Microcosmos is an excellent book and its author was a fascinating woman.
In keeping with the name of this blog, I will try to have my first post about any given book focus on the parts that I disagree with. Microcosmos is so full of unorthodox perspective that any disagreement I could have with it, will necessarily be more orthodox than the perspectives in the book. Hence, I will have to revisit some of the topics in order to express how strongly I am persuaded by Margulis's insights.
The only major points in the book that I find highly dubious have to do with its assertions relating to meiosis. Margulis believes that eukaryotic sexual reproduction is maladaptive, and that meiosis is valuable because it provides increased stability by serving as an elaborate biochemical roll call. She disagrees with the belief that it serves to increase diversity.
Many species exhibit parthenogenesis. Some reptiles are entirely parthenogenic. They survive. They exhibit phenotypic diversity within the species. Sex is not biologically necessary even for Eukaryota. Hurrah!
Cnemidophorus neomexicanus is entirely parthenogenic. There are no males in the entire species and the species doesn't mate. It's more numerous than its close relatives, indicating that parthenogenesis can clearly be adaptive if it can be pulled off. It's close relatives mate normally.
The same is true of other species that reproduce parthenogenicly. Their close relatives very seldom do.
Parthenogenesis has clearly evolved independently on many different occasions. It is extremely unlikely that all of those occasions occurred in the past ten million years. Rather it has been happening for as long as their have been animals. Yet these species almost never go on to form clades. Some clades, like rotifers, have many species that exhibit both male-female mating and parthenogenesis. But there is no existing clade of more than five species that is entirely parthenogenic.
The available evidence strongly indicates that parthenogenesis evolves frequently and easily in some classes of animals. However, the species that develop it rarely issue new lineages that survive past the million years or so of environmental consistency that typically constitutes a given species's time in the sun. As the world changes, these species remain the same. Hence, they go extinct. Their mating relatives much more successful issue progeny adapted to the evolving world.
In the case of Cnemidophorus neomexicanus, we know with certainty that the species didn't somehow evolve into its sexually reproducing near relatives. Its two closest relatives can hybridize to create new female, always female, Cnemidophorus neomexicanus. (I.e. this is a hybrid species of two sexually reproducing species.)
Let's pick another species.
Komodo dragons can reproduce parthanogenically, but all offspring so produced are male (which can then mate with the female Komodo dragons to produce female or male Komodo dragons).
Clearly, they aren't leveraging parthanogenesis as much of a survival strategy. There are plenty of males on the Galapagos, and we have no reason to believe that males randomly die out while the females survive. Instead we see that under reptilian reproduction, parthenogenesis is an easy adaptation. Komodo dragons exhibit this trait simply because they can. It doesn't help them survive. It hasn't helped them colonize new territory (they are naturally confined to a very small area). It's simply something they do because Komodo dragons can hatch with a haploid chromosome count, and then develop diploidy as they mature which means that whenever a female can lay unfertilized eggs, the males of that batch can survive. (Komodo dragons do not have X and Y chromosomes to determine gender like humans do. They have ZW chromosomes. Males are ZZ; females are ZW. Only the males survive birth in parthenogenic reproduction because WW is not viable, and they they start out haploid.)
Other animals that display parthenogenesis display it because it has been similarly easy for them to evolve it, not because it is a winning strategy. It might be winning in the short term for some species, but these species that develop extreme dependence on this strategy as a means of reproduction rarely speciate into new lineages, and typically fail to survive the environmental shifts that cause new species to emerge among the mating animals.
If it was easy and adaptive, it would be ubiquitous. We have strong evidence that parthenogenesis is an easy adaptation for many species. We know that it is not ubiquitous. Far more species reproduce sexually. Therefore, we have very strong reason to conclude that sexual reproduction, not parthenogenesis, is adaptive, at least in the long term.
Hermaphrodism among animals and plants would lead us to the same conclusion. Many hermaphroditic species have adaptations that prevent them from mating with themselves, again indicating that sexual reproduction is adaptive for Eukaryota. Hermaphrodism is a very easy mutation. A huge proportion of plants and animals have some hermaphroditic mutants in the species. It's likely that all do. Yet very few species are predominantly hermaphroditic. The clades that are, e.g. many groups of plants as well as some animals like snails and slugs, have also developed traits to prevent autosexual reproduction. Again, this gives us strong evidence to believe that the genetic exchanges from mating produce variety that helps species adapt to changes in their environment and evolve accordingly. Autosexual reproduction is obviously less expensive in the short term, therefore it is in some sense adaptive. It's pretty obvious that animals that can clone themselves without going to the trouble of finding a mate can reproduce to fill their evolutionary niche a lot more easily than their less fully endowed relatives. However, such species rapidly go extinct once their niche goes away and they leave no daughter species after them.
Margulis may be right about a lot of things. I strongly believe she is. However, the biological evidence (not just the entrenched consensus of academia) is overwhelmingly against her in her claim that sexual reproduction is a maladaptive vestige of early Eukaryotic evolution that will vanish when faced with the superior reproductive methods of Eukaryotic species that can clone themselves. Locally, when the ability to essentially clone oneself develops in an individual that is very well-suited to a particular niche, yes, that individual and its clones will go on to out compete their relatives and fill the niche. However, brief survey of Eukaryotic life will quickly tell you that those species will not survive to produce daughter species; whereas the mating organism occupying other parts of the world and its other ecological niches much more frequently will.
No comments:
Post a Comment