Teaching evolution using a card game: negative frequency-dependent selection

Figures & data

Figure 1. Illustration of negative frequency-dependent selection leading to protected polymorphism.

Notes: Grey line indicates allele fitness (which is defined as the average over the fitnesses of individuals carrying that allele in a population), black line allele frequency. When an allele’s frequency becomes low, its fitness goes up which protects it from going extinct.

Figure 2. Schematic view of Sporophytic Self Incompatibility (SSI) in plants.

Notes: A monoecious diploid plant with genotype S₁S₂ produces pollen grains which carry either the S₁ or the S₂ allele. Phenotypically, however, each pollen grain carries both the S₁ and S₂ recognition proteins on its outer wall (this is different in Gametophytic SI, where the pollen grain carries only the recognition protein produced by its own genotype). If a pollen grain lands on the stigma of its parental plant (or one with a similar genotype) it may germinate but growth of the pollen tube down the style is stopped, because the S-proteins of the stigma and the pollen grain fit like a lock-and-key. A pollen grain from a plant with a different genotype, in contrast, such as S₃S₄, will germinate and its pollen tube will grow down to the style to the ovary, allowing fertilisation between the male and female gametes. If the phenotype of pollen grain and stigma have at least one expressed parental allele in common, which can be the case both in case of selfing and with certain outcrossed combinations, fertilisation is prevented

Table 1. Learning goals and starting questions of game with regard to negative frequency-dependent selection.

	Learning goals	Starting questions
Part 1		how many homozygotes and heterozygotes do you expect
		in a natural population with SSI?
	outcrossing promotes heterozygosity
	1a) in SSI, offspring are only produced when sexual partners share no alleles
	1b) homozygotes should not be found in the population
Part 2		what is the consequence for alleles if their fitness
		depends on how common they are?
	negative frequency-dependent selection maintains allelic diversity
	2a) fitness of an allele depends on its frequency in the population
	2b) expected equilibrium frequencies in the population are equal
Part 3		what would happen if we increased
		the number of alleles carried by each individual?
	ploidy level influences the phenotype
	3a) polyploidy is detrimental to pollination success in SSI
Part 4		will allele frequencies be different if some alleles are dominant over others?
	dominance affects fitness
	4a) with dominance among alleles, there will be more successful pollinations
	4b) the more recessive an allele, the higher its equilibrium frequency
Part 5		how will allelic diversity change if we add a mutant that can self-fertilize?
	reproduction via selfing carries substantial costs and benefits
	5a) selfing can be very advantageous
	5b) inbreeding depression is a disadvantage of selfing

Note: SSI = sporophytic self-incompatibility (sporophytic means that both alleles are expressed on the pollen grain).

Figure 3. Explanation of why recessive alleles reach a higher frequency at equilibrium than dominant alleles in SSI plants. Panels A–C show the dynamics of a newly evolved recessive allele, panels D–F that of a new dominant one. Top panels (A, D: focal plants black, rest of population grey) depict the critical time phase when a new allele has increased and is found in several heterozygotes. Dashed lines indicate potential fertilisation partners. Panel D shows that heterozygotes carrying a new dominant allele already begin to miss out on non-self crosses, while this is not yet the case for the recessive mutant (black dotted line in panel A). Middle panels (B, E) show the frequency increase of the mutant alleles through time. The exponential increase ends sooner for the dominant (E) than for the recessive allele (B). Bottom panels give equilibrium frequency distributions: dominant alleles will have lower frequencies at equilibrium than recessive alleles.