While it’s not the reason we have 2 sexes, there is an evolutionary reason for 2 partners to make an offspring, which is likely a big contributing factor
While asexual reproduction is easier, the downside is that the child is almost an exact genetic match of the parent. There’s a lot of reasons why this is not ideal (I don’t remember them lol, its complicated), so having 2 organisms mix their genes to create a genetically unique offspring is extremely advantageous. Having 2 sexes is likely easier than doing this with a unisex species, but as another commenter pointed out: evolution doesn’t make sense, it just does what works.
It seems to be the demands of large terrestrial mammals. Land is a harsher environment where more genetic diversity from sexual reproduction is more advantageous. Being larger and having longer lifespans makes asexual reproduction rarely advantageous. One parent needing to gestate the offspring would exacerbate the pressures for sexual dimorphism. Almost no mammals asexually reproduce, in fact it might actually be none, but I’m not sure.
Sexual reproduction is more advantageous than asexual reproduction because it makes genes more varied and prone to mutations and changes. If, for some reason, all beings of your species without characteristic “X” die because, for example, they do not have enough resistance to cold, in the end someone with resistance to cold survives because, through the mixing of genes and other factors, this advantageous characteristic has emerged and prevails.
Whereas in asexual species, everyone is born with almost the same genes as their parents, so if it starts to get very cold, eventually everyone dies because no one has developed resistance to cold, as it is rarer for this to happen.
PS: I am not an expert and I try my best to explain what I remember.
While it’s not the reason we have 2 sexes, there is an evolutionary reason for 2 partners to make an offspring, which is likely a big contributing factor
While asexual reproduction is easier, the downside is that the child is almost an exact genetic match of the parent. There’s a lot of reasons why this is not ideal (I don’t remember them lol, its complicated), so having 2 organisms mix their genes to create a genetically unique offspring is extremely advantageous. Having 2 sexes is likely easier than doing this with a unisex species, but as another commenter pointed out: evolution doesn’t make sense, it just does what works.
It seems to be the demands of large terrestrial mammals. Land is a harsher environment where more genetic diversity from sexual reproduction is more advantageous. Being larger and having longer lifespans makes asexual reproduction rarely advantageous. One parent needing to gestate the offspring would exacerbate the pressures for sexual dimorphism. Almost no mammals asexually reproduce, in fact it might actually be none, but I’m not sure.
Sexual reproduction is more advantageous than asexual reproduction because it makes genes more varied and prone to mutations and changes. If, for some reason, all beings of your species without characteristic “X” die because, for example, they do not have enough resistance to cold, in the end someone with resistance to cold survives because, through the mixing of genes and other factors, this advantageous characteristic has emerged and prevails.
Whereas in asexual species, everyone is born with almost the same genes as their parents, so if it starts to get very cold, eventually everyone dies because no one has developed resistance to cold, as it is rarer for this to happen.
PS: I am not an expert and I try my best to explain what I remember.