Atomic Nerds

Having gone into a thumbnail of the history of evolutionary theories of the advantage of sexual reproduction, the question as it stands nowadays is not “why sex”, but rather “why NOT” sex, for species that have entirely given it up* and gone to all-female lines of parthenogetically reproducing individuals.

When I initially set out with the idea of this post, I expected it to be very simple, and I turned out to be dramatically wrong. I will start out with the simple bit.

Both parthenogenesis in sexual species and especially completely parthenogenetic lines do share a common pattern, which is that they excel in a specialized niche as colonizers. Colonizing species are what you get first in an environment that has either recently had its previous ecosystem wiped out or has yet to be exploited; these species excel at rapidly moving in and filling what niches are available, but tend to eventually be out-competed by other species as the ecosystem matures. In some cases they may never be booted out due to poor habitat or general inaccessibility- for this reason islands don’t normally tend to have terribly complex ecosystems and harbor a number of species that would be outcompeted by mainland species- and often are, either over a very long period or a much shorter one if humans move in toting their mainland species with them.

Parthenogenesis offers an obvious advantage to a colonizer; there doesn’t need to be more than one of the animal in question to begin colonizing, the genotype that is successful enough to move into a harsh environment in the short term is probably a good bet to replicate rather than recombine and radiate rapidly into the entirety of the available niche, and their clean-slate situation is generally a perfect one for the short-term-strategy, high-payoff investor I mentioned in the previous post. Aside from colonizing islands, like the completely parthenogenetic Mourning Gecko of Hawaii (now declining as they are apparently outcompeted by a mainland species, the house gecko) and the parthenogenesis-capable Komodo dragons** of the Indonesian archipelago, this applies to any habitat that has been recently disturbed or is prone to recurring disturbance.

Most of the all-female lines of whiptail lizards (and there are several) seem to predominate in riparian areas of the southwestern US, which are prone to frequent destruction by flash flood. Aside from just washes, the southwest has a fairly long and recent history of disruption; human habitation, even by native Americans, was a little more than some of the environment could handle, and the Anasazi in particular turned a lot of forest into arid grassland. Combine this with the fire-dependent ecosystem of Ponderosa forest, and you have a rapidly changing environment in which a species that happens to be suited to the environment the changes create could spread very rapidly, more rapidly without sex than with it- we are in the middle of Ponderosa forest, surrounded by many canyons, and all of our little yellow striped yard lizards are of the local all-female race of whiptails.

This was where I expected the post to conclude originally: in a genus of lizards with many sexual species and many independently evolved lines of all-female parthenogenetic lizards, the parthenogenetic lizards win where the environment has either been recently disturbed or is constantly disturbed, and their sexual cousins win in more stable habitats where the advantages of sex have the chance to win the long game.

What I did NOT expect was to discover was that, in vertebrates, parthenogenesis is heavily associated with hybridization events to the point where the paper I eventually tracked down trying to figure out what on earth was going on asserted that all known vertebrate cases of parthenogenesis are associated with hybridization events, while a few more cases since then have emerged and don’t appear to have any link to any hybridization***.

Kearny makes the case that the hybridization is not incidental, but rather a hand-in-hand strategy with the parthenogenesis itself for species moving into a disturbed habit. He ties everything around glaciation events (which is, I suppose, why our local whiptails weren’t included), but glaciation is itself a very specific type of disturbance whose patterns can be geologically tracked and which tends to open up broad swathes of newly available, but inhospitable, habitat. He demonstrates that hybridization and gene flow from sometimes even multiple members of the same genus is associated with successful colonizing and radiating in disturbed/harsh territory even with sexual species.

This is where parthenogenetic lines- and both the geckos and the whiptails I’ve been talking about are known to have originated from multiple independent hybridization events- tie back into the portfolio-balancing theory of the advantage of sex. (He specifically cites Roughgarden coming to his conclusion, in fact.) Hybridization does not necessarily produce an intermediate result; it can be a tremendous dice-throw when it comes to the result, and there can be consequences like dwarfism or gigantism****, extreme color morphs, and other results dramatically different from both parent species; if sex produces a wide range of genes that work well in combination with each other within a species’s gene pool, hybridization means all bets are off with how well or poorly said genes will work in combinations that haven’t been fine-tuned by selection for the last million years.

For related species on the ragged edge of new potential territory, or coming back from a major environmental shakeup, they can be in the same relatively disadvantageous position: possibly a small population with a reduced gene pool and not particularly adapted for the new world order. Hybridization with the strange neighbors would result mostly in offspring that were even more poorly adapted- but for the few equally wild-card results that WERE, a parthenogenetic line would “fix” the new type not present in either population in a position of advantage as a colonizer. With low or no gene flow from the parent species (sometimes parthenogenetic species will occasionally be sexual outside the species), its advantage could persist without immediate dilution from either original, markedly less-well-adapted species.

Shuffle the cards, shuffle the cards… then run with the first really good hand you get.

*Given up sexual reproduction, that is. They still have sex. They are demonstrably more fecund when sharing a burrow or laboratory habitat with a mate, with whom they trade off taking the “male part”. They are unusual among reptiles and even within their own genus for this chumminess. No evidence of ice cream or “The L Word” has been found, however.

**Interestingly, the dragons are also unusual among reptiles and lizards in general in that they seem to form some kind of pair bond with their mates. I have no idea what this means but it’s worth noting.

***The Komodo dragons again, also some shark species, and also snakes. The snakes actually were discovered prior to the paper’s publication, though. I have no idea if its author simply hadn’t heard or discounted it because it did not result in a unisexual line. As a purely taxonomic footnote, the Varanid monitor lizards to which Komodos belong are arguably more closely related to snakes than to the geckos and whiptails, and they and the snakes share the same ZW sex-determination system, which is why Komodos couldn’t go fully parthenogenetic even if it were to their advantage to do so- all the offspring are male. Reptilia is one fucked up clade.

****Back to the dragons, so far as I could determine no hybridization event is known for them, and their likeliest ancestors include lizards as large or even larger than them… but they do stand out as quite odd among island colonizers for their sheer size. The predominating theory is that they began smaller and grew larger to exploit stegodonts and deer as a food source. Pure and rampant speculation, but food for thought.

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