Mechanisms of Microevolution, Hardy-Weinberg Equilibrium, and Speciation

Comprehensive practice flashcards reviewing microevolutionary mechanisms, population genetics calculations, Hardy-Weinberg equilibrium assumptions and tests, and macroevolutionary speciation modes.

Microevolution

A change in the frequencies of alleles or genotypes within a population across successive generations.

Macroevolution

The evolutionary pattern occurring at or above the species level, including the origin of new species.

Mutation

A permanent change to DNA sequence that generates new alleles randomly with respect to organismal fitness at a low frequency.

Lactose Tolerance Mutation

A beneficial point mutation involving a $C \rightarrow T$ substitution in the lactase gene that enables lactase persistence in adults.

Natural Selection

A non-random process wherein individuals with favorable phenotypes survive and reproduce at higher rates than individuals with other phenotypes.

Adaptive Evolution

Evolutionary change driven exclusively by natural selection that results in a better match between organisms and their current environment.

Non-Random Mating

A form of selection in which individuals possessing a specific phenotype are more or less likely to mate with one another than expected by chance.

Assortative Mating

A non-random mating pattern in which individuals choose mates based on phenotypic similarity or dissimilarity to themselves.

Positive Assortative Mating

Mating where individuals choose phenotypically similar mates, which decreases heterozygosity and can promote disruptive selection.

Negative Assortative Mating

Mating where individuals choose phenotypically dissimilar mates, which increases heterozygosity and can maintain balancing selection.

Inbreeding

The mating of genetically similar relatives that increases homozygosity and exposes deleterious recessive alleles.

Inbreeding Depression

A marked reduction in biological fitness within a population resulting from elevated homozygosity caused by matings between close relatives.

Self-Fertilization

Sexual reproduction occurring within a single organism via meiosis and fertilization, which decreases heterozygosity across generations.

Gene Flow

The transfer of alleles or traits into or out of a population via migration, which is non-adaptive but can alter the rate of selection.

Immigration (Gene Flow)

The movement of individuals or gametes and their associated alleles into a population, often increasing local genetic diversity.

Emigration (Gene Flow)

The movement of individuals or gametes and their associated alleles out of a population, often decreasing local genetic diversity.

Genetic Drift

Any change in allele or genotype frequencies in a population that is caused purely by random chance events, having larger effects in small populations.

Fixation

The state in which only a single allele remains at a specific gene locus in a population, reaching a frequency of $1.0$.

Allele Loss

The complete disappearance of an allele from a population's gene pool due to drift or selection, bringing its frequency to $0$.

Population Bottleneck

A severe drop in population size due to an environmental catastrophe or event, leading to the random loss of genetic diversity.

Founder Effect

A form of genetic drift occurring when a small subset of individuals separates from a larger population to establish a new colony carrying limited genetic variation.

Ellis-van Creveld Syndrome

A recessive condition with dwarfism and polydactyly occurring at a $0.5\%$ frequency in Lancaster County Amish descended from $3$ couples, compared to $< 0.002\%$ globally.

Greater Prairie Chicken Bottleneck

An Illinois population collapse from $1{,}000\text{--}25{,}000$ birds to $< 50$ in 1993, decreasing alleles per locus from $5.2$ to $3.7$ and egg hatch rates from $93\%$ to $< 50\%$.

Tiburon Island Desert Bighorn Sheep

A population founded by $20$ individuals in 1975 that grew to $500$, displaying lower heterozygosity across $11$ surveyed genes than mainland sheep.

Evolutionary Compromise in Guppies

An evolutionary tradeoff in male Poecilia reticulata where bright coloration increases mating success via sexual selection but increases predation mortality via natural selection.

Habsburg Jaw

A distinctive mandibular prognathism overrepresented in European royal lineages such as Charles II of Spain as a result of repeated familial inbreeding.

Florida Panther Genetic Rescue

An outbreeding management program introducing Texas cougars into the Florida panther population to increase heterozygosity and alleviate inbreeding depression.

Random Sampling of Gametes

A stochastic mechanism of genetic drift wherein gametes unite by chance in proportions that deviate slightly from expected Mendelian ratios.

Northern Elephant Seal Bottleneck

A historic reduction in Northern Elephant Seal numbers that severely depleted genetic variation across the surviving population via genetic drift.

Snow Goose Plumage Morphs

A phenotypic system consisting of white and blue morphs that exhibits positive assortative mating through mate preferences for birds of the same color.

White-throated Sparrow Polymorphism

A plumage and behavioral system in which white-striped and tan-striped morphs engage in negative assortative mating, maintaining balancing selection.

Inbreeding Avoidance Mechanisms

Evolved behavioral and life-history adaptations, such as sex-biased natal dispersal and negative assortative mating, that reduce matings between relatives.

Cheetah Genetic Uniformity

The near-total absence of genetic variation observed in cheetah populations caused by ancient population bottlenecks and prolonged genetic drift.

Non-Adaptive Evolutionary Mechanisms

Evolutionary forces—specifically mutation, gene flow, and genetic drift—that alter allele frequencies without necessarily conferring an adaptive advantage.

Monohybrid Cross F2 Ratio Deviation

The slight variation from an exact $3:1$ phenotypic ratio observed in Mendel's breeding experiments, caused by the random sampling of gametes.

Hardy-Weinberg Theorem

A foundational theorem demonstrating that allele frequencies in diploid populations remain constant across generations in the absence of microevolutionary forces.

Hardy-Weinberg Equation

The mathematical relationship $p^2 + 2pq + q^2 = 1$, used to calculate expected genotype frequencies from measured allele frequencies.

Gene Pool

The complete collection of all alleles for every gene locus present within a specified interbreeding population.

Genetic Structure

The combination of allele frequencies, genotype distribution, and total population size ($N$) for a specific gene locus in a population.

Actual Genotype Frequency

The proportion calculated as the observed count of individuals of a particular genotype divided by the total population size ($N$).

Actual Allele Frequency

The proportion calculated as the total copies of a specific allele divided by the total number of alleles at that locus in the population ($2N$).

Diploid Total Alleles ($2N$)

The total number of gene copies at a single locus in a population of diploid organisms, calculated as two times the population size ($N$).

$p$ (Hardy-Weinberg)

The variable representing the frequency of the dominant allele for a given gene locus in a population.

$q$ (Hardy-Weinberg)

The variable representing the frequency of the recessive allele for a given gene locus in a population.

$p^2$ (Hardy-Weinberg)

The term representing the expected frequency of homozygous dominant individuals ($AA$) in a population at Hardy-Weinberg equilibrium.

$2pq$ (Hardy-Weinberg)

The term representing the expected frequency of heterozygous individuals ($Aa$) in a population at Hardy-Weinberg equilibrium.

$q^2$ (Hardy-Weinberg)

The term representing the expected frequency of homozygous recessive individuals ($aa$) in a population at Hardy-Weinberg equilibrium.

Allele Frequency Sum Rule

The principle stating that the sum of all allele frequencies at a two-allele locus must equal one ($p + q = 1$), regardless of equilibrium status.

Genotype Frequency Sum Rule

The principle stating that the sum of all genotype frequencies in a population must equal one ($p^2 + 2pq + q^2 = 1$).

Infinite Population Size Assumption

The Hardy-Weinberg condition requiring an infinitely large population to ensure that allele frequencies are unaffected by genetic drift.

Population Isolation Assumption

The Hardy-Weinberg condition requiring an absence of gene flow, meaning no alleles enter or leave through migration.

No Net Mutation Assumption

The Hardy-Weinberg condition requiring that no mutations alter existing alleles or produce novel alleles at the locus being analyzed.

Random Mating Assumption

The Hardy-Weinberg condition requiring that mate choice is entirely random with respect to the genotypes and phenotypes under study.

Equal Survival and Reproduction Assumption

The Hardy-Weinberg condition requiring that all genotypes have equal fitness, meaning natural selection is not operating on the locus.

Chi-Square ($\chi^2$) Test in HWE

The statistical test utilized to determine whether differences between observed genotype counts and expected Hardy-Weinberg counts are significant.

$p$-value (HWE Test)

The statistical probability that deviations between actual and expected genotype numbers are due solely to random sampling chance.

Significance Threshold ($p < 0.05$)

The statistical result indicating a significant difference from expected frequencies, demonstrating that the population is evolving and not in HWE.

Non-Significance Threshold ($p \ge 0.05$)

The statistical result indicating no significant difference between observed and expected frequencies, confirming the population is in HWE.

MN Blood Group System

A human codominant erythrocyte antigen system governed by two alleles ($M$ and $N$), commonly used to evaluate Hardy-Weinberg equilibrium.

North American Caucasian MN Study

A sample of $6{,}129$ individuals ($1{,}787\text{ M}$, $3{,}039\text{ MN}$, $1{,}303\text{ N}$) with $p = 0.54$ and $q = 0.46$, confirmed in HWE with a $p$-value of $0.888$.

Pennsylvania Town MN Study

A population of $131$ individuals ($84\text{ MM}$, $26\text{ MN}$, $21\text{ NN}$) with $p = 0.740$ and $q = 0.260$, rejected from HWE with a $p$-value of $0.0013$.

Hartnup Disease

An autosomal recessive amino acid transport disorder affecting $1$ in $14{,}000$ newborns ($q^2 \approx 0.0000714$), with an expected carrier frequency ($2pq$) of approximately $1.7\%$.

Carrier Frequency Calculation

The method of estimating heterozygote frequency ($2pq$) in a population in HWE by taking the square root of the recessive disease incidence ($\sqrt{q^2} = q$) and calculating $2(1-q)q$.

Foraging Gene (for)

A behavioral gene in Drosophila melanogaster larvae that dictates exploration distance, occurring as dominant Rover (+$) and recessive Sitter (-$$) alleles.

Rover Phenotype

The dominant behavioral phenotype of Drosophila melanogaster larvae (genotypes $+/+$ or $+/-$) characterized by high movement across multiple food patches.

Sitter Phenotype

The recessive behavioral phenotype of Drosophila melanogaster larvae (genotype $-/-$) characterized by remaining stationary within a single food patch.

Species

A group of related organisms that maintain a distinct set of shared traits in nature.

Species Concept

A biological framework or set of criteria used by scientists to distinguish one species from another.

Morphological Species Concept

An approach distinguishing species primarily by unique structural, anatomical, and physical characteristics.

Ecological Species Concept

An approach distinguishing species based on their ecological niches, roles, and habitat resource utilization.

Phylogenetic Species Concept

An approach identifying species as distinct lineages defined by unique molecular, genetic, or evolutionary differences.

Biological Species Concept

An approach identifying species as populations whose members interbreed in nature to produce viable, fertile offspring, and are reproductively isolated from other groups.

Reproductive Isolation

The presence of biological barriers that prevent members of two different populations from successfully interbreeding and producing fertile offspring.

Biological Species Concept Limitations

The inherent inability of interbreeding criteria to classify extinct fossil organisms or species that reproduce exclusively via asexual mechanisms.

Reproductive Isolating Mechanisms (R.I.M.s)

Any biological or ecological barriers that prevent gene flow between diverging populations and promote reproductive isolation.

Pre-Zygotic Isolating Mechanisms

Reproductive barriers that prevent mating attempts or impede fertilization, thus blocking the formation of a zygote.

Post-Zygotic Isolating Mechanisms

Reproductive barriers that operate after fertilization to prevent the development of a viable, fertile adult offspring.

Habitat Isolation

A pre-zygotic barrier wherein two populations encounter each other rarely or not at all because they occupy different ecological habitats.

Temporal Isolation

A pre-zygotic barrier wherein two populations breed during different times of day, different seasons, or different years.

Behavioral Isolation

A pre-zygotic barrier wherein distinct courtship displays, pheromone cues, or vocalizations are recognized only by conspecific mates.

Mechanical Isolation

A pre-zygotic barrier wherein anatomical differences prevent successful copulation or the physical transfer of gametes.

Gametic Isolation

A pre-zygotic barrier wherein sperm and ova fail to unite, or biochemical recognition mechanisms prevent successful fertilization.

Reduced Hybrid Viability

A post-zygotic barrier wherein genetic incompatibilities disrupt normal embryonic development, resulting in weak or nonviable hybrid offspring.

Reduced Hybrid Fertility

A post-zygotic barrier wherein interspecific hybrids develop into healthy, robust adults but are sterile or produce inviable gametes.

Hybrid

An offspring resulting from the successful interbreeding between individuals of two genetically distinct varieties, subspecies, or species.

Mule Sterility

A sterile hybrid produced by crossing a female horse ($2n = 64$) with a male donkey ($2n = 62$), possessing $63$ chromosomes that cannot pair during meiosis.

Thamnophis Habitat Isolation

Pre-zygotic isolation between two garter snake species in the same geographic region because one resides in water and the other occupies terrestrial habitats.

Bufo Temporal Isolation

Pre-zygotic isolation between the American toad (Bufo americanus), which breeds in early summer, and Fowler's toad (Bufo fowleri), which breeds in late summer.

Spilogale Temporal Isolation

Pre-zygotic isolation between the Western spotted skunk (Spilogale gracilis), which mates in the fall, and the Eastern spotted skunk (Spilogale putorius), which mates in late winter.

Ipomoea Temporal Isolation

Pre-zygotic isolation between Lavender Moonvine (Ipomoea muricata), which opens at night, and Morning Glory (Ipomoea purpurea), which blooms in the morning.

Bradybaena Mechanical Isolation

Pre-zygotic isolation in snails whose opposite shell coiling directions (dextral versus sinistral) prevent genital apertures from aligning during copulation.

Star Orchid and Darwin's Hawk Moth

An evolved mechanical mutualism between Angraecum sesquipedale and the hawkmoth Xanthopan morgani praedicta, which possesses a proboscis matching the flower's long spur.

Sea Urchin Gametic Isolation

Pre-zygotic isolation between sympatric giant red and purple sea urchins whose broadcast-spawned gametes fail to fuse due to incompatible surface bindin proteins.

Allopatric Speciation

The divergence of a parent population into distinct new species following geographic separation by a physical environmental barrier.

Sympatric Speciation

Speciation that takes place within populations that inhabit the exact same geographic area without physical isolation.

Grand Canyon Ground Squirrel Speciation

An example of allopatric speciation where Ammospermophilus harrisii on the south rim and Ammospermophilus leucurus on the north rim diverged after canyon formation.

Gambusia Pond Speciation

Allopatric divergence in mosquitofish where predator presence selected for burst swimming while predator absence favored endurance, leading to reinforcing assortative mating.

Apple Maggot Fly (Rhagoletis pomonella)

A demonstration of sympatric speciation via habitat differentiation, with populations adapting their life cycles to either native hawthorn fruit or introduced domestic apples.

Pundamilia Cichlid Speciation

Sympatric divergence in Lake Victoria cichlids (P. pundamilia and P. nyererei) driven by female visual mate choice, which breaks down under monochromatic orange light.

Dusky Seaside Sparrow Subspecies Conflict

A taxonomic controversy where the endangered Ammodramus maritimus nigrescens was crossed with morphologically similar A. m. peninsulae, despite phylogenetic data indicating closer kinship to Atlantic coast subspecies.