Stable environments favor slower lives and lower reproduction levels, with competitive efficiency, where there gets to be some degree of competition between individuals of the same species, as a population tends to increase to the habitat’s carrying capacity.
Carrying capacity is the population size that a habitat might sustain. Given stable conditions, animal populations tend to grow toward carrying capacity.
As a life-history strategy, both sexuality and its orientation are options.
Asexuality was the original mode of reproduction and remains de rigueur for all prokaryotes. Many fungi and plants reproduce asexually. Genetic diversity is the facile drawback of asexual reproduction, overcome by genic transfer and pickup.
Sexual reproduction arose 1.2 BYA. Bacterial conjugation – DNA transfer between 2 consenting bacteria – has similar mechanics.
One hypothesis explaining the evolution of sexual reproduction is the ability of a population to more rapidly radiate in response to a changing habitat than parthenogenesis affords. Expansion speed could be advantageous in competition with other species for limited resources.
Simultaneous hermaphrodites have the advantage of sexual reproduction but don’t have separate sexes. Hermaphroditism yields sexual flexibility at the slight expense of expressing both sex organs. Besides grasses and some other plants, earthworms, certain snails and slugs, and some fish are simultaneous hermaphrodites.
A form of hermaphroditism in botany is monoecy: a plant with both staminate (pollen-producing male) and carpellate (ovule-producing female) parts. Monoecy is common in conifers, but only 7% of angiosperms are monoecious. Compared to long-lived conifers, flowering plants generally prefer greater genetic diversity as an edge in evolvability.
Sequential hermaphrodites change sex. A male that changes into a female practices protandry; clownfish are exemplary. With protogyny, a born female may change into a male. Wrasses and groupers practice protogyny after maturing. Sequential hermaphroditism allows populations to respond to ecological pressures by ensuring sufficient numbers of reproductive members in reasonable ratios.
Banana Slug Sex
The hermaphroditic banana slug savors sex. It has an outsized male member and a proportionately petite female organ.
In the act, slug penises becoming spirally wrapped; an entanglement not easily undone. If post-coital vigorous wiggling fails to separate the big bananas, one slug gnaws a penis off: either its own or its partner. This apophallation leaves one slug permanently memberless, though able to enjoy sex in the future as a female.
If partner sex is not possible, banana slugs practice safe sex and reproduce by self-fertilization.
Ram Horns and Longevity
There is often considerable genetic variation underlying sexually selected traits. ~ English evolutionary biologist Susan Johnston et al
Ample horns are a ram’s ticket to reproductive dominance. During breeding season males butt heads for access to females. Big horns triumph – at least for Soay sheep living on Hirta, an island off the coast of Scotland. Yet this has not launched an evolutionary escalation cycle. Some male sheep have short horns or none at all.
A single gene controls horn size, with various alleles that determine a male’s headset destiny. Those with the biggest horns father nearly twice as many lambs each year: 3 versus 1.6 for the less horny. But big-horned rams are less likely to last the harsh Hirta winter. Lesser-horned sheep live longer, and so breed for more years.
Lechwe Reproductive Strategy
Nile lechwe are an African antelope that live up to 12 years. Almost all adult females breed. As a female gets older, the odds of her producing a son increment.
Yearling first-time mothers bear sons 57% of the time. This rises to 67% by the time a female lechwe is 7 years old. The biomechanics of ratcheting the sex ratio is not known.
A female lechwe is 3 times more likely to die from bearing a son than a daughter. Sons are heavier than daughters, and so more taxing and risker to birth.
Sons are risky in other ways. Only dominant males have breeding rights. Hence, sons are genetic gambles, whereas daughters are an insurance policy for keeping a lineage alive.
Having played it safe while younger, older females risk getting more bang for their bucks.