2.3 Variation and sexual reproduction

• Sexual reproduction is the production of new living organisms by combining genetic information from two individuals of different types (sexes)
• Costs of sexual reproduction include males being unable to produce offspring; only half of each parent’s genome passed onto offspring, ​

  disrupting successful parental genomes. 
• (sexual reproduction)

  Benefits outweigh costs due to an increase in genetic variation in the population .​

  Genetic variation provides the raw material required for adaptation giving sexually reproducing organisms a better chance of survival under changing selection pressures.
• Co-evolutionary interactions between parasites and hosts may select for sexually reproducing hosts. ​

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  Hosts that better able to resist and tolerate parasitism will have greater  fitness Parasites better able to feed , reproduce and find new hosts have greater  fitness .
• If hosts reproduce sexually, the genetic variability in their offspring reduces the chances that they will be susceptible to infection by parasites.
• Asexual reproduction is a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametes.
• (asexual reproduction)

  Maintaining the genome of the parent is an advantage particularly in very narrow , stable  niches or when recolonising  disturbed  habitats .
• Parthenogenesis is reproduction from a female  gamete without fertilisation .  It is more common in cooler climates, which are disadvantageous to parasites, or regions of low parasite  density  or diversity . 
• (asexual reprodution)

  Offspring can be reproduced more  often and in larger  numbers with asexual reproduction.
• Organisms that reproduce principally by asexual  reproduction also often have mechanisms for horizontal gene  transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts.
• Prokaryotes can exchange genetic material horizontally, resulting in faster  evolutionary change than in organisms that only use vertical transfer. 
• In diploid cells, chromosomes typically appear as homologous pairs. 
• Meiosis is split into two stages: Meiosis I  and Meiosis II  

    
• Meiosis I: The chromosomes , which have replicated prior to meiosis I, each consist of two genetically identical chromatids attached at the centromere . 
• Meiosis I:

  2. Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged . 
• Meiosis I:

  4. Spindle  fibres attach to the homologous pairs and line them up at the equator of the spindle . The orientation of the pairs of homologous chromosomes at the equator is random .
• Meiosis I:

  5. The chromosomes of each homologous pair are separated and move towards opposite  poles .  ​

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  Cytokinesis occurs and two  daughter cells form.
• Meiosis II:

  Each of the two  cells produced in meiosis I undergoes a further division during which the sister chromatids of each chromosome are separated . A total of four  haploid cells are produced. 
• In homogametic  females (XX) one of the two X chromosomes present in each cell is randomly  inactivated at an early stage of development.
• X  chromosome  inactivation is a process by which most of one X chromosome is inactivated  X chromosome inactivation prevents a double  dose of gene products, which could be harmful to cells.
• X carriers are less likely to be affected by any deleterious  mutations on these X chromosomes. ​

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  As the X chromosome inactivated in each cell is random , half of the cells in any tissue will have a working  copy of the gene in question.
• Hermaphrodites are species that have functioning male and female reproductive organs in each individual. They produce both male and female  gametes and usually have a partner with which to exchange gametes.
• Hermaphroditism:

  The benefit to the individual organism is that if the chance of encountering a partner is an uncommon  event, there is no  requirement for that partner to be of the opposite sex.
•  Sex can also change within individuals of some species as a result of size , competition , or parasitic  infection .

  In some species the sex ratio of offspring can be adjusted in response to resource  availability.