BCH447 lecture 7
The evolution of sex... We are not talking about sex. We are talking about
the reasons behind why sex developed.
With yeast we get haploid yeast... And they can replicate and make more
haploids.... Furthermore, the yeast also have a sequence that defines a
sex... Some are a type and others are alpha type sex.
It isimagined that in the beggining it was two different genes....
But the two different mating types can come together, fuse and form a diploid
cell. And those diploid cells can also divide mitotically and keep surviving.
The diploid cell will undergoe meiosis and form 4 spores... Usually form
during difficult conditions.
The question is, what is the advantage of going diploid and then going
What we are talking about is outcrossing. Redistributing alleles. We are also
talking about recombination. Are diploid cells better off then haploid cells.
Keep in mind that prokaryotes are still here... So the question is... Why do
eukaryotes mainly do sexual haploid diploid stuff.... Perhaps being diploid
helps mask recessive genes... But this is not an explanation of sex... Its an
explanation for diploidy.
Dont get confused between the evolution of sex and the maintanance of sec.
Once evolved... Things may have happened to make sex irreversible... We wanna
know the origin and evolution of sex.
Note begging the question means avoiding the question.
Mixing of genes allows for a population to survive because it contains more
variation in the proteins formed.... The assumption is that sex was an
accident... That a diploid evolved by accident and was maintained because of
genes allowing populations to survive.... Mixing of genes may also allow for
the evasion of deleterious mutation buildup... This is mullers ratchet...
Muller said that the accumulation of slightly deleterious mutations over time
in a population could cause the extinction of that population... Perhaps
sexual reproduction allows the reshuffling of genes... So that mullers
ratchet is no longer a problem.
Sometimes the slightly deleterious population may survive over wildtype... As
this happens over and over again... We will get a species with too much
Mixis... Sex is advantageous because sex provides more variability.
The assumption is that either sex was an accident. Orit was faster and better
at mixing up alleles.
With mixis argument it may be faster to mix two alleles that have recessive
regions that will complement each other. But, there is a lot of chance
involved... And as such it may not be such a good argument.
Is mixis and recombination enough to induce a selective advantage for sex?.. No.
The probelm with these explanations is that we are selecting for the future.
Sex is important because we can bring together alleles that are on different
homologous chromosomes... Here we are not talking about the advantage of sex,
but the advantage of recombination.
Recombination may help increase diversity in a population. In order for
recombination to occur is so that the alleles must be on the same
chromosome... And close enough on the chromosomes that they segregate
together... They are linked. If they are on different chromosomes then we are
not talking about recombination, bu