DP Biology SL A4.1 Evolution and Speciation

DP Biology SL A4.1 Evolution and Speciation - Full notes

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DP Biology SL | A4.1 Evolution and Speciation

DP BIOLOGY · THEME A: UNITY AND DIVERSITY

A4.1

Evolution and Speciation

Full notes for SL · IB-style

Contents

1	What evolution is
2	Evidence from base and amino acid sequences
3	Evidence from selective breeding
4	Evidence from homologous structures and convergent evolution
5	Speciation by reproductive isolation and differential selection
6	Allopatric speciation
7	Evolution defined
8	Evidence from DNA and protein sequences
9	Evidence from selective breeding
10	Evidence from homologous structures
11	Convergent evolution and analogous structures
12	Vestigial structures and embryology
13	The fossil record
14	Speciation: the role of isolation
15	Allopatric speciation
16	Sympatric speciation
17	Adaptive radiation
18	Hybridization and barriers
19	Mixed review: worked problems
20	Additional exam-style practice
21	Rates of evolution
22	Evolution observed today
23	Common misconceptions and links
24	Exam-style question with markscheme
25	Check your understanding
26	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
define evolution as change in heritable characteristics of a population
explain how sequence data provide evidence for evolution
explain how selective breeding provides evidence for evolution
distinguish homologous and analogous structures, using the pentadactyl limb
explain speciation through reproductive isolation and differential selection
explain allopatric speciation with an example

This booklet covers the SL content of this subtopic. Additional Higher Level material is not included.

Guiding questions

What is the evidence that species change over time and share common ancestors? How does one species split into two?

How this topic is examined

A4.1 is examined through definitions (evolution, speciation), evidence questions (sequences, homologous structures, selective breeding) and case studies of speciation. Paper 2 often provides sequence data or a map of populations to interpret.

Precise use of the words 'population', 'heritable', 'reproductive isolation' and 'gene pool' is essential.

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 What evolution is

Change in the heritable characteristics of a population over time.

Think about it

A giraffe stretches its neck all its life to reach high leaves. Will its offspring be born with longer necks? Why or why not?

Evolution is the change in the heritable characteristics of a population over time. 'Heritable' means passed on in genes from parents to offspring. Characteristics acquired during an individual's life, such as stronger muscles from exercise or a stretched neck, are not inherited and do not cause evolution. This was the flaw in Lamarck's earlier theory of the inheritance of acquired characteristics.

Evolution happens to populations, not individuals. An individual's genes do not change during its life; what changes is the frequency of alleles in the population from generation to generation. The theory of evolution by natural selection, proposed by Charles Darwin and Alfred Russel Wallace in 1858, explains how this happens (D4.1).

Evolution is supported by many independent lines of evidence. This subtopic covers molecular evidence, selective breeding and comparative anatomy.

Common mistake

Saying 'an animal evolved to survive the cold'. Individuals do not evolve; populations do, as alleles that improve survival become more frequent over generations.

Quick check

1. Define evolution.

2. Why are acquired characteristics not part of evolution?

Answers: 1. change in heritable characteristics of a population over time 2. they are not passed on in genes

2 Evidence from base and amino acid sequences

The more similar the sequences, the more recent the common ancestor.

All organisms use DNA and the same genetic code, which suggests that they share a common ancestor. When we compare the base sequences of genes, or the amino acid sequences of proteins, between species, we find that closely related species have very similar sequences, while distantly related species have more differences.

Differences accumulate gradually as mutations occur and are passed on. So the number of differences reflects how long ago two species shared a common ancestor. For example, the haemoglobin of humans and chimpanzees is identical, that of humans and gorillas differs by one amino acid, and that of humans and horses by many more.

Sequence evidence is powerful because it is objective, can be gathered for thousands of genes, and is independent of anatomical evidence. When different genes give the same pattern of relationships, this strongly supports common ancestry.

Species compared with human	Amino acid differences in haemoglobin β chain (146 amino acids)
Chimpanzee	0
Gorilla	1
Rhesus monkey	8
Dog	15
Frog	67

Quick check

1. Why do closely related species have more similar sequences?

2. Why is sequence evidence considered strong?

Answers: 1. less time since divergence for mutations to accumulate 2. objective, many genes, independent of anatomy

3 Evidence from selective breeding

Humans have caused rapid evolution in domesticated species.

Selective breeding (artificial selection) is the deliberate choice by humans of which individuals reproduce, so that desired heritable characteristics become more common. Over many generations, this has produced large changes in domesticated species.

Wild cabbage (Brassica oleracea) has been bred into cabbage, broccoli, cauliflower, kale, kohlrabi and Brussels sprouts, each by selecting for enlargement of a different part of the plant. All modern dog breeds descend from wolves. Modern maize was bred from a wild grass, teosinte, with tiny cobs.

Selective breeding shows that the heritable characteristics of a population can change over time when some individuals reproduce more than others, and that such changes can be large and fast. Natural selection works in the same way, but the 'selector' is the environment rather than humans.

In the real world

Teosinte has ears about 5 cm long with 5 to 12 hard seeds. Over about 9 000 years of selective breeding by farmers in Mexico, it became modern maize, with cobs 20 to 30 cm long carrying hundreds of soft kernels.

Quick check

1. Define selective breeding.

2. How does selective breeding provide evidence for evolution?

Answers: 1. humans choose which individuals breed for desired heritable traits 2. shows heritable traits of populations change when reproduction is unequal

4 Evidence from homologous structures and convergent evolution

Same bones, different uses.

Homologous structures are structures that have the same basic structure because they were inherited from a common ancestor, even though they may now have different functions. The classic example is the pentadactyl limb (five-fingered limb) of vertebrates: the arm of a human, the wing of a bat, the flipper of a whale and the leg of a horse all contain the same bones (humerus, radius and ulna, carpals, metacarpals and phalanges), adapted for different uses. This shared plan is best explained by descent from a common ancestor that had this limb structure; the differences arose by adaptive radiation as descendants adapted to different ways of life.

Analogous structures have similar functions but different structures and origins. They arise by convergent evolution: unrelated organisms evolve similar features because they face similar selection pressures. The wings of insects and birds, or the streamlined bodies of sharks (fish) and dolphins (mammals), are analogous. Analogous structures do not indicate close relationship.

Figure 1. Bird, bat and insect wings: bird and bat wings are homologous as forelimbs; insect wings are analogous.

	Homologous structures	Analogous structures
Origin	common ancestor	independent evolution
Structure	same basic plan	different
Function	may differ	similar
Process	divergent (adaptive radiation)	convergent evolution
Example	pentadactyl limb	insect and bird wings

Quick check

1. Define homologous structures.

2. What is convergent evolution?

3. Give an example of analogous structures.

Answers: 1. same structure from a common ancestor, may have different functions 2. unrelated organisms evolving similar features under similar pressures 3. insect and bird wings; shark and dolphin body shape

5 Speciation by reproductive isolation and differential selection

A species splits when populations stop interbreeding and diverge.

Speciation is the formation of a new species from an existing one. It requires two things: reproductive isolation between populations, so that genes no longer flow between them; and differential selection (or genetic drift), so that the separated populations change in different ways.

Once populations no longer interbreed, mutations, selection and drift act independently on each gene pool (all the alleles in a population). Over many generations the populations diverge, until they could not interbreed even if they met again. At that point they are separate species.

Speciation adds to the total number of species. Extinction removes species. The current diversity of life results from the balance between them over billions of years.

Figure 2. The stages of speciation.

Quick check

1. Define speciation.

2. What two conditions are needed for speciation?

Answers: 1. formation of a new species from an existing one 2. reproductive isolation and differential selection

6 Allopatric speciation

A geographical barrier divides the population.

Allopatric speciation happens when a population is split by a geographical barrier, such as a mountain range, a river, a stretch of ocean or a glacier. The separated populations cannot interbreed, experience different environments and selection pressures, and diverge.

Example: bonobos and chimpanzees. The Congo River formed about 1 to 2 million years ago and separated an ancestral ape population. Chimpanzees evolved north of the river and bonobos south of it. Neither species can swim, so the river acted as a barrier. Different food supplies and competition on each side are thought to have led to differences in behaviour and body form.

Other examples include the many species of finches on different Galápagos islands, and species separated by the formation of the Isthmus of Panama, which split marine populations into Pacific and Caribbean groups.

In the real world

Snapping shrimp (Alpheus) on either side of the Isthmus of Panama form 'twin' species: pairs that look alike but rarely mate successfully when brought together, because they have been separated for about 3 million years.

Common mistake

Saying the barrier itself creates a new species. The barrier only isolates the populations; the divergence is caused by different selection pressures and drift over many generations.

Quick check

1. What causes reproductive isolation in allopatric speciation?

2. Give an example of allopatric speciation.

Answers: 1. a geographical barrier 2. bonobos and chimpanzees separated by the Congo River

7 Evolution defined

Change in heritable characteristics.

Evolution is the change in the heritable characteristics of a population over generations. Only heritable changes (in DNA) count; characteristics acquired during a lifetime (such as muscles from exercise) are not inherited and do not cause evolution.

WORKED EXAMPLE 1 Acquired traits (standard)

Explain why a giraffe stretching its neck does not cause evolution of longer necks.

Solution

Stretching does not change the DNA of gametesR1

Acquired characteristics are not inheritedR1

Quick check

1. Define evolution.

Answers: 1. change in heritable characteristics of a population over time

8 Evidence from DNA and protein sequences

Molecular similarities.

Closely related species have more similar base sequences and amino acid sequences. The universal genetic code and shared genes (such as those for ribosomal RNA) point to common ancestry.

WORKED EXAMPLE 2 Interpreting sequences (standard)

Species A differs from B by 3 amino acids and from C by 18 in a protein. Which pair shares the more recent ancestor?

Solution

A and B: fewer differencesR1

9 Evidence from selective breeding

Artificial selection.

Humans have produced great variety in crops and domestic animals in a short time by selecting individuals with desired traits for breeding: wild mustard gave cabbage, broccoli, cauliflower and kale; wolves gave all dog breeds. This shows that selection can cause substantial change.

WORKED EXAMPLE 3 Artificial selection (standard)

Explain how selective breeding provides evidence for evolution.

Solution

Large heritable changes occur over relatively few generationsR1

Shows selection acting on variation changes populationsR1

In the real world

All modern broccoli, cabbage, kale, cauliflower and Brussels sprouts were bred from a single wild species, Brassica oleracea.

10 Evidence from homologous structures

Pentadactyl limbs.

The pentadactyl limb has the same basic bone arrangement (humerus, radius and ulna, carpals, metacarpals, phalanges) in mammals, birds, reptiles and amphibians, despite different functions (grasping, flying, swimming, running). This suggests descent from a common ancestor with adaptive radiation.

WORKED EXAMPLE 4 Pentadactyl limb (exam level)

Explain how the pentadactyl limb is evidence for evolution.

Solution

Same basic structure in different groupsR1

Adapted for different functionsR1

Best explained by common ancestry and divergent evolutionR1

11 Convergent evolution and analogous structures

Similar solutions.

Unrelated organisms in similar environments can evolve similar features (analogous structures): wings of birds and insects, streamlined bodies of sharks and dolphins, eyes of octopuses and vertebrates. These do not indicate close relationship.

WORKED EXAMPLE 5 Analogous (standard)

Explain why the similar shape of sharks and dolphins is not evidence of close relationship.

Solution

Sharks are fish; dolphins are mammalsR1

Similar selection pressures in water produced similar shapes (convergent evolution)R1

12 Vestigial structures and embryology

Traces of ancestry.

Vestigial structures are reduced and have lost most or all of their original function: whale pelvic bones, the human appendix and coccyx. Embryos of vertebrates share features such as pharyngeal pouches and tails, reflecting shared ancestry.

Quick check

1. Give an example of a vestigial structure in humans.

Answers: 1. coccyx (or appendix, wisdom teeth)

13 The fossil record

Changes through time.

Fossils show that organisms have changed over time and that groups appear in a sequence consistent with evolution (fish before amphibians before reptiles before mammals). Transitional fossils, such as Tiktaalik (fish–tetrapod) and Archaeopteryx (dinosaur–bird), link major groups.

WORKED EXAMPLE 6 Transitional fossil (standard)

Explain why Archaeopteryx is called a transitional fossil.

Solution

Features of reptiles (teeth, bony tail, claws) and birds (feathers, wings)R1

Shows an intermediate stage between groupsR1

Quick check

1. Give one limitation of the fossil record.

Answers: 1. incomplete: soft-bodied organisms rarely fossilize

14 Speciation: the role of isolation

Splitting gene pools.

Speciation is the formation of new species. It requires reproductive isolation: gene flow between populations stops, so they evolve independently through natural selection, mutation and genetic drift until they can no longer interbreed.

WORKED EXAMPLE 7 Isolation (standard)

Explain why isolation is necessary for speciation.

Solution

Gene flow would mix alleles between populationsR1

Without it, differences cannot accumulateR1

15 Allopatric speciation

Geographical separation.

In allopatric speciation, populations are separated by a physical barrier (mountains, rivers, sea). Different selection pressures and drift change allele frequencies until reproductive isolation develops.

WORKED EXAMPLE 8 Example (exam level)

Outline allopatric speciation in Galapagos finches.

Solution

Finches colonized separate islands (geographical isolation)R1

Different food sources selected different beak shapesR1

Populations became reproductively isolated: new speciesR1

16 Sympatric speciation

Same place, separate gene pools.

In sympatric speciation, populations in the same area become reproductively isolated by behavioural (different courtship), temporal (different breeding times) or ecological isolation, or by polyploidy in plants.

Isolating mechanism	Example
behavioural	different courtship songs in crickets
temporal	flowering at different times of year
ecological	apple and hawthorn maggot flies
polyploidy	new wheat species

WORKED EXAMPLE 9 Temporal isolation (standard)

Explain how different flowering times can cause speciation.

Solution

Plants flowering at different times cannot cross-pollinateR1

Gene flow stops; populations divergeR1

17 Adaptive radiation

One ancestor, many niches.

Adaptive radiation is the rapid evolution of many species from a common ancestor to fill different niches. Examples: Darwin's finches, Hawaiian honeycreepers, cichlid fish in African lakes, and mammals after the extinction of the dinosaurs.

WORKED EXAMPLE 10 Cichlids (standard)

Explain why Lake Victoria has hundreds of cichlid species.

Solution

Many different niches (feeding styles, depths)R1

Rapid speciation from a few ancestorsR1

18 Hybridization and barriers

Pre- and post-zygotic.

Pre-zygotic barriers prevent mating or fertilization (habitat, timing, behaviour, mechanical, gametic). Post-zygotic barriers reduce hybrid viability or fertility (mules are sterile). Hybrids can sometimes be fertile, especially in plants.

Quick check

1. Classify the infertility of mules as a pre- or post-zygotic barrier.

Answers: 1. post-zygotic

19 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 11 Mixed problem (exam level)

Two populations of a frog species are separated by a new river. After many generations, they cannot interbreed. Explain how this happened.

Solution

Geographical isolation stops gene flowR1

Different selection pressures, mutations and drift change allele frequenciesR1

Reproductive isolation: allopatric speciationR1

20 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 12 Paper 1 question (standard)

Which is evidence of convergent evolution? A. pentadactyl limbs B. wings of bats and insects C. human coccyx D. similar cytochrome c sequences

Solution

BA1

WORKED EXAMPLE 13 Paper 2 question (exam level)

Distinguish between allopatric and sympatric speciation.

Solution

Allopatric: geographical barrier separates populationsR1

Sympatric: same area; isolation by behaviour, timing or polyploidyR1

21 Rates of evolution

Gradualism and punctuated equilibrium.

Gradualism proposes slow, steady change; punctuated equilibrium proposes long periods of little change interrupted by rapid speciation. The fossil record shows evidence of both patterns.

Quick check

1. What evidence in the fossil record supports punctuated equilibrium?

Answers: 1. species appear suddenly and remain unchanged for long periods

22 Evolution observed today

Evolution in action.

Evolution can be observed directly: antibiotic resistance in bacteria, pesticide resistance in insects, the peppered moth during the industrial revolution, and beak size changes in Galapagos finches after droughts.

In the real world

During a 1977 drought on Daphne Major, finches with larger beaks survived better because only large, hard seeds remained; average beak depth increased in the next generation.

Quick check

1. Explain why bacteria evolve resistance quickly.

Answers: 1. very short generation times and large populations

23 Common misconceptions and links

Students often think…	Actually…
individuals evolve	populations evolve
acquired characteristics are inherited	only heritable (genetic) characteristics evolve
similar structures always show relationship	analogous structures arise by convergent evolution
the barrier causes speciation directly	isolation allows divergence by selection and drift

How A4.1 connects to the rest of the course

Topic	Connection
A3.2 Classification (HL)	cladistics and sequence evidence
D4.1 Natural selection	the mechanism of evolution
A3.1 Species	species concepts
D3.2 Inheritance	gene pools and alleles
A4.2 Conservation	extinction and biodiversity

24 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 3. Amino acid differences in haemoglobin β chain compared with humans.

(a) Define evolution. [2]

(b) Using the data, identify the species most closely related to humans. [1]

(d) Explain how the pentadactyl limb provides evidence for evolution. [3]

(e) Outline how allopatric speciation occurs. [3]

Markscheme

Part	Answer	Marks
(a)	change in heritable characteristics	A1
	of a population over time	A1
(b)	chimpanzee	A1
(c)	fewer differences in closely related species	R1
	differences accumulate over time from a common ancestor	R1
(d)	same bone structure in different vertebrates	R1
	used for different functions	R1
	best explained by a common ancestor; adaptive radiation	R1
(e)	geographical barrier separates populations	R1
	different selection pressures	R1
	gene pools diverge until interbreeding is impossible	R1

Examiner's comments. In (a), 'population' and 'heritable' are both required. In (e), weaker answers did not explain why isolation leads to divergence.

25 Check your understanding

Section A: multiple choice

1. Evolution is a change in

A. heritable characteristics of a population B. characteristics of an individual C. acquired characteristics D. the environment

2. The pentadactyl limb is an example of

A. analogous structures B. vestigial structures C. homologous structures D. convergent evolution

3. Wings of insects and birds are

A. homologous B. analogous C. vestigial D. identical

4. Selective breeding is evidence for evolution because it shows

A. acquired traits are inherited B. species never change C. mutations do not occur D. heritable traits in populations can change

5. Allopatric speciation requires

A. polyploidy B. a geographical barrier C. the same habitat D. no isolation

6. Chimpanzees and bonobos were separated by

A. the Congo River B. the Sahara C. the Andes D. the Atlantic

Section B: structured

7. Explain how sequence data provide evidence for common ancestry. [3]

8. Distinguish homologous and analogous structures. [3]

9. Explain the role of differential selection in speciation. [3]

Answers

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

7. Universal code; similar sequences in related species; differences accumulate over time.

8. Homologous: common ancestor, same structure, different functions; analogous: similar function, different origin (convergent).

9. Isolated populations face different pressures; different alleles favoured; gene pools diverge.

26 Summary and topic map

Idea	Remember
Evolution	change in heritable characteristics of a population
Sequence evidence	fewer differences = closer relationship
Selective breeding	rapid change in domesticated species
Homologous structures	common ancestor; pentadactyl limb
Analogous structures	convergent evolution
Speciation	reproductive isolation + differential selection
Allopatric	geographical barrier; bonobos and chimpanzees

Before the exam, make sure you…

•use 'population' and 'heritable' in definitions
•distinguish homologous from analogous
•explain why isolation leads to divergence
•give named examples of speciation

Evolution and Speciation 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 4. Topic map for Evolution and Speciation.

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