BIO A01 Lecture 4: Selection and Maintaining Variation

Vocabulary flashcards covering Lecture 4 of BIO A01 on selection mechanisms, maintaining genetic variation, sexual selection, balancing selection, and constraints on adaptation.

Mutation

A heritable change in DNA that introduces new genetic variation into a population.

Gene Flow

A change in allele frequencies that occurs as individuals join a population and reproduce, which may introduce genetic variation from another population.

Genetic Drift

Random changes in allele frequencies caused by chance events; it reduces genetic variation (particularly in small populations) and can eliminate alleles.

Natural Selection

Differential survivorship or reproduction of individuals possessing different genotypes, resulting in an increase in the frequency of one allele or the preservation of allelic variation.

Nonrandom Mating

The choice of mates based on their phenotypes and genotypes; it does not directly alter allele frequencies, but typically prevents genetic equilibrium.

Eurosta solidaginis

The goldenrod gall fly (family Tephritidae), whose larvae induce the formation of spherical stem galls on goldenrod plants and are subject to opposing selective forces.

Eurytoma gigantea

A species of parasitic wasp that selectively parasitizes goldenrod gall fly larvae inside small galls.

Picoides pubescens

The Downy Woodpecker, a predatory bird that selectively preys upon goldenrod gall fly larvae situated within large galls.

Stabilizing Selection (Goldenrod Gall Size)

Selection maintaining intermediate gall diameter driven by opposing mortality pressures: parasitic wasps (Eurytoma gigantea) parasitize small galls, while downy woodpeckers (Picoides pubescens) consume larvae in large galls.

Sexual Selection

A form of natural selection defined ultimately as the competition for mates, which can take place between rivals or for the affection of a potential mate.

Sexual Cannibalism

An adaptive male mating strategy occurring when the selective benefits of being eaten (such as increased paternity and reduced likelihood of female remating) exceed the male's expected future reproductive value.

Latrodectus hasselti

The Australian Redback spider, an extreme example of sexual cannibalism where males exhibit low costs for cannibalistic sacrifice because they are unlikely to mate more than once even if they survive.

Good Genes Hypothesis

The hypothesis tested by Alison Welch in gray tree frogs showing female preference for males with long calls (LC), whose offspring develop faster and show greater survival and growth compared to offspring of short call (SC) males.

Balancing Selection

Various forms of natural selection that lead to the active maintenance of genetic variation in natural populations by bringing allele frequencies to a stable equilibrium state.

Balanced Polymorphism

The maintenance of multiple distinct genotypes or morphs in a stable frequency within a population by natural selection.

Heterozygote Advantage

A form of balancing selection in which heterozygous individuals have higher relative fitness than either homozygote; exemplified by $HbA/Hba$ individuals having protection against malaria without suffering severe sickle cell anemia.

Dr. William Warrick Cardozo

Physician and researcher (1905–1962) who demonstrated that sickle cell anemia was an inherited condition found almost exclusively among people of African descent, was not always fatal, and did not cause anemia in all individuals with sickle cells.

Sickle Cell Anemia Control Act of 1971

Legislation passed by the U.S. Congress providing federal funding for research and treatment of sickle cell disease, following pediatric newborn screening research by Dr. Roland Scott.

Negative Frequency-Dependent Selection

A mechanism of balancing selection in which the relative fitness of a genotype is not constant but increases as its frequency decreases, resulting in the highest fitness when a morph is rare.

Perissodus microlepis

A scale-eating cichlid exhibiting negative frequency-dependent selection, wherein 'right-mouthed' individuals attack prey from the left side and 'left-mouthed' individuals attack from the right side, stabilizing the morph frequencies around $0.5$.

Adaptive Traits

Heritable phenotypes produced by natural selection that increase an organism's relative fitness.

Adaptation

The accumulation of adaptive traits in a population over successive generations.

Competitive Optimum

The level of trait perfection favored by natural selection, which makes an organism as good as or better than existing alternatives rather than achieving an absolute theoretical maximum.

Historical Constraints

Evolutionary limits on adaptation arising because natural selection can only act on preexisting ancestral structures and variation (e.g., the panda's false thumb modifying wrist bones rather than producing a true thumb).

Vestigial Structures

Useless or reduced structures retained as 'evolutionary leftovers' from ancestors because natural selection does not eliminate every non-harmful, unnecessary feature.

Adaptation Lag

The constraint wherein traits in an existing population reflect past selection pressures because each generation is adapted to its parents' environment and natural selection never anticipates future environmental change.