6 Flashcards
draw a tree diagram for the various reproductive modes of organisms on earth
reproductive system - sexual/asexual
sexual system - hermaphrodite/dioecious
mating system - self fertilisation/cross fertilisation
define parthogenesis
asexual reproduction in which an embryo develops from an egg without fertilisation
define clonal propagation
asexual reproduction not involving an egg
define a hermaphrodite
individual has both male and female reproductive parts.
define dioecious
a characteristic of certain species that have distinct unisexual individuals
3 characteristics of sexual reproduction
- 2 parents contribute genetic material to offspring
- meiotic, reductive division to form gametes
- fusion of gametes
3 characteristics of asexual reproduction
- 1 parent contributes genetic material
- no meiotic reductive division.
- offspring are genetic replicas (clones) of parents
give 2 examples of organisms which can reproduce either sexually or asexually
water fleas (daphnia)
water hyacinth (pontederia)
two-fold cost of meiosis
- compared to an asexual female, a sexual female contributes only 50% of her gene copies to the next generation
- this transmission bias favours asexuals in competition with sexual females
whereas asexual reproduction maintains favourable combinations of alleles, sexual reproduction
can continually recreate unfavourable combinations of alleles
list the 6 costs of sex
- time and energy to find and attract mates
- increased energetic costs of mating
- risk of predation and infection
- cost of producing males
- 50% less genetic transmission
- break up of adaptive gene combinations through segregation and recombination
state the 2 main benefits of sex
it allows for the bringing together of favourable mutations and the elimination of harmful mutations (independent assortment recombination)
what are the benefits of genetic variation in variable/unpredictable environments known as?
lottery models
- tangled bank hypothesis
- red queen hypothesis
tangled bank hypothesis
an evolutionary theory that suggests sexual reproduction increases genetic diversity, which is advantageous in complex and variable environments.
in what environments is the tangled bank hypothesis applicable?
in spatially heterogeneous environments
in what environments is the red queen hypothesis applicable?
in temporally heterogeneous environments
red queen hypothesis
an evolutionary theory that suggests species must constantly adapt and evolve in order to survive while competing against other organisms that are also evolving.
describe the evening primrose
- 30% of Oenothera species are functionally asexual
- over time, there have been many independent sexual-asexual transitions
describe what experiments on asexual Oenothera have shown
asexual oenothera have:
- more premature stop mutations which lead to dysfunctional proteins
- higher rates of protein sequence evolution
- this implies a greater accumulation of deleterious mutations
what is the effect of spatial heterogeneity in selection?
it can facilitate the evolution of sex
describe what experiments were carried out on the rotifer Brachionus calyciflorus
- planktonic freshwater animal
- facultatively sexual with genetic variation for the propensity to reproduce sexually or asexually
- each population consisted of two subpopulations, with migration performed manually between them
- 10 replicate populations evolved under each of these conditions for ~70 generations
draw diagrams for the experiments conducted on rotifer
what were the results of experiments conducted on rotifers?
higher rates of sex are maintained in populations evolving in heterogenous habitats:
- sex declined rapidly over 12 weeks (70 generations) in homogeneous environments
- sex persisted at a much higher level with spatial heterogeneity
describe the distribution of asexuality by parthenogenesis
- sporadically distributed across the animal kingdom
- more common in invertebrates, rare in vertebrates
describe the distribution of asexuality by clonal propagation
- much more common in plants
- few species (if any) are exclusively asexual
why are asexual species usually t the tips of phylogenies?
- macroevolutionary pattern indicates higher extinction rate
- low chance of long term evolutionary persistence
- probably due to extremely low genetic variation and accumulation of deleterious mutations
describe a rare case of ancient asexuality
- bdelloid rotifers
- males are unknown
- no sex for millions of years
outbreeding
mates are less closely related than random
inbreeding
mates are more closely related than random
in practice, there is a —– between outbreeding and inbreeding
continuum
outcrossing
- mating with someone else either by outbreeding or inbreeding
- fusion of gametes from 2 parents, where gametes derive from meiotic reductive division
selfing (self-fertilisation)
- mating with yourself
- most extreme form of inbreeding
- NOT asexual reproduction
- fusion of gametes from 1 parent
- gametes derive from meiotic reductive division
why is there plenty of potential for inbreeding?
- local population substructure enhances mating among relatives
- hermaphroditic organisms have the potential for self fertilisation (most plants, many animals)
- in small populations, even random mating can lead to mating among relatives
describe two inbreeding avoidance traits in flowering plants
- 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
- eg spacing of anther and stigma
describe inbreeding avoidance behaviours in animals
- dispersal by one sex
- delayed maturation
- extra pair copulation
- kin recognition and avoidance
what are the population genetic effects of inbreeding?
- changes genotype frequencies: increases homozygosity, decreases heterozygosity (H)
- does not directly change allele frequencies; does not change polymorphism (P)
the effect of inbreeding on the rate of heterozygosity decline depends on
mating patterns
define inbreeding depression
the reduction in fitness of inbred offspring compared to outcrossed offspring
effects of inbreeding depression
- lower viability (survival)
- lower fertility (reproductive output)
strong inbreeding depression disfavours inbred offspring
- thus favouring outcrossed mating systems
why can inbreeding reduce fitness?
due to homozygosity of recessive deleterious alleles
what can reduce polymorphism?
competition between homozygous genotypes (selection) and genetic drift of small populations
describe the transition from inbreeding depression to selfing
- very common
- associated with extensive phenotypic evolution
- roughly 20% of plants and hermaphroditism animals are highly selfing
describe the frequency in nature of selfing and outcrossing in 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 and exported pollen
-> low inbreeding depression - but harmful effects of inbreeding depression encourage outcrossing
describe the frequency in nature of selfing and outcrossing in 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
describe the theory of automatic selection of a selfing gene by R.A fisher
table
- selfing variant has a transmission advantage
