Reproductive Strategies

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

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

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

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

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

• 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

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

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

• 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

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

• 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