Posted by Belphegorite at 12:29am Aug 4 '09I don't remember all my details here, so at least google some of this stuff before you go believing it, or even worse quoting it.
Anyway, I'll start with plants because a) they're simpler and 2) I remember them better. Actually, I'll start with fungi, which aren't plants but may share an evolutionary path. Fungi reproduce by spores. Most spores (I think) are asexual, meaning they contain the exact genetic material of the parent. There is no need for other genetic material to be combined before the spore is ready to create new offspring. But some spores have a sort of primitive "sex" known as "+" and "-". Like sexual gametes (gamete is fancy word for reproductive cell) a + needs a -, and vice versa, before it can produce offspring. Unlike sexual gametes, there is no structual difference between + and - like there is between male and female cells in true sexual organisms. So anyway, we're seeing the beginning of the move from asexual to sexual reproduction; The focus has shifted from efficiency to genetic diversity.
As we move into plants, we see a further refinement into a true sexual species. There are distinct male (pollen) and female (seeds) gametes, as well as specialized sex organs for each cell (stamens and pistils and whatnot). Some plants still retain an asexual reproductive mechanism (such as rhizomes in ferns), but the more complex organisms are purely sexual. However, all plants are hermaphroditic, so we still need to address the development of seperate sexes.
Let's move on to animals. I don't think there are any animals which reproduce entirely asexually. If so, they're stupid things like tunicates or nematodes and we don't care about them. Anyway, just like in plants sexual reproduction becomes the dominant form of reproduction very early in animal evolution. But like plants, organisms (earthworms and such) are still hermaphroditic. As we get into higher complexity (fish, insects) we see a shift from not just distinct male/female organs but distinct male/female organisms. This follows the general trend of reduction in redundant systems/organs as complexity increases. We also see the development of ovaparism (egg laying) which seems to be related, although I don't know if ovaparism forced distinct sexes or if having distinct sexes made ovaparism viable. Heck, they may just be purely coincidental.
Again, I'm going off 7+ year old memory so try to verify some of this stuff before you accept it. This should at least give you ideas of what to look for.
Anyway, I'll start with plants because a) they're simpler and 2) I remember them better. Actually, I'll start with fungi, which aren't plants but may share an evolutionary path. Fungi reproduce by spores. Most spores (I think) are asexual, meaning they contain the exact genetic material of the parent. There is no need for other genetic material to be combined before the spore is ready to create new offspring. But some spores have a sort of primitive "sex" known as "+" and "-". Like sexual gametes (gamete is fancy word for reproductive cell) a + needs a -, and vice versa, before it can produce offspring. Unlike sexual gametes, there is no structual difference between + and - like there is between male and female cells in true sexual organisms. So anyway, we're seeing the beginning of the move from asexual to sexual reproduction; The focus has shifted from efficiency to genetic diversity.
As we move into plants, we see a further refinement into a true sexual species. There are distinct male (pollen) and female (seeds) gametes, as well as specialized sex organs for each cell (stamens and pistils and whatnot). Some plants still retain an asexual reproductive mechanism (such as rhizomes in ferns), but the more complex organisms are purely sexual. However, all plants are hermaphroditic, so we still need to address the development of seperate sexes.
Let's move on to animals. I don't think there are any animals which reproduce entirely asexually. If so, they're stupid things like tunicates or nematodes and we don't care about them. Anyway, just like in plants sexual reproduction becomes the dominant form of reproduction very early in animal evolution. But like plants, organisms (earthworms and such) are still hermaphroditic. As we get into higher complexity (fish, insects) we see a shift from not just distinct male/female organs but distinct male/female organisms. This follows the general trend of reduction in redundant systems/organs as complexity increases. We also see the development of ovaparism (egg laying) which seems to be related, although I don't know if ovaparism forced distinct sexes or if having distinct sexes made ovaparism viable. Heck, they may just be purely coincidental.
Again, I'm going off 7+ year old memory so try to verify some of this stuff before you accept it. This should at least give you ideas of what to look for.
Not sure, but... (Warning: Science) - Belphegorite