Asexual & Sexual Reproduction

• rapid production of large numbers of individuals having an identical genetic composition
• clones
• binery fission, budding, bulbs runners tubers

sexual

• two parents 
• less rapid
• offspring are genetically varied
• diploid body cells produce haploid gametes, fusion of two haploids makes a zygote

asexual

• lack of variety to environmental change
• however if an individual has genetic makeup suited ta particular set of conditons large numbers of this successful type may be built up

sexual

• slower
• increase in genetic variety for adaptation
• development of resistant stage in life cycle for wthstanding adverse conditions
• dispersal of offspring reduces competition enabling genetic variety

• diploid body cells / haploid sex cells/gametes
• body cells with full chromosome numbers produced by mitosis
• haploid cells have half produced by meiosis
• at fertilisation male gamete sperm fuses with female egg gamete producing a dipoid zygote which further divides by mitosis for growth
• male gamete is very small and motile 
• female gamete is large and sedentary due to presence of stored food
• mammalian eggs contain very little stored food as materials used for growth are obtained from the mother via the placenta

• aquatic organisms discharge their gametes directly into the surrounding water, as gametes are quickly dispersed theres a likely chance many eggs will not be fertilised this is external fertilisation
  • large wastage so large numbers of gametes produced to increase chances 
• in frogs the joining of sperm and egg is facillitated by sexual coupling
  • eggs laid by female and male immediately releases sperm directly over them
  • fertilisation is still external for amphibians
• terrestial animals mainly use internal fertilisation inside the females body requiring the use of an intromittent organ to introduce the sperm
  • less waste as increased chance of fertilisation
  • male gamete is independant of water for movement
  • fertilised agg can be enclosed in protective covering before leaving female body (eggs) or can develop within the mother and recieve nourishment via a placenta like in mammals

• zygote usually undergoes development outside the body 
• easy prey= many produced to increase chances of survival
• internal fertilisation increases chance of fertilisation
• evolution of eggs in reptiles and birds with a fluid filled cavity with a protective shell membrane enclosing the embryo within a yolk sac
• birds incubate eggs 
• in mammals the young are incubated inside the womb and nourished from the mothers blood via placenta and born highly developed

• laying fertilised eggs and leaving them unattended is little/ no parental care
• male stickleback looks after the egss and defends his territory and fans them to give them oxygen until hatching
• parental care is mst common with birds and mammals 
• providing shelter, feeding, protection and training prepares the offspring for adult life
• more parental care leads to less offspring being produced
• fish spawn thousands while humans spawn one

• the zygote develops into a nymph or larva juvinile
• these insects have a hard exoskeleton that must be shed in order to grow, which is done several times 
• this is incomplete metamorphis
• the nymph simply grows to full size e.g. a locust
• advanced insects go through complete metamorphis 
• here the insect larva grows and eats 
• then enters a pupa stage where it undergoes considerable change 
• it emerges as a specialised adult for dispersal and reproduction e.g a butterfly

• pollen grains have a hard coat and contain the male gamete to be transferred to the female part by wind or insects
• male gamete travels down the pollen tube to the egg, gametes no longer need to travel through water
• the ovule develops into a seed with a food store
• key aspet of their success is their link with animals
• plant flowers attract insects and exploit their mobility for pollenation and seed dispersal
• enclosure of the eggs in an ovary and evolution of a seed containing food reserves and a resistant coat allowing withstance against adverse conditions
• why so successful?
  • time between flower production and seed setting is a couple of weeks
  • seed with a food store allows embryo to develop until leaves can reach above ground to photosynthesise and also protects the seed from harm
  • leaves decaying on the ground rapidly allows rapid recycling of ions