The
evolutionary origin of sex is a puzzle
-Sex often means spending huge amounts of
energy finding and wooing desirable partners.
- male and female
genomes must recombine without major mistakes.
-conflicting interests of each sex must be
delicately negotiated in order to benefit the species as a whole.
-Asexual
reproduction less messy and more efficient, but sexual reproduction is by far
the most dominant mode
1. In asexual reproduction an individual inherits all chromosomes from one parent.
2.
Bacterial cells reproduce by binary fission, producing two daughter cells
containing the same genetic information.
3. Most
protists divide asexually unless under stress, then
they switch to sexual reproduction.
4.
Plants and multicellular organisms frequently
reproduce asexually. So animals may reproduce by budding off localized masses
of cells which grows by mitosis to form a new individual.
Even
when sex is possible there may still be reproduction without sex. Development from an unfertilized egg is called
parthenogenesis
Most scientists in this field agree that the advantage of sexual reproduction must lie in genetic recombination. Most theories centre on one or both of two theories:
1. sex and recombination remove harmful mutations and
2. sex allows new combinations of genes to come together
-Sex is of great evolutionary advantage to species or populations but evolution occurs because of changes at level of the individual.
-Survival
and reproduction of the individual is what matters. There is no obvious
advantage to the offspring of a sexually reproducing individual.
-Recombination of genetic material is evolutionarily both constructive (providing new combinations and variation) and destructive.
-Complex adaptations are less likely to benefit from recombination.
-A successful individual should (on the surface) do better by creating identical copies of its successful self rather than risking less adapted combinations.
-High cost of sex: cost of producing males.
Theories
on the Origin and Maintenance of Sex
The Red Queen hypothesis
-the
advantage of sex is that it may allow populations to “store” recessive (hidden)
alleles. Under current conditions the recessive alleles are bad, but they may
prove useful in future.
-stated another way, sex serves to assemble beneficial mutations and create a well adapted lineage in the face of a rapidly changing environment.
-An example in the real world? host-parasite interactions.
-This
theory proposes the evolution of sexual species in order to keep pace with
ever-changing constraints. Always have backup sheltered recessives to call on
when conditions change. Also known as the treadmill hypothesis: the name “Red
Queen” comes from the Queen of Hearts in Alice in Wonderland who tells
Muller’s ratchet
-mutations happen and most of them are bad. Sex produces loser and winner offspring by re-sorting the mutations.
-Asexual populations incorporate a mutational ratchet mechanism. The harmful mutations arise and because there is no way to eliminate them, they accumulate over time.
-Sex wins out because sex brings together and then purges the bad mutations
-Most
scientists will agree that both theories are involved in the maintenance of
sexual reproduction, and probably in the origin as well.