Reproductive Strategies

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  • Reproductive Strategies
    • Reproduction
      • Asexual
        • Advantages
          • It requires less energy as a mate is not needed
          • It produces clones, so if one is successful they are all successful
          • 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 individuals susceptible to environmental changes or new diseases
        • 1 Parent
        • Genetically Identical Offspring
        • Involves Mitosis
        • Diploid (2n) Somatic cells produced
      • Sexual
        • 2 Parents
        • Genetically different offspring
        • Involves meiosis
        • Haploid gametes produced
        • Advantages
          • It produces genetic variation in a population
          • It is a good strategy when environmental conditions are changeable
          • Some offspring will survive unfavourable conditions
          • 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 high % of gametes are wasted
    • Structure of Gametes
      • A human ovum
        • Large cells
        • Non motile
        • Cytoplasm includes many yolk droplets which contain lipids and proteins and act as food reserves for the developing embryo
        • Fewer ovum produced than sperm
      • A human sperm
        • Small cells
        • Have a flagellum which they use to swim to the ovum
        • Contain many mitochondria to provide the ATP needed for swimming
        • Motile
        • Large numbers produced
    • Gametes
      • In normal body cells chromosomes exist in homologous pairs (one from the male and one from the female)
        • A normal body cell with the full compliment of paired chromosomes is said to be diploid. The diploid number of chromosomes =2n
      • Gametes undergo a second cell division, meiosis which halves the diploid number of chromosomes to the haploid number
        • 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
    • Fish
      • Live in aquatic environments and exhibit external fertilisation
      • Gametes are released into the water
        • The 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 dependant on the yolk supply for it's development
      • Generally no paternal care is shown by fish so survival of fertilised embryos is low
    • Amphibians
      • Although amphibians have colonised to land, they must return to water to reproduce
      • Amphibians also exhibit external fertilisation
      • They 'couple' during reproduction to increase the chance of fertilisation as the sperm and the egg are released in close proximity
      • 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 entirely dependant on the yolk for their development
    • Reptiles and Birds
      • Both these groups have successfully colonised to land and do not need to return to the water to reproduce
      • They both exhibit internal reproduction
      • Fusion of gametes occurs within the females body
        • This greatly increases the chance of fertilisation so fewer gametes are reproduced
      • The developing embryo is enclosed in an amniotic egg which is laid by the female
        • The egg is permeable to gases and waterproof so prevents desiccation of the embryo
          • The embryo is dependant on the yolk for it's development
            • The % of yolk in the egg gives an indication of the degree of development of the offspring before hatching and the degree of parental care given
              • A duck egg has a large yolk (35% of egg mass) whereas a starling's egg has a smaller yolk (17% of egg mass)
      • Reptiles produce a number of eggs (but fewer than amphibians) and show little parental care
      • Birds lay fewer eggs and show a large amount of parental care which increases the chance of their offspring surviving
    • Mammals
      • Exhibit internal fertilisation
      • Low numbers of gametes are produced as the chances of fertilisation are high
      • In placental animals (humans) the embryo gains nourishment from it's mothers blood via the placenta
      • The embryo develops inside the mothers uterus and is protected from predators or adverse conditions
      • The young of mammals are born relatively well developed
      • Mammals show a high degree of parental care which increases the chance of the offspring reaching sexual maturity. Therefore, there are low numbers of offspring produced
    • As vertebrates evolved from water to land the following changes occured;
      • A change from external 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 offspring
    • Insects
      • Developed many strategies to colonise terrestrial habitats, one of which being the use of internal fertilisation
      • Most species lay eggs covered with a waterproof layer
      • The life cycle of insects differs. They either show;
        • Complete metamorphosis e.g. water beetle
          • The adults produce eggs which develop into larvae
            • Larvae are usually structurally different from the adult and are designed for feeding
              • Next comes the pupal stage, during this complete reorganisation of the tissues takes place
                • Finally the adult emerges ready for reproduction
        • Incomplete Metamorphosis e.g. Dragonfly
          • The adults produce eggs which develop into nymphs
            • The nymphs resemble the adults and progress through a series of moults to become fully grown
              • Thre is no pupa stage

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