BIO lecture 2

Relative Fitness ($w$)

A standardized measure of biological fitness calculated as an individual's or genotype's absolute reproductive contribution divided by that of the most successful genotype:

$$\text{Relative Fitness } (w) = \frac{\text{Absolute Fitness}}{\text{Maximum Absolute Fitness}}$$

Relative fitness is inherently context-dependent and assesses reproductive success relative to other phenotypes in a given environment.

Negative Frequency-Dependent Selection

A form of balancing selection in which the fitness of a phenotype decreases as it becomes more common and increases when rare, maintaining polymorphism.

• Example: Bumblebee pollinators preferentially visit the rarer color morph (yellow vs. purple) in elderflower orchids (Dactylorhiza sambucina), maintaining both floral morphs.

Heterozygote Advantage

A selective mechanism where individuals heterozygous at a locus have higher relative fitness than either homozygote under specific ecological conditions.

• Example: The sickle-cell hemoglobin allele ($Hb^S$) is maintained at high frequencies where malaria is endemic because heterozygotes have lower susceptibility to malaria while avoiding the fatal complications of full sickle-cell disease.

Clinal Variation

A continuous, gradual phenotypic or allele frequency gradient across the geographic range of a species, typically correlated with an environmental factor.

• Example: European white clover exhibits a temperature cline in cyanide production; cyanide deters slugs and mice in warmer climates, but frost causes cell damage that releases cyanide into the plant's own tissues in colder areas.

Cryptic Species

Two or more distinct, independently evolving lineages that are morphologically identical or nearly indistinguishable, yet are reproductively isolated and do not interbreed.

Biological Species Concept (BSC)

Ernst Mayr's definition of species as groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

• Focuses on reproductive isolating barriers and irreversible divergence.

• Foundational for population genetics.

• Cannot be applied to asexual organisms or extinct fossil taxa.

Evolutionary Species Concept

G. G. Simpson's definition of a species as an ancestor-descendant lineage evolving separately from other lineages, maintaining its own unitary evolutionary role, historical continuity, and tendencies.

• Readily applies to asexual taxa and chronological lineages in the fossil record.

• Essential for macroevolutionary and historical biogeographic analysis.

General Lineage Concept

Kevin de Queiroz's unified concept defining a species as a separately evolving metapopulation lineage.

• Lineage independence is the sole defining criterion of species status.

• Traditional operational criteria (e.g., phenetic diagnosability, reciprocal monophyly, reproductive incompatibility) represent secondary properties acquired asynchronously along the divergence continuum.

Oriole Speciation Paradox (Baltimore vs. Bullock's Orioles)

A case demonstrating conflict between the BSC and phylogenetic history:

• Baltimore Orioles (Icterus galbula) and Bullock's Orioles (I. bullockii) interbreed freely in a secondary contact zone across the Great Plains and were lumped under the BSC as the Northern Oriole.

• Phylogenetically, Baltimore Oriole is sister to the Black-backed Oriole (I. abeillei), and Bullock's Oriole is sister to the Streak-backed Oriole (I. pustulatus).

• Interbreeding capacity is an ancestral symplesiomorphy retained through multiple prior speciation events.

Allopatric Speciation

The divergence of populations into distinct species following geographical separation that eliminates gene flow between non-overlapping ranges.

Vicariance

A mechanism of allopatric speciation where a newly developed geographic barrier physically divides a previously contiguous ancestral population.

• Example: Glaciation dividing highland stream populations; the uplift of the Isthmus of Panama splitting Pacific and Caribbean marine populations.

Dispersal in Allopatric Speciation

A mechanism of allopatric divergence where individuals cross an already existing extrinsic barrier to establish a new, isolated colony.

• Colonizing groups are often tiny founder populations that undergo severe genetic bottlenecks, amplifying the evolutionary influence of genetic drift alongside novel selection pressures.

Hawaiian Picture-Winged Drosophila Radiation

An extensive adaptive radiation driven by island dispersal:

• As volcanic hotspot activity formed Hawaiian islands sequentially, flies dispersed from older northwestern islands to younger southeastern islands.

• Dispersal events and speciation trajectories correlate with island age (Kauai being oldest, Hawaii youngest) and trade wind directions.

Ring Species

A continuous chain of interbreeding populations wrapped around an uninhabitable geographic barrier, where the two terminal populations at the ends of the ring overlap in sympatry without interbreeding.

• Examples: Ensatina salamanders circling California's Central Valley, and Larus gulls around the Arctic Ocean.

Circumpolar Divergence in Larus Gulls

A ring speciation system encircling the northern oceans:

• Adjacent populations interbreed along a circumpolar continuum from western Europe eastward through Siberia and North America.

• When the terminal populations—the herring gull (Larus argentatus) and lesser black-backed gull (Larus fuscus)—overlap in northern Europe, they act as reproductively isolated biological species.

Sympatric Speciation

The evolution of reproductive isolation and divergence into separate species within the same geographic area without physical barriers dividing the ancestral population.

• Requires internal mechanisms (such as disruptive selection paired with assortative mating or polyploidy) to eliminate gene flow.

Threespine Stickleback Limnetic vs. Benthic Ecomorphs

Ecological divergence in postglacial lakes of the Pacific Northwest:

• Limnetic morph: Open-water form with slender body, large eyes, upturned mouth, and dense, fine gill rakers adapted for plankton feeding.

• Benthic morph: Vegetated-zone form with stout, broad body, small eyes, horizontal mouth, and coarse gill rakers adapted for macroinvertebrates.

Assortative Mating in Sticklebacks

A behavioral isolating mechanism where individuals preferentially mate with phenotypically similar partners.

• In postglacial lakes, limnetic females preferentially spawn with limnetic males, and benthic females spawn with benthic males, enforcing reproductive isolation between the two sympatric ecomorphs.

Sympatric Divergence in Howea Palms

Speciation on Lord Howe Island between Howea forsteriana and Howea belmoreana:

• Diverged ~$1.9$ million years ago without geographic separation.
• H. forsteriana adapted to novel, alkaline calcareous soils, whereas H. belmoreana remained on volcanic soils.
• Disruptive soil selection became genetically coupled with a six-week displacement in flowering times, creating strong pre-zygotic temporal isolation.

Autopolyploidy

A form of polyploid speciation resulting from the duplication of chromosome sets within a single ancestral species.

• Caused by meiotic non-disjunction producing unreduced diploid ($2n$) gametes, which fuse to produce an instantaneous tetraploid ($4n$) species.

Allopolyploidy

A mode of polyploid speciation initiated by hybridization between two distinct species followed by chromosome doubling.

• Chromosome duplication restores homologous pairing during meiosis, yielding fertile, reproductively isolated polyploid offspring.

Meiotic Non-Disjunction

The failure of homologous chromosomes or sister chromatids to disjoin properly during meiotic division.

• Generates gametes that receive either two copies of a chromosome set (unreduced $2n$ gametes) or zero copies, serving as the cytological mechanism generating polyploids.

Post-Zygotic Isolation in Tetraploid-Diploid Crosses

The reproductive barrier isolating newly formed tetraploids ($4n$) from their diploid ($2n$) parents:

• Mating between $4n$ and $2n$ individuals yields triploid ($3n$) hybrids.

• Triploids are sterile because their three sets of chromosomes cannot synapse and segregate evenly during meiosis, resulting in aneuploid, non-viable gametes.

Triploid Sterility in Cultivated Bananas

Commercial bananas are seedless triploid ($3n$) cultivars (such as AAB, AAA, ABB) produced by crosses between Musa acuminata ($\text{AA}$) and Musa balbisiana ($\text{BB}$).

• Their triploid state prevents normal homologous pairing during meiosis, resulting in reproductive sterility and seedless edible fruit.

Extreme Polyploidy in Ophioglossum reticulatum

The adder's tongue fern (Ophioglossum reticulatum) possesses the highest recorded chromosome number of any organism, with $2n = 1440$ chromosomes ($n = 720$), highlighting the capacity for repeated rounds of polyploid speciation in plants.

Pre-Zygotic Isolating Mechanisms

Barriers that impede mating or prevent fertilization before zygote formation:

• Temporal isolation: Differences in breeding seasons or daily activity.

• Habitat isolation: Populations utilize distinct microhabitats.

• Behavioral isolation: Lack of mate recognition or courtship compatibility.

• Mechanical isolation: Structural mismatches of genitalia or floral morphology.

• Gametic isolation: Incompatibility between sperm and egg cell receptors.

Post-Zygotic Isolating Mechanisms

Barriers that reduce hybrid fitness after fertilization occurs:

• Hybrid inviability: Zygotes suffer developmental failure or mortality prior to reproductive maturity.

• Hybrid sterility: Offspring survive to adulthood but produce non-viable gametes (e.g., mules with $63$ chromosomes resulting from a donkey with $2n = 62$ and a horse with $2n = 64$).

• Hybrid breakdown: First-generation ($F_1$) hybrids are fertile, but $F_2$ or backcross generations have reduced survival or fertility.

Dobzhansky–Muller Incompatibilities

A genetic model demonstrating how post-zygotic isolation evolves in allopatry without passing through low-fitness intermediates:

• New mutations arise and fix independently at distinct interacting loci within allopatric lineages without causing harm on their native background.

• When secondary contact occurs, hybrid offspring inherit incompatible allele combinations, leading to epistatic conflicts that produce inviable or sterile hybrids.

Relationship Between Genetic Divergence and Reproductive Isolation

Reproductive isolation increases as an empirical function of genetic divergence and time since divergence from a common ancestor:

• Genetically similar sister pairs generally show weak pre- and post-zygotic isolation.

• Genetically distant lineages consistently demonstrate strong or total reproductive isolation.

Reinforcement

The evolutionary reinforcement of pre-zygotic isolating mechanisms driven by natural selection against the production of unfit or sterile hybrid offspring upon secondary contact.

• Example: Phlox drummondii produces blue-pink flowers in allopatric zones, but produces red flowers in areas of sympatry with Phlox cuspidata to minimize pollinator sharing and maladaptive hybridization.