Reproductive Strategies

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Reproduction

Sexual reproduction

  • 2 parents
  • Genetically different offspring
  • Involves 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 unfavourable 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 seed, which can be dispersed far away, reducing competition between parent and offspring
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Reproduction

Disadvantages

  • It is slow
  • It requires a lot of energy and a high % of gametes are wasted

Asexual reproduction

  • One 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
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Reproduction

Disadvantages

  • Lack of genetic variation within the population leaves all individuals susceptible to environmental changes or new diseases entering the population

Gametes

  • These are specialised cells, they 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)
  • A normal body cell with the full complement of paired chromosomes is said to be diploid
  • The diploid number of chromosomes for a species is 2n
  • Gametes undergo a second cell division, meiosis, which halves the 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
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Reproduction

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

Fish

  • 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 as 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 parental care is shown by fish, so survival of fertilised embryos is low
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Reproduction

Amphibians

  • Although amphibians have colonised the land, they must return to water to reproduce
  • Aphibians also 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 entirely dependant on the yolk for their development

Reptiles and Birds

  • Both these groups have successfully colonised the land and do not need to return to water to reproduce
  • They both exhibit internal fertilisation
  • Fusion of gametes occurs within the females body
  • This greatly increases the chance of fertilisation and therefore fewer gametes are produced
  • The developing embryo is enclosed in an amniotic egg which is laid by the female
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Reproduction

  • The egg is permeable to gases and waterproof, so prevents dessication of the embryo
  • The embryo is dependant on the yolk for it's development
  • The % of yolk in a birds egg gives a good indication of the degree of development of the offspring before hatching and the level of parental care given
  • A duck egg has a large egg yolk (35% of egg mass). When the duckling hatches, it is covered in down and can walk and leave it's nest immediately
  • A starlings egg has a smaller yolk (17% of egg mass) and when the chick hatches it is naked and unable to walk, so needs more parental care

Mammals

  • Exhibit internal fertilisation
  • Low number of gametes are produced as the chances of internal fertilisation are high 
  • In placental animals (humans), the embryo gains nourishment from from it's mothers blood via the placenta
  • The embryo develops inside the mothers 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 chance of the offspring reaching sexual maturity. Therefore, low numbers of offspring are produced
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Reproduction

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

  • 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 e.g. Water Beetle

  • The adults produce eggs which develop into larvae
  • Larvae are 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, designed for reproduction
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Reproduction

Incomplete Metamorphosis e.g Dragonfly

  • 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

Reproductive strategies in flowering plants (angiosperms)

  • Flowering plants are the most sucessful group of plants. Their colonisation of terrestrial habitats is closely linked to the evolution of insects
  • Flowering plants can reproduce sexually and asexually, which has contributed to their success
  • To be able to colonise terrestrial habitats, plants have had to evolve strategies for the male gamete to reach and fuse with, the female gamete without the need for water
  • The reproductive organ of an angiosperm is the flower
  • The 'male part' of the flower is the stamen, where the male gametes are formed in the anther
  • The 'female part' of the flower is the carpel, where the female gamete is formed within the ovule
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Pollination

  • Pollination is the transfer of pollen from one plant to another
  • The pollen grains have a hard outer coat to prevent dessicaiton
  • Pollination can occur by wind or animals

Wind Pollination

  • Wind pollination is inefficient, so wind pollinated plants must produce large quantities of pollen to ensure pollination

Insect Pollination

  • Insect pollinated flowers have large colourful petals and attractive scents. They also produce large quantities of pollen or nectar to attract insects
  • As they feed, pollen gets stuck to the insects bodies. When the insect moves to another plant of the same species, the pollen will be deposited on the stigma
  • The pollen grains then travel down the pollen tubes to reach the egg cell
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Seed formation and dispersal

After fertilisation;

  • The fertilised egg cell develops into a plant embryo
  • The ovule develops into a seed
  • The seed is surrounded by a testa (a tough resistant coat) which protects the seed against adverse environmental conditions
  • The seed also contains a food store which enables the embryo to develop and grow large enough to emerge above ground and produce leaves so that it can begin to photosynthesise
  • In many plant species, the seed is enclosed in fruit which results in the seeds being dispresed by animals
  • Animals eat the fruit, but cannot digest the seeds due to the protective testa, so they pass out in the faeces
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Why are angiosperms so successful

  • A key feature to their success is their relationship with animals for pollination and seed dispersal
  • Plants with large, brightly coloured flowers/nectar/scent are insect pollinated , while plants with small inconspicuous flowers are wind pollinated
  • It is only a matter of weeks between the time that the plant produces a flower and the time that the seed is set in the ground
  • The evolution of a food store in the seed enables the embryo plant to develop until it produces leaves above the ground and begins to photosnythesise to produce energy
  • The seed has a resistant coat (testa) that enables it to withstand adverse conditions
  • When a plant dies, it decomposes in the soil so that it's mineral ions can be recycles and taken up by the roots of other plants
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