Evolution Exam 2 Revised Again

Natural Selection (in Adaptive Landscapes)

Always moves a population up a fitness peak if able; it never moves a population down

Genetic Drift (in Adaptive Landscapes)

Moves a population in any direction; it is always random and always occurring

Local Maxima

A fitness peak that is not the global optimum; natural selection can trap populations here because it is myopic (short-sighted)

Myopic Selection

Natural selection only "sees" the immediate fitness increase; it will not permit a population to travel through a fitness valley to reach a higher peak

Sickle Cell Allele A (Genotype AA)

Normal phenotype but susceptible to malaria; fitness value of 0.9

Sickle Cell Allele S (Genotype AS)

Sickle trait conferring malarial resistance; highest fitness in malarial environments (1.0)

Sickle Cell Allele S (Genotype SS)

Causes severe anemia; lowest fitness value (0.2)

Sickle Cell Allele C (Genotype CC)

Normal phenotype and resistant to malaria; the global fitness maximum (1.3)

The C Allele Trap

The C allele is often trapped in low-fitness heterozygotes (AC or SC) unless genetic drift or inbreeding allows it to reach a high enough frequency for selection to "see" the CC genotype

Shifting Balance Theory Phase 1

Genetic drift in local populations (demes) shifts allele frequencies, potentially moving them toward a new fitness peak

Shifting Balance Theory Phase 2

Intrademic selection (mass selection) pushes the population up the local peak

Shifting Balance Theory Phase 3

Interdemic selection where high-fitness demes export more migrants, eventually spreading the "best" allele frequencies to all demes

Deme

A subdivision of a population consisting of closely related individuals that typically breed within the group

Polymorphism

The occurrence of two or more possibilities of a trait on a gene within a population

Substitution

The product of a mutation leading to polymorphism and fixation, where the wild-type allele in a population is replaced by a new one

The Classical School

Argues that most new alleles are deleterious, the "wild type" is best, and selection primarily removes variation

The Balance School

Argues that most alleles can be beneficial depending on the environment and that selection often preserves multiple alleles

Neutral Theory of Molecular Evolution

Proposed by Motoo Kimura; states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants rather than Darwinian selection

Rate of Neutral Evolution (K)

K = u; the rate of neutral evolution is equal to the neutral mutation rate

Probability of Neutral Fixation

The probability that a new neutral mutation will eventually become fixed in a population is equal to its initial frequency, 1/2Ne

Effective Population Size (Ne)

The number of individuals in a population that actually contribute to the gene pool

Divergence (Div)

The number of differences between two sequences divided by the length of the sequences (n_diffs / L)

Evolutionary Rate (R_evo)

The divergence divided by the time since the most recent common ancestor (Div / tMRCA)

Lineage Rate Calculation

When calculating the evolutionary rate for a single lineage from total divergence, the total must be divided by two

Synonymous Sites

Nucleotide positions where differences do not change the amino acid; these are often called silent sites and usually evolve neutrally

Nonsynonymous Sites

Nucleotide positions where differences result in amino acid replacement, affecting the phenotype and subject to selection

dN/dS Ratio = 1

Indicates neutral evolution

dN/dS Ratio > 1

Indicates positive (adaptive) selection; nonsynonymous changes are favored (eg, antigen-binding sites)

dN/dS Ratio < 1

Indicates purifying (negative) selection; nonsynonymous changes are eliminated to preserve function

Functional Constraint

The degree to which a protein's function limits its ability to tolerate mutations; strong constraint (eg, histones) leads to very slow evolution

Predictability

An organism's physiological ability to sense and prepare for future environmental changes (eg, circadian clocks)

Environmental Grain

The ratio between the frequency of an organism's reproduction and the frequency of environmental fluctuation

Coarse-grained Environment

An environment where each generation experiences only one dominant state, though it may change between generations

Fine-grained Environment

An environment where individuals experience multiple states or rapid fluctuations within a single generation

Conservative Bet-hedging (CBH)

A strategy where organisms always use the same phenotype regardless of environmental cues to minimize long-term variance

Diversifying Bet-hedging (DBH)

A strategy where parents produce offspring with multiple different phenotypes at once

Phenotypic Plasticity

The ability of a single genotype to alter its phenotype (behavior, physiology, or morphology) in response to environmental changes

Norms of Reaction

The array of different phenotypes displayed by a single genotype across a range of environments

Canalization

The resistance of a genotype to phenotypic change, resulting in a constant phenotype across different environments

G x E Interaction

Occurs when two different genotypes respond to environmental variation in different ways; graphically, their reaction norms are not parallel

Quantitative Trait Locus (QTL) Mapping

A statistical method used to identify which molecular markers are linked to genes that contribute to quantitative (continuous) traits

Genome-Wide Association Study (GWAS)

A method of scanning the genome for associations between genetic markers and traits without requiring experimental crosses

Linkage Equilibrium (LE)

When the genotype at one locus is independent of the genotype at another locus on the same chromosome

Linkage Disequilibrium (LD)

A non-random association between loci where genotypes are dependent; it is increased by drift and decreased by recombination

Additive Model of Gene Effects

A model where alleles from multiple loci are "added together" to determine the final phenotype

Heritability (h^2)

The proportion of phenotypic variation in a population that is due to genetic variation (h^2 = Vg / Vp)

The Breeders Equation

R = h^2 * S; calculates the response to selection based on heritability and the selection differential

Selection Differential (S)

The difference between the mean of the selected parents and the mean of the entire parental population (P* - Pbar)

Response to Selection (R)

The difference between the mean of the offspring from selected parents and the mean of offspring from the original population (O* - Obar)

Two-fold Cost of Sex

The observation that asexual populations can grow twice as fast as sexual populations because every individual can produce offspring

Muller's Ratchet

The process by which asexual populations accumulate deleterious mutations over time that cannot be removed without recombination

Red Queen Hypothesis (Sex)

The theory that sexual reproduction is favored because it generates novel genotypes that can keep up with rapidly evolving parasites

Anisogamy

A reproductive system with different-sized gametes: small, low-investment sperm and large, high-investment eggs

Bateman's Principle

Sexual selection is generally stronger in the sex with higher variance in reproductive success (typically males)

Intra-sexual Selection

Competition between members of the same sex for access to mates (eg, combat or sperm competition)

Inter-sexual Selection

Mate choice, where the high-investment sex (typically females) chooses mates based on quality or displays

Good Genes Hypothesis

The theory that females choose males with elaborate displays because those traits are honest signals of high genetic quality

Runaway Sexual Selection

A positive feedback loop where a male trait and the female preference for it become genetically correlated (LD), leading to exaggerated traits}