Vocabulary Flashcards: Evolution, Speciation, Phylogeny, and Animal Form & Function
1. Evolutionary Processes: Natural Selection, Adaptation, and Modes
Natural Selection describes how populations change over time, leading to adaptations that improve survival and reproduction in a specific environment.
It depends on three principles:
Inheritance: Traits are passed from parents to offspring (e.g., a cactus passes on its inherited genes, not new ones it 'needed').
Overproduction & Competition: More offspring are produced than can survive, leading to competition for limited resources.
Variation & Differential Survival: Individuals vary in traits; those with advantageous heritable traits are more likely to survive and reproduce, passing those traits on.
Adaptation: A heritable trait that enhances an organism's fitness (reproductive success) in a particular environment (e.g., hummingbird beaks for nectar).
Environmental Dependence: The fitness of a trait is specific to the environment (e.g., a trait helpful in one environment might be harmful in another).
Evolutionary Trade-off: When a beneficial trait (like colorful plumage for attracting mates) also comes with a cost (like increased predation risk).
Modes of Natural Selection:
Directional Selection: Favors one extreme phenotype, shifting the population mean over time (e.g., snow voles becoming smaller due to early snowfall, or larger if late snowfall makes larger size advantageous).
Stabilizing Selection: Favors intermediate phenotypes, reducing variation (e.g., optimal human birth weight).
Diversifying (Disruptive) Selection: Favors two or more extreme phenotypes over intermediate ones, increasing variation.
Frequency-dependent selection: Fitness of a phenotype depends on its frequency in the population; can favor common or rare phenotypes.
2. Genetic Variation and Population Dynamics
Genetic Diversity: The variety of alleles and genotypes within a population.
Heritability: The proportion of phenotypic variation in a population that is due to genetic variation (e.g., male guppy coloration is heritable if offspring resemble fathers).
Genetic Drift: Random changes in allele frequencies, more pronounced in small populations.
Consequences: typically decreases genetic diversity, can cause rare alleles to become more common or be lost, and can increase the frequency of deleterious recessive alleles. It does not increase genetic diversity.
Founder Effect: A new population is established by a small number of individuals whose gene pool differs from the source population (e.g., turtles separated by a highway starting a new population with unique allele frequencies).
Bottleneck Effect: A drastic, random reduction in population size that alters the genetic makeup and reduces diversity.
Mutation: The ultimate source of new genetic variation (new alleles).
E. coli Experiment: Even in genetically identical populations with unlimited resources, mutations will occur randomly over generations, and if any of these mutations provide a slight fitness advantage, natural selection can differentiate the populations.
3. Speciation and Reproductive Isolation
Speciation: The process by which one original species splits into two or more descendant species.
Biological Species Concept: Defines a species as groups of interbreeding natural populations that are reproductively isolated from other such groups, producing fertile, viable offspring.
Limitations: Cannot be applied to asexual organisms or extinct species (e.g., Q25).
Reproductive Isolation: Mechanisms that prevent gene flow between different species.
Prezygotic Barriers (before fertilization):
Behavioral Isolation: Differences in courtship rituals or other behaviors (e.g., distinct bird courtship dances).
Temporal Isolation: Species breed at different times.
Habitat Isolation: Species live in different habitats and rarely encounter each other.
Mechanical Isolation: Incompatible reproductive structures.
Gametic Isolation: Sperm and egg are incompatible.
Postzygotic Barriers (after fertilization):
Hybrid Inviability: Zygote fails to develop.
Hybrid Sterility: Hybrids are viable but infertile (e.g., mules).
Hybrid Breakdown: First-generation hybrids are fertile, but subsequent generations (F2 or later) are infertile or suffer reduced viability (e.g., Senecio plant example).
4. Phylogeny and Classification
Phylogenetic Tree: A diagram showing the evolutionary relationships and history among organisms.
Rooted Tree: Has a common ancestor at the base.
Node: Represents a common ancestor where lineages diverge.
Clade (Monophyletic Group): A group that includes a common ancestor and all of its descendants (e.g., a group of fish or a group including turtles, lizards, snakes, crocodiles, and birds if it includes their most recent common ancestor and all its descendants).
Taxon: Any group of organisms that has been formally named (can be a clade, but not always).
Traits on Phylogenetic Trees:
Ancestral Trait: A trait present in the common ancestor of a group and shared by its descendants (e.g., lungs are ancestral for mammals, turtles, lizards, snakes, crocodiles, and birds as they evolved early in the lineage).
Derived Trait: A trait that evolved relatively recently in a specific lineage, not present in the common ancestor of a larger group.
Homologous Traits: Shared traits due to common ancestry (e.g., forelimbs of bats and humans).
Convergent Traits (Analogous Traits): Traits that evolve independently in distantly related species due to similar selective pressures, not common ancestry (e.g., flight in bats and insects, or potentially traits like nocturnal activity or specialized diving among different bird groups if they evolved separately).
Reading Phylogenetic Trees:
Relatedness: Organisms that share a more recent common ancestor are more closely related and are expected to have fewer genetic differences (e.g., sister taxa like flamingo and grebe).
Divergence: After a node, lineages evolve independently, subject to different evolutionary forces.
5. Molecular Evolution
Gene Duplication: A mechanism where a gene is duplicated, providing a redundant copy that can then evolve new functions.
Neofunctionalization: When a duplicated gene evolves an entirely new function (e.g., a trypsinogen gene duplicating and becoming an antifreeze protein).
Selection on Genes:
Synonymous vs. Nonsynonymous Changes: Synonymous substitutions are DNA changes that do not alter the amino acid sequence; nonsynonymous changes do.
If Nonsynonymous/Synonymous (N/S) > 1: Indicates positive selection, where beneficial amino acid changes are favored.
If N/S < 1: Indicates purifying selection, where deleterious amino acid changes are removed.
If N/S = 1: Indicates neutral evolution, where mutations accumulate randomly and there's no strong selection pressure (e.g., in pseudogenes, which are non-functional gene copies).
Molecular Clock: The concept that mutations accumulate in genomes at a relatively constant rate, allowing researchers to estimate the time since two species or lineages shared a common ancestor (e.g., dating the appearance of HIV in humans).
6. Organismal Physiology: Gas Exchange, Thermoregulation, and Diffusion
Gas Exchange Surfaces: Efficient gas exchange requires:
Large Surface Area: To maximize the amount of gas that can diffuse (e.g., ~75extm275extm2 in human lungs).
Short Diffusion Distance (Thin Membrane): To allow gases to pass quickly (e.g., alveoli and capillaries are one cell thick).
Fick's Law of Diffusion: The rate of diffusion is directly proportional to the surface area and the concentration difference, and inversely proportional to the thickness of the membrane. (RateimesDimesAimes(P1−P2)/TRateimesDimesAimes(P1−P2)/T where D=diffusion coefficient, A=surface area, (P1-P2)=concentration difference, T=thickness).
Countercurrent Exchange (e.g., in fish gills): A highly efficient mechanism where two fluids flow in opposite directions, maintaining a favorable diffusion gradient along the entire length of the exchange surface, maximizing transfer of a substance (like oxygen into blood).
Pulmonary Embolism: A blood clot blocking an artery to the lungs immediately affects perfusion (blood flow to the lung tissues for gas exchange), rather than ventilation or structural aspects like surface area or path length.
Thermoregulation: The process by which organisms maintain internal body temperature.
Endotherms (Homeotherms): Maintain a stable internal body temperature through metabolic heat and insulation. Their body temperature remains the same even as environmental temperature increases.
Ectotherms (Poikilotherms): Rely on external heat sources. Their body temperature increases as environmental temperature increases.
Thermoneutral Zone: The environmental temperature range where an organism does not need to expend metabolic energy to maintain its core body temperature.
7. Biology of Skin Color (Not in original notes, but implied by exam questions)
Polygenic Trait: Skin color is determined by the cumulative effect of many genes, leading to a continuous spectrum of shades.
UV Radiation Effects:
Vitamin D Synthesis: UV light is necessary for vitamin D production in the skin. Lighter skin in low-UV environments allows sufficient vitamin D synthesis.
Folate Degradation: UV light degrades folate (a B vitamin essential for fetal development and sperm production). Darker skin in high-UV environments protects folate.
Skin Cancer Risk: Exposure to UV light increases the risk of skin cancer.
Evolutionary Balance: Human skin color evolved as a balance between adequate vitamin D production and protection against folate degradation and skin cancer, tailored to local UV levels. If UV no longer affected vitamin D, there would be less selective pressure for lighter skin, and darker skin (for folate protection) might become more prevalent globally.
Implications for Future Evolution: As climate change alters UV radiation levels and lifestyle changes reduce sun exposure, human populations may continue to adapt in terms of skin pigmentation to optimize health.