I'm very much looking forward to reading this! Not only is it about plants, I also quite enjoy your writing style. Curious what non-plant-people will get out of it though, unless I've missed some introduction
Thanks! I'll add the links to the previous discussions of kin vs. group selection on LW. That is the introduction I am building on. Forgot to do it being too excited:)
(There's no such thing as non-plant-people, they just need to come to terms with it, it can be rough, waking up to the plant-peopleness for the first time. We ought to be gentle...)
Why is there no alternation of generations in animals? Is it an evolutionary accident or is there a deeper reason for this?
Naively, alternation of generations seems like a more complex structure than the lack of it. So it's not surprising if most classes don't have it.
The benefit of alternation of generations is (apparently?) to disseminate offspring to remote locations and to encourage outbreeding. Most animals are mobile enough as adults that they don't need any special adaptations for that. The problem of sessile adults with mobile offspring is mostly unique to plants.
Sorry, I should be more specific. We could mean two separate things by "alternation of generations":
(a) Different chromosome numbers and different reproductive strategies for different parts of the organism's lifecycle. For instance, diploid (reproducing asexually) and haploid (reproducing sexually) forms in some plants.
(b) Different parts of the lifecycle look different, live in different environments, and eat different things (but are composed of the same kind of cells with the same chromosome count). This happens in animals a lot. For example insects invented metamorphosis possibly so worm-like larva and winged adults would not compete for the same food (also perhaps for dispersal?). Sea squirts invented a motile larva with a spiral chord to solve the dispersal problem. Some time later, highly specialized neotenous versions of that larva are walking around on the moon...
The fact that these kinds of lifecycles are "more complex" should not influence much our belief in how likely they are. Evolution doesn't care about Occam's razor at all, it will develop all sorts of crazy elaborate things if there are local reasons for it (and in fact frequently does so). Occam's razor is something of a kung fu style for scientists, not an organizing principle of the Universe.
I am mostly curious about the lack of (a) in animals. Generally by "alternation of generations" biologists mean (a).
There are some theories that dominant diploid forms arose because they are more resistant to mutations. OP sounded like a plant biologist (or someone in graduate school studying plant biology), so I thought I would ask an expert's opinion on this question.
AFAIU, in 'lower animals' alternating modes of reproduction are very common. In 'higher animals', it's not so. Has to do with when meiosis happens in the life cycle and what stages are for dissemination. So no, it's never an accident, but for a deeper reason you should ask someone else.
it's never an accident
Not sure what you mean -- lots of things are "accidents" in the sense of encoding a possibility that could have gone the other way. Any time a similar type of thing gets reinvented by evolution in a different way, that thing is a type of accident. For example, the vertebrate eye (molluscs use something a bit different), the vertebrate jaw as a modified branchial arch (invertebrates use modified limbs for jaws), etc. etc.
An example of something that I don't think is an accident is that animals do not photosynthesize. I think this is not an accident because you can't support the energy requirements of a multicellular thing that moves around on our timescales with photosynthesis -- you have to ingest high energy-density things (e.g. other organisms) directly.
Some plants and fungi have alternation of generations, and I think all animals do not. But animals differ a lot from fungi and plants in being highly mobile heterotrophs. I am wondering if something about animal characteristics channels them to avoid alternation of generations, just as multicellular mobility channels them to avoid photosynthesis, or whether it is an accident in the set of protista-like ancestors in the animal lineage. I could certainly imagine an animal with alternation of generations (isn't that what Giger's aliens are?)
Ah. Now I see. What you call 'accident' I call 'event', because such a great thing as alternation has consequences for most areas of an organism's life. It might have gone the other way, but that it is still so widespread is exactly as accidental as the placement of a brick in the middle of the wall.
If I am not mistaken (I can look it up sometime), practically all plants and fungi have it. In lower animals, some stages of a life cycle can be mobile founders, and later ones would live in tightknit colonies. Asexual/sexual alternation is pretty common, too. See wiki on trematodes.
No idea about Ginger's aliens, except that they just might be accidentally transmitted to humans when humans eat their 'rightful' hosts. Some frog parasites go on living in snakes... (Is that what you mean?)
Related: http://www.nature.com/nature/journal/v351/n6324/pdf/351314a0.pdf
(an article on the relative merits of haploidy and diploidy)
Here I offer a very broad review of knowledge about ferns & 'fern allies' we have today. The topic is possible organism-level adaptations for kin selection and population structuring.
Imagine a flowering plant (as a child would draw, to generalize.)
It has roots, a stem, leaves, flowers and fruit. In its flowers, anthers and pistils negotiate pollination like the adults they are. In its fruit, baby plants dream, covered by integuments. The plant is one complex, breathing thing.
On no level it is perfect or 'harmonious'. There are resource allocation problems, meaning all seeds do not have equal chances to mature. There is competition between fathers. There are herbivores and malformations and volcanic eruptions, and the plant has pretty little defences against them, but -
- when you look at the seed -
- not entirely powerless. The tender sperm meets the egg inside a complex organ, protected from drying out and capable of recognizing specific pollen. The mother plant provides nutrition and encapsules the embryo in layers optimized for both survival and germination. Once established, it becomes an environment for its own progeny: the sporophyte (the adult specimen producing spores) hosts plenty of gametophytes (i.e. the sperm and eggs developing from the spores). The sporophyte is a mighty diploid (or triploid, or more...), while the gametophyte is haploid (generally), meaning it has but one set of chromosomes - half as much inherent genetic diversity to meet all the various demands posed by the environment.
The layout is not so bad, after all.
...and now imagine a fern.
Their spores fall down wherever chance takes them, and they are already haploid. They give rise to small gametophytes, whose sperm must swim for torturous centimetres to reach the ovule of another, or risk adding to genetic load of itself if (and that is a big if) inbreeding is actually possible.
With seeds, you have a fortress with nurseries hidden behind the most capable biochemical locks that have evolved in plants. Without seeds, you have farflung holdings struggling for survival, which your own fronds might deprive of sunlight. And oh, there's the problem of promoting own genomes without going extinct because of homozygosity.
Parts overview
(It would involve pheromones/hormones, ploidy, reticulate evolution, embrionic selection, climate and maybe some Hard Botany, though I will try to keep it to a minimum.)
In Part One, we will recall the four major groups of seedless vascular plants of today and see the difficulties they have to overcome to reproduce at all, not to mention differentially support their own genes.
In Part Two, we will learn how kin selection should be able to work for them, from Wilson (1981) to Greer et alia (2009) and beyond, and see for ourselves how difficult it is to crack the riddle of inbreeding vs outbreeding in the wild. This might take up some space if done rigorously.
In Part Three, we will return to species level and see hybrids competing with their parents, fertilizing and being fertilized and escaping the race by producing vegetative offspring. Yet why, oh why is it seen in some taxa and not in others?
In Part Four, we will see how environment promotes some features of population organisation. The only reason why it is thinkable as a blog-post is... lack of studies, certainly, otherwise we'd be buried under the sheer variety of ecological niches.
Part Five will round up the series, hopefully with some conclusions about how reproduction constraints anatomy and biochemistry and how gamete- and spore-producing generations coexist and flourish.
I might change the plan later on. Sorry for not discussing mosses; they are just too different from the rest, and I don't know them well enough.
(I cannot tell how often I will be able to post, and if there are any mistakes, please point them out to me.)