Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Behavioural Ecology
Evolution of sex
| Question | Answer |
|---|---|
| Gametogenesis | The production of haploid sperm and eggs via meiosis. Involves recombination and isolation of one set of chromosomes |
| Fertilisation | Fusion of haploid gametes from two different individuals to produce diploid embryos/offspring |
| Sexually producing females | Share offspring relatedness with a male. Offspring relatedness is 0.5 rather than 1 Have to produce offspring who won’t produce any themselves (sons) This is inefficient compared with asexually reproducing females |
| Asexual vs sexual | In a resource-limited environment, asexual lineages can rapidly outcompete sexual lineages |
| Costs | Sharing genetic relatedness of offspring Production of males |
| Benefits | Breakup of coadapted gene complexes through recombination Requirement to find mates Sexual conflict |
| Sexual reproduction is old | - 1.2 billion years old Therefore selected early on in evolution of ‘life’ = advantageous phenomenon? |
| Why is it old? | 2 hypotheses: Drift-based model: Fisher-Muller Hypothesis Selection-based model: Red Queen Hypothesis |
| Fisher-Muller hypothesis | In asexual lineages, any mutations that arise are passed on to clonal offspring and so accumulate in the lineage Mutations are almost always detrimental to fitness – essentially all mutations are accidents |
| Fisher-Muller hypothesis | In asexual reproduction the only mechanism to purge a mutation is a randomly occurring back-mutation. This is very rare So essentially, mutations go one way |
| Inheritance under sexual reproduction | Sexual reproduction can break this ratchet, as in a diploid organism, inheritance is not 100%. Parents are likely to come from different lineages so unlikely to carry the same mutation |
| Inheritance under sexual reproduction | 1 in 4 of the offspring have both mutations purged This enables selection to purge these mutations |
| Evidence for drift-based models | Although asexual species exist, they do not tend to persist over evolutionary timescales That is, they could be referred to as ‘twigs on the tree of life’ |
| Red Queen Hypothesis | Over time and space, biotic AND abiotic selection pressures vary Examples of this include climate change/global warming; host-parasite coevolution, or predator-prey coevolution |
| Red Queen Hypothesis | Due to recombination, sexual reproduction increases variation, which increases adaptability This allows for more rapid evolutionary change/adaptation to new conditions |
| Evidence for selection-based models | This example uses a host-parasite symbiosis system Potamopyrgus are abundant in freshwater habitats throughout New Zealand - 2 forms: sexual and asexual Asexuals can produce twice as many daughters as sexuals – so why do sexuals exist? |
| Evidence for selection-based models | Positive association between proportion of males in a population and parasite load Why? Suggests that sexually reproducing snails (measured by proportion of males in a population) are more able to cope with parasite load |
| Why has sexual reproduction evolved? | 1. Drift-based models: Sex can undo mutation accumulation and thus the associated costs (purge bad genomes) |
| Why has sexual reproduction evolved? | 2. Selection-based models: Sexual reproduction allows increased variability to combat changing selection pressures, e.g. parasites. (spread good genomes) |
| Evolution of anisogamy | Consider the initial evolution of sexual reproduction: a prehistoric population of aquatic sexual reproducers, producing isogamous protogametes |
| Evolution of anisogamy | These isogamous protogamtes shed into the water and seek to fuse with another Naturally, there will be small amounts of variation in size/number of protogametes produced |
| Evolution of anisogamy | Protogametes need to do 2 things Fuse with other protogametes Provision for the resulting zygote |
| Evolution of anisogamy | An individual has finite resources, they can make numerous very small protogametes to be competitive for maximising the number of fusions, or they can make fewer large protogametes to produce fitter offspring |
| 1. | Lots of resources for offspring BUT collisions very rare |
| 2. | Collisions very common BUT few resources for offspring |
| 3. | Collisions not very common AND few resources |
| 4. | Lots of resources AND collisions common |
| More sperm = more competition for fusion | Monogamy vs promiscuity Species at higher risk of experiencing sperm competition produce more sperm Males within a species produce more sperm when there is a risk of competition |
| Sperm number & sperm competition experiment | There is natural variation in the sperm length of male crickets Within a male, sperm size is fairly consistent across spermatophores Within a male, sperm number is fairly consistent across spermatophores |
| Sperm number & sperm competition experiment | We know sperm size and sperm number are relatively consistent across spermatophores within an individual Screen sperm number and length from spermatophore. Mate two screened males with a female. Identify who wins fertilisations |
| Evidence | Larger eggs = fitter offspring Experiment in thick-billed murre. Eggs translocated between nests within a colony Larger eggs: Fledged earlier Wing feathers developed faster Higher weight maintenance after fledging |
| Evidence | Larger eggs = fitter offspring Experiment in Brook Trout Larger eggs: Larger juveniles at hatching Increased hatchling survival Effects compounded when juvenile diet was limited (e.g. under stress) |
| Anisogamy as an Evolutionarily Stable Strategy | Doubling sperm volume = insignificant increase - to 0.00002% of zygote investment Would result in = halving of ejaculate size = significant loss of ~500 million sperm cells and likelihood of fertilisation success |
Created by:
rose.coo
Popular Ecology sets