Chapter 17 Evolution 08. Offspring that are the result of sexual reproduction are not genetically identical

08.1 Describe how the events in meiosis and fertilisation contribute to variation in offspring There may be resemblance, sexually produced offspring are not identical to parents or siblings Only identical organisms are those from one egg/sperm- identical twins Four main factors which contribute/cause variation in offspring form sexually reproductive parents: MUTATIONS: Spontaneous Random- changes to base sequence in DNA Often fatal but can confer an advantageous gene trait-e.g more melanin-darker skin-protection against UV light

MEIOSIS: Occurs in the ovaries/testes 1st stage homologous chromosomes line up close to each other and join at points called chiasmata Enzymes then cut the DNA at certain points and sections of chromosomes will actually “cross over” Result- a particular chromosome has different DNA- new sequence/ different combination of genes INDEPENDENT ASSORTMENT: At the end of the first stage of meiosis the homologous chromosomes pulled apart Random manner Result- mixture of maternal and paternal chromosomes in daughter cells Many millions of different chromosome combinations

RANDOM FERTILISATION: Many millions of sperm are produced each day/hundreds of eggs in a lifetime Eggs are released at random by the female and sperm produced at random Only one sperm/one egg are fertilised normally- also random process Huge variation in offspring

09 Not all offspring will survive to reproduce 09.1 Understand that some genetically controlled characteristics increase the chances of survival and reproduction. Life is usually a struggle for existence Sometimes resources are plentiful-pop increases Most often- resources limited- organisms die The “limiting factor” may not always be food- could be nesting space/penguins: sense of hearing in dingoes Its not just about staying alive/but having the right conditions to reproduce and pass on particular genes to the next generation Primary purpose of all organisms except humans is to reproduce

Charles Darwin Travelled the world in the “beagle” 150 years ago Wrote “Origin of species” Explains life is a struggle/organisms fight for survival/the fittest or most well adapted to their environment survive/reproduce Process of natural selection- basis of his theory of evolution evolution- does not only refer to the origin of life on the planet-means the gradual change that occurs within a species and the development of a new species over time. Read pg 164…..giraffes etc

Summary: Variation always occurs within a species As resources/environment changes-come organisms better suited to survive Therefore those characteristics will be the ones reproduced and passed on (natural selection) Gradual changes occur over time then in a species ???? Leads to evolution The features that allow the changes – ADAPTATIONS structural Functional behavioural