It has always been said that sexual reproduction produces offsprings which are superior to their parents, due to the variations which they acquire causing them to survive better in their environment. That's because we can think of meiosis occurring at some level of their life cycle resulting in the variations in the final offspring. But what when we talk about bisexual organisms, might they be plants, animals or any other life form? Can't there be variations in their offsprings if they produce them asexually? It's like when they undergo gamete formation in different male and female structure present in that single parent, they'll surely undergo meiosis(if the parent is not haploid undoubtedly) forming the gametes which are dissimilar in their genetic makeup. When these fertilize, shouldn't they show variations?
2 Answers
The question is rooted in a misunderstanding of what sexual reproduction is and is not.
Any good intro book to evolution has a chapter on the diversity of reproductive systems. Consider for example Evolution: Making Sense of Life. If you speak french, the book évolution biologique has a very chapter on the diversity of reproductive systems. If you want something more complete and more advanced, you can have a look at The Evolution of Sex Determination.
It has always been said that sexual reproduction produces offsprings which are superior to their parents, due to the variations which they acquire causing them to survive better in their environment.
Who said that? The common simplistic idea and sex increases mean fitness because it increases genetic diversity is both wrong because sex does not necessarily increase genetic diversity and wrong because increase in genetic diversity does not necessarily result in increasing mean fitness.
You can have a look at the work of Nick Barton, Sally Otto and many others on the subject. The above misconception is well explained in the introduction of this talk from S. Otto's
[..] But what when we talk about bisexual organisms, might they be plants, animals or any other life form?
I'm not sure what you meant here. You probably did not want to use the term bisexual. Maybe you meant dioceous.
Please note that sexual reproduction does not mean dioecy. Sexual reproduction does not even mean anisogamy and doe snot necessarily involved more than one individual (see selfing).
Can't there be variations in their offsprings if they produce them asexually?
There is a whole range of reproduction mode and saying sexually vs asexually is making gross categories that make it sometimes hard to really answer the question. It is also hard to know what you mean by "variations" here. But shortly speaking:
Asexual reproduction does not necessarily mean total absence of recombination (see for example Narra and Ochman 2006) and definitely does not mean absence of mutations (see for example Sniegowski et al. 1997).
It's like when they undergo gamete formation in different male and female structure present in that single parent, they'll surely undergo meiosis(if the parent is not haploid undoubtedly) forming the gametes which are dissimilar in their genetic makeup. When these fertilize, shouldn't they show variations?
If there is fusion, then there is sexual reproduction, even if it is selfing and even is they don't have genders (anisogamy).
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$\begingroup$ Thanks for the answer, it had literally cleared many of my misconceptions. I agree with your point that sexual reproduction doesn't always result in better offspring but can you please explain what you meant by 'sex does not necessarily increase genetic diversity'? I mean it does create variations. By variations I meant genetic recombination. I'm not sure what you meant here. You probably did not want to use the termbisexual. Maybe you meant dioceous. No I mean the use of term bisexual and an explanation to this was selfing as I well understood it. $\endgroup$ Commented Aug 12, 2017 at 4:40
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$\begingroup$ Maybe you meant hermaphrodite by saying bisexual. Bisexual is a term mainly used for humans in sociology and does not mean being able to reproduce with oneself (selfing when there is fertilization) and does not mean having both sexes (hermaphroditism, usually called intersex in humans) either. $\endgroup$– Remi.bCommented Aug 12, 2017 at 4:51
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$\begingroup$ Recombination does not necessarily increase genetic variance in the population. It rather diminish it actually. It is a common misconception that sex necessarily increase genetic variation. Have a look at the link to Sally Otto's talk, that will probably clear things up. $\endgroup$– Remi.bCommented Aug 12, 2017 at 4:52
The problem here is simply that you are considering self-fertilization as an example of assexual reproduction, when it's not the case.
Here is an oversimplified explanation, that may help you:
Sexual reproduction is a form of reproduction in which there is genetic recombination (unlike assexual reproduction).
Regarding the origin of the gametes, sexual reproduction can be classified as:
- Cross fertilization: each gamete comes from a different individual.
- Self-fertilization: both gametes come from the same individual.
Contrary to common belief, most hermaphrodite animals (what you call bisexual) and most monoecious plants perform cross fertilization, not self-fertilization, but some of them do perform self-fertilization. However, self-fertilization is a type of sexual reproduction, not assexual.
Therefore, next time you read about a hermaphrodite (like the tapeworm Taenia, for instance) doing self-fertilization, remember: that is sexual reproduction, not asexual.