BIOL 243 - Theme 4: Mendelian Genetics, Population Genetics, & Ways of Change: Drift, Selection, & Speciation

Explain what alleles and genes are, and how their expression results in different
phenotypes

• a gene is a section of DNA on a chromosome that codes for a specific trait (e.g., flower colour, height)

• an allele is a version of a gene (e.g., tall vs short for the height gene)

• your genotype is the combination of alleles you have (e.g., AA, Aa, or aa)

• the phenotype is the observable trait (e.g., purple flowers, tall plant), which depends on the genotype and whether the alleles are dominant or recessive

takeaway: genotype determines phenotype—the alleles you carry influence how traits appear

Explain how meiosis determines the frequency and genotype of gametes of
homozygous and heterozygous individuals

• during meiosis (anaphase I), homologous chromosomes (each with one allele) are separated into different gametes

• A homozygous individual (e.g., AA or aa) will produce gametes that all carry the same allele

• A heterozygous individual (e.g., Aa) will produce two types of gametes in equal frequency—50% a, 50% a

• this random separation explains the genotypic ratios observed in offspring

takeaway: meiosis physically separates alleles, which determines what combination gametes—and eventually zygotes—receive

Explain Mendel’s genetic crosses (monohybrid cross and test cross) and the
associated genotypic and phenotypic ratios

• a monohybrid cross is between two heterozygous individuals (e.g., Aa x Aa) for one gene

  • Genotypic ratio: 1:2:1 (AA: Aa: aa)

  • Phenotypic ratio: 3:1 (dominant: recessive)

• A test cross is between an individual showing the dominant phenotype (could be AA or Aa, we don’t know yet) and a homozygous recessive individual

  • if offspring show any recessive phenotype, the dominant parent was heterozygous (Aa)

  • if all offspring show the dominant phenotype, the dominant parent was homozygous dominant (AA)

takeaway: monohybrid crosses revel expected inheritance patterns of heterozygous individuals; test crosses reveal genotype.

Explain the difference between dominant and recessive alleles

• dominant alleles are expressed in the phenotype when present—even if only one copy is inherited

• recessive alleles are only expressed when two copies are present

at a molecular level:

• dominant alleles often produce a functional protein that results in a trait

• recessive alleles may be non-functional or defective, so the trait is only seen when both alleles are recessive (no functional protein produced)

takeaway: dominance usually reflects whether the allele codes for a working version of a gene product

Explain the difference between incomplete dominance and co-dominance and how
these affect phenotypic ratios

• incomplete dominance: one allele is not fully dominant over the other

  • the heterozygous phenotype is intermediate between the two homozygous phenotypes

  • example: red (CRCR) x white (CWCW) —> Pink (CRCW)

  • phenotypic ratio is 1 red: 2 pink: 1 white

  • genotypic and phenotypic ratios are the same (1:2:1)

• Co-dominance: both alleles are equally dominant and fully expressed in the heterozygote

  • example: Blood type AB (A allele for antigen and B allele for antigen are both expressed)

  • heterozygotes show both traits clearly, not a blend

takeaway: in incomplete dominance, phenotypes mix; in co-dominance, both traits show up side by side

Describe the difference between discrete and continuous traits

• Discrete traits: traits that fall into distinct categories, often controlled by a single gene (e.g., purple vs. white flowers)

  • Just means there are separate, countable categories

  • only a few possible phenotypes, no blending

  • follows simple Mendelian inheritance, co-dominance, incomplete dominance. not to strict on that.

  • examples: blood type, flower colour

• continuous traits: traits that show a range of values along a spectrum, influenced by multiple genes (polygenic) and environmental factors

  • no clear-cut categories

  • examples: height (milk as an example of an environmental factor), skin colour, body size

takeaway: discrete = either/or; continuous = on a spectrum

Be able to explain why HWE is a null hypothesis for evolutionary change

• HWE assumes that no evolutionary forces are acting on a population

• if all conditions are met, allele frequencies will stay constant across generations

• this represents the “no change” scenario — so it’s used as a null hypothesis for evolution

• if observed data deviates from HW expectations, it suggest that evolutionary change may be occurring (reject the null hypothesis in this case)

takeaway: HWE = baseline expectation; any departure suggests something is causing evolution

Describe in their own words the major assumptions of Hardy-Weinberg
equilibrium and how violations of assumptions affect testing for HWE

• five expectations for a population to stay in HWE:

  • no mutation — alleles don’t change or new ones aren’t introduced

  • no migration (gene flow) — no individuals enter or leave the gene pool

  • large population size (no genetic drift) — prevents random allele changes + the effect of random events (ex. flood)

  • no natural selection — all genotypes have equal survival and reproduction

  • random mating — no preference based on genotype or phenotype

• if any assumption is violated, allele frequencies can change, meaning:

  • the population is evolving

  • it is no longer in equilibrium

  • we reject the null hypothesis (HWE no longer applies)

takeaway: violating HWE assumptions means the population may be undergoing evolutionary change)

Distinguish the general differences between disruptive, directional and stabilizing selection

Disruptive selection

• both extreme phenotypes are favoured, and intermediate phenotypes are selected agains

• lead to a bimodal distribution of traits (when on a graph of the population’s trait distribution, there are two distinct peaks—so instead of one average/typical trait, you have two dominant forms that are both common/being selected for and the middle/average trait is rare)

  • ex. finches with either large or small beaks surviving better than medium-sized ones

Directional selection

• one extreme phenotype is favoured

• the average trait value shifts in one direction over time

  • ex. darker-coloured moths become more common due to pollution (peppered moths)

Stabilizing selection

• intermediate phenotype is favoured 9the average)

• both extremes are selected against

• trait variation decreases, but the mean stays the same

  • ex. human birth weight—too low or too hight = lower survival

Explain what sexual selection is

sexual selection is a form of natural selection that acts on traits that increase an individuals chance of mating, not necessarily surviving or producing more viable offspring.

• mating success = how often an individual gets to reproduce

• sexual selection favours traits that improve access to mates, not traits that guarantee more or healthier offspring

two types:

• intrasexual selection: competition within a sex for mates (e.g. male-male fighting)

• intersexual selection (e.g. females choosing colourful or dominant males)

takeaway: traits can increase mating success even if they reduce survival—because mating=step one in passing on genes

Explain Bateman’s Principle

Bateman’s principle states that:

• males experience more variation in reproductive success than females

• mating success increases fitness more for males than for females

this happens because:

• males produce man cheap sperm — fitness limited by how many mates they get

• females produce fewer, costly eggs — fitness limited by resources and care, not the number of mates

takeaway: sexual selection tends to act more strongly on males than females meaning males experience experience more intense evolutionary pressure to compete for mates, because their fitness increases the more they mate. Females don’t gain as much from having more mates—their reproductive success depends more on quality, so selection tends to favour choosiness in females, and males evolve traits to compete or impress for mating access (leading to more competition and variation in male reproductive success)

Explain what drift and inbreeding are and why they are more important in small populations

• genetic drift is the random change in allele frequencies due to chance events (e.g. random survival or reproduction)

  • strongest in small populations, where random effects can cause alleles to become fixed or extinct/lost

  • leads to reduced genetic variation

  • this is heavily related to Hardy-Weinberg equilibrium because one of the conditions to keeping HWE is a large population so the effects of genetic drift aren’t as great/prominent—remember the m&m’s demonstration

• inbreeding is mating between relatives, which increases homozygosity and the risk of inbreeding depression

  • this doesn’t change allele frequencies directly, but increases the chance that harmful recessive alleles are expressed

  • inbreeding depression: a drop in fitness due to increased expression of deleterious recessive traits (e.g., genetic diseases, lower survival or fertility)

takeaway: in small populations, drift and inbreeding both reduce variation and can lead to reduced fitness and adaptability over time

Describe the major forms of reproductive isolation and modes of speciation

Reproductive isolation prevents gene flow and leads to speciation

• prezygotic barriers (before fertilization)

  • temporal (different mating times ex. ovulating cycles)

  • behavioral (courtship differences)

  • mechanical (incompatible anatomy ex. big dog, small dog)

  • habitat isolation (geographical barrier)

  • gametic isolation (sperm can’t fertilize eggs ex. lecture slide about the plants having their gametes in different areas of the plant (bottom vs. top))

• postzygotic barriers (after fertilization)

  • Reduced hybrid viability

  • reduced hybrid fertility

  • hybrid breakdown (offspring of hybrids have reduced fitness)

modes of speciation:

• Allopatric speciation: populations split by a physical barrier, evolve separately

• sympatric speciation: occurs without geographic separation, often due to ecological specialization or polyploidy

  • ecological isolation (e.g., niche specialization)

  • polyploidy in plants (instant speciation via chromosome doubling—offspring are instantly a new species by definition as they are genetically different from parents)

takeaway: isolation (pre or post) blocks gene flow, and over time this leads to speciation

Describe the Biological Species Concept and distinguish it from the
Morphological Species Concept

• the biological species concept (BSC) defines species as groups of individuals that can interbreed and produce fertile, viable offspring

  • based on reproductive isolation — if two groups can’t reproduce successfully, they’re different species

  • pro: more biologically meaningful

  • con: hard to apply in the field—doesn’t work well with fossils, asexual species, or when reproduction can’t be directly observed

• the morphological species concept defines species based on physical traits (e.g., size, shape, colour, wings)

  • pro: easy to apply in practice—especially with fossils

  • con: can be inaccurate—some species look alike (cryptic species), and some vary a lot within the same species

takeaway: BSC=based on who can mate, morphological=based on how they look