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How fast can sexual traits evolve?

Experimental populations of hermaphroditic plants evolved a significant increase in male function in only three generations.

Many plant species are hermaphrodites, with each individual producing both pollen and seeds. Others species have separate sexes, as mammals and birds do, while still others have mixtures of unisexuals and hermaphrodites. Based on the distribution of these traits in the family tree of life, evolutionary transitions among these "lifestyles" appear to have been fairly common. This week's paper shows how hermaphrodites can evolve to be more female or, in this case, more male. Hermaphroditic Sex Allocation Evolves When Mating Opportunities Change was just published in Current Biology by Marcel Dorken and John Pannell.

What does it mean to be “more male?” Dorken and Pannell previously showed that hermaphrodites of the plant they study, Mercurialis annua, make more pollen when they are spaced farther apart (i.e., when they need their own pollen because they are unlikely to get it from neighbors). This is an individual response, rather than an evolutionary change in the genetic composition of a population. But can greater maleness also evolve?

When there are lots of male-only plants around, there is plenty of pollen. (Males make at least 10 times as much pollen as hermaphrodites the same size, because pollen grains are smaller than seeds and therefore cheaper.) But when males are scarce, hermaphrodites that make more pollen should sire a larger fraction of the next generation (on themselves or neighboring hermaphrodites), so genes for this greater male function should increase in frequency.

And that is what the authors found. Experimental hermaphrodite-only populations increased the fraction of their resources spent on pollen, rather than seeds, by 10% in only three generations.

Meanwhile, hermaphrodites mixed with about 50% males showed no evolutionary change. I think they may have been expecting these populations to evolve in the opposite direction, with hermaphrodites becoming more female – perhaps eventually all-female, leading to a population with separate male and female plants. But, given the higher cost of seeds, relative to pollen, it may take stronger selection or a longer time to evolve greater seed production, relative to greater pollen production.

Other recent papers that looked interesting:

Accommodating natural and sexual selection in butterfly wing pattern evolution

Communal Nutrition in Ants

Flat lizard female mimics use sexual deception in visual but not chemical signals

Two fungal symbioses collide: endophytic fungi are not welcome in leaf-cutting ant gardens

Phylogenomics Revives Traditional Views on Deep Animal Relationships

Non-breeding season events influence sexual selection in a long-distance migratory bird

No actual conflict over colony inheritance despite high potential conflict in the social wasp Polistes dominulus

Sequencing and Analyses of All Known Human Rhinovirus Genomes Reveal Structure and Evolution

Cooperation and virulence of clinical Pseudomonas aeruginosa populations

New Guinea highland origin of a widespread arthropod supertramp

Flight speeds of swifts (Apus apus): seasonal differences smaller than expected

The cultural and chronological context of early Holocene maize and squash domestication in the Central Balsas River Valley,

Sexual selection drives weak positive selection in protamine genes and high promoter divergence, enhancing sperm competitiveness

Coevolution of diet and prey-specific venom activity supports the role of selection in snake venom evolution

The evolution of primate visual self-recognition: evidence of absence in lesser apes

Mimicry, colour forms and spectral sensitivity of the bluestriped fangblenny, Plagiotremus rhinorhynchos

Reliabilities of identifying positive selection by the branch-site and the site-prediction methods

Plants with double genomes might have had a better chance to survive the Cretaceous–Tertiary extinction event

Ecomorphological selectivity among marine teleost fishes during the end-Cretaceous extinction

Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico

A microraptorine (Dinosauria–Dromaeosauridae) from the Late Cretaceous of North America

Natural variation in a neural globin tunes oxygen sensing in wild : Caenorhabditis elegans

It takes two to tango: reproductive skew and social correlates of male mating success in a lek-breeding bird

Trill consistency is an age-related assessment signal in banded wrens

Influence of major histocompatibility complex genotype on mating success in a free-ranging reptile population

Seasonal host dynamics drive the timing of recurrent epidemics in a wildlife population

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