18) evolution

What is biological evolution?

Descent with modification from a common ancestor

What is microevolution?

A change in allele frequencies in a population over successive generations, occurring over a few generations, with no new species formed (e.g. antibiotic resistance in bacteria, peppered moth pigmentation)

What is macroevolution?

A change in allele frequencies over successive generations occurring over thousands/millions of generations, resulting in the formation of new species (e.g. Darwin's Galapagos finches)

How are microevolution and macroevolution linked?

Microevolution may eventually lead to macroevolution given enough time; both involve changes in allele frequencies due to the 5 agents of evolutionary change

What are the 5 agents/mechanisms of evolutionary change?

Natural selection, disruption of gene flow, mutations, non-random mating, genetic drift

Who proposed the theory of natural selection?

Charles Darwin

What is natural selection?

A process in which individuals with certain inherited traits tend to survive and reproduce at higher rates than others because of those traits

What is "overproduction of offspring" in natural selection theory?

All organisms produce a large number of offspring, which can lead to exponential population growth if all survive

What is "constancy of numbers" in natural selection theory?

Population size stays relatively constant because many offspring die before reaching reproductive age

What is "struggle for existence" in natural selection theory?

Individuals compete for limited resources like food, mates and shelter, and factors like disease and predators limit population numbers

What is "variation within a population" in natural selection theory?

Individuals differ genetically and phenotypically due to different alleles; this variation is a prerequisite for natural selection

What is "survival of the fittest" in natural selection theory?

Individuals with favourable characteristics better adapted to the environment are selected for and survive to reproduce, while less well-adapted individuals are selected against

What does "like produces like" mean in natural selection theory?

Individuals with advantageous characteristics breed successfully and pass on their alleles, increasing favourable allele frequencies (microevolution)

How does natural selection lead to formation of a new species?

Over many generations, reproductive isolation can occur, leading to macroevolution and new species formation

What is variation, and where does it arise from?

Phenotypic differences between individuals of the same species due to different alleles; arises from mutations, meiosis, and sexual reproduction

Why is variation important for natural selection?

It is the raw material natural selection acts on; when environmental changes occur, phenotypic variation allows best-adapted individuals to survive and reproduce better, increasing favourable allele frequencies

Do meiosis and random fusion of gametes create new alleles?

No, they only reshuffle existing alleles among individuals, resulting in different phenotype combinations

What is the only source of new alleles?

Mutations, which introduce new genotypes and can create new phenotypes, increasing genetic variation

In the peppered moth example, what happened before the industrial revolution?

The lighter form was more common as it was well-camouflaged against light, lichen-covered tree bark, so it was selected for

In the peppered moth example, what happened after the industrial revolution began?

Soot darkened tree bark, so the melanic form became better camouflaged and was selected for, increasing in frequency

What was the selection pressure in the peppered moth example?

Predation

Is it correct to say industrialisation caused the melanic moth form to appear?

No, the melanic form arose by spontaneous mutation that existed before industrialisation; industrialisation only provided the selection pressure

In the antibiotic resistance example, why do resistant bacteria survive antibiotic treatment?

They have a selective advantage in the presence of antibiotics, allowing them to survive and pass on the resistance allele to offspring

Why is it important to complete a full course of antibiotics?

So that susceptible bacteria are killed and the immune system can eliminate resistant bacteria, reducing chances of resistant mutations spreading

Why does using a combination of antibiotics reduce the spread of resistance?

Bacteria are unlikely to have mutations making them resistant to all antibiotics simultaneously, so the combination kills them before resistance can spread

Why is a population considered the smallest unit that can evolve?

Because allele frequency changes over successive generations can only be measured in a population, not in a single individual

What is the biological species concept?

A species is a group of organisms capable of interbreeding and producing fertile, viable offspring, and are reproductively isolated from other species

What is an advantage of the biological species concept?

Organisms can be interbred to test if they produce fertile, viable offspring

What are two limitations of the biological species concept?

It cannot be applied to asexually reproducing organisms or extinct species whose breeding cannot be observed, or to artificial hybridisation of plant species producing fertile polyploid hybrids (e.g. bread wheat)

Why is a mule (from a horse and donkey) viable but not fertile?

Homologous chromosomes do not exist between horse and donkey chromosomes, so they cannot pair up during prophase I of meiosis, preventing gamete formation in mule.

—> A horse (2n=64) and donkey (2n=62) can interbreed to produce a mule (2n=63), which is viable but sterile.

What is speciation?

A process by which one or more new species arise from a previously existing species

What is gene flow?

The transfer of alleles from one population to another due to movement of fertile individuals or their gametes

What must happen to gene flow for speciation to occur?

It must be disrupted between two populations so evolutionary changes can occur independently in each subpopulation

What are the 3 isolating mechanisms that disrupt gene flow?

Geographical isolation, physiological isolation, behavioural isolation

What is geographical isolation?

A physical barrier (e.g. body of water) between subpopulations that blocks migration and disrupts gene flow

What is physiological isolation?

Unique physiology (e.g. different flowering times due to soil conditions) of individuals in the same area that disrupts gene flow

What is behavioural isolation?

Unique mating rituals or preferences (e.g. different bird song) that cause a group to isolate themselves from the main population, disrupting gene flow

What is allopatric speciation?

Speciation that occurs due to geographical isolation

What is sympatric speciation?

Speciation that occurs within the same geographical area due to physiological or behavioural isolation

Give an example of allopatric speciation.

Caribbean porkfish and Panamic porkfish, separated by the formation of the Isthmus of Panama

Give an example of sympatric speciation.

Howea forsteriana and Howea belmoreana palms on Lord Howe Island, isolated by different flowering times due to different soil types

Describe the 7 steps of allopatric speciation.

An ancestral population is separated by a geographical barrier; gene flow is disrupted; each subpopulation faces different selection pressures; favourable traits are selected for in each; allele frequencies change independently due to mutation, drift, and selection; over many generations subpopulations become reproductively isolated; new species form via macroevolution

What is the binomial system of nomenclature?

A classification system giving every organism a genus name and species name/specific epithet in modern Latin

What is biological classification?

The organisation of species according to shared characteristics in a hierarchical manner into increasingly inclusive groups/taxa, using morphological/anatomical characteristics

What is a taxon?

A recognisable group of organisms at any particular level of classification

List the 8 levels of the taxonomic hierarchy in order.

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

What is a disadvantage of using morphological characteristics for classification?

Two different species that look superficially similar due to convergent evolution can be wrongly classified as closely related

Why do sharks and dolphins look similar despite not being closely related?

They live in similar habitats with similar selection pressures, so their similar dorsal fins are analogous structures, not due to shared ancestry

What is phylogeny?

The organisation of species in a phylogenetic tree to show their evolutionary relationships (ancestor-descendant relationships)

How is evolutionary history established for phylogeny?

By comparing anatomical and molecular homologous characters due to shared ancestry

What is an advantage of using homologous characters to classify organisms?

Morphologically indistinguishable organisms that arose due to convergent evolution can be distinguished

What is a disadvantage of using homologous characters to classify organisms?

The process is time-consuming, and fossils may have missing/incomplete or damaged DNA

Why are genome sequences important in reconstructing phylogenetic relationships?

More closely related species have a greater number of similar nucleotide and amino acid sequences than less closely related species, since evolutionary changes are captured in nucleic acids

Describe the 4 steps of using molecular sequences to establish evolutionary relationships.

Choose a homologous gene common to the group (e.g. cytochrome C); amplify and sequence the gene using PCR; carry out multiple sequence alignment; compare differences/similarities to establish relationships

List 4 advantages of molecular methods over morphological methods in classification.

Nucleotide data can compare all organisms sharing common genes; it can distinguish convergently evolved organisms; it is objective and unambiguous; it is quantitative and amenable to statistical analysis

Why can nucleotide sequence differences help estimate speciation timing?

Changes in nucleotide sequences accumulate over time with clockwork regularity

What type of specimens can be used for molecular classification?

Specimens need not be complete or alive, so long as the molecules (e.g. mitochondrial DNA) survive degradation

How does classification differ from phylogeny in basis of grouping?

Classification groups organisms by overall morphological similarity; phylogeny groups organisms by evolutionary history/ancestor-descendant relationships

How does classification differ from phylogeny in inference of common ancestors?

Classification does not allow inference of common ancestors; phylogeny allows inference of common ancestors as descendants are shown in the same branch

What is homology?

Similar anatomical and molecular characteristics found in different species due to common ancestry, providing evidence for descent with modification

Why is DNA preferred over mRNA or amino acid sequences to infer relationships?

The genetic code is degenerate so amino acid sequence doesn't reflect DNA exactly, and mutations in non-coding DNA regions aren't seen in mature mRNA or polypeptides

What is the pentadactyl limb an example of?

A homologous structure in tetrapod forelimbs (e.g. humans, cats, whales) that was modified by natural selection for different functions, providing evidence for descent with modification

What are vestigial structures?

Previously functional anatomical structures that became burdensome or non-contributory to fitness and are now degenerate/rudimentary, evidencing common ancestry (e.g. pelvic bones in whales)

What is biogeography?

The study of the past and present geographical distribution of organisms, supporting evolutionary deductions based on homologies

What are fossils, and how is their age determined?

Relics or impressions of past organisms preserved in rock; their age is determined using radioactive/carbon dating techniques

What do horse fossils show about evolution?

An ordered sequence of limb lengthening, toe reduction, and increased tooth size over time, coinciding with the change from dense forest to open grasslands

What is a transitional fossil, and what does Tiktaalik demonstrate?

A fossil sharing characteristics of both modern descendants and a prehistoric ancestor; Tiktaalik shows features of both fish (gills, scales) and tetrapods (leg bones, lungs, upward eyes, mobile neck)

What do lungfish fossils reveal about evolution and plate tectonics?

Lungfish lived across Pangea; when it split, lungfish evolved independently on different landmasses; African and South American lungfish are more genetically similar to each other than to Australian lungfish, matching the timeline of continental separation

What is the Wallace Line?

A biogeographical boundary separating the animals of Asia from Australia, running through the Lombok and Makassar Straits

Why are animals in Bali and Lombok evolutionarily distinct despite being only 35km apart?

Deep water, strong ocean currents, and specific niche adaptations have prevented migration, keeping them geographically isolated with different ancestries (Asian vs Australian)

What is adaptive radiation, as shown by Galapagos finches?

The rapid diversification of a single ancestral line into many new species adapted to different ecological niches

How do heterozygote protection preserve recessive alleles?

Dominant alleles mask recessive alleles in heterozygotes, so recessive alleles persist without being selected against since the disadvantageous trait isn't expressed

What is heterozygote advantage, using sickle cell anaemia as an example?

A form of balancing selection where heterozygotes (HbAHbS) have greater fitness than either homozygote, avoiding both sickle cell anaemia and increased malaria risk, preserving both alleles

What is frequency-dependent selection, using scale-eating fish as an example?

A form of balancing selection where the rarer phenotype is favoured, keeping the frequency of both "left-mouthed" and "right-mouthed" fish oscillating near 50%

What are the two categories of mutations that create genetic variation?

Gene mutations and chromosomal mutations

What is polyploidy?

A chromosomal number mutation where more than 2 homologous sets of chromosomes are present (e.g. triploid 3n, tetraploid 4n)

What is aneuploidy, and give an example.

When one or more chromosomes are over- or under-represented due to non-disjunction during meiosis I or II; example is Trisomy 21

List the 4 types of chromosome structural mutations.

Deletion, duplication, inversion, translocation

What is a deletion chromosomal mutation, and give an example disease.

When a segment of a chromosome is missing; e.g. cri-du-chat disease

What is a duplication chromosomal mutation?

When an extra segment of a chromosome is present

What is an inversion chromosomal mutation?

When a chromosome segment is detached, flipped 180 degrees, and reattached to the same chromosome

What is a translocation chromosomal mutation?

When a segment from one chromosome is detached and reattached to a different chromosome

How does meiosis contribute to genetic variation?

Independent assortment and segregation of homologous chromosomes and sister chromatids, plus crossing over between non-sister chromatids, create many allelic combinations in gametes

How does sexual reproduction contribute to genetic variation?

Random fusion of gametes adds to the variety of genotypes, which act as raw material for natural selection

What is the Hardy-Weinberg principle?

A population's allele and genotype frequencies remain constant from generation to generation as long as certain conditions are met

List the 5 conditions required for Hardy-Weinberg equilibrium.

No natural selection, no gene flow, no mutations, random mating, and a very large population

What is the Hardy-Weinberg allele frequency equation?

p + q = 1, where p is the frequency of the dominant allele and q is the frequency of the recessive allele

What is the Hardy-Weinberg genotype frequency equation?

p² + 2pq + q² = 1, where p² is homozygous dominant frequency, 2pq is heterozygous frequency, and q² is homozygous recessive frequency

What are the 5 agents that cause evolutionary change (changes in allele frequency)?

Natural selection, disruption of gene flow, mutations, non-random mating, genetic drift

What is genetic drift?

A change in allele frequencies due to chance events, with greater impact in smaller populations, resulting in some alleles being over- or under-represented

What is the founder effect?

When a small group separates from a larger population to establish a new colony, causing certain alleles to be over- or under-represented, usually reducing genetic variation

What is the bottleneck effect?

When a population is dramatically reduced by catastrophe and then rebounds, with certain alleles over- or under-represented among survivors, usually reducing genetic variation even after numbers recover

What is directional selection? Give an example.

Selection for one extreme phenotype; e.g. the melanic form of peppered moth having an advantage in polluted, soot-covered areas

What is disruptive selection? Give an example.

Selection for both extreme phenotypes while selecting against intermediate phenotypes; e.g. squirrels with very long or very short tails being favoured over mid-length tails

What is stabilising selection? Give an example.

Selection for intermediate phenotypes while selecting against extreme phenotypes; e.g. babies of optimum birth weight having a survival advantage over those heavier or lighter

Why is mitochondrial DNA maternally inherited?

The egg contributes the cytoplasm (including mitochondria) while the sperm only contributes the nucleus

Why is mtDNA useful for estimating timing of speciation?

It does not undergo recombination or crossing over, so changes are solely due to mutations accumulating at a constant rate over time

Why does mtDNA have a high mutation rate?

It has a poor proof-reading mechanism relative to nuclear DNA, and may be damaged by reactive oxygen radicals from the mitochondrion

What is the difference between prezygotic and postzygotic barriers?

Prezygotic barriers prevent mating or fertilisation; postzygotic barriers cause zygote fatality or prevent hybrids from developing into fertile adults.

Peppered Moth (Biston betularia) — Directional Selection

Before the industrial revolution, the lighter form of the peppered moth was more common because it was well-camouflaged against light, lichen-covered tree bark, giving it a survival advantage over the melanic (dark) form, which arose by spontaneous mutation. After industrialisation, soot darkened the tree bark, so the melanic form became better camouflaged instead and was selected for, while the lighter form became more visible to predators. The frequency of the melanic allele increased and the lighter allele decreased — an example of microevolution driven by predation as the selection pressure. Note that industrialisation didn't cause the mutation; it only changed which pre-existing form had the advantage.