D4.1Theme D — Continuity and change
Natural selection — IB Biology SL Revision Notes & Topical Questions
5 min read 12 parts Lesson 10 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
D4.1 Natural selection — IB BIOLOGY SL notes: Explain how variation, overproduction and selection pressures cause evolution by natural selection, including sexual selection.Evolution = changes in the heritable characteristics of organisms over generationsNatural selection = the p…
- Darwin's deductions
- Paradigm shift
- Variation needed
- Pressure and variation
- Crossing over
- Random orientation
Section 1 of 12Natural selection
Evolution
Evolution changes in the heritable characteristics of organisms over generations
Natural selection
Natural selection the process by which organisms better adapted to their environment survive, reproduce and pass on advantageous alleles
- Advantageous characteristics increase in frequency in the population.
- Natural selection acts on phenotypes but changes allele frequencies.
- Populations evolve, not individuals.
Darwin's deductions
- Darwin's deductions from On the Origin of Species in 1859, after his five-year voyage
- Individuals with advantageous alleles survive better and so reproduce more.
- A favourable characteristic increases in frequency and can become the most common.
- Unfavourable characteristics decrease in frequency and may be lost.
Paradigm shift
Paradigm shift a major change in scientific thinking when research contradicts earlier assumptions
- Darwin's theory replaced Lamarckism, which claimed acquired characteristics are inherited.
- Nearly 200 years of genetic research supports it.
Section 2 of 12Example: rabbits
1
A rabbit population varies in fur colour, with one allele for brown fur and another for white.
2
Foxes act as a selection pressure through predation.
3
Brown rabbits are better camouflaged, so more survive and breed.
4
They pass on the brown allele to their offspring.
5
Over many generations the frequency of the brown allele rises and that of the white allele falls.
Variation needed
- Variation needed if all individuals were identical, none would be favoured and the population could not adapt
Pressure and variation
- Pressure and variation variation exists before a pressure acts and is not produced in response to it
Section 3 of 12Sources of variation
| Source | When it happens | Creates new alleles? | Effect |
|---|---|---|---|
| Gene mutation | Any time DNA is copied or damaged | Yes | New allele in the gene pool |
| Crossing over | Prophase I of meiosis | No | New combinations on one chromosome |
| Random orientation | Metaphase I of meiosis | No | Mixes maternal and paternal chromosomes |
| Random fertilisation | Fusion of gametes | No | Unique allele pairings in the zygote |
Mutation
- Mutation a copying error in DNA replication is the original source of variation
- Most mutations are neutral or harmful and a few are beneficial.
- Only mutations in the cells of the sex organs reach gametes.
- A somatic mutation has no evolutionary effect.
- It is the only source of variation in asexual species.
Crossing over
- Crossing over non-sister chromatids break and rejoin, swapping alleles between homologues
Random orientation
- Random orientation the pairs line up independently at metaphase I, giving 2n combinations
- Humans with n = 23 give 223 = 8 388 608.
Random fertilisation
- Random fertilisation any male gamete can fuse with any female gamete
- Siblings from successive matings are almost never genetically identical.
Section 4 of 12Overproduction and competition
1Overproduction of offspring
- Overproduction of offspring most species make more young than the environment supports
- An elephant usually has one calf, whereas some ants lay millions of eggs.
- Food, water, mates, space and light run short, which limits the carrying capacity.
2Competition
- Competition many offspring fail to survive and reproduce
- Overproduction and competition promote natural selection.
3Intraspecific competition
- Intraspecific competition it plays a greater role than interspecific competition
- Members of a species share the same niche.
- They face the same abiotic and biotic factors.
- They meet more often.
4Differential reproduction
- Differential reproduction well-adapted individuals are more likely to survive to adulthood, find a mate and leave more offspring
- Faster, stronger lions catch more prey and raise more cubs.
- Better-adapted individuals are more likely to survive, not guaranteed to.
Section 5 of 12Heritable traits and abiotic pressures
Heritable traits and abiotic pressures
Heritable characteristics
- Heritable characteristics determined by alleles and passed on in gametes
- Physical examples are the length of a giraffe's neck.
- Behavioural examples are a woodlouse moving to a dark hiding place.
Non-heritable characteristics
- Non-heritable characteristics acquired during life and not passed on, such as autumn fat gain or an injury from a predator
Abiotic factors
- Abiotic factors non-living factors that can act as selection pressures
- Temperature and rainfall are density-independent.
- They affect survival and so make population size fluctuate.
- Salt-tolerance alleles help plants survive and spread on salty soils.
Biotic pressures
- Biotic pressures predators, parasites, pathogens and competitors
Section 6 of 12Sexual selection
Sexual selection
Sexual selection selection due to the preference of one sex for certain characteristics in the other
- Males vary in physical and behavioural traits that females can see.
- Females may take these traits as signs of fitness, such as healthy tail feathers suggesting a low disease burden.
- Competition between males for mates favours large bodies and weapons such as antlers.
Effects
- Reproductive isolation, for example altered courtship makes some males unattractive so they no longer breed with the rest.
- Sexual dimorphism, a distinct difference in size or appearance between the sexes, as in birds of paradise.
Peacocks
- Peacocks females prefer long, elaborate tails, which makes males more prone to predation
Islands
- Islands plentiful resources and few predators make female choice the main pressure
Significance
- Significance sexual selection can amplify selection and maintain variation, giving rapid change
- It is viewed as Darwin's second-greatest insight.
Section 7 of 12Natural and sexual selection compared
| Feature | Natural selection | Sexual selection |
|---|---|---|
| What is favoured | Survival in the environment | Success in getting mates |
| Main pressure | Predators, climate, food and disease | Mate choice or rival fights |
| Typical traits | Camouflage and drought tolerance | Bright colours, large antlers and displays |
| Effect on survival | Raises it | May lower it |
| Outcome | Better adaptation to the habitat | Sexual dimorphism |
- Competition type
- natural selection comes from competition for resources and sexual selection from competition for mates
Section 8 of 12Endler's guppy experiments
Guppies
- Guppies fish of Trinidad streams with variable colour and spot patterns
- Males are bright and females are dull.
- Spots mimic the gravel of the streambed, which camouflages the fish.
Hypotheses
- With predators, the type of streambed changes survival and so spot brightness.
- With low predation, sexual selection makes male spots brighter.
- As predation rises, spot brightness decreases.
Laboratory design
- Guppies bred for six months in tanks with no predation, and their spots were counted and measured.
- They were randomly assigned to ten greenhouse ponds, five with coarse gravel and five with fine gravel.
- Two ponds had no predators, four had a weak predator (killifish) and four had a dangerous predator (pike cichlid).
- Spots were counted again after five months and after a further nine months.
Results
- With high predation the mean number of spots decreased.
- With low predation the number of spots kept increasing.
- With predators, spot size matched the gravel size, so coarse gravel gave larger spots.
- With no predation the opposite occurred, as males that did not match the background were more conspicuous to females.
Field test
- Field test dull males moved to a low-predation stream became more colourful after about 15 generations
Trade-off
- Trade-off brightness helps mating but raises predation risk
Models
- Models the ponds are controlled, simplified systems for testing a hypothesis
Section 9 of 12Tracing selection through generations
1
Individuals in a population vary and some of the variation is heritable.
2
More offspring are produced than the environment can support.
3
Competition and other pressures mean many die before breeding.
4
Those with better-suited traits survive and reproduce more.
5
They pass their favourable alleles to their offspring.
6
Allele frequencies shift over many generations.
- Finch example
- in a drought deep-beaked birds crack hard seeds, survive and breed, so the mean beak depth rises in the next generation
Section 10 of 12More from the specification
1Endler's guppy experiments
- Classic ecological experiments demonstrating that natural selection occurs rapidly when predation pressures and sexual selection balance in stream populations.
2Directional vs stabilising vs disruptive selection
- Natural selection modes altering phenotypic distribution depending on environmental selective pressures.
3Selective pressure dynamics
- Biotic and abiotic constraints that favor differential survival and reproductive success of better-adapted phenotypes.
4Evolutionary adaptation
- Gradual accumulation of beneficial inheritable alleles within a population gene pool over successive generations.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Natural selection | The greater survival and reproduction of individuals whose heritable traits suit the environment. |
| Selection pressure | A factor in the environment that affects survival and reproduction. |
| Overproduction | The production of more offspring than the environment can support. |
| Sexual selection | Selection due to one sex preferring certain traits in the other. |
| Sexual dimorphism | A distinct difference in size or appearance between the sexes of a species. |
| Paradigm shift | A major change in scientific thinking when new evidence overturns accepted ideas. |
| Heritable trait | A characteristic determined by alleles that can be passed to offspring. |
| Variation | Differences between individuals caused by differences in DNA and environment. |
Section 12 of 12Exam technique
1
Saying individuals evolve — populations evolve over generations
2
Saying organisms develop traits because they need them — selection acts on existing variation
3
Saying the fittest are certain to survive — they are only more likely to survive and reproduce
4
Saying natural selection produces the best possible organism — it favours traits that suit the current environment
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