"Sexual evolution"

[gohanick]

If we did evolve, how did we change from asexual reproduction to sexual. The chances for a male and a female specieces "mutating" seems astronomically impossible.

[ecoli]

not if they live relatively close to each other, ie. operate within the same biological niche.

[tejaswini]

we did not change suddenly from sexual to asexual phase. there were intermediate stages between the two like for example in conjugation there is no cleare demarcation between the male and female cell. but it's kind of hypothetical.i hope u understood what i'm trying to say.

[Moonquake]

The persistence of life has always relied on its successful self-replication; a main barrier to this has inherently been inaccuracy in the creation of new DNA and therefore the creation of imperfect copies.

When life began the mutation rate may have been about 1 mistake every 100 letters of code, this was mainly due to them using RNA as their master-copy, a much more error prone molecule than DNA.

Through other mechanisms as well, including about 50 catalysts, by the time organisms as sophisticated as bacteria evolved, the error rate had been reduced to less than one mistake in every thousand million letters. Bacteria have many fewer genes than multicellular organisms, we have about a thousand times as many, and also our DNA has to be copied 100 times per generation, against the bacteria’s once per generation. A bacterium therefore only makes a mistake once in every 1000 offspring, whereas we make over 100 mistakes in every offspring. Also the production of a faulty bacterium represents much less resource cost.

 

This was a massive problem for the evolution of complex multicellular organisms and it is likely the main reason sexual reproduction evolved.

 

As most detrimental mutations are recessive alleles, the inheritance of two sets of genetic information, one from each parent, will allow for the mutation to be concealed and not be expressed phenotypically because it is unlikely both parents will have the mutation unless the population is small with high relatedness, even in this situation the mutations would be concentrated in some of the offspring leaving the others relatively healthy, these mutant offspring can then be purged from the gene pool through natural selection.

Another benefit from sexual reproduction was the ability to concentrate different beneficial mutations from different parents in some offspring; however this was likely to be a much less significant factor in its evolution as the vast majority of mutations are harmful to fitness.

 

Sexual reproduction would have originally evolved in single celled eukaryotes; whole individuals would have joined to exchange genes much like many bacteria do today.

Once they evolved multicellularity however, different cells would have specialised for different functions. Some cells would take the roll of fusing with cells of others to produce new individuals, as it would be difficult for whole multicellular organisms to fuse.

Initially these reproductive cells (or gametes) would have been the same size in all individuals; however a mutation that made some of them produce relatively smaller gametes would have reduced their resource cost and allowed them to produce more gametes. However if two of these smaller gametes fused, there would be insufficient nutritional resources for the new individual to develop successfully, therefore they would evolve to only fuse with the larger sized gametes of normal members of the species.

There would be further competition between smaller gamete producers to produce even smaller ones to out produce each other. Because of the significantly greater number of smaller gametes, the larger gametes would almost exclusively fuse with them, by chance.

Larger gamete producers would then experience a selection pressure to produce even larger ones to compensate for the lack of nutrients.

 

‘Male’ refers to the members of the species which produce the smaller gametes, ‘Female’ to the larger gametes.

 

This was critical for the development of complex multicellular organisms. In each eukaryotic organism’s cells there are multiple sets of DNA, the main sexually reproducing set that lies within the nucleus, and also the smaller asexually sets that lie within the organelles.

In the earliest sexually reproducing organisms, offspring would have inherited organelles from both parents because gametes were the same.

Instead of performing their function effectively, these organelles would have competed with each other to out reproduce each other.

There would be a selective pressure for mitochondria to be better at replicating themselves to ensure their survival in the host over the other parent’s mitochondria; they would do this instead of producing energy for the host as efficiently as they could.

However once gametes evolved to be different sizes, the larger gamete could produce mechanisms by which it would prevent the organelles of the smaller gamete from becoming part of the zygote. All organelles with their own DNA such as mitochondria and chloroplasts are therefore inherited only from maternal gametes.

 

Competition between organelles still occurs however. As our lives go on our mitochondria are still replicating, any mitochondria which shortens it’s DNA through mutation in our cells will be able to replicate faster than the others and therefore spread, even though it will be much less efficient at functioning, this is proposed as the main reason we age, as this mitochondrial competition occurs our cells become decreasingly efficient at functioning.

 

Mitochondria have 37 genes compared to the 30,000 genes of the nucleus and therefore have an insignificant mutation rate, therefore it doesn’t matter that they don’t reproduce sexually. This is due to the initial creation of eukaryotes through the fusion of different cells, since then most of the genes that code mitochondrial proteins have been relocated to the nucleus; otherwise there would be competition between nuclear and mitochondrial genomes. The mitochondrial genome hasn’t been transferred to the nucleus entirely because mechanisms by which some mitochondrial proteins can be transferred through its membrane are difficult to evolve.

In animals such as humans where the mitochondrial genome has been reduced to an extremely low number of genes, the genetic code has actually changed in relation to proteins coded for thus preventing the transfer of the last few genes to the nucleus, this is unusual as the genetic code is otherwise almost universal, because no other self-replicating genome has been small enough to change it, since life’s common ancestor.

 

In higher organisms, further differences between these genders occurred. Because of the much larger gamete size of females, their resource investment in each offspring is vastly greater than that of males, whereas males are therefore driven to get their gametes to fuse with as many female gametes as possible, females would benefit from being selective about which genes they allow to combine with theirs, the development of sophisticated sensory perception allowed for females to select which male they allowed to fertilise their gametes which they were able to protect internally or by choosing when to release them.

This female choice of males allowed for the accelerated spread of beneficial mutations through a population and provided a huge advantage for these species.

 

If a new adaptive trait develops, not only will it spread because it will be naturally selected for, but also because genes that make females attracted to that trait will also be naturally selected for as the females who choose to mate with males which have this trait will have fitter offspring. This allows the new trait to spread faster than if mating occurred only by chance.

This is how paternal care evolved in animals as offspring survival rates of a species would have benefited from female selection of mates who would contribute more to the development of their offspring.

 

As you would expect, in species with paternal care in monogamous relationships there is a roughly 1:1 ratio of males to females. However, even in species where there is no paternal care, in most cases there is still this 1:1 ratio of males to females, even though a species will produce more offspring if there are many more females, all of which a smaller number of males could fertilise.

This phenomenon is due to a natural tendency towards equilibrium. If male births were less common than female births, a newborn male has better mating prospects than a newborn female, and is therefore likely to have more offspring.

Therefore in this situation parents genetically disposed to produce males tend to have greater numbers of grandchildren.

Therefore the genes for male producing tendencies will spread and male births become commoner. The advantage the genetic disposition to produce more males will only be eliminated once the 1:1 ratio is reached.

 

It seems that in these species genes that produce males are almost parasitic in that the organism with them doesn’t reproduce its own self but imposes these genes on female reproduction.

However although females of these species invest significantly more resources into reproduction, the males invest significantly more in competition with each other through fighting or attractive displays to advertise their genetic health often at the expense of their own survival rates, for example due to being more obvious to predators or received damage from mating fights.

The exclusion of genetically less healthy males is an important factor in significantly reducing the likelihood of a population wasting resources in the production of mutant offspring which have less chance of survival and therefore further reproduction.

[Milken]

The main issue is that asexual reproduction seems like it would be favored by natural selection because it's more efficient (less of a challenge, less satisfying, makes some people feel lonely). In a population where both are, the asexual reproduction ones will quickly outnumber the sexual reproductive ones.

 

Granted sexual reproduction has the advantage of diversity, it's a future advantage, not a present one NS would select.

[Airmid]

Thanks Moonquake, that was a good read!

[Moonquake]

The main issue is that asexual reproduction seems like it would be favored by natural selection because it's more efficient (less of a challenge' date=' less satisfying, makes some people feel lonely). In a population where both are, the asexual reproduction ones will quickly outnumber the sexual reproductive ones.

 

Granted sexual reproduction has the advantage of diversity, it's a future advantage, not a present one NS would select.[/quote']

 

In a population where there is competition, higher quality offspring are more successful than a larger number of poor quality offspring.

[Milken]

In a population where there is competition, higher quality offspring are more successful than a larger number of poor quality offspring.

 

What kind of competition? I agree, I think. I used diversity(instead of quality) because they'd have a better gene flow and each bacteria wouldn't be susceptible to the samething. Prok were around for about 1bil yrs before Euk. I concede it is possible there was a local population they could have come from. If it happened it's also probable a cyan and arch bacteria had a symbiotic relationship to from the first euk. What makes you think RNA was used instead of DNA in the begining?

[Moonquake]

What kind of competition? I agree, I think. I used diversity(instead of quality) because they'd have a better gene flow and each bacteria wouldn't be susceptible to the samething. Prok were around for about 1bil yrs before Euk. I concede it is possible there was a local population they could have come from. If it happened it's also probable a cyan and arch bacteria had a symbiotic relationship to from the first euk. What makes you think RNA was used instead of DNA in the begining?

 

Any competition which leaves the less adapted individuals less able to reproduce, for example because they're killed.

 

It is theorised that RNA was used because in simulations of early Earth, RNA bases have been producable in these non-biological conditions, whereas the DNA base Thymine hasn't been. RNA is technically capable of self-replication in a primordial soup of these bases. It only takes slight modification through biological means to convert Uracil to Thymine. Which would have been how DNA came about. Look up "RNA World".

However this part of my post has little to do with sexual evolution but was intended to give a background to early life and mutation rates.

[Moonquake]

Thanks Moonquake, that was a good read!

 

You're welcome. I wrote it for Uni.

[aguy2]

The persistence of life has always relied on its successful self-replication; a main barrier to this has inherently been inaccuracy in the creation of new DNA and therefore the creation of imperfect copies...

This was a massive problem for the evolution of complex multicellular organisms and it is likely the main reason sexual reproduction evolved.

 

It would seem that 'biological life' has rejected random, accidental mutation as an agent of 'change', in favor of a sexual reproduction that emphasizes 'purposeful volition' over 'accidental randomness'.

 

aguy2

[Moonquake]

Simply, if a feature allows an organism to replicate more effectively, that feature itself will be replicated more effectively and will therefore spread faster than a feature that does not have this effect.

[aguy2]

Simply, if a feature allows an organism to replicate more effectively, that feature itself will be replicated more effectively and will therefore spread faster than a feature that does not have this effect.

 

If the prodigy of a system where 1/2 the species is almost 'expendable', eats all the rest of the possible systems for breakfast, lunch, and dinner it certrainly passes any test of pragmatism.

aguy2