First-year courses · Sciences

Biology: genetics, evolution and ecology

Natural selection, Mendelian crosses, Hardy-Weinberg, drift, speciation, phylogenies, population growth and ecosystem energy flow in first-year flashcards.

45 cards6 topicsAbout 20 minutes a roundTypical first-year introductory biology, organismal term (genetics, evolution, ecology)

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About this deck

This deck covers the half of first-year biology that deals with populations rather than single cells. It begins with natural selection and the evidence for evolution, moves to Mendelian genetics (monohybrid and dihybrid crosses, test crosses, incomplete dominance, codominance and sex linkage), then population genetics with the Hardy-Weinberg equation, genetic drift and gene flow. It continues with species concepts, reproductive barriers and speciation, reading phylogenetic trees, and ends with population growth models, species interactions, trophic levels and ecological succession.

Background from Grade 12 biology is enough, along with comfort with fractions, percentages and basic probability; Punnett squares and Hardy-Weinberg problems are really probability problems.

Try to answer each conceptual card with a real organism as your example, not just a definition, because exam questions are usually set in a scenario (a bird population on an island, a plant crossed in a greenhouse). For calculation cards, redo the numbers with your own made-up values. After you know the tags individually, mix Mendelian genetics with Population genetics, since many students can do a Punnett square but freeze when the same idea appears as allele frequencies.

Common mistakes: describing evolution as individuals adapting during their lifetime (populations evolve; individuals do not); taking p or q directly from the percentage of dominant phenotypes in Hardy-Weinberg problems instead of starting from the homozygous recessive frequency q^2; and reading phylogenetic trees left to right as a ladder of progress, when the order of tips along the edge carries no meaning. Remember too that fitness means reproductive success, not strength or health.

Before you start: Grade 12 biology and basic probability (fractions and percentages).

All 45 cards

Cover the definitions with a sheet of paper and test yourself, or use the study modes above.

Natural selection and evidence

Natural selection: Process in which heritable traits that improve survival and reproduction in a given environment become more common over generations. Requires variation, inheritance and differential reproductive success.Example: Peppered moths: dark forms increased in soot-darkened forests during industrial England.
Evolutionary fitness: An individual's relative contribution of offspring to the next generation compared with others in the population; reproductive success, not strength.Example: A small bird raising 6 chicks has higher fitness than a large one raising 2.
Adaptation: A heritable trait that increases fitness in a particular environment, shaped by natural selection over many generations.Example: Thick fur and small ears in Arctic foxes reduce heat loss.
Homologous structures: Features in different species that share an underlying structure because they were inherited from a common ancestor, even if their functions differ.Example: Human arm, whale flipper and bat wing share the same bone pattern.
Analogous structures: Features with similar function but different evolutionary origins, produced by convergent evolution under similar pressures.Example: Wings of insects and wings of birds.
Modes of selection: Directional selection favours one extreme; stabilizing selection favours intermediate values; disruptive selection favours both extremes over the middle.Example: Human birth weight shows stabilizing selection: very low and very high weights had higher mortality.
Sexual selection: Selection arising from differences in mating success, through competition within one sex or mate choice by the other; it can favour costly traits.Example: The peacock's large tail attracts mates despite making escape harder.

Mendelian genetics

Allele: One of the alternative versions of a gene at a particular locus.Example: A pea colour gene has a yellow version (Y) and a green version (y).
Genotype versus phenotype: Genotype is the combination of alleles an organism carries; phenotype is the observable trait that results (with environmental influence).Example: YY and Yy peas both look yellow.
Law of segregation: Each individual has two alleles for a gene, and they separate during gamete formation so each gamete gets only one.Example: A Yy plant makes Y and y gametes in equal numbers.
Law of independent assortment: Alleles of genes on different chromosomes (or far apart on one) are sorted into gametes independently of each other.Example: A YyRr plant makes YR, Yr, yR and yr gametes, each 1/4.
Monohybrid cross: A cross between individuals heterozygous for one gene; with complete dominance it gives a 1:2:1 genotype ratio and a 3:1 phenotype ratio.Example: Aa x Aa: 1 AA : 2 Aa : 1 aa, so 3/4 show the dominant trait.
Dihybrid cross: A cross of two individuals heterozygous at two independently assorting genes; it gives a 9:3:3:1 phenotype ratio.Example: YyRr x YyRr: 9 yellow round, 3 yellow wrinkled, 3 green round, 1 green wrinkled out of 16.
Test cross: Crossing an individual with a dominant phenotype to a homozygous recessive one to reveal whether it is homozygous or heterozygous.Example: If any offspring show the recessive trait, the parent was heterozygous.
Incomplete dominance versus codominance: Incomplete dominance: heterozygotes show an intermediate phenotype. Codominance: heterozygotes fully show both alleles.Example: Red x white snapdragons give pink; IA IB gives type AB blood.
Sex-linked inheritance: Inheritance of genes on a sex chromosome (usually the X); males (XY) express any X-linked recessive allele they carry.Example: Carrier mother x unaffected father: half of their sons are expected to have red-green colour blindness.
Product rule in genetics: The probability of independent events all happening is the product of their separate probabilities.Example: AaBb x AaBb: chance of aabb = (1/4)(1/4) = 1/16.

Population genetics

Gene pool and allele frequency: The gene pool is all alleles in a population; an allele frequency is the proportion of gene copies that are a particular allele.Example: 100 diploid individuals have 200 copies; if 60 are A, the frequency of A is 0.30.
Hardy-Weinberg equation: For two alleles with frequencies p and q (p + q = 1), genotype frequencies at equilibrium are p^2 (AA), 2pq (Aa) and q^2 (aa).Example: p = 0.7, q = 0.3: AA 0.49, Aa 0.42, aa 0.09.
Hardy-Weinberg conditions: Allele and genotype frequencies stay constant only with no mutation, random mating, no selection, a very large population and no gene flow.Example: Real populations rarely meet all five, so departures signal evolution.
Solving a Hardy-Weinberg problem: Start from the recessive phenotype frequency, which equals q^2; take the square root for q, then p = 1 - q.Example: 16% show the recessive trait: q = 0.4, p = 0.6, carriers 2pq = 0.48.
Genetic drift: Random changes in allele frequencies from chance events in which individuals reproduce; strongest in small populations and can fix or eliminate alleles.Example: In a population of 10 plants, a storm may wipe out most carriers of an allele by chance.
Bottleneck and founder effects: Two forms of drift: a sharp population reduction (bottleneck) or a small group starting a new population (founder) leaves an unrepresentative gene pool.Example: Northern elephant seals have very low genetic diversity after hunting reduced them to a few dozen.
Gene flow: Transfer of alleles between populations through migration of individuals or gametes; it makes populations more similar.Example: Pollen blown between meadows carries alleles across.
Mutation: A change in DNA sequence; it is the original source of all new alleles, although it changes allele frequencies only slowly by itself.Example: A new mutation for antibiotic resistance spreads once selection favours it.

Speciation and phylogeny

Biological species concept: A species is a group of populations whose members can interbreed in nature and produce fertile offspring, but cannot do so with other such groups.Example: It does not apply to asexual organisms or fossils.
Prezygotic barriers: Reproductive barriers that prevent fertilization: habitat, timing, behaviour, mechanical incompatibility and gamete incompatibility.Example: Two frog species that breed in different months are separated by temporal isolation.
Postzygotic barriers: Barriers acting after fertilization: hybrid offspring die early, are sterile, or have fertile hybrids whose later generations fail.Example: A mule (horse x donkey) is usually sterile.
Allopatric speciation: Formation of new species after a population is split by a geographic barrier, so the parts evolve separately.Example: Harris's and white-tailed antelope squirrels live on opposite sides of the Grand Canyon.
Sympatric speciation: Formation of new species without geographic separation, for example through polyploidy or strong habitat or mate preferences.Example: Many plant species arose from polyploid individuals that cannot breed with the parent species.
Phylogenetic tree: A branching diagram of hypothesized evolutionary relationships; each node is a common ancestor, and branch tips are taxa.Example: Humans and chimpanzees share a more recent common ancestor than either does with gorillas.
Clade: A group made of an ancestor and all of its descendants (a monophyletic group).Example: Birds plus all other dinosaurs form one; reptiles excluding birds do not.
Shared derived character: A trait that evolved in a common ancestor and is shared by its descendants; used to define clades (also called a synapomorphy).Example: Hair is one for mammals.
Maximum parsimony: Principle of choosing the phylogenetic tree that requires the fewest evolutionary changes to explain the observed traits.Example: A tree needing 5 trait changes is preferred over one needing 9.

Population ecology

Exponential growth: Growth when resources are unlimited: dN/dt = rN, so the population increases faster as it gets larger (J-shaped curve).Example: N = 100, r = 0.1 per year: growing at 10 individuals per year at that moment.
Logistic growth: Growth that slows as the population nears carrying capacity K: dN/dt = rN(K - N)/K (S-shaped curve).Example: N = 500, K = 1000, r = 0.2: dN/dt = 0.2(500)(0.5) = 50 per year.
Carrying capacity (K): The maximum population size an environment can sustain over time given its food, space and other resources.Example: Growth is fastest at N = K/2 in the logistic model.
r-selected and K-selected species: Life-history strategies: r-selected species have many small offspring with little care (unstable habitats); K-selected species have few offspring with much care (near capacity).Example: Mice and dandelions lean r-selected; elephants and whales lean K-selected.
Density-dependent factors: Influences on birth or death rates that intensify as population density rises, such as competition, disease and predation.Example: Disease spreads faster in a crowded deer herd. A wildfire, by contrast, is density-independent.
Survivorship curves: Plots of surviving individuals versus age: Type I (most die old), Type II (constant death rate), Type III (most die young).Example: Humans are Type I; many fish and plants are Type III.

Communities and ecosystems

Species interactions: Competition (both harmed), predation and parasitism (one gains, one harmed), mutualism (both gain) and commensalism (one gains, other unaffected).Example: Bees and flowers are mutualists.
Competitive exclusion principle: Two species competing for exactly the same limiting resources cannot coexist indefinitely in the same place; one will be eliminated or the niches will diverge.Example: Gause's Paramecium cultures: one species outcompeted the other.
Keystone species: A species whose effect on its community is far larger than its abundance would suggest; removing it reshapes the ecosystem.Example: Sea otters on the Pacific coast keep sea urchins in check, protecting kelp forests.
Trophic levels and the 10% rule: Feeding levels from producers to consumers; on average only about 10% of energy at one level is passed to the next, limiting food chain length.Example: 10 000 kJ in producers supports about 1000 kJ in herbivores and 100 kJ in carnivores.
Ecological succession: Gradual change in community composition after a disturbance: primary succession starts on bare ground without soil, secondary succession where soil remains.Example: Lichens and mosses colonize bare rock left by a retreating glacier.