كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

D4.1Theme D — Continuity and change

Natural selection — IB Biology HL Revision Notes & Topical Questions

10 min read 12 parts Lesson 11 of 18 2 quizzes

📚 IB Biology HL Hub / Topical Revision

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.

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Lesson summary

D4.1 Natural selection — IB BIOLOGY HL notes: Explain natural selection, sources of variation, selection pressures, gene pools, types of selection, the Hardy-Weinberg principle and artificial selection.Evolution = changes in the heritable characteristics of organisms over generat…

Nature of science
Selection needs variation
Sources of DNA sequence differences
Crossing over
Random orientation (independent assortment)
Random fertilisation

Part 1 of 10Natural selection and evolution

Evolution

Evolution changes in the heritable characteristics of organisms over generations.

Natural selection

Natural selection the process by which better adapted organisms survive, reproduce and pass on advantageous alleles,

so advantageous characteristics increase in frequency.

Darwin

Darwin he proposed the theory after a five-year expedition around the world and published On the Origin of Species in 1859.
Individuals with advantageous alleles are more likely to survive and reproduce, so those alleles are passed on.
The frequency of the favourable characteristic rises until it is the most common, so the population is adapted to its environment.
Unfavourable characteristics fall in frequency and are eventually lost.

Nature of science

Darwin's theory caused a paradigm shift,
which happens when research contradicts previous assumptions, and it replaced Lamarckism.
- (inheritance of acquired characteristics)
Nearly 200 years of genetic research support it.

Rabbit example

Fur colour varies, with one allele for brown and one for white.

Predators such as foxes act as a selection pressure.

Brown rabbits are better camouflaged, survive and reproduce more.

The frequency of brown alleles rises and that of white alleles falls over generations.

Part 2 of 10Variation: mutation and variation: meiosis and fertilisation

Variation

Variation differences between organisms of one species, such as coat colour in mammals, body length in fish and flower colour in plants.

Selection needs variation

Selection needs variation if all individuals were identical, none would be favoured and the population could not adapt.

Sources of DNA sequence differences

Sources of DNA sequence differences mutation, meiosis and random fertilisation.

Mutation

Mutation the original source of variation, a change in DNA from a copying error in replication, which produces new alleles.
Mutations in dividing cells of sex organs change the alleles of gametes passed on.
A new allele may be advantageous, disadvantageous or neutral, and advantageous alleles are more likely to be passed on.
Somatic mutations are not passed on and have no effect on natural selection.
Mutation is the only source of variation in asexual species.

Variation: mutation and variation: meiosis and fertilisation

Crossing over

Crossing over in meiosis I homologous chromosomes pair, and non-sister chromatids break and rejoin, giving new allele combinations.

Random orientation (independent assortment)

Random orientation (independent assortment) homologous pairs line up at the equator in metaphase I, each pair independently with either chromosome on either side.
The number of combinations is 2ⁿ for n haploid chromosomes, so humans have 2²³ = 8 388 608.

Random fertilisation

Random fertilisation any male gamete can fuse with any female gamete,
so each zygote has a unique allele combination, and successive sexual offspring are almost never genetically identical.

Part 3 of 10Overproduction and competition

Overproduction and competition

Overproduction of offspring

Overproduction of offspring species produce few young or many (some ants lay 3–4 million eggs), and usually more than the environment can support.
- (elephants, one at a time)
Limits such as food, space and light set the carrying capacity.
Many offspring fail to survive and reproduce, promoting natural selection.

Competition

Competition it can be interspecific or intraspecific.
Intraspecific competition matters more in evolution because members of a species meet more often, share the same niche and face the same factors.

How it promotes selection

Variation is present.

Some individuals are better adapted, such as lions that are stronger and faster and so more likely to catch prey ('survival of the fittest').

Well-adapted survivors are more likely to find a mate and have many offspring.

Poorly adapted individuals reproduce less or not at all.

Better adapted organisms are more likely, not guaranteed, to survive.

Part 4 of 10Heritable traits and selection pressures: abiotic factors and sexual selection

Heritable traits and selection pressures: abiotic factors and sexual selection

Heritable characteristics

Heritable characteristics they are set by alleles and can be physical or behavioural (a woodlouse's innate move to dark places).
- (a giraffe's neck length)

Individuals with beneficial heritable traits leave more offspring and pass on the alleles.

Non-heritable characteristics

Non-heritable characteristics they are acquired in a lifetime and are not passed on.
- (such as gaining weight on autumn nuts or an injury)

Abiotic factors

Abiotic factors non-living factors such as temperature or rainfall, sometimes density-independent, can act as selection pressures that change population size.

Sexual selection

Sexual selection selection due to one sex's preference for certain characteristics in the other sex.

Variation in visible physical and behavioural traits can signal fitness, so it affects success in attracting a mate.
Effects include reproductive isolation and sexual dimorphism.

Reproductive isolation

Reproductive isolation changes in alleles and phenotypes stop some individuals breeding with others,
- such as a courtship behaviour change that females no longer find attractive.

Sexual dimorphism

Sexual dimorphism a distinct difference in size or appearance between the sexes,

as in birds of paradise.
- (bright, displaying males and grey-brown females)
A trait such as a peacock's vibrant, healthy tail suggests low disease burden.
Females choose males with the trait, and the genes become more common.

Part 5 of 10Natural versus sexual selection

Feature	Natural selection	Sexual selection
Competition	For resources	For mates
Result	Individuals well adapted to the environment	Enhanced mating success, not always adaptation

On islands with plentiful resources and few predators, females are often the main selection pressure on males.

Peacocks

Peacocks long, elaborate tails improve mating success but reduce agility and flight and make the bird easier to spot, increasing predation risk.

Strength

Strength sexual selection can be stronger than natural selection because variation in mating success amplifies selection and maintains genetic variation,
giving rapid change.

Darwin regarded it as his second greatest insight, to explain traits natural selection does not favour.

Part 6 of 10Case study: guppies

Guppies

Guppies fish of mountain streams in Trinidad and Tobago with colour variation and sexual dimorphism, with bright males and dull females.
Spots camouflage them against the gravel of the streambed, which is finer or coarser in different streams.

John Endler's hypotheses (1970s)

With predators, substrate type affects survival and changes spot brightness.
With low predation, male spot brightness rises through sexual selection.
As predation increases, spot brightness falls.

Laboratory experiment

Guppies bred freely without predation for six months in large tanks, and spots were counted and measured.

They were assigned randomly to ten greenhouse ponds, five with coarse and five with fine gravel.

Two ponds had no predators, four had a dangerous predator (pike cichlid) and four had a weak predator (killifish).

After five months the spots were counted, and again after nine more months.

Results

High predation reduced the mean number of spots, and low predation allowed it to keep rising.
Coarse gravel favoured larger spots and fine gravel smaller spots, so spot size mimicked gravel size.
With no predation the opposite occurred, as not matching the background made males conspicuous and attractive to females.

Field experiment

Field experiment dull males moved from high to low predation areas for 15 generations (two years) became more colourful.

Interpretation

Interpretation evolution is a dynamic trade-off of natural and sexual selection.
Predators see bright males more easily, so natural selection favours duller fish.
Sexual selection favours bright, large spots for mates, but with high predation bright males survive less whatever their reproductive advantage.

Part 7 of 10Gene pools and allele frequency

1Gene pool

Gene pool all the genes and alleles in an interbreeding population, which governs the next generation's genomes.

Isolated populations of one species have separate gene pools.

2Stable gene pool conditions

Stable gene pool conditions a large population, equal chance of mating, random mating and no selection on phenotype.
A stable gene pool means the population is not evolving.

3Neo-Darwinism

Darwin knew nothing of genetics, and modern science explains evolution at the molecular level.
Allele frequencies change by natural selection, sexual selection and genetic drift.

4Allele frequency

Allele frequency the relative frequency of an allele at a locus.

Alleles can differ by one nucleotide, and the frequencies of all alleles of a gene add up to 1.
Geographic separation causes differences, but isolated human populations are rare because of travel.
Alleles are polymorphisms, most commonly SNPs, and online databases list human allele frequencies.
The Hardy-Weinberg formula converts between phenotype and allele frequencies.

5Uses

Uses finding genetic associations with disease, estimating disease susceptibility, estimating drug resistance and evolutionary or anthropological studies.

6Counting alleles

Counting alleles each individual has two alleles per locus, so a population has twice as many alleles as individuals.
- (not for X or Y genes)
Homozygotes contribute two copies of an allele and heterozygotes one.

Part 8 of 10Types of natural selection

Type	Effect	Conditions	Example
Directional	Population shifts to one extreme	Environmental change	Frost-resistant alleles after a fall in temperature; warming seas select smaller fish because large fish need more oxygen
Stabilising	Favours the average individual, discards extremes	Stable environment, most common	Mouse coat colour matching the forest floor; human birth mass
Disruptive	Selects against the average	Habitat or resource change, rarest	Beak sizes of Darwin's finches in the Galápagos

Types of natural selection

Directional selection

Directional selection a disadvantageous allele need not be fatal but must stop successful reproduction, so it leaves the gene pool.
An allele might code for a protein that lowers the freezing point of cell contents and prevents damaging ice crystals.

Stabilising selection

Stabilising selection it reduces diversity, acts mostly on polygenic traits and gives a bell-shaped curve.

Disruptive selection

Disruptive selection it can lead to speciation, so it is called diversifying selection.
Fifteen finch species arose from a common ancestor with different beaks for different food (seeds, fruit, leaves, arthropods).
Diets overlap when food is abundant, and specialisation reduces competition when food is scarce, giving each species a niche.

Part 9 of 10Hardy-Weinberg principle

Principle

Principle allele frequencies of a gene stay constant between generations if the conditions are met.

Conditions

Organisms are diploid.
Reproduction is sexual only.
Generations do not overlap.
Mating is random.
The population is large.
There is no migration, mutation or selection.
Allele frequencies are equal in both sexes.

If genotype frequencies do not fit, one or more conditions are not met, and the conditions are rarely all met in nature.

Equations (for alleles B and b)

p = frequency of the dominant allele and q = frequency of the recessive allele, and p + q = 1.
p² = homozygous dominant, 2pq = heterozygous and q² = homozygous recessive, so p² + 2pq + q² = 1.
Frequencies are proportions of 1, such as 120 dominant alleles out of 200 giving p = 0.6.

Calculation method

Find q², the recessive phenotype frequency, because it is the only phenotype whose genotype is known.

Take the square root to get q.

Find p

1 − q.

Calculate p² and 2pq.

Check that the three genotype frequencies add up to 1.

Principle versus equations
- the equations estimate frequencies, and the principle states they remain in equilibrium.

Part 10 of 10Artificial selection

1Artificial selection (selective breeding)

Artificial selection (selective breeding) humans choose organisms with desirable traits and breed them together over many generations.

Humans have done this for thousands of years before genetics was understood.
Breeders choose by phenotype, so they may unknowingly enhance linked traits that harm health.

Process

The population shows phenotypic variation.

The breeder selects an individual with the desired phenotype.

A second individual with the phenotype is selected, not closely related.

The two are bred together.

Mature offspring are tested, and those showing the trait most are selected for further breeding.

Repeat for many generations until all offspring show the trait.

Animals

Animals higher milk or meat yield in cows, goats and sheep, large eggs in chickens, gentle domestic dogs, good wool in sheep and fine, fast horses.
- (all descended from wolves)

Plants

Plants disease resistance, higher yield, hardiness such as drought tolerance, better-tasting fruit and large flowers.
Wild brassica gave cauliflower, cabbage, broccoli, Brussels sprouts, kale and kohlrabi.

Natural versus artificial selection

Artificial selection involves deliberate human choice of traits, while natural selection does not.
Antibiotic resistance arises by natural selection, as an unintended result of antibiotic overuse.
Natural selection produces animals well adapted to their environment, while artificial selection can produce animals poorly suited to it.

Key terms

Term	Meaning
Natural selection	survival and reproduction of better adapted organisms, raising the frequency of their alleles.
Sexual selection	selection due to mate choice for certain traits.
Gene pool	all the alleles of all genes in an interbreeding population.
Directional selection	selection favouring one extreme phenotype.
Stabilising selection	selection favouring the average phenotype.
Disruptive selection	selection favouring both extreme phenotypes.
Allele frequency	the proportion of all alleles of a gene that are of one form.
Hardy-Weinberg equation	p² + 2pq + q² = 1, relating allele and genotype frequencies.
Artificial selection	human selection of breeding individuals for desired traits.

Avoid these mistakes

Individuals evolve during their lifetime — populations evolve as allele frequencies change.

Better adapted organisms are certain to survive — they are only more likely to survive and reproduce.

Sexual selection always improves survival — traits such as long peacock tails can lower survival.

Hardy-Weinberg holds in real populations — its conditions are rarely all met.

Last updated: 18 October 2026