IMAT Biology · Evolution and Ecology
Speciation, Evidence for Evolution and Classification
New species form when populations stop interbreeding; fossils, anatomy and DNA show that all life shares ancestors, and classification groups it into three domains.
Why this matters
The papers asked in 2015 what evolution can lead to (new species, changed allele frequencies, more biodiversity) and in 2020 which kind of isolation a case of a new finch species shows. Evidence for evolution and classification are syllabus topics not yet asked.
Concept 1 of 3: Species and speciation: isolation barriers, allopatric and sympatric
Two groups of one species can only drift apart genetically if their genes stop mixing. Once something stops them interbreeding, each group changes on its own path, and eventually they could not interbreed even if they met. That point is a new species.
Definition
A species (biological species concept) is a group of populations whose members can interbreed in nature and produce fertile offspring.
- Speciation needs reproductive isolation: gene flow between the groups stops.
- Allopatric speciation: a geographical barrier (sea, mountain, river) splits the population.
- Sympatric speciation: a new species forms in the same area, for example by polyploidy in plants or by a change in mating behaviour or food.
- Prezygotic barriers stop a zygote forming; postzygotic barriers act on the hybrid.
- Adaptive radiation: one ancestor gives many species suited to different niches (Darwin's finches on the Galápagos).
- Outcomes of evolution: changed allele frequencies, adaptation, new species and so more biodiversity (and also extinction).
| Barrier | Before or after the zygote | Example |
|---|---|---|
| Habitat | Before (prezygotic) | Two species live in the same area but in different places, such as water and land |
| Temporal | Before (prezygotic) | Breeding seasons or flowering times differ |
| Behavioural | Before (prezygotic) | Different courtship songs or displays, so mates are not recognised |
| Mechanical or gametic | Before (prezygotic) | Reproductive parts do not fit, or sperm cannot fertilise the egg |
| Hybrid inviability | After (postzygotic) | The hybrid embryo dies early |
| Hybrid sterility | After (postzygotic) | The mule, from a horse and a donkey, is sterile |
Practice this conceptself-check · 4 quick reps
Producing offspring is not enough to be one species
The test is fertile offspring in nature. Horses and donkeys produce mules, but mules are sterile, so horses and donkeys are separate species. A barrier that acts after the zygote forms is still reproductive isolation.
Concept 2 of 3: Evidence for evolution: fossils, anatomy, embryos and molecules
If all living things descend from shared ancestors, the traces should be everywhere: in rocks, in body plans, in embryos and in DNA. They are, and the different lines of evidence agree with each other.
Definition
The main lines of evidence:
- Fossils: older rock layers hold simpler forms; transitional fossils (Archaeopteryx, between reptiles and birds) link groups. Radioactive dating gives their ages.
- Homologous structures: same basic structure, different uses, from a common ancestor (divergent evolution).
- Analogous structures: same use, different structure and origin (convergent evolution). They do not show close relationship.
- Vestigial structures: reduced remnants with little or no function.
- Molecular evidence: the genetic code is nearly universal; the more closely related two species are, the more similar their DNA and protein sequences.
- Biogeography: island species resemble those of the nearest mainland. Evolution is also observed directly, as in antibiotic resistance.
| Evidence | What it shows | Example |
|---|---|---|
| Homologous structures | Common ancestry | Pentadactyl limb: human arm, bat wing, whale flipper |
| Analogous structures | Similar pressures, separate origins | Wings of birds and of insects |
| Vestigial structures | Ancestors used the organ | Human coccyx, hip bones inside whales |
| Comparative embryology | Shared developmental plan | Vertebrate embryos all have pharyngeal pouches and a tail |
| DNA and protein sequences | Degree of relationship | Human and chimpanzee DNA differ by only a few per cent |
Practice this conceptself-check · 3 quick reps
Similar function does not mean close relatives
Structures that do the same job but are built differently (analogous) evolved separately under similar conditions. Shared structure, not shared function, is what points to a common ancestor.
Concept 3 of 3: Classification: the three domains, the ranks and binomial names
Classification sorts living things by relationship, from very broad groups to single species. The broadest split, found by comparing ribosomal RNA, is into three domains: two of prokaryotes and one of everything with a nucleus.
Definition
Taxonomy sorts organisms into nested ranks: domain, kingdom, phylum, class, order, family, genus, species.
- Binomial nomenclature (Linnaeus): genus plus species, written in italics with a capital only on the genus, as in Homo sapiens.
- The three domains (Woese, from ribosomal RNA): Bacteria, Archaea and Eukarya.
- Eukarya include protists, fungi (cell walls of chitin, feed by absorption), plants and animals.
- Viruses are not cells and belong to no domain.
- Humans: Eukarya, Animalia, Chordata, Mammalia, Primates, Hominidae, Homo, sapiens.
| Domain | Cell type | Key features | Examples |
|---|---|---|---|
| Bacteria | Prokaryote | No nucleus; cell wall of peptidoglycan; 70S ribosomes | E. coli, Streptococcus, cyanobacteria |
| Archaea | Prokaryote | No nucleus; no peptidoglycan; unusual membrane lipids; many live in extreme places | Methanogens, salt-loving and heat-loving archaea |
| Eukarya | Eukaryote | Nucleus and membrane-bound organelles; 80S ribosomes in the cytoplasm | Protists, fungi, plants, animals |
Practice this conceptself-check · 4 quick reps
Archaea are not a kind of bacteria
Archaea look like bacteria under a microscope, but they form their own domain. They lack peptidoglycan and in several molecular features (such as how they copy and read DNA) they are closer to eukaryotes than to bacteria.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Species and speciation: isolation barriers, allopatric and sympatric6 rows
| Barrier | Before or after the zygote | Example |
|---|---|---|
| Habitat | Before (prezygotic) | Two species live in the same area but in different places, such as water and land |
| Temporal | Before (prezygotic) | Breeding seasons or flowering times differ |
| Behavioural | Before (prezygotic) | Different courtship songs or displays, so mates are not recognised |
| Mechanical or gametic | Before (prezygotic) | Reproductive parts do not fit, or sperm cannot fertilise the egg |
| Hybrid inviability | After (postzygotic) | The hybrid embryo dies early |
| Hybrid sterility | After (postzygotic) | The mule, from a horse and a donkey, is sterile |
Evidence for evolution: fossils, anatomy, embryos and molecules5 rows
| Evidence | What it shows | Example |
|---|---|---|
| Homologous structures | Common ancestry | Pentadactyl limb: human arm, bat wing, whale flipper |
| Analogous structures | Similar pressures, separate origins | Wings of birds and of insects |
| Vestigial structures | Ancestors used the organ | Human coccyx, hip bones inside whales |
| Comparative embryology | Shared developmental plan | Vertebrate embryos all have pharyngeal pouches and a tail |
| DNA and protein sequences | Degree of relationship | Human and chimpanzee DNA differ by only a few per cent |
Classification: the three domains, the ranks and binomial names3 rows
| Domain | Cell type | Key features | Examples |
|---|---|---|---|
| Bacteria | Prokaryote | No nucleus; cell wall of peptidoglycan; 70S ribosomes | E. coli, Streptococcus, cyanobacteria |
| Archaea | Prokaryote | No nucleus; no peptidoglycan; unusual membrane lipids; many live in extreme places | Methanogens, salt-loving and heat-loving archaea |
| Eukarya | Eukaryote | Nucleus and membrane-bound organelles; 80S ribosomes in the cytoplasm | Protists, fungi, plants, animals |
Watch out for (3)
- Producing offspring is not enough to be one species→ Species and speciation: isolation barriers, allopatric and sympatric
- Similar function does not mean close relatives→ Evidence for evolution: fossils, anatomy, embryos and molecules
- Archaea are not a kind of bacteria→ Classification: the three domains, the ranks and binomial names
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