Revised Midterm 2 biol 213

Mechanisms Changing Allele Frequencies

Selection, genetic drift, gene flow, and mutation (the 4 mechanisms of evolution).

Mechanism Changing Genotype Frequencies Only

Non-random mating (alters genotype frequencies and heterozygosity without changing allele frequencies on its own).

Heterozygosity vs. Homozygosity

Heterozygosity is high frequency of heterozygous genotypes (high genetic variation); Homozygosity is high frequency of homozygous genotypes (low variation).

Genetic Drift

Unpredictable, random changes in allele frequencies across generations due to chance sampling of gametes; strongest in SMALL populations.

Effects of Genetic Drift

Loss of genetic variation (decreases heterozygosity, increases homozygosity), allele fixation or loss, and population divergence over time.

Bottleneck Effect

An extreme form of genetic drift resulting from a sudden, drastic reduction in population size (e.g., natural disaster).

Founder Effect

An extreme form of genetic drift occurring when a small group colonizes a new area, carrying only a fraction of the original gene pool's diversity.

Neutral Evolution

Evolutionary changes in the frequency of selectively neutral traits (no fitness impact), driven entirely by genetic drift rather than natural selection.

Gene Flow

The transfer of alleles into (immigration) or out of (emigration) a population.

Effects of Gene Flow

Homogenizes allele frequencies between populations (reduces divergence), restores genetic diversity lost to drift, but can slow down local adaptation.

Unidirectional vs. Bidirectional Gene Flow

Unidirectional = movement of alleles into a population without return movement; Bidirectional = allele exchange in both directions.

Conditions for Evolution by Natural Selection

1. Phenotypic variation in a population; 2. Trait variation is heritable; 3. Variation causes differential survival/reproduction (fitness differences).

Adaptive Evolution

Consistent evolutionary changes driven by natural selection that increase an organism's suitability/fitness for its specific environment.

Directional Selection

Selection favoring phenotypes at ONE extreme of a distribution, shifting the average trait value up or down.

Stabilizing Selection

Selection favoring INTERMEDIATE phenotypes and selecting against extreme traits, narrowing phenotypic variation.

Disruptive (Divergent) Selection

Selection favoring BOTH EXTREMES over intermediate phenotypes, which can split a population into distinct trait groups.

Positive Frequency-Dependent Selection

Selection where a phenotype's fitness INCREASES as it becomes MORE common in the population.

Negative Frequency-Dependent Selection

Selection where a phenotype's fitness INCREASES as it becomes RARER (e.g., host-parasite immune dynamics, predator search images).

Case Study: Finch Beak Evolution (Daphne Major)

Peter & Rosemary Grant showed that drought (1977) selected for deeper beaks (heritable h²=0.74) because larger seeds remained, demonstrating directional selection in real time.

Sexual Selection

Differential reproductive success resulting specifically from variation in ability to obtain mates.

Bateman's Principle (Asymmetry of Sex)

Reproductive success in females is limited by high-cost eggs/resources; male success is limited by access to mates, making sexual selection stronger in males.

Intersexual Selection

Mate choice where individuals of one sex (usually females) choose mates of the opposite sex based on specific traits/displays.

Intrasexual Selection

Direct competition between individuals of the same sex (usually male-male combat/territory defense) for mating access.

Sexual Selection vs. Survival Trade-offs

Exaggerated male traits (e.g., bright plumage) can increase mating success enough to compensate for reduced survival/increased predation risk.

Positive Assortative Mating

Mating between phenotypically similar individuals; INCREASES homozygosity across associated loci.

Negative Assortative Mating

Mating between phenotypically dissimilar individuals; INCREASES heterozygosity across associated loci.

Inbreeding & Inbreeding Depression

Mating between close relatives (common in small populations); increases homozygosity across the entire genome, leading to reduced fitness by exposing harmful recessive alleles.

Speciation

The evolutionary process by which one original lineage splits over time to form two or more distinct species.

Why Defining a Species is Challenging

No single species concept works universally across all living, extinct, sexual, and asexual organisms.

Morphological Species Concept

Defines a species by shared physical/structural features; Pros: simple, applies to sexual/asexual/fossils; Cons: subjective, fails with cryptic species or convergent evolution.

Phylogenetic Species Concept

Defines a species as the smallest group of individuals forming a distinct clade on a phylogenetic tree; Pros: objective/molecular, applies to sexual/asexual; Cons: expensive, genetic differences don't always equal biological differences.

Biological Species Concept

Defines a species as groups of interbreeding populations that produce viable, fertile offspring; Pros: testable, reflects gene flow; Cons: cannot evaluate fossils or asexual organisms.

Reproductive Isolation

The existence of biological barriers that prevent members of different species from producing viable, fertile offspring.

Pre-zygotic Barriers

Reproductive barriers that act BEFORE fertilization by preventing mating or preventing successful fertilization if mating occurs.

Habitat Isolation (Pre-zygotic)

Two species occupy different habitats within the same geographical range and rarely encounter one another (e.g., oak vs. maple tree insect populations).

Temporal Isolation (Pre-zygotic)

Species breed at different times of day, different seasons, or different years.

Behavioral Isolation (Pre-zygotic)

Unique courtship rituals or behavioral displays prevent mating between different species.

Mechanical Isolation (Pre-zygotic)

Structural differences in genitalia or floral anatomy prevent successful mating or pollen transfer.

Gametic Isolation (Pre-zygotic)

Sperm of one species is biochemically unable to fertilize the egg of another species.

Post-zygotic Barriers

Reproductive barriers that act AFTER fertilization, preventing hybrid zygotes from developing into viable, fertile adults.

Reduced Hybrid Viability (Post-zygotic)

Hybrid zygotes fail to develop properly or do not survive to reproductive maturity.

Reduced Hybrid Fertility (Post-zygotic)

Hybrids survive and are healthy, but are completely sterile or have severely reduced fertility (e.g., mules).

Hybrid Breakdown (Post-zygotic)

First-generation hybrids are viable and fertile, but their offspring accumulate lethal defects or sterility.

Core Mechanism of Speciation

Occurs when populations become isolated (no gene flow), allowing mutation, genetic drift, and natural selection to independently diverge gene pools over time.

Allopatric Speciation

The formation of new species occurring when a physical geographic barrier (e.g., river, mountain range) splits a population, halting gene flow.

Sympatric Speciation

The formation of new species occurring WITHOUT geographic barriers while populations inhabit the same overlapping area.

Sympatric Speciation via Habitat Differentiation

Occurs when a subpopulation utilizes a distinct host plant, food source, or microhabitat within the same geographic range.

Sympatric Speciation via Polyploidy

An unreduced chromosome error during meiosis resulting in extra set(s) of chromosomes; instantly creates a reproductive barrier with diploid ancestors.

Polyploidy Reproductive Isolation Mechanism

Mating between polyploid and diploid individuals produces triploid offspring whose chromosomes cannot pair properly in meiosis, resulting in sterility.

Phylogeny & Phylogenetic Tree

Phylogeny is the evolutionary history of a group of species; a phylogenetic tree is a branching diagram representing this history as a hypothesis[cite: 11].

Tree Anatomy: Nodes & Branches

Terminal nodes = studied taxa/species at tips; Internal nodes = hypothetical common ancestors/speciation events; Branches = lineages over time; Root = common ancestor of all taxa in tree[cite: 11].

Sister Groups

Two taxa or clades that split from the exact same immediate common ancestor, making them each other's closest evolutionary relatives[cite: 11].

Equivalent Trees & Node Rotation

Trees sharing the exact same branching pattern (topology); rotating nodes around a branch point does not alter evolutionary relationships[cite: 11].

Cladogram

A phylogenetic tree showing patterns of shared ancestry and lineage relationships without scaled branch lengths[cite: 11].

Phylogram

A phylogenetic tree where branch lengths are proportional to the amount of evolutionary change (e.g., genetic mutations)[cite: 11].

Chronogram

A phylogenetic tree where branch lengths represent absolute physical time elapsed along an explicit timescale[cite: 11].

Monophyletic Group (Clade)

A group consisting of a common ancestor and ALL of its descendants (passes the single "snip test")[cite: 11].

Paraphyletic Group

A group consisting of a common ancestor and SOME, but NOT ALL, of its descendants[cite: 11].

Polyphyletic Group

A group composed of taxa that excludes their most recent common ancestor[cite: 11].

Homology vs. Homoplasy

Homology = traits similar due to shared common ancestry (phylogenetically useful); Homoplasy = traits similar due to convergent evolution (not useful for relatedness)[cite: 11].

Goal of Phylogenetic Reconstruction

Maximize the reliance on homologous traits (homologies) and minimize the impact of convergent traits (homoplasies)[cite: 11].

Synapomorphy

A shared derived trait that arose in the common ancestor of a clade and is present in all its descendants; used to identify evolutionary branch points[cite: 11].

Outgroup

A distantly related taxon used in cladistics as a reference point to distinguish ancestral traits from derived traits[cite: 11].

Maximum Parsimony Method

An algorithmic approach assuming the tree requiring the fewest total evolutionary changes/mutations is the most likely hypothesis[cite: 11].

Limitations of Maximum Parsimony

Evolution does not always follow the simplest path; when multiple topologies require similar step counts, parsimony alone cannot resolve the true tree[cite: 11].

Distance-Based Reconstruction

Estimates relatedness by quantifying pairwise genetic distance (% sequence difference); the outgroup shows the highest distance to all other taxa[cite: 11].

Anagenesis vs. Cladogenesis

Anagenesis = evolutionary change within a single unbranched lineage over time; Cladogenesis = branching speciation where a parent species splits into distinct lineages[cite: 11].

Phyletic Gradualism vs. Punctuated Equilibrium

Gradualism = continuous, steady evolutionary change over long periods; Punctuated Equilibrium = long periods of stasis interrupted by brief, rapid bursts of speciation[cite: 11].

Adaptive Radiation

The rapid evolution of many diverse species filling open ecological niches from a single ancestral lineage over a short timeframe[cite: 11].

Allometry

The study of how biological traits (e.g., metabolic rate, lifespan, organ size) change in relation to body size

Allometric Power Function

Mathematical formula y = b * x^\alpha (log y = \alpha * log x + log b), where y is a trait, x is body mass, and \alpha is the scaling exponent

Isometric Scaling (\alpha = 1)

Scaling where a biological parameter increases in direct proportion to body mass (e.g., heart growth)

Hypermetric Scaling (\alpha > 1)

Scaling where a parameter increases at a greater proportion than body mass (e.g., male fiddler crab claw size)

Hypometric Scaling (\alpha < 1)

Scaling where a parameter increases at a lesser proportion than body mass (e.g., brain growth, basal metabolic rate)

Basal Metabolic Rate (BMR) Scaling

BMR scales hypometrically across organisms (\alpha \approx 0.75); larger animals require LESS energy per unit mass per time than smaller animals

Energy Allocation Categories

Organisms divide limited incoming energy among five competing demands: Maintenance, Growth, Reproduction, Storage, and Defense

Evolutionary Trade-Off

A fitness compromise where allocating energy to one biological trait (e.g., reproduction) reduces energy available for another (e.g., survival or growth)

Offspring Number vs. Offspring Size Trade-Off

The inverse relationship between producing many small offspring (low investment per young) versus few large offspring (high investment per young)

Semelparity vs. Iteroparity

Semelparity = reproducing once in a lifetime followed by death (e.g., salmon); Iteroparity = reproducing multiple times across a lifespan

Fast Life-History Strategy

"Live fast, die young": small body size, early sexual maturity, high fecundity, low parental investment, semelparity, short lifespan (favored in unstable habitats)[

Slow Life-History Strategy

"Live slow, die old": large body size, late maturity, low fecundity, high parental investment, iteroparity, long lifespan (favored in stable habitats)

Cohort (x) & Number of Females (n_x)

A cohort (x) is a group of individuals of the same age followed from birth; n_x is the number of surviving females at age x

Survival Rate (S_x) vs. Survivorship (l_x)

S_x = probability an individual of age x survives to age x+1; l_x = fraction of the original cohort still alive at age x

Fecundity (m_x)

The average number of female offspring produced per living female in age class x

Net Reproductive Rate (R_0) Formula

R_0 = \sum (l_x * m_x); represents the average total female offspring produced per female over her entire lifespan[

Net Reproductive Rate (R_0) Thresholds

R_0 > 1 = population is growing; R_0 = 1 = population size is stable; R_0 < 1 = population is declining

Type I Survivorship Curve

High survival throughout early/middle life, followed by steep mortality in old age (e.g., humans, large mammals)

Type II Survivorship Curve

Constant mortality rate and equal probability of survival throughout the entire lifespan (e.g., songbirds, lizards)

Type III Survivorship Curve

Extremely high initial juvenile mortality, but high survival rates for the few individuals reaching adulthood (e.g., trees, marine invertebrates)

Two-Fold Cost of Males

In sexual populations, only females produce offspring, causing sexual populations to grow at half the rate of asexual populations

Cost of Meiosis

Sexual parents transmit only 50% of their alleles to each offspring, whereas asexual parents transmit 100% of their genome to all offspring

Costs of Sexual Structures and Behavior

The significant time and energy expended on developing sexual ornaments, courting, and locating mates

Evolutionary Benefits of Sex

Recombination allows sexual species to purge harmful mutations faster, combine beneficial alleles into one lineage, and adapt rapidly to coevolving parasites