Getting plant sex wrong (2)

I’ve been reading Horseshoe Crabs and Velvet Worms by Richard Fortey. Overall, I’ve been finding it enjoyable. However, portions of the book cause me to cringe. Here’s an example near the beginning of chapter two:

These Gondwanan coniferous trees, with their relatively large leaves and bright berries, do have a very special appearance, at least to a European accustomed to pines and firs with their dry-looking cones. A botanist would remind me I should really describe the berries as “fleshy peduncles” because they carry exposed seeds at their tips.

What’s wrong with that? Well, the trees (podocarps in New Zealand, specifically totara, Podocarpus totara and rimu, Dacrydium cupressinum) Fortey is talking about do not have berries. He is apparently aware of this, but the way he presents it suggests that these things really are berries and that describing them as “fleshy peduncles” is some kind of obscure scientific quibbling. It isn’t: “fleshy peduncle” is right, “berry” is wrong. This follows an irritatingly predictable tendency in science writing about plants: present a misleading description as though it were true, restrict the correct information to some kind of afterthought, and don’t explain it enough for readers to actually figure out what’s going on. This both misinforms readers and gives the impression that the situation is hopelessly complex and cannot be understood by anyone who doesn’t have a PhD and a labcoat.

So, OK, here’s what a fleshy peduncle is. In conifers, a peduncle is a stem that bears one or more cones. In this case we’re talking about seed cones, analogous to the seed cones you’d see on pine, spruce, Douglas fir, etc. You know, these things. In the podocarps Fortey is discussing, the seed cones are very small, having one or two small leaves each with a single exposed seed. In these species, the peduncles bearing the seed cones are swollen, fleshy, edible, and look somewhat like berries. Why aren’t they berries? A berry is a kind of fruit. Fruits are found only in flowering plants (not conifers) and instead of leaves with exposed seeds they have carpels. Carpels are leaves that have been folded and fused into little chambers with seeds inside of them, protected from the environment.

That is more complicated than just saying “these trees have berries; well, technically they’re fleshy peduncles”. However, it’s also informative. Authors writing about science should aim to inform readers about science rather than reinforcing misconceptions and presenting science in a dismissive and uninformative manner.

Here’s a similar example from near the end of chapter 3:

Despite their apparent simplicity, Porphyra and Bangia have quite complex life histories. Cells of the “weed” contain only one package of genetic information; they are described as haploid. A second phase in the life history of these seaweeds is called the Conchocelis stage, which makes miniature branching plants, some varieties of which inhabit the borings they make inside seashells. These plants are so different from their “parents” that they were once given the separate generic name, Conchocelis, which is now only retained as a label for one stage in the life cycle. Conchocelis plants are the diploid, or the sexual, stage of the red algae. The leafy stage releases gametes that mate with one another, thereby doubling up the genetic content; this produces spores that can germinate into Conchocelis.

Some of the basic ideas here are correct. However, the definitions of “haploid” and “diploid” are misleading, the processes of fertilization and sporulation are conflated, and describing Conchocelis as “the sexual stage” is incorrect.

First, what does “one package of genetic information” mean? A package of genetic information could refer to a few different things. It could possibly refer to a gene, or a chromosome, or a nucleus. When discussing haploidy and diploidy, we’re talking about chromosomes, which are long strands of DNA, each containing many genes as well as regulatory sequences and non-coding “junk” DNA, bound up with proteins. So why not just say “one chromosome”? This would be less ambiguous, but it would also be wrong. One vs. two chromosomes is not the distinction between haploid and diploid cells. Instead, the distinction is how many copies of each chromosome is present. In a haploid cell, each chromosome present as a single copy. In a diploid cell, chromosomes are present in pairs. We could easily reword the second sentence of this quote to say “Cells of the ‘weed’ contain only one copy of each chromosome; they are haploid.”

Second, Fortey says that “the leafy stage releases gametes that mate with one another, thereby doubling up the genetic content; this produces spores”, which conflates fertilization and spore production. The union of two gametes is fertilization, the process by which we move from haploid cells to diploid cells. The cell produced by fertilization is not a spore, it is a zygote. The diploid zygote then goes through several mitotic cell divisions to produce diploid spores. By suggesting that fertilization produces spores, Fortey is simply skipping this stage of the life cycle and implying that fertilization directly produces spores, which is incorrect.

Third, Fortey describes Conchocelis as “the diploid, or the sexual” stage of Porphyra and Bangia. Sex consists of the production of gametes and their subsequent union in fertilization. The Conchocelis stage is not directly involved in this process: it doesn’t produce gametes and it isn’t produced by fertilization. Instead, the diploid spores mentioned above can grow into the Conchocelis stage, and the Conchocelis stage produces haploid spores by meiosis (Porphyra and Bangia produce multiple types of spores, which is rather odd). The Conchocelis stage is part of the whole life cycle in these algae and the whole life cycle includes sex, but that’s as close as it gets. It’s like describing a human liver as “the sexual stage” in humans. A functional liver is a necessary component of the human life cycle and the human life cycle includes sex, but the liver doesn’t have any direct role in sex.

The life cycles of Porphyra and Bangia are fairly complex and difficult to describe clearly and accurately. Fortey had two good choices here: either don’t bother with it since it’s not really necessary for the narrative of the chapter or go into the detail needed to convey what’s going on. Instead he makes a third, bad choice: discuss the topic briefly, confusingly, and incorrectly. All readers are likely to get from this passage is that something weird and confusing is happening.

That said, Fortey is doing far better at botany than Bernd Heinrich, who in The Trees in My Forest repeatedly refers to “flowers” of conifers. This is just wrong. Pines do not have flowers. At least Fortey tells us that the “berries” of podocarps are in fact fleshy peduncles, even if he does so in a rather unhelpful fashion. And maybe we can cut him some slack, since his book isn’t primarily focused on plants, much less trees or conifers in particular. Heinrich, on the other hand, wrote a book about trees, with long discussions of conifers, and gets it completely wrong. He tells us that conifers have flowers and provides no explanation nor any indication that this might not be consistent with what we actually know about botany.

Misunderstanding group selection (1)

I’m not sure why, but group selection seems to be a topic that inspires vociferous but poorly-considered critique. An example from Steven Pinker:

Human beings live in groups, are affected by the fortunes of their groups, and sometimes make sacrifices that benefit their groups. Does this mean that the human brain has been shaped by natural selection to promote the welfare of the group in competition with other groups, even when it damages the welfare of the person and his or her kin? If so, does the theory of natural selection have to be revamped to designate “groups” as units of selection, analogous to the role played in the theory by genes?

No, groups do not play a role analogous to genes. But that isn’t what group selection is about. Here’s the very short version:

Genes are the basic units of heritable information. Genotypes, however, are not directly exposed to natural selection. Genotypes are exposed to selection via the phenotypes to which they give rise. The question we are concerned with in the group selection debate boils down to “Phenotypes at which level?” Genes are expressed at varying levels of organization. Any particular cell has a phenotype. If we’re talking about multicellular organisms, we can talk about the phenotype of the organism. If we’re talking about multicellular organisms that occur in groups, we might also talk about the phenotype of the group. At which levels can natural selection apply, and, for any particular trait of interest, which level is most appropriate and enlightening? The “pro” argument on group selection boils down to: In some cases, discussing selection at the level of group phenotypes is both accurate and the best way of understanding what’s going on. The “con” side boils down to: It is never appropriate to talk about selection at the level of group phenotypes.

There’s a lot more nuance to it than that, of course, but the basic idea is that we’re talking about gene expression at different levels of organization. Steven Pinker gets it wrong in the third sentence of this article, and perpetuates that error throughout the article. As a result, I find Pinker’s criticisms largely unintelliglble. This is too bad, as in his other writings I’ve found him cogent and compelling.

Jerry Coyne has also been making criticisms of group selection that I find confused or poorly-expressed, but his errors are more sophisticated. I may get to them later.