolation within one area via polyploidy, niche differentiation, or sexual selection.