Reproductive Strategies

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  • Reproductive Strategies
    • Asexual Reproduction
      • 1 parent
      • Genetically identical offspring
      • Involves mitosis
      • Diploid somatic cells produced
      • Advantages
        • It produces clones - if one is successful, they are all successful
        • It requires less energy as a mate is not needed
        • It is fast and can produce rapid population growth
        • It allows species to colonise a stable environment and is a good strategy when environmental conditions are favourable
      • Disadvantages
        • Lack of genetic variation within the population leaves all the individuals susceptible to environmental changes or new diseases entering the population
    • Sexual Reproduction
      • 2 parents
      • Genetically different offspring
      • Involved meiosis
      • Haploid gamete cells produced
      • Advantages
        • It produces genetic variation in a population
        • It is a good strategy when environmental conditions are changeable
        • Some offspring will survive unfavorable conditions, which allows the species to adapt by natural selection
        • It can involve the production of a resistant phase in a life cycle, such as seeds which can be dispersed far away, reducing competition between parent and offspring
      • Disadvantages
        • It is slow
        • It requires a lot of energy and a large % of gametes are wasted
    • Gametes
      • These are specialised cells and develop differently from normal body cells that divide by mitosis
        • In normal body cells, chromosomes exist in homologous pairs (one from the male and one from the female)
      • During fertilisation, the haploid sperm fuses with the haploid egg to produce a diploid fertilised egg
        • The zygote formed then divides many times by mitosis to grow into a new individual
    • Reproductive strategies in vertebrae
      • All vertebrae reproduce sexually, however different groups of vertebrates have different strategies in terms of;
        • How fertilisation occurs
        • The development of the embryo
        • The degree of parental care given to the offspring
        • As vertebrates evolved from water to land the following changes occured;
          • A change from external fertilisation to internal fertilisation
          • A reduction in the number of gametes produced
          • A reduction in the number of offspring produced
          • An increase in the degree of parental care given to the offspring
      • Fish
        • Fish live in aquatic environments and exhibit external fertilisation
        • Gametes are released into the water. Water provides a medium in which the sperm can swim
          • Large numbers of gametes are produced because the chance of fertilisation is low - many gametes are eaten or carried away by the current
        • If fertilisation does occur, the embryo is entirely dependent on the yolk supply for it's development
      • Amphibians
        • Although amphibians have colonised to land, they must return to water in order to reproduce
        • Amphibians exhibit external fertilisation
          • They 'couple' during reproduction to increase the chance of fertilisation as the sperm and ova are released in close proximity
        • Amphibians do not produce as many gametes, because fewer are wasted
        • Most amphibians show no parental care and therefore produce large numbers of embryos which are entirley dependent on the yolk for their development
      • Reptiles and Birds
        • Both of these groups have successfully colonised to land and do not need to return to water to reproduce
        • They both exhibit internal reproduction
        • Fusion of gametes occurs within the female's body
          • This greatly increases the chance of fertilisation and therefore fewer gametes are produced
        • Reptiles produce large numbers of eggs and show little parental care
        • Birds lay fewer eggs and show a large amount of parental care which increases the chances of offspring reaching their reproductive age
      • Mammals
        • Exhibit internal fertilisation
        • Low numbers of gametes are produced as the chances of internal fertilisation are high
        • In placental animals (humans), the embryo gains nourishment from it's mother's blood via the placenta
        • The embryo develops inside the mother's uterus and is protected from predators or adverse weather
        • The young of mammals are born relatively well developed
        • Mammals show a high degree of parental care which increases the of offspring reaching sexual maturity. Therefore, there are low numbers of offspring produced
      • Insects
        • Insects have developed many strategies to colonise terrestrial habitats and one of these is the use of internal fertilisation
          • Most species lay eggs covered with a waterproof layer
        • The life cycle of insects differ. species either show complete or incomplete metamorphosis
          • Complete metamorphosis
            • The adults produce eggs which develop into larvae
              • Larvae are structurally different from the adult and are designed for feeding
                • Next comes the pupal phase. During this, complete reorganisation of the tissues takes place
                  • Finally, the adult emerge, it designed for reproduction
          • Incomplete metamorphosis
            • The adults produce eggs which develop into nymphs
              • The nymphs resemble the adult and progress through a series of moults to become fully grown
                • There is no pupa stage

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