DP Biology HL D4.1 Natural Selection

DP Biology HL D4.1 Natural Selection - Full notes

RevisionPrep

DP Biology HL | D4.1 Natural Selection

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D4.1

Natural Selection

Full notes for HL · IB-style

Contents

1	Darwin, Wallace and the theory of natural selection
2	Variation: the raw material
3	Overproduction and the struggle for survival
4	Selection pressures and adaptation
5	Sexual selection
6	Evidence: antibiotic resistance and other examples
7	Gene pools and the Hardy-Weinberg principle (HL only)
8	Types of selection: stabilizing, directional, disruptive (HL only)
9	Darwin's theory of natural selection
10	Sources of variation
11	Overproduction and competition
12	Fitness and adaptation
13	Sexual selection
14	Types of selection
15	Antibiotic resistance as natural selection
16	Peppered moths
17	Artificial selection compared
18	Gene pools and allele frequencies
19	Genetic drift and founder effects
20	Hardy–Weinberg principle (HL)
21	Mixed review: worked problems
22	Additional exam-style practice
23	Misconceptions about natural selection
24	Practical: simulating natural selection
25	Speed of evolution
26	Common misconceptions and links
27	Exam-style question with markscheme
28	Check your understanding
29	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain the theory of natural selection and the contributions of Darwin and Wallace
explain the sources of variation
explain overproduction and competition
explain how selection pressures lead to adaptation
explain sexual selection with examples
explain the evolution of antibiotic resistance and other examples of natural selection
distinguish natural selection from artificial selection
define gene pool and allele frequency
use the Hardy-Weinberg equation to calculate allele and genotype frequencies
distinguish stabilizing, directional and disruptive selection

Sections marked HL only (Sections 7 and 8) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How does natural selection cause populations to change over time? What evidence shows natural selection in action?

How this topic is examined

D4.1 is examined through explanations of natural selection (an extended-response favourite), case studies such as antibiotic resistance and the peppered moth, and data on changes in populations. HL questions include Hardy-Weinberg calculations and graphs of selection types.

A full explanation includes: variation, heritability, overproduction, competition, differential survival and reproduction, and change in allele frequencies over generations.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Darwin, Wallace and the theory of natural selection

Variation plus selection produces change.

Think about it

Why did the giraffe's long neck evolve? Try to explain it without saying the giraffe 'needed' or 'tried' to grow a longer neck.

In 1858 Charles Darwin and Alfred Russel Wallace independently proposed that species evolve by natural selection. Darwin's book On the Origin of Species (1859) set out the evidence, from his voyage on HMS Beagle, including the Galápagos finches, and from selective breeding.

The theory in outline: individuals in a population show variation, much of which is heritable. Populations overproduce offspring, more than the environment can support, so there is competition for limited resources. Individuals with variations that make them better adapted to their environment are more likely to survive and reproduce (differential survival and reproduction). They pass on their heritable characteristics to their offspring. Over many generations, the frequency of the favourable characteristics (and the alleles that cause them) increases in the population. The population evolves.

Natural selection is not random: it consistently favours characteristics that improve survival and reproduction in a particular environment. But the variation it acts on arises randomly, by mutation.

Figure 1. The process of natural selection.

Quick check

1. Who proposed the theory of natural selection?

2. What is meant by differential survival and reproduction?

Answers: 1. Darwin and Wallace 2. better-adapted individuals survive and reproduce more

2 Variation: the raw material

Without variation, selection has nothing to choose between.

Natural selection can only act on heritable variation: differences in genotype that are passed to offspring. The sources of genetic variation are: mutation, which creates new alleles (D1.3); meiosis, through crossing over and random orientation of chromosomes (D2.1); and random fertilization, combining alleles from two parents (D3.1).

Variation caused only by the environment (such as muscles developed by exercise) is not heritable and cannot be selected. This is why Lamarck's idea of the inheritance of acquired characteristics is rejected.

Common mistake

Saying organisms mutate in order to adapt. Mutations occur randomly; selection then favours the useful ones that happen to exist.

3 Overproduction and the struggle for survival

More offspring are born than can survive.

Most species produce far more offspring than can survive to reproduce. A single cod lays millions of eggs; a pair of rabbits can produce dozens of young per year. Yet population sizes tend to stay roughly stable over time, because resources (food, space, mates, nesting sites) are limited. There is therefore competition, both within species (intraspecific) and between species, as well as death from predators, disease and harsh conditions.

This 'struggle for existence' (Darwin's phrase, borrowed from Malthus) means that only some individuals survive long enough to reproduce. Which ones survive is not entirely chance: those with advantageous variations are more likely to.

4 Selection pressures and adaptation

The environment decides which variations are favourable.

A selection pressure is any factor in the environment that affects which individuals survive and reproduce: predators, disease, climate, food availability, competition. Different environments impose different pressures, so different characteristics are favoured.

An adaptation is a heritable characteristic that makes an organism suited to its environment (B4.1). Adaptations accumulate gradually over many generations as natural selection repeatedly favours individuals with them.

The peppered moth (Biston betularia) is a classic example. Before the Industrial Revolution, most moths in England were pale, camouflaged against lichen-covered tree bark. Pollution killed lichens and blackened trees with soot; dark (melanic) moths were then better camouflaged from bird predators and became common (over 90% in some industrial areas by 1900). After clean-air laws in the 1950s, trees became paler again and the frequency of dark moths fell.

Figure 2. The rise and fall of dark peppered moths.

Quick check

1. Define selection pressure.

2. Explain the rise of dark peppered moths.

Answers: 1. an environmental factor affecting survival and reproduction 2. soot-darkened trees; dark moths camouflaged; less predation; more reproduction

5 Sexual selection

Characteristics that help win mates.

Sexual selection is a form of natural selection in which individuals with certain characteristics are more successful at obtaining mates. It explains features that do not help survival, or even reduce it, but increase reproductive success.

Mate choice (usually by females) favours conspicuous traits that may signal good genes or health: the peacock's tail, bright plumage of birds of paradise, elaborate courtship displays. Competition between males favours size, strength and weapons such as antlers in deer.

Sexual selection explains why males and females of many species look different (sexual dimorphism).

In the real world

Male birds of paradise in New Guinea perform elaborate dances and display brilliantly coloured plumage. Females choose mates based on these displays, so the genes for showy plumage spread even though they make males more visible to predators.

6 Evidence: antibiotic resistance and other examples

Evolution observed in real time.

Antibiotic resistance in bacteria is natural selection that can be observed within years. In a population of bacteria, a random mutation may make a few resistant to an antibiotic. When the antibiotic is used, susceptible bacteria are killed but resistant ones survive and reproduce rapidly, so the population becomes mostly resistant. Resistance genes also spread between bacteria on plasmids. Overuse of antibiotics in medicine and livestock increases the selection pressure (C3.2).

Other examples: pesticide resistance in insects; resistance to myxomatosis in Australian rabbits; changes in beak size of Galápagos finches after droughts (studied by Peter and Rosemary Grant), where birds with larger beaks survived because only large, hard seeds were available.

Natural selection vs artificial selection: in artificial selection (selective breeding, A4.1), humans choose which individuals reproduce for characteristics they want; in natural selection, the environment determines which individuals survive and reproduce.

Figure 3. Rising proportion of antibiotic-resistant bacterial isolates in a hospital.

Quick check

1. Explain how antibiotic resistance evolves.

2. Distinguish natural and artificial selection.

Answers: 1. random mutation; antibiotic kills susceptible; resistant survive and reproduce 2. environment selects vs humans select

7 Gene pools and the Hardy-Weinberg principle (HL only)

Measuring evolution as changes in allele frequency.

HL only

This section is Additional Higher Level content. SL students do not need it.

A gene pool is all the genes and their different alleles in an interbreeding population. Allele frequency is the proportion of all copies of a gene that are a particular allele. Evolution can be defined as a change in allele frequencies in a gene pool over time.

The Hardy-Weinberg principle describes a population that is not evolving. For a gene with two alleles, with frequencies p (dominant) and q (recessive): p + q = 1 and p² + 2pq + q² = 1, where p² is the frequency of homozygous dominant individuals, 2pq heterozygotes and q² homozygous recessive.

The principle holds only if: the population is large; mating is random; there is no migration; there are no mutations; and there is no natural selection. If observed genotype frequencies differ from those predicted, one of these conditions is not met: the population may be evolving.

WORKED EXAMPLE 1 Hardy-Weinberg calculation

Cystic fibrosis (recessive) affects 1 in 2 500 people in a population. Calculate the frequency of carriers.

Solution

q² = 1 ÷ 2 500 = 0.0004, so q = 0.02M1 A1

p = 1 − 0.02 = 0.98A1

Carriers = 2pq = 2 × 0.98 × 0.02 = 0.039 (about 1 in 25)A1

8 Types of selection: stabilizing, directional, disruptive (HL only)

Selection can shift, narrow or split a population.

HL only

This section is Additional Higher Level content. SL students do not need it.

Stabilizing selection favours intermediate phenotypes and selects against extremes. The variation narrows; the mean stays the same. Example: human birth weight: very small and very large babies historically had lower survival.

Directional selection favours one extreme, so the mean shifts in that direction. Examples: antibiotic resistance; the peppered moth; increasing beak size of finches after drought.

Disruptive selection favours both extremes over intermediates, which may eventually lead to two separate populations and speciation (A4.1). Example: in some African finches (black-bellied seedcrackers), birds with small or large beaks feed efficiently on soft or hard seeds; intermediates do poorly.

Figure 4. Types of selection: the dashed line shows the original distribution, the solid line after selection.

Quick check

1. State the two Hardy-Weinberg equations.

2. Give the conditions for Hardy-Weinberg equilibrium.

3. Which type of selection can lead to speciation?

Answers: 1. p + q = 1; p² + 2pq + q² = 1 2. large population, random mating, no migration, mutation or selection 3. disruptive

9 Darwin's theory of natural selection

The key observations.

Darwin and Wallace observed: populations produce more offspring than can survive; individuals vary; much variation is heritable; there is competition for limited resources. Individuals with advantageous traits are more likely to survive and reproduce, passing on their alleles. Over generations, allele frequencies change.

WORKED EXAMPLE 2 Steps (exam level)

Outline the process of natural selection.

Solution

Variation exists in a population; overproduction of offspringR1

Competition; better adapted individuals survive and reproduceR1

Advantageous alleles passed on; frequencies change over generationsR1

10 Sources of variation

The raw material.

Variation arises from mutation (new alleles), meiosis (crossing over and independent assortment) and random fertilization. Only heritable variation can be acted on by natural selection.

Quick check

1. Why is mutation the ultimate source of variation?

Answers: 1. it creates new alleles; meiosis and fertilization only reshuffle existing ones

11 Overproduction and competition

A struggle for existence.

Most species produce far more offspring than survive: a cod releases millions of eggs. Limited resources (food, space, mates) cause competition, so only some individuals survive to reproduce.

WORKED EXAMPLE 3 Overproduction (standard)

Explain why overproduction is essential to natural selection.

Solution

Not all offspring can surviveR1

Creates competition so selection can actR1

12 Fitness and adaptation

Reproductive success.

Biological fitness is the ability to survive and produce fertile offspring. Adaptations are heritable traits that increase fitness in a particular environment; what is advantageous depends on the environment.

Quick check

1. Define biological fitness.

Answers: 1. the ability to survive and pass on alleles to the next generation

13 Sexual selection

Competing for mates.

Sexual selection favours traits that increase mating success, even if they reduce survival: peacock tails, deer antlers, bird songs. Females choose mates (intersexual selection) or males compete (intrasexual selection).

WORKED EXAMPLE 4 Peacocks (standard)

Explain how the peacock's tail evolved although it makes escape harder.

Solution

Females prefer males with large, colourful tailsR1

These males have more offspring, spreading tail allelesR1

In the real world

Experiments on guppies in Trinidad showed males became more colourful in streams with few predators and duller where predators were common.

14 Types of selection

Stabilizing, directional, disruptive.

Type	Effect	Example
stabilizing	favours average phenotype; reduces variation	human birth mass
directional	favours one extreme; mean shifts	antibiotic resistance, beak size in droughts
disruptive	favours both extremes	beak sizes in seed-cracker finches

WORKED EXAMPLE 5 Classifying (standard)

Babies of very low or very high birth mass have higher mortality. Classify the selection.

Solution

Stabilizing selectionA1

15 Antibiotic resistance as natural selection

Evolution in action.

A random mutation gives a bacterium resistance. When antibiotics are used, susceptible bacteria die and resistant ones survive and reproduce. Resistance genes spread by vertical (reproduction) and horizontal (plasmids) transfer.

WORKED EXAMPLE 6 MRSA (exam level)

Explain how overuse of antibiotics has led to MRSA.

Solution

Random mutation produced resistant bacteriaR1

Antibiotics killed susceptible bacteria (selection pressure)R1

Resistant bacteria multiplied and spreadR1

16 Peppered moths

A classic example.

During the industrial revolution, soot darkened tree trunks in Britain; dark (melanic) moths were better camouflaged from birds and increased in frequency. After clean-air laws, pale moths increased again.

WORKED EXAMPLE 7 Peppered moth (standard)

Explain the change in frequency of dark moths during the industrial revolution.

Solution

Dark moths camouflaged on sooty trees, less predationR1

More survived to reproduce; dark allele frequency increasedR1

17 Artificial selection compared

Humans as the selective agent.

In artificial selection, humans choose which individuals breed, based on desired traits. It works in the same way as natural selection but the selective agent is human choice, and it can produce rapid change.

Quick check

1. Give an example of artificial selection.

Answers: 1. breeding of high-yield wheat or dog breeds

18 Gene pools and allele frequencies

Population genetics.

A gene pool is all the alleles in an interbreeding population. Evolution is a change in allele frequencies. Allele frequency = number of copies of the allele/total number of alleles for that gene.

WORKED EXAMPLE 8 Allele frequency (standard)

In 100 individuals, 36 are AA, 48 Aa and 16 aa. Calculate the frequency of a.

Solution

a alleles = 48 + 2 × 16 = 80 out of 200M1

= 0.40A1

19 Genetic drift and founder effects

Chance changes.

Genetic drift is random change in allele frequencies, strongest in small populations. The founder effect occurs when a few individuals start a new population; the bottleneck effect when a population is drastically reduced. Both reduce genetic diversity.

Quick check

1. Why is genetic drift stronger in small populations?

Answers: 1. chance events affect a larger proportion of the gene pool

20 Hardy–Weinberg principle (HL)

Testing for evolution.

If a population is large, randomly mating, with no mutation, migration or selection, allele frequencies stay constant: p + q = 1 and p2 + 2pq + q2 = 1. Deviations indicate that evolution is occurring.

WORKED EXAMPLE 9 Carriers (exam level)

1 in 2500 people has cystic fibrosis (recessive). Estimate the carrier frequency.

Solution

q2 = 1/2500: q = 0.02; p = 0.98M1

2pq = 2 × 0.98 × 0.02 = 0.039 (about 1 in 25)A1

21 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 10 Mixed problem (exam level)

Explain why a population of insects becomes resistant to a pesticide over several generations.

Solution

Variation: a few insects carry a resistance allele (mutation)R1

Pesticide kills susceptible insectsR1

Resistant survivors reproduce; allele frequency increasesR1

22 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 11 Paper 1 question (standard)

Which type of selection reduces variation around the mean? A. directional B. disruptive C. stabilizing D. sexual

Solution

CA1

WORKED EXAMPLE 12 Paper 2 question (exam level)

Distinguish natural selection from artificial selection.

Solution

Natural: environment determines survival and reproductionR1

Artificial: humans choose which organisms breedR1

23 Misconceptions about natural selection

What it is not.

Organisms do not evolve because they need to; individuals do not evolve during their lifetime; natural selection does not produce perfect organisms; evolution has no goal. Populations evolve as allele frequencies change across generations.

WORKED EXAMPLE 13 Correcting (standard)

Correct the statement: 'Giraffes grew long necks because they needed to reach high leaves'.

Solution

Variation in neck length existed; longer-necked giraffes survived and reproduced moreR1

Over generations, alleles for long necks became more commonR1

Common mistake

Writing 'the bacteria became resistant to survive'. Resistance arose by random mutation; the antibiotic selected the resistant bacteria.

24 Practical: simulating natural selection

Camouflage model.

Scatter coloured paper 'prey' on a patterned background and let 'predators' pick them quickly. Prey colours that match the background survive more; 'reproduce' survivors and repeat over several generations.

WORKED EXAMPLE 14 Simulation (standard)

What does the simulation show?

Solution

Better camouflaged prey survive; their frequency increasesR1

Try this

Run a camouflage simulation with coloured paper dots on newspaper and record how colour frequencies change over four generations.

25 Speed of evolution

Generations matter.

Evolution is fastest in organisms with short generation times and large populations (bacteria, insects). Large mammals evolve more slowly, although strong selection can cause rapid change, such as reduced tusk size in elephants under ivory poaching.

Quick check

1. Why has tusklessness increased in some elephant populations?

Answers: 1. poachers kill tusked elephants; tuskless ones survive and reproduce

26 Common misconceptions and links

Students often think…	Actually…
organisms adapt because they need to	random variation is selected; no intention
individuals evolve	populations evolve over generations
mutations are caused by the antibiotic	resistant mutants already exist and are selected
Hardy-Weinberg describes evolution	it describes a population that is not evolving

How D4.1 connects to the rest of the course

Topic	Connection
A4.1 Evolution and speciation	evidence and speciation
D1.3 Mutations	source of variation
D2.1 Meiosis	variation
C3.2 Defence	antibiotic resistance
D3.2 Inheritance	alleles

27 Exam-style question with markscheme

Try this under exam conditions (about 18 minutes) before reading the markscheme.

Figure 5. Antibiotic-resistant isolates in a hospital.

(a) Describe the change in resistant isolates from 2000 to 2020. [2]

(b) Explain the evolution of antibiotic resistance. [4]

(d) Distinguish natural selection from artificial selection. [2]

(e) A recessive phenotype occurs in 16% of a population. Calculate the frequency of heterozygotes. [3]

Markscheme

Part	Answer	Marks
(a)	increased from 5% to 38%	A1
	steady increase	A1
(b)	variation: random mutation gives resistance	R1
	antibiotic kills susceptible bacteria	R1
	resistant survive and reproduce	R1
	resistance allele frequency increases; plasmid transfer	R1
(c)	traits increase mating success	R1
	mate choice / male competition	R1
	e.g. peacock tail / birds of paradise	A1
(d)	natural: environment selects	A1
	artificial: humans select	A1
(e)	q² = 0.16, q = 0.4	M1
	p = 0.6	A1
	2pq = 0.48	A1

Examiner's comments. In (b), answers that said bacteria 'became resistant to survive' scored poorly: resistance pre-exists and is selected.

28 Check your understanding

Section A: multiple choice

1. Natural selection acts on

A. heritable variation B. acquired characteristics C. all variation equally D. individuals' needs

2. The original source of new alleles is

A. natural selection B. competition C. mutation D. overproduction

3. The peppered moth is an example of

A. sexual selection B. directional selection C. artificial selection D. genetic drift

4. Sexual selection explains

A. antibiotic resistance B. camouflage C. beak size after drought D. the peacock's tail

5. Overproduction leads to

A. mutation B. competition C. speciation directly D. less variation

6. Antibiotic resistance spreads between bacteria on

A. plasmids B. ribosomes C. spores D. flagella

7. If q = 0.3, the frequency of homozygous recessive individuals is

A. 0.3 B. 0.42 C. 0.49 D. 0.09

8. Stabilizing selection

A. favours one extreme B. favours both extremes C. favours intermediate phenotypes D. has no effect

Section B: structured

9. Explain the theory of natural selection. [6]

10. Explain how the peppered moth provides evidence for natural selection. [4]

11. Explain why variation is essential for natural selection. [2]

12. Distinguish stabilizing, directional and disruptive selection. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. Variation; heritability; overproduction; competition; better adapted survive and reproduce; alleles passed on; frequencies change.

10. Pale and dark forms; pollution changed bark colour; predation; frequency changes; reversal after clean air.

11. Selection needs differences to act on; without heritable variation, no change in allele frequencies.

12. Stabilizing: intermediates favoured; directional: one extreme; disruptive: both extremes.

29 Summary and topic map

Idea	Remember
Theory	Darwin and Wallace, 1858
Steps	variation, overproduction, competition, differential survival, inheritance
Variation	mutation, meiosis, fertilization
Selection pressure	predators, disease, climate
Sexual selection	mate choice, competition
Evidence	antibiotic resistance, peppered moth, finches
HL	gene pool; Hardy-Weinberg; stabilizing, directional, disruptive

Before the exam, make sure you…

•include every step in natural selection answers
•avoid 'need' and 'try' language
•use allele frequency in explanations
•give named examples

Natural Selection on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 6. Topic map for Natural Selection.

revisionprep.comIB-style notes · question bank · mock papers