A4.1Theme A — Unity and diversity
Evolution and speciation — IB Biology HL Revision Notes & Topical Questions
5 min read 11 parts Lesson 12 of 14 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 HL notes: Explain evolution by natural selection, the evidence for evolution, and how speciation occurs by isolation, hybridisation barriers and polyploidy.Evolution = changes in the heritable characteristics of organisms over generation…
- Lamarck's theory
- Why it is wrong
- Nature of science
- Common ancestry
- Conserved sequences
- Selective breeding (artificial selection)
Section 1 of 11Evolution and natural selection
Evolution
Evolution changes in the heritable characteristics of organisms over generations.
- Only inherited changes count; a plant having its leaves eaten does not cause evolution.
- Heritable characteristics are determined by alleles, which can change by random mutation.
Darwin's theory
1
Individuals vary because of random mutations in DNA.
2
Individuals compete for survival under selection pressures.
3
Those best suited to the environment survive and reproduce more.
4
Advantageous alleles are passed to offspring.
5
Over many generations these alleles become more frequent.
- Diversity
- a single original species divided repeatedly over millions of years into millions of species.
Section 2 of 11Lamarck and the nature of the theory
Lamarck's theory
- Lamarck's theory characteristics acquired during life are inherited, and frequently used features become stronger while unused ones disappear.
- His giraffe example said stretching for high leaves lengthened the neck and the elongated neck was inherited.
Why it is wrong
- Why it is wrong acquired characteristics are not heritable, though epigenetics may give limited exceptions that are unlikely to drive natural selection.
Nature of science
- Nature of science natural selection explains and predicts a broad range of observations and is unlikely to be falsified.
- Minor ideas were falsifiedevolution is not always slow,
- as antibiotic resistance evolves quickly,
- and the fossil record does show intermediate species.
- The geological timescale means formal proof is impossible, so it remains a theory.
Section 3 of 11Evidence from sequence data
1Sources
- Sources the base sequence of DNA, RNA, and the amino acid sequence of proteins.
- (nucleus, mitochondria, chloroplasts)
2Common ancestry
- Common ancestry similar sequences in different species suggest shared ancestry.
3Conserved sequences
- Conserved sequences comparisons use the same, highly conserved regions of DNA.
- Like is compared with like.
- There are few differences, so they are easy to identify.
- They exist in many species, for example genes for haemoglobin or respiratory enzymes.
4
Method
1
Extract DNA from blood, skin or fossil remains.
2
Process it and obtain the base sequence.
3
Compare it with other organisms, where more similarity means closer relationship.
1Example
- Example the chimpanzee genome (2005) shares almost 99% with the human genome, making chimpanzees our closest living relatives.
2Certainty
- Certainty comparing several genes raises certainty, and the data build evolutionary trees.
Section 4 of 11Evidence from selective breeding and homologous structures
Evidence from selective breeding and homologous structures
Selective breeding (artificial selection)
Selective breeding (artificial selection) humans choose organisms with desirable traits and breed them repeatedly.
- It uses natural variation, such as higher yield or disease resistance in crops, or heavy wool and high milk output in animals.
- Humans choose which alleles pass on, rather than survival, and change is faster because only selected individuals breed.
- It shows that evolution results from accumulation of small changes in DNA.
Process
1
Identify variation in the population.
2
Select individuals with the desired characteristics.
3
Breed the selected individuals together.
4
Test the mature offspring and select the best for further breeding.
5
Repeat over many generations.
Evidence from selective breeding and homologous structures
Wild brassica variation gave many crop plants.
Homologous structures
Homologous structures body parts with different functions but similar structure from a common ancestor.
- The pentadactyl limb has five digits and the same bone layout in humans, whales, birds, frogs and alligators despite different uses.
- Adaptive radiation explains them as adaptation of a common ancestor to different environments, though it is not proof.
Section 5 of 11Convergent evolution
Convergent evolution
Analogous structures
Analogous structures features with similar function but different evolutionary origin.
Cause
- Cause distantly related species face similar selection pressures, so advantageous traits evolve separately.
Examples
- Dolphins (mammals) and sharks (fish) have streamlined bodies.
- Cacti in the Americas and euphorbias in Africa both have spines and succulent stems.
- Butterfly and bat wings.
Difference
- Difference homologous structures show shared ancestry, while analogous structures show natural selection acting on advantageous traits.
Section 6 of 11Speciation and isolation
1
Speciation
Speciation the development of new species from pre-existing species.
2
Gene flow
Gene flow exchange of genes between populations, which isolation can stop.
- Differences accumulate faster when selection pressures differ.
3
Reproductive isolation
Reproductive isolation individuals can no longer interbreed to produce fertile offspring, and it is required for speciation.
4
Causes of reproductive isolation
- Seasonal changes in mating or flowering times.
- Changes in courtship behaviour.
- Geographical isolation by barriers such as water, mountains or a motorway.
5
Differential selection
Differential selection natural selection acting differently on isolated populations.
6
Bonobos and chimpanzees
- Bonobos and chimpanzees the Congo river likely separated ancestral populations, with chimpanzees north and bonobos south.
- Chimpanzees became more aggressive with male-dominated groups and bonobos have dominant females.
7
Extinction reduces diversity, as in the passenger pigeon and woolly mammoth.
Section 7 of 11Allopatric and sympatric speciation
| Feature | Allopatric | Sympatric |
|---|---|---|
| Isolation | Geographical barrier | No barrier; alleles and phenotypes change |
| Frequency | Most common | Less common |
| Mechanism | Differential selection and genetic drift change allele frequencies | Temporal or behavioural isolation, or a change such as food preference, stops interbreeding |
| Example | A new mountain range splits a tree population | A mutation changes the diet of laboratory fruit flies |
1Common sequence
- Common sequence no gene flow, allele frequencies diverge, phenotypes diverge, and the populations become reproductively isolated.
2
Geographical isolation can be temporary, while reproductive isolation means speciation has occurred even in the same habitat.
Section 8 of 11Preventing hybridisation and adaptive radiation
Hybrid
Hybrid offspring of two species, usually rare and infertile.
Barriers
- Incompatible chromosomes — gametes with different numbers produce non-viable zygotes or sterile hybrids because chromosomes cannot pair,
- as in the mule.
- Incompatible courtship — differences in visual, chemical or auditory rituals prevent mating, as in birds of paradise.
Adaptive radiation
Adaptive radiation rapid evolution of many species from a common ancestor.
- The new species share features but fill different ecological niches, so they can live together.
- Examples are Darwin's finches in the Galapagos and the more than 50 Hawaiian honeycreeper species.
Section 9 of 11Speciation in plants by polyploidy
Abrupt speciation
- Abrupt speciation plants can speciate in one generation because polyploid cells remain viable.
- (more than two chromosome sets, such as 3n or 4n)
Autopolyploidy
Autopolyploidy extra chromosome sets from within one species, through nondisjunction in meiosis giving a 2n gamete.
Allopolyploidy
Allopolyploidy extra sets from two species, when diploid gametes of different species fuse into a polyploid hybrid.
Advantages of polyploidy
- Otherwise infertile hybrids can undergo meiosis.
- Polyploid plants are often larger and more vigorous.
- Extra gene copies mask harmful alleles.
New species
- 4n plants make 2n gametes, and crossing with 2n parents gives infertile 3n offspring.
Examples
- Persicaria: foliosa is 2n, japonica is 4n and puritanorum is 6n.
- Fallopia: Japanese knotweed is 8n, giant knotweed 4n and Bohemian knotweed 6n, a hybrid of the two.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Evolution | change in the heritable characteristics of organisms over generations. |
| Homologous structure | a similar structure inherited from a common ancestor but used differently. |
| Analogous structure | a structure with similar function but different evolutionary origin. |
| Reproductive isolation | inability to interbreed to produce fertile offspring. |
| Allopatric speciation | speciation caused by geographical isolation. |
| Sympatric speciation | speciation within the same area without a geographical barrier. |
| Adaptive radiation | rapid evolution of many species from one ancestor filling different niches. |
| Polyploidy | having more than two sets of chromosomes. |
Section 11 of 11Exam technique
1
Individuals evolve during their lifetime — populations evolve as allele frequencies change over generations.
2
Acquired characteristics are inherited — only alleles are heritable, so Lamarck's idea is wrong.
3
Geographical isolation is the same as reproductive isolation — geographical isolation stops gene flow, but only reproductive isolation means separate species.
4
Similar structures always show close ancestry — analogous structures arise by convergent evolution.
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