Title: InkBiology / Evolution and speciation
On every note of this deck, unless a note says otherwise:
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Front: DEFINE
Define: Genetic bottleneck.
Back: A sharp reduction in population size that leaves the survivors carrying only a fraction of the original genetic diversity.

Front: DEFINE
Define: Gene flow.
Back: The movement of alleles between populations, by migration of individuals or of gametes, which keeps their gene pools similar.

Front: DEFINE
Define: Founder effect.
Back: The reduced and unrepresentative genetic diversity of a population established by a small number of individuals separated from a larger population.

Front: DEFINE
Define: Reproductive isolation.
Back: The state in which gene flow between two populations has stopped, because of some feature of the organisms or of where and when they breed.

Front: DEFINE
Define: Gene pool.
Back: All the alleles of all the genes in a population at a given time.

Front: DEFINE
Define: Vertical gene transfer.
Back: Passing genes from a parent cell to its daughter cells by binary fission.

Front: DEFINE
Define: Natural selection.
Back: The process by which individuals with phenotypes better suited to the environment reproduce more successfully, so the alleles responsible become more frequent.

Front: DEFINE
Define: Speciation.
Back: The formation of a new species from an existing one, through reproductive isolation followed by genetic divergence of the isolated populations.

Front: DEFINE
Define: Selection pressure.
Back: An environmental factor that causes individuals with certain phenotypes to survive and reproduce more successfully than others.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Evolution is survival of the fittest, and the fittest are the strongest, fastest and biggest.
Back: Fitness in biology is not athletic. It is the number of offspring an individual leaves that themselves survive to breed, relative to everyone else. A small unimpressive organism raising eleven surviving young is fitter than a magnificent one raising none.

Front: DEFINE
Define: Adaptation.
Back: A characteristic that increases an organism's chance of surviving and reproducing in its environment. Adaptations may be anatomical, physiological or behavioural.

Front: DEFINE
Define: Antibiotic resistance.
Back: The ability of a bacterium to survive and reproduce in a concentration of an antibiotic that would kill or inhibit others of its species.

Front: CHECK YOURSELF
A species of moth rests on tree trunks. In an industrial area the trunks are darkened by soot, and over fifty years the proportion of dark moths in the population rises from 2% to over 90%. Explain this change, and then explain why the same alleles were present in the population before the trunks darkened.
Back: Mutation had already produced an allele for dark colouring, so both phenotypes were present before the soot arrived. Mutation happens whether or not the allele is useful.
Darkened trunks are the selection pressure. Against a dark background, pale moths are more conspicuous to birds hunting by sight and are eaten more often; dark moths are better camouflaged.
Dark moths therefore reproduce in greater numbers and pass on the allele. Over many generations its frequency in the gene pool rises, so the proportion of dark moths rises. That change in allele frequency is the evolution.
Before the trunks darkened the dark allele was present but rare, because against pale lichen-covered bark the dark phenotype was the conspicuous one: held at low frequency by selection, not absent. This is Biston betularia, and the detail is unusually well documented: the carbonaria form went from under 2% in Manchester in 1848 to about 98% by 1895, and the mutation has been identified as a transposable element in the cortex gene, dated to around 1819.
The move that scores is refusing to say the soot caused the mutation.

Front: DEFINE
Define: Antibiotic.
Back: A substance produced by a microorganism, or a synthetic derivative of one, that kills or inhibits the growth of bacteria.

Front: DEFINE
Define: Mutation.
Back: A change in the base sequence of DNA. The only source of new alleles.

Front: DEFINE
Define: Genetic variation.
Back: Differences between individuals in a population that are caused by differences in their alleles, and can therefore be inherited.

Front: CHECK YOURSELF
Two populations of the same species of bird live on neighbouring islands. Ornithologists find that the two have distinct songs and that females of each population do not respond to the song of the other. When birds from the two islands are kept together in an aviary, a few pairs form and the offspring are healthy and fertile. Are these two species? Explain your answer, and predict what would happen if the two populations came into contact naturally.
Back: On the biological species concept they are not yet two species, because they can interbreed to produce fertile offspring. The aviary result shows that no postzygotic barrier has developed.
But they are close, and the barrier that exists is behavioural and prezygotic. Song is a species-recognition signal, so if females do not respond to the other population's song, mating between the populations will be very rare in the wild even though it is possible in principle. Reproductive isolation in nature can be effectively complete while the physiological capacity to interbreed remains.
That is what allopatric divergence caught partway looks like. The islands separate the gene pools; different mutations, different selection pressures and drift have made the populations diverge; song is one of the traits that diverged.
On natural contact, one of three things follows. If the song difference is enough, they coexist without interbreeding and are treated as two species. If it is not, hybrids form, gene flow resumes and the gene pools merge back into one. Or hybrids are produced but do less well than either parent type, perhaps because their song is intermediate and attracts nobody, so selection favours birds that discriminate most sharply, the song difference sharpens and the separation completes. That third outcome is reinforcement.
The structure that earns marks: apply the definition, name the isolating mechanism and when it acts, then use it to make the prediction.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Two organisms belong to different species if they look different enough.
Back: Appearance is evidence about a boundary, not the boundary. Male and female eclectus parrots were described as separate species for a century and are one; some pairs of gulls are near-identical and never interbreed. What matters is whether the gene pools are still connected.

Front: CHECK YOURSELF
A hospital finds that the proportion of Staphylococcus aureus isolates resistant to a particular antibiotic has risen from 4% to 61% over eight years. A newspaper reports that 'the bacteria have learned to fight off the drug'. Explain what has actually happened, and describe two different processes that could have contributed to the rise.
Back: No bacterium learned or changed anything. The alleles conferring resistance were already present in a small minority of cells, or arose by random mutation during the eight years; either way they were not produced in response to the drug, and the rise in the proportion of resistant isolates is a change in allele frequency produced by selection.
Use of the antibiotic in the hospital is the selection pressure. Susceptible cells are killed and resistant cells survive and reproduce, so each course of treatment leaves behind a population in which the resistance allele is commoner. Repeated over eight years, that raises the proportion of resistant isolates.
First process: vertical transfer. A resistant cell divides by binary fission and both daughter cells inherit the allele, so resistant lineages increase in number within the hospital's bacterial population.
Second process: horizontal transfer. Resistance genes are often carried on plasmids, which can be passed by conjugation to unrelated bacteria, including bacteria of other species. A previously susceptible cell can therefore become resistant without any mutation of its own, and this can move resistance into the population far faster than division alone.
A third contribution worth a mark if you have room: resistant strains may be brought in from outside, which is migration rather than change within the population.

Front: DEFINE
Define: Genetic drift.
Back: A change in allele frequencies from one generation to the next caused by chance sampling of gametes, rather than by selection. Its effect is greatest in small populations.

Front: DEFINE
Define: Horizontal gene transfer.
Back: Passing genes between bacteria that are not parent and offspring, most often on a plasmid transferred by conjugation.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Bacteria become resistant because the antibiotic makes them mutate.
Back: The mutation was already there, in a few cells out of billions, and it arose while nothing was threatening them. The antibiotic kills everything that lacks it, so the population that grows back descends from the survivors. No cell changed its genotype in response.

Front: DEFINE
Define: Species.
Back: A group of organisms with similar characteristics that can interbreed to produce fertile offspring, and which are reproductively isolated from other such groups.

Front: DEFINE
Define: Fitness.
Back: The contribution an individual makes to the gene pool of the next generation, relative to other individuals in the population, usually counted as offspring that themselves survive to reproduce.

Front: DEFINE
Define: Environmental variation.
Back: Differences between individuals caused by the conditions they have experienced, which are not passed to offspring.