Lecture 18 Flashcards
Sexual reproduction:
- 2 parents contribute genetic material to offspring
- Meiotic, reductive division to form gametes
- Fusion of gametes
Asexual reproduction:
1 parent contributes genetic material
* No meiotic reductive division
* Offspring are genetic replicas (clones) of parents
The Costs of Sex
Time and energy to find and attract mates
* Increased energetic costs of mating
* Risk of predation & infection
* Cost of producing males
* 50% less genetic transmission
* Break up of adaptive gene combinations
– Segregation, recombination
What are the benefits?
* This is the big question
* Known as ‘the paradox of sex’
* Can they overcome the costs?
The Two-fold Cost of Meiosis
Compared to asexual females,
sexual females contribute
only 50% of her gene copies
to the next generation
This transmission bias
favours asexuals in
competition with females
Hypotheses for the
Advantages of Sex
Bringing together favourable mutations
* Eliminating harmful mutations
* Benefits of genetic variation in variable environments
– “Lottery models” given environmental unpredictability
– Spatially heterogeneous environments
* ‘Tangled Bank hypothesis’
– Temporally heterogeneous environments
* ‘Red Queen hypothesis’
Many theoretical models, but
only limited experimental evidence
Advantage of Sex in Evening Primrose:
Elimination of Harmful Mutations
Asexual Oenothera have:
* More “premature” stop codon
mutations
* Leads to dysfunctional proteins
* Higher rates of protein
sequence evolution
* Implies greater accumulation
of deleterious mutations
Higher rates of sex maintained in populations
evolving in heterogeneous habitats
Sex declined rapidly over
12 weeks (70 generations) in
homogenous environments
* Sex persisted at a much
higher level with spatial
heterogeneity
Macroevolutionary history of asexuality
- Asexuality by parthenogenesis:
- Sporadically distributed across the animal kingdom
- More common in invertebrates, rare in vertebrates
- Asexuality by clonal propagation:
- Much more common in plants
- Few species (if any) are exclusively asexual
- Asexual species are usually at the tips of phylogenies
- Macroevolutionary pattern indicates higher extinction rate
- Low chance of long-term evolutionary persistence
- Probably due to extremely low genetic variation & accumulation of
deleterious mutations
Mystery of the Bdelloid rotifers:
No Sex for Millions of Years
A rare case of ancient asexuality
* Males are unknown
* Species diversification has led to > 300 spp
Mating patterns:
Who mates with who, and how often
Mates are less closely related than random
= Outbreeding
* Mates are more closely related than random
= Inbreeding
In practice there is a continuum between
outbreeding & inbreeding
Outcrossing
Mating with someone else!
* Either by outbreeding or inbreeding
* Fusion of gametes from 2 parents
* Gametes derive from meiotic reductive division
Selfing (self-fertilization):
Mating with yourself!
* Most extreme form of inbreeding
* But NOT asexual reproduction
* Fusion of gametes from 1 parent
* Gametes derive from meiotic reductive division
Plenty of potential for inbreeding
Local population substructure enhances
mating among relatives
* Hermaphroditic organisms have potential
for self-fertilization
– Most plants, many animals
* In small populations, even random mating
can lead to mating among relatives
Inbreeding Avoidance Traits in
Flowering Plants
Large, showy flowers attract pollinators
* Timing offset between male and female
reproduction
– Pollen vs. ovule maturation within a flower
– When male vs. female flowers open
* Diverse morphological & physiological
mechanisms to avoid selfing
– Self-incompatibility
– E.g. spacing of anther and stigma
Inbreeding Avoidance Behaviors
in Animals
- Dispersal by one sex
- Delayed maturation
- Extra pair copulation
- Kin recognition and avoidance
Population genetic effects of inbreeding
Changes genotype frequencies
* Increases homozygosity
* Decreases heterozygosity (H)
* Does not directly change allele
frequencies
* Does not change polymorphism (P)
Inbreeding Depression
The reduction in fitness of inbred offspring compared
to outcrossed offspring
* Lower viability (survival)
* Lower fertility (reproductive output)
* Strong inbreeding depression disfavors inbred offspring
* Thus favoring outcrossed mating systems
Why can inbreeding reduce fitness?
Homozygosity of recessive deleterious alleles
The genetic consequences
of inbreeding
Genotypic frequencies changed
– Heterozygosity (H) reduced by 50% per generation with
self-fertilization
– Competition between homozygous genotypes (selection)
& genetic drift of small pop’ns can reduce P
* Homozygosity for deleterious recessive alleles
– Results in inbreeding depression
Inbreeding Depression Causes Reduced Fitness
…Yet Selfing has Evolved many times
One of the most common
evolutionary transitions
* Associated with extensive
phenotypic evolution
* Roughly 20% of plants and
hermaphroditic animals are
highly selfing
Understanding the Frequency in Nature
of Selfing and Outcrossing
Over the short term:
* If conditions are favourable selfing can spread
via natural selection
* Lack of “reproductive assurance” due to rarity of pollinators or mates
* Transmission advantage from self + exported pollen
* Low inbreeding depression
* BUT harmful effects of inbreeding depression
encourage outcrossing
* Over the long term:
* Selfing leads to low diversity and inefficient selection
* Can drive higher extinction rates in selfing species
* Macroevolutionary pattern of greater prevalence of outcrossing
