Biol 321 The Evolution of Sex
Terms in this set (35)
In multicellular eukaryotes, asexual reproduction is typically defined as the
production of offspring from unfertilized gametes
two basic forms of asexual reproduction
apomixis and automixis. In both cases,
reproduction involves a single, female parent
In apomixis, an unfertilized
gamete undergoes a single mitosis-like cell division, producing two "daughter cells."
Each daughter cell has an unreduced number of chromosomes and is genetically
identical to its mother. In plants, apomixis is sometimes referred to as apogamy.
involves the production of haploid gametes via meiosis, but diploidy is
usually restored by the fusion of haploid nuclei from the same meiosis (some biologists
consider this a form of sexual reproduction).
Offspring from automictic asexual reproduction are genetically different
from their parent and their siblings, but much less genetic variation is generated here
than in sexual reproduction.
types of automixis
(A) In gamete duplication, a full round of meiosis is
followed by replication of the haploid genome. Even in the presence of recombination, the resulting
offspring will be entirely homozygous. (B) In terminal fusion, meiotic products containing two sister
chromatids fuse to form an individual that is homozygous except for any recombination events
that occur during meiosis. (C) In central fusion, meiotic products containing homologous chromatids
fuse, leading to a heterozygous offspring that would be identical to the parent were it not for
recombination events that may have occurred during meiosis
sexual reproduction involves the joining together of
genetic material from two parents to produce an offspring that has genes from each
sexual selection is characterized by amphimixis
1. Recombination: the crossover between homologous chromosomes, which
produces new chromosomal variants.
2. Gamete production: the production of haploid gametes by diploid
individuals via reductive meiotic division.
3. Gamete fusion: the gametic exchange between (usually) unrelated
individuals, in which haploid gametes fuse to produce a diploid offspring.
distinguishing between sexual and asexual reproduction
researchers used natural history ( searching for direct and indirect physical clues of sexual reproduction) to classify a species as reproducing sexually or asexually until recently.
these clues can be
observing two individuals mating, courtship implies sexual reproduction as well as sexual organs. in asexual organism presence of males indicate seual
The molecular machinery and cellular processes associated with various components
of sexual reproduction
reproduction—recombination, gamete production, and gamete fusion—are
complex and involve many genes operating simultaneously we can use the presence ans absence of these genes and their homologs to infer whether reproduction is sexual or asexual.
a comparison of phylogenetic trees to help understand if a species reproduces primarily through sexual or asexual
To see how, recall that mitochondrial
DNA (mtDNA) is inherited only through females, but nuclear DNA is inherited
through both parents. This means that in asexual species—which contain only
females—phylogenetic trees based on mtDNA and nuclear DNA should be fairly
congruent—that is, they should be similar to one another. But a comparison of
phylogenetic trees based on mtDNA and nuclear DNA genes is predicted to be less
congruent in sexual species (although this comparison can be skewed by population
bottlenecks and population expansions). Evolutionary biologists can then use the
degree of phylogenetic incongruity between nuclear DNA- and mtDNA-based
trees to infer mode of reproduction (Figure 16.7).
phylogenetic overview of sexual and asexual reproduction
Evidence also suggests that asexual taxa are short-lived compared to
sexual taxa. Although there is much debate as to how to calculate "short-lived" in
absolute time—that is, how many thousand years is considered short?—in general,
the consensus is that asexual species go extinct more quickly than sexual species
Indeed, to date, evidence suggests that
all species of eukaryotes that reproduce only asexually are derived from an ancestral
sexual species, strongly suggesting that sexual reproduction is the ancestral state in
The costs of sexual reproduction
diploid sexual females produce haploid gametes containing only one of the two sets of chromosomes that they possesAs such,
any haploid gamete, should it successfully fuse
with another haploid gamete, will produce a
diploid offspring that contains only one set of
its mother's chromosomes (and one set of its
assumin no inbreeding
asexual females are twice as genetically related to their offspring as are sexual females
the twofold cost of sex
John Maynard Smith made the following argument: Consider a
population that is made up of asexually reproducing females, as well as sexually
reproducing males and females the number of asexuals in sucha population wil grow at twice the rate of the sexually reproducing individuals
asexual females avoid the cost of producing males.
by producing offspring that do not need to beinseminated in order to reproduce themselves ony females
twofold cost of sex (Maynard Smith)
sexual reproduction less efficient for females as males dont produce offspring directly. only half the females genes are present in each offspring
twofold cost ctd
courtship, mating takes time energy and is dangerous. parasites etc
Sex can break up favorable gene combination
recombination can potentially break up associations between gene combinations that have been favored by natural selection.
favorable gene combination
allele at one locus is fav ored when it occurs in the presence of a specific allele at another locus but not otherwise
accumulation of slightly deleterious mutations
no chromosome free of deleterious mutations first turn of the ratchet. eachturn of the ratchet is quickly followed by the fixation of a deleterious mutations and these are the chromosomes that will populate other classes.
background selection- accumulation of deleterious mutations
strongly deleterious mutation, selection removes mutations leading to a smal effective population sizel and deleterious mutations fix as selection is not efficient against slightly deleterious mutations
beneficial mutation increases in freqency but the fixation of the beneficial mutation fixes also deleterious mutations. this occurs becuasee this is the best chromosome in the population despite carrying deletrious mutations
fisher muller theory
sexual populations evolve faster because independently arising beneficial mutations combine more quickly in the same genotype however most mutations are deleterious while beneficial ones are rare. so an asexual lineage will outcompete sexuals before a potential longterm benefit from the combination of suc rare beneficial mutations becomes important.
in asexual reproduction the mutations must be accumulated sequentially in the descendants of a single mutant individual.
If frog asexual it must wait for each mutation to arise in the
germ line in which the previous mutation arose.
In the meantime frogs with each single adaptation compete.
If frog sexual then wherever
each mutation arises,
recombination will bring them
together into one very well
muller ratchet ctd
'Muller's Ratchet' is another classical explanation for sex. It is based on deleterious mutations and the effect
of genetic drift (the random loss of genotypes in finite populations). It denotes a mechanism operating in
finite asexual populations whose affect is that the number of deleterious mutations can only increase over
time Recombination is beneficial,
because it can re-create the group with the lowest
variation in offspring
asexually will not do as well the environment is stable unlike sexual reproduction because it produces variable offspring
sexual species survive longer
fossil record suggests asexual species go extinct more often than sexual. assumed thats because they cant adapt when environment changes. all these benefits of sex are benefits to the species and not the individual so cannot be the cause of the evolution of sex.
sex and mullers ratchet
As highest fitness groups lost
by drift, average fitness
Loss of highest fitness group
by drift exceeds recreation by
Burden called genetic load.
Sex allows recreation of zero deleterious
mutation subpopulation each generation by
• With asexual reproduction, the zero
deleterious mutation genotypes are missing
(i.e. there is linkage disequilibrium)
mullers ratchet outcomes
Very dependent on population size
- Small pop' affected
• Mildly deleterious mutations more likely to accrue
- Strongly deleterious selected out
• Studies with endosymbiotic bacteria demonstrate
- But over long term
- Short term asex strongly favored
red queen hypothesis
coevolution, particularly between hosts and parasites, could lead to sustained oscillations in genotype frequencies
This idea forms the core for one of the leading hypotheses for the persistence of sexual reproduction
In species where asexual reproduction is possible (as in many plants and invertebrates), coevolutionary interactions with parasites may select for sexual reproduction in hosts as a way to reduce the risk of infection in offspring
According to the Red Queen hypothesis, sexual reproduction persists because it enables host species to evolve new genetic defenses against parasites that attempt to live off them.
two fold cost of sex
number of asexuals grow at twice the rate o the sexually reproducing individuals because male only produces sperm never have offspring directly and asexual females avoid the cost of produces males by producing offspring that dont need to be inseminated inorder to reproduce themselves, asexual females
one type of gamete medium games when they fuse are together the size of the large gamete produced by anisogamy
two diferent types of gamete
parasites ans sexual reproduction
proportion ofmales is a measure of the prevalence of sexual reproduction red queen hypothesis predicts that the frequency of sexual reproduction will be related to the level of parasitic inection if parasites infect an asexual lineage the parasites are most likely successfcul since in each generation the host genome remains unchanged but in sexual genetic variation occasionally produces parasite resistant groups so may fav or sexual when parasites are abundant
muller ratchet is not the same as fixation of deleterious allele
the ratchet can turn without any deleterious mutation becoming fixed similarly a deleterious mutation can be fixed without causing the ratchet to turn