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Unit 7: Natural Selection flashcards

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7.1 Introduction to Natural Selection

What is evolution, and what evolves?
A change in the heritable characteristics of a population over generations. Populations evolve; individuals do not.
What is Darwin's argument for natural selection?
More offspring are produced than can survive; resources are limited, so individuals compete; individuals vary; much variation is inherited; so individuals with favourable phenotypes survive and reproduce more, and their traits become more common.
How is fitness measured?
By reproductive success: offspring that themselves survive to breed. Not strength, size or lifespan. A sterile animal that lives 20 years has a fitness of zero.
How do you calculate relative fitness?
w = mean offspring of a phenotype ÷ mean offspring of the most successful phenotype (which has w = 1; no units). 1.5 ÷ 2.5 = 0.60.
What is needed for natural selection to work?
Variation, heritability of that variation, and differences in reproduction. Without variation there is nothing to choose; if it is not heritable offspring do not share it; if all reproduce equally nothing changes.
Why can the rate and direction of evolution change?
Biotic (predators, prey, parasites, mates) and abiotic (temperature, sunlight, rock colour) conditions fluctuate, so different variants can be favoured in different generations.
Did traits arise because organisms needed them?
No. Variation arises first; selection then favours variants that suit the current environment. Individuals do not evolve or adapt by changing their genes.

7.2 Natural Selection

Does natural selection act on genotype or phenotype?
On phenotypes. Genotypes are affected only through the phenotypes they produce.
Which variation can natural selection pass on?
Only genetic variation (from mutation and new combinations in meiosis and fertilisation). Environmentally caused differences are not passed to the next generation.
What is a selective pressure?
A feature of the environment that makes some phenotypes survive or reproduce better. The same phenotype can raise fitness in one environment and lower it, or make no difference, in another.
Why is the sickle-cell allele common where malaria is common?
Carriers of one Hb S allele resist severe malaria, so they have the highest fitness there, even though two copies cause disease.
How can molecular variation affect fitness?
Differences in the types and numbers of molecules change cell function, which changes phenotype and so survival and reproduction in this environment. Bacteria moved to the cold benefit from more unsaturated membrane tails that keep the membrane fluid.
Is insecticide resistance artificial selection?
No. It is natural selection: the insecticide is the pressure, but humans do not choose which individuals breed. The insecticide does not cause the resistant variation.

7.3 Artificial Selection

What is artificial selection?
Humans decide which individuals reproduce, choosing traits they want, so those traits become more common. It needs variation, heritability and differential reproduction, like natural selection; only the selecting agent differs.
What are the effects of artificial selection on a population?
The mean shifts in the chosen direction, variation within the population falls, and differences between breeds or varieties grow.
Give examples of artificial selection.
Kale, cabbage, Brussels sprouts, kohlrabi, broccoli and cauliflower from wild cabbage; maize from teosinte; dog breeds from wolves.
Why does artificial selection eventually slow or stop?
Selection uses existing heritable variation. Wanted alleles become fixed and others are lost, so little variation remains and the response plateaus. Breeders do not create new alleles.
Why is a uniform crop or herd at risk?
Every individual has the same or similar genotype, so one pathogen that infects one can spread through nearly all.
How do you show a selected line has responded genetically?
Compare it with an unselected control kept in the same conditions. The trait must be at least partly heritable; changes from diet or training are not passed on.

7.4 Population Genetics

When does a population evolve?
When its allele frequencies change between generations. Natural selection is not the only cause: mutation, genetic drift and gene flow also change frequencies.
What is genetic drift?
Chance change in allele frequencies because each generation is a random sample of the last. It is strongest in small populations (but acts in all) and can fix or lose alleles whatever their effect.
How do the bottleneck and founder effects differ?
Bottleneck: the whole population shrinks to a few individuals; lost alleles stay lost after regrowth. Founder effect: a few individuals start a separate population whose frequencies reflect the founders, not the source.
What does gene flow do?
Migration adds or removes alleles, making populations more alike and preventing them from diverging into separate species.
What role does mutation play in evolution?
Mutation is random and is the only source of brand-new alleles. It supplies the variation selection acts on but alone changes frequencies very slowly.
How do you calculate an allele frequency from genotype counts?
Copies of B = 2 × (BB) + (Bb); total copies = 2 × individuals. 30 BB, 50 Bb, 20 bb: (60 + 50) ÷ 200 = 0.55. Count alleles, not individuals, and double homozygotes.

7.5 Hardy–Weinberg Equilibrium

What do the Hardy–Weinberg equations state?
p + q = 1 for two alleles. p² + 2pq + q² = 1 for genotype frequencies AA, Aa, aa when gametes combine at random. Heterozygotes are 2pq, not pq.
What five conditions keep a population in Hardy–Weinberg equilibrium?
Large population, no migration, no new mutations, random mating, no natural selection. No real population meets all five, so the prediction is used as a null hypothesis.
How do you find allele frequencies from the recessive phenotype?
q² is the recessive phenotype frequency, so q = √(q²), assuming equilibrium. 16% recessive: q = 0.4, p = 0.6, carriers 2pq = 0.48. Do not treat q² as q.
Which force acts when each Hardy–Weinberg condition is broken?
Small population: drift. Migration: gene flow. Mutation: new alleles. Non-random mating: genotype frequencies shift (inbreeding gives fewer heterozygotes, not new allele frequencies). Selection: fitter alleles rise.
How do you test whether a population is in Hardy–Weinberg equilibrium?
Compare observed genotype numbers with expected numbers using χ² = Σ (o − e)² ÷ e. With the allele frequency known in advance, df = 2 (critical value 5.99 at p = 0.05). Failing to reject does not prove no evolution.
Does equilibrium mean p = q = 0.5?
No. Equilibrium means frequencies stay constant between generations, at any values. Dominant alleles also do not become more common on their own.

7.6 Evidence of Evolution

How are fossils dated?
Relative dating: in undisturbed sedimentary rock, lower layers are older. Radiometric dating: isotopes decay at a constant rate; older fossils are bracketed by dating volcanic layers above and below them with longer-lived isotopes.
How is carbon-14 used to date remains?
Half-life about 5,700 years; useful for once-living remains up to about 50,000 years, not millions. Fraction left = (1/2)ⁿ. 25% left = 2 half-lives = 11,400 years.
How do homologous, vestigial and analogous structures differ as evidence?
Homologous: same basic parts, different functions (mammal forelimbs), evidence of common ancestry. Vestigial: reduced remnants (whale pelvis), also evidence. Analogous: same function, different build, not evidence of common ancestry.
How does molecular evidence show relatedness?
Differences in DNA or amino acid sequences build up after lineages split, so fewer differences mean a more recent common ancestor. Differing at 3 amino acids is closer than differing at 12.
What does biogeography show about evolution?
Island species resemble the nearest mainland species, and the same fossil species on now-separate continents shows they were once joined.
Does vestigial mean useless?
No. A vestigial structure is a reduced version that has lost its main function; it may still have a minor use.

7.7 Common Ancestry

What features are shared by all life?
DNA, a nearly universal genetic code, ribosomes, a phospholipid membrane and ATP. These link all three domains.
What three features do all eukaryotes share?
Membrane-bound organelles (nucleus, endomembrane system, mitochondria), linear chromosomes with telomeres, and genes containing introns removed by a spliceosome. These point to a common ancestor of eukaryotes.
When does a shared feature count as evidence of common ancestry?
Only if it is homologous: inherited and built from related molecules, not just similar in job. Cell walls built from different molecules are analogous.
Why are shared intron positions strong evidence of common ancestry?
An intron is unlikely to be inserted at exactly the same codon in separate lineages, so inheritance from a shared ancestor explains it.
Did humans evolve from yeast?
No. Living species are relatives sharing common ancestors, not ancestors of each other.
Would a spliceosome-blocking drug harm bacteria?
No. Introns are rare in prokaryotes and they have no spliceosome.

7.8 Continuing Evolution

Are species still evolving?
Yes, all species, humans included. New mutations arise each generation, environments change, hosts, parasites and predators evolve too, and drift acts in every population. Change is fastest in large populations with short generations.
How does resistance to antibiotics or pesticides spread?
Resistant variation exists before the chemical is used. The chemical kills susceptible individuals, resistant survivors reproduce (bacteria may transfer resistance genes), and repeated use makes the resistant type common. The chemical does not cause the mutations.
Why do combination treatments slow resistance?
A cell needs two independent mutations, and the chance is the product of two small rates, so such cells are very unlikely to exist at the start.
How do you estimate the number of resistant mutants expected?
Population size × mutation rate for that trait. 3 × 10⁸ cells × 2 × 10⁻⁸ per cell = 6 resistant cells.
What evidence shows evolution is continuing?
New mutations in long-term lab bacterial populations, gradual trait shifts through rock layers, spreading resistance to antibiotics, pesticides, herbicides and chemotherapy, and emergent diseases.
Why do flu vaccines need updating?
The virus's surface proteins keep changing, so antibodies to older forms bind less well.

7.9 Phylogeny

What do the root, nodes and clades on a phylogenetic tree represent?
Root: ancestor of all lineages shown. Node: the most recent common ancestor of the lineages beyond it, a speciation event. Clade: an ancestor plus all its descendants.
How do you judge relatedness on a tree?
By the most recent shared node, not by which tips are printed side by side. Branches can rotate at a node without changing relationships; living tips are never ancestors of each other.
What is a shared derived character?
A new state (gained or lost) present in two or more lineages; it marks a clade. Ancestral characters, present in the outgroup, cannot separate groups.
What is the role of an outgroup?
A lineage less closely related to the rest; it shows which character state is ancestral. It is not the ancestor of the others.
How do cladograms and phylogenetic trees differ?
A cladogram shows branching order only; branch length means nothing. A phylogenetic tree shows branching order plus time or amount of change.
How does a molecular clock work?
Rate = differences ÷ time since a fossil-dated split; time = differences ÷ rate. At 4 differences per million years, 40 differences means a split 10 million years ago. It assumes a steady rate, so dates are estimates.
Why are molecular data usually more reliable for building trees?
They give many more characters and are less often misled by convergent evolution than morphology. Trees are hypotheses, revised by new evidence (DNA placed whales next to hippos).

7.10 Speciation

What is the biological species concept?
A species is a group that can interbreed in nature and produce viable, fertile offspring. It applies to sexually reproducing organisms, not to asexual ones or fossils.
How do allopatric and sympatric speciation differ?
Allopatric needs geographic isolation. Sympatric happens in the same area, for example by polyploidy in plants or a host shift (apple maggot fly, hawthorn to apple).
What are the pre-zygotic barriers to reproduction?
Habitat, temporal (different breeding times), behavioural (signals not recognised), mechanical (parts do not fit) and gametic isolation (sperm cannot fuse with egg). Gametic isolation acts after mating but before a zygote forms.
What are the post-zygotic barriers to reproduction?
Reduced hybrid viability (hybrids die young), reduced hybrid fertility (sterile hybrids such as the mule, 32 + 31 = 63 chromosomes) and hybrid breakdown (fertile hybrids with weak or sterile offspring).
How do gradualism and punctuated equilibrium differ?
Gradualism: slow, steady change over very long times. Punctuated equilibrium: long stasis, then rapid change (rapid in geological terms, not overnight).
How do divergent and convergent evolution differ?
Divergent: related populations adapt to different habitats and spread apart, fastest in adaptive radiation into empty niches. Convergent: similar pressures give similar traits in unrelated lineages (Anolis ecomorphs).
Why is a tetraploid × diploid hybrid usually sterile?
Gametes give 2 sets + 1 set = 3 sets (triploid), which cannot pair properly in meiosis. For a diploid with 2n = 20, the triploid hybrid has 10 + 20 = 30 chromosomes.

7.11 Variations in Populations

What is genetic diversity and how is it measured?
The variety of alleles and genotypes within a population or species, measured by alleles per gene, heterozygosity (expected 2pq for two alleles) or the spread of a heritable trait.
What gains and loses genetic diversity?
Gained by mutation, meiosis and fertilisation, and gene flow. Lost by selection, chance in small populations and inbreeding.
Why does low genetic diversity put a population at risk?
One disease or change in conditions can affect almost every individual, as with one potato variety and blight in 1840s Ireland. High diversity makes it more likely that some already carry alleles to survive.
What is genetic rescue?
Adding individuals from another population of the species, which restores alleles by gene flow and can raise resilience.
What fraction survives if only rr individuals die?
1 − q². If q = 0.9, 1 − 0.81 = 0.19 survives. Use q², not q, for the homozygous recessive frequency.
Does inbreeding change genetic diversity?
It makes more individuals homozygous, so harmful recessive alleles are expressed more often.
Is a large population always genetically diverse?
No. Size and diversity differ; a large clonal crop has almost no variation. Also, the spread of a trait, not just its average, decides who survives an extreme.

7.12 Origins of Life on Earth

When did life on Earth begin, according to geological evidence?
Earth formed about 4.6 bya and was too hostile for life until about 3.9 bya. The earliest fossil evidence, such as stromatolites, is about 3.5 bya. The plausible window is 3.9 − 3.5 = 0.4 billion years.
What does the oldest fossil date tell you about the origin of life?
Only a minimum age: life existed by then. It is not the moment life began; the end of the hostile period sets the earliest plausible date.
What is the RNA world hypothesis?
That RNA was the earliest genetic material: it stores information in its base sequence, is copied by base pairing (A–U, G–C), and can fold into catalysts (ribozymes), so proteins were not needed for catalysis.
What evidence supports the RNA world hypothesis?
Ribozymes exist, including the core of the ribosome, so RNA can catalyse reactions; and lab RNAs can copy templates by base pairing. These show what is possible, not what did happen.
Why could RNA, but not DNA alone, start the first genetic system?
RNA can both carry information and act as a catalyst; DNA needs protein enzymes to be copied. Later DNA took over storage (more stable) and proteins most catalysis.
What is stromatolite evidence?
Layered mounds built by microbial mats; among the earliest fossil evidence of life, about 3.5 bya.

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