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.
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?)
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.
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)