A4.1Theme A — Unity and diversity
Evolution and speciation — IB Biology SL Revision Notes & Topical Questions
5 min read 13 parts Lesson 10 of 11 2 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
A4.1 Evolution and speciation — IB BIOLOGY SL notes: Define evolution, evaluate the evidence for it, and explain how reproductive isolation leads to speciation.Evolution = changes in the heritable characteristics of organisms over generationsIt acts on populations, not on single …
- Requirement
- Why Lamarck fails
- Status of the theory
- Like with like
- Selective breeding (artificial selection)
- Faster than nature
Part 1 of 11Evolution
Evolution
Evolution
Evolution changes in the heritable characteristics of organisms over generations
- It acts on populations, not on single individuals.
- Only heritable variation is passed on, so a plant having its leaves eaten does not lead to evolution.
Heritable characteristics
- Heritable characteristics they are determined by the alleles of genes
- Alleles change by random mutation and become more or less advantageous.
- Advantageous alleles are more likely to be passed on, so a species changes gradually.
Speciation
- Speciation changes in heritable characteristics can also produce completely new species
- About 1.8 million species have been named, and the true total is far higher.
Part 2 of 11Darwinian evolution
1
Individuals in a species vary because of random mutations in their DNA.
2
Selection pressures make individuals compete for survival.
3
Individuals best suited to the environment are more likely to survive and reproduce.
4
Advantageous alleles are passed to the offspring.
5
Over many generations the advantageous alleles become more frequent in the population.
- Requirement
- characteristics must be heritable
Part 3 of 11Lamarck compared with Darwin
| Feature | Lamarck | Darwin |
|---|---|---|
| Era | Early 1800s | Mid 1800s |
| Source of change | Use or disuse during life | Random variation from mutation |
| What is inherited | Acquired characteristics | Alleles present at birth |
| Giraffe neck | Stretching lengthened necks | Longer-necked variants fed and bred more |
| Status today | Rejected, with epigenetics a minor exception | Accepted, with updates |
Why Lamarck fails
- Why Lamarck fails acquired changes do not alter the DNA of the gametes, so they are not inherited
- Epigenetic effects are too small to drive evolution.
Status of the theory
- Status of the theory natural selection explains and predicts a broad range of observations
- Evolution can be fast, as with antibiotic resistance in bacteria.
- Fossils do show intermediate forms.
- Over geological time it cannot be formally proven, so it is still called a theory.
Part 4 of 11Sequence data as evidence
1Sources
- DNA from the nucleus, mitochondria and chloroplasts, RNA from expressed genes, and the amino acid order of proteins
2Principle
- Principle the more similar the sequences, the more closely related the species
- Humans and chimpanzees share about 99% of their DNA.
- Closer relatives show fewer base differences in shared genes.
3Like with like
- Like with like compare the same region of DNA, often a conserved region
- Conserved regions give few, clear differences and exist in many species.
- Examples are genes for haemoglobin and respiratory enzymes.
4Method
- Method extract DNA from blood, skin or fossil remains, sequence it and compare the base sequence with other organisms
- Compare several genes to raise certainty.
- The results are used to build evolutionary trees.
Part 5 of 11Selective breeding as evidence
- Selective breeding (artificial selection) = humans choose organisms with desired characteristics and breed them repeatedly
- Targets include crop yield, disease resistance, wool quality and milk volume.
1
Identify variation in the population.
2
Choose individuals showing the desired trait most strongly.
3
Breed the chosen individuals together.
4
Raise the offspring to maturity and test for the trait.
5
Select the best offspring as the next parents.
6
Repeat over many generations until the trait is reliable.
Faster than nature
- Faster than nature only chosen individuals breed, so less suited alleles are excluded
Evidence value
- Evidence value it shows that evolution can occur by the accumulation of small changes to DNA
- Wild brassica gave kale, broccoli and cauliflower.
Part 6 of 11Homologous structures
Homologous structures
Homologous structures
Homologous structures body parts with the same underlying structure that may look and work very differently
Pentadactyl limb
- Pentadactyl limb a limb with five digits, found in mammals, birds, amphibians and reptiles
- The same bone layout is used for walking, swimming, flying and jumping.
- The human foot is for upright walking and running.
- The whale flipper is for swimming.
- The bird wing is for flight.
- The frog limb is for walking, jumping and swimming.
- The alligator limb is for walking and swimming.
Adaptive radiation
- Adaptive radiation descendants of one ancestor adapt to different environments
- It explains homologous structures but is not proof of common ancestry.
Part 7 of 11Convergent evolution
Convergent evolution
Analogous structures
Analogous structures characteristics with similar form and function but different evolutionary origin
Convergent evolution
- Convergent evolution distantly related species in habitats with similar selection pressures evolve similar features separately
- Each advantageous feature evolves separately and not from a single mutation.
Examples
- Dolphins (mammals) and sharks (fish) both have streamlined bodies.
- Cacti in the Americas and euphorbias in Africa both have spines and succulent stems.
- The wings of butterflies, birds and bats are analogous.
Compared with homology
- Compared with homology homologous structures show shared ancestry and analogous structures show the effect of natural selection
Part 8 of 11Evidence compared
| Feature | Molecular sequences | Selective breeding | Homologous structures |
|---|---|---|---|
| What is compared | DNA, RNA or amino acid sequences | Domesticated varieties and wild ancestors | Bone plans of limbs |
| Example | Humans and chimpanzees share almost identical cytochrome c | Wild cabbage giving kale, broccoli and cauliflower | Bat wing, whale flipper and human arm |
| Shows | Common ancestry, with relatedness measurable | Selection can change species quickly | Divergence from a shared ancestor |
| Limitation | Mutation rates vary | Humans choose the trait | Similarity may be convergent |
Part 9 of 11Speciation
1Speciation
Speciation the development of new species from pre-existing species over time
- It increases the diversity of species.
- In theory life began as one species that split again and again over millions of years.
2Extinction
- Extinction it reduces the number of species, as with the passenger pigeon and the woolly mammoth
3Gene flow
- Gene flow speciation can occur when gene flow between populations is prevented, for example on different islands
- Genetic differences then accumulate, faster if the selection pressures differ.
4Reproductive isolation
Reproductive isolation changes in alleles or phenotypes that stop individuals breeding successfully with others
- A speciation split has occurred when the populations cannot interbreed to produce fertile offspring.
- Gradual change alone is not enough without reproductive isolation.
5Species concept
- Species concept the biological concept defines a species as organisms that interbreed to produce fertile offspring
- It cannot be applied to fossils or asexual organisms.
Part 10 of 11Barriers and allopatric speciation
| Feature | Geographical | Behavioural | Seasonal |
|---|---|---|---|
| Location | Separated populations | Same area | Same area |
| Barrier | River, mountain, sea or motorway | Different courtship or songs | Breeding at different times |
| Example | Snapping shrimps either side of Panama | Lacewing species with different mating songs | Plants flowering in different months |
| Permanent? | Not necessarily, as populations could rejoin | Yes, once the mating signals differ | Yes, once the timings no longer overlap |
1Geographical isolation versus reproductive isolation
- Geographical isolation versus reproductive isolation geographical isolation may be temporary,
- whereas reproductive isolation means speciation has occurred
2Differential selection
- Differential selection different selection pressures acting on separated populations drive them apart
1
A single population shares one gene pool.
2
A geographical barrier splits the population into two.
3
Gene flow between the groups stops.
4
Different mutations and selection pressures act on each group.
5
Allele frequencies drift apart over many generations.
6
If the groups meet again they can no longer produce fertile offspring.
Sympatric speciation
- Sympatric speciation new species arise in the same area, for example through behavioural or temporal isolation
Bonobos and chimpanzees
- Bonobos and chimpanzees the Congo River separated their ancestors
- Chimpanzees live north of the river and bonobos south of it.
- Chimpanzees are more aggressive with male-dominated groups, and bonobos have dominant females.
- The two groups eventually became reproductively isolated.
Part 11 of 11More from the specification
1Natural selection mechanism
- Evolutionary process where inheritable variations best suited to ambient environmental pressures enhance survival and reproductive success.
2Artificial selection
- Selective breeding by humans for specific desirable characteristics across successive generations, demonstrating evolutionary divergence.
3Fossil record evidence
- Preserved anatomical remains in sedimentary rock strata provide chronological evidence of structural transitions throughout Earth's history.
4Speciation
- The evolutionary divergence of geographically or reproductively isolated populations into distinct, non-interbreeding species generating great biological diversity.
Key terms
| Term | Meaning |
|---|---|
| Evolution | A change in the heritable characteristics of a population over generations. |
| Natural selection | The greater survival and reproduction of individuals whose inherited traits suit the environment better. |
| Homologous structures | Body parts with a shared underlying plan inherited from a common ancestor. |
| Analogous structures | Parts with a similar function that evolved separately in unrelated groups. |
| Convergent evolution | Independent evolution of similar features in unrelated lineages under similar selection pressures. |
| Speciation | The formation of new species from existing ones. |
| Reproductive isolation | A barrier that stops gene flow because individuals can no longer breed successfully. |
| Adaptive radiation | The diversification of one ancestor into many species suited to different niches. |
Avoid these mistakes
1
Saying acquired characteristics are inherited — only heritable alleles are passed on
2
Confusing geographical isolation with reproductive isolation — only reproductive isolation means speciation has happened
3
Saying homologous structures have the same function — they share structure and may have very different functions
4
Saying evolution happens to an individual — populations evolve
Marked practiceQuizzes for this lesson
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