Topic 3: Evolution of SexBefore we go into why sexual reproduction exists and there are different sexes, a bit of context is useful.
We can break the tree of life into 3 main domains:
- Archaea (e.g. some chemosynthetic organisms)
- Bacteria (e.g.
Thermos aquaticus famous for taq polymerase & cyanobacteria which is sometimes [misleadingly] called blue-green algae)
- Eucarya (e.g. plants and animals)
We can also break organisms into two different groups:
- Prokaryotes (bacteria and archaea)
- Eukaryotes (eucarya)
[fun fact: archaea is more closely related to eucarya than to bacteria]
There are some differences between prokaryotes and eukaryotes, with the most important one being the prokaryotes don't have a nucleus or membrane bound organelles as a rule. Basically, us eukaryotes like to make different segmented off areas within our cells and prokaryotes don't do that. As you may have guessed, that's not the difference between prokaryotes and eukaryotes we'll be examining in this post.
Prokaryotes | Eukaryotes |
No membrane bound organelles | Yes membrane bound organelles |
single celled | can be multicellular |
big circle of DNA plus plasmids | linear chromosomes |
Reproduction is asexual (only) | Reproduction can be asexual or sexual |
Before I get too carried away with myself for coding a table in the forums let's move on.
You might then think that sexual reproduction evolved at some point in the eukaryotes, it was clearly superior and thus all its descendants are sexual while those that descended from a different ancestor didn't have such luck and here we are. That isn't the case, and it
can't be the case because organisms which reproduce asexually isn't a monophyletic group (i.e. the most recent common ancestor of all sexually reproducing organisms has some descendants which reproduce asexually). This means that sexual reproduction and/or asexual reproduction has to have evolved multiple times.
So why might asexual reproduction evolve? What are the benefits?
Well, consider 2 populations each with four individuals and the difference being sexual vs asexual reproduction. Each generation the (semelparous) organisms have 2 offspring then the parents die
In the sexual one we'll assume even sex ratios (i.e. 50/50 male/female):
- 1 male + 1 female produces 2 offspring, 1 male + 1 female produces 2 offspring : total of 4 offspring
- now the offspring are parents, process repeats and 4 offspring are produced
- now the offspring are parents, process repeats and 4 offspring are produced
ok, so after 3 generations we still have 4 organisms. cool
What about with asexual reproduction?
- 4 individuals (often classed as females) produce 2 offspring each: total of 8 offspring
- 8 females produce 2 offspring each: total of 16 offspring
- 16 females produce 2 offspring each: total of 32 offspring
after 3 generations we have 32 organisms. I think asexual reproduction wins.
This demonstrates the twofold cost of sexual reproduction. Since the amount of offspring in a sexually reproducing population is constrained by the number of females, this can be described as asexual females producing twice as many daughters as sexual females.
So given that asexual individuals can spread their genes into a population
much faster than sexual individuals, why is it that sexual reproduction exists?
There are 3 major ideas around this:
(note: I'm just using these names so I can remember them - I've made them up)
1. Addressing Muller's Ratchet
2. Wisemann's adaptation benefit of 1904
3. Mortiz et.al 1991 & the Red Queen
Before I can explain what these ideas are, you need to understand recombination (i.e. crossing over) - a process which occurs in meiosis.
It goes like this: you have 2 copies of each chromosome - one from each of the gametes (e.g. sperm, egg) which combined to form the zygote that developed into you. When you make/made your gametes, those two copies can cross over, switching alleles (i.e. versions of a gene) and recombing into two new chromosomes with a unique combination of alleles compared to your parents chromosomes.
The benefits of this fall into two categories:
- Removing deleterious ("bad") alleles without getting rid of everything attached to it (addressing Muller's Rachet)
- Increasing genotypic diversity (Wisemann's adaptation benefit of 1904, Moritx et. al 1991 and the Red Queen)
1. Addressing Muller's Ratchet- Recall that mutations are random changes in DNA
- Most of these random changes will not be beneficial to the organism - and there will be more changes which reduce fitness than increase it
- Over time, these mutations will accumulate (even though selection opposes them - remember that stochasticity/randomness plays a role)
- In an asexual population you can't get rid of these deleterious mutations without removing/killing all organisms with this trait
This is Muller's Ratchet
In a sexual population, however, recombination means that you can chuck the deleterious mutations on one chromosome (which has a 50% chance of being passed down to each of your offspring) thus allowing you to have offspring without the deleterious mutations (and with the beneficial mutations originally on the same chromosome as a deleterious one)
2. Wisemann's adaptation benefit of 1904 &
Mortiz et.al 1991 & the Red QueenSince we can get all of these funky new combinations of alleles on chromosomes and you have 2 different copies of each chromosome, that more genotypic diversity.
What's good about genotypic diversity? It gives more things for natural selection to act on.
At this point I'm going to split off and focus just on Wisemann's adaptation benefit of 1904
This means that the chance of a beneficial genotype (combination of alleles) existing in the population is increased. In natural selection, the proportion of that genotype is increased in the population across time - benefitting future generations.
This is supported by research showing that sexual strains of yeast had higher fitness than asexual strains when evolving under stressful environmental conditions. There were no differences under benign conditions. (Goddard et al 2005
Nature)
Going back to
Mortiz et.al 1991 & the Red QueenThis is focused not so much on being
better but rather on being
ok. The Red Queen Hypothesis describes co-evolution where populations need to keep evolving just to not be disadvantaged by the evolution of other populations. (It's named after a conversation between Alice and the Red Queen in one of Lewis Carrol's books where the Red Queen says you need to run to stay in the same place.)
In regards to the evolution of sex, this is focused on parasites. Parasites tend to have a shorter lifespan than their hosts, which means they can undergo adaptive evolution faster than hosts e.g. bacteria often go through 70 generations in just one day. This means that they'll be able to adapt to whatever resistance the host has fast. Therefore, to compensate for this disadvantage, the host wants to make sure that when they do increase their genotypic variation they do it by a lot (more variation = more likely to have lots of new ways of being resistant). Since sexual reproduction creates many novel/new genotypes, this favours sexual reproduction.
A study into a species of Australian geckos (
Heteronotia binoei) found that sexually reproducing geckos had fewer parasites than parthenogenetic geckos (Moritz et al. 1991) - supporting this theory.
Note: a parthenogenetic gecko is a gecko that developed from an unfertilised egg.