meiosis

• continuous variation = variation in which organisms do not fall into distinct catergories but show gradations from one extreme to the other
• crossing over= the process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that their alleles are exchanged
• diploid= a term applied to cells in which the nucleus contains two sets of chromosomes
• discontinous variation = variation which organism falls into distinct catergoeries
• fertilisation= the fusion of two different gametes bringing together the dna from each
• gamete= reproductive sec cell that fuses with another gamete during fertilisation
• genetic variation = differences in dna base sequence between two or more organisms witht he possibility of resulting in different alleles
• haploid- term referring t cells that contain only a single copy of each chromosome e.g the sex cells (gamete)
• homologous chromosome

  a pair of chromosomes, one maternal and one paternal that have the same gene loci and therefore determine the same features. they are not necessarily identical, however as individual alleles of the same gene may vary, eg one chromosome may carry the allele for blue eyes, the other the allele for brown eyes. homologous chromosomes are capable of pairing during meiosis
• independent assortment= the process by which maternal and paternal copies of homologous chromosomes are randomly distributed to gametes during meiosis
• interspecific variation is the variation between individuals of different species, and intraspecific variation is the variation within a species
• meiosis - a type of nuclear division that reduces the number of chromosomes from diploid to haploid so that gametes can be produced.
• random fertilisation is when sperm and egg cells are randomly mixed and fertilisation occurs
• random sample = a sample that is selected from the population using a random method.
• two nuclear divisions results usually in the formation of four haploid daughter cells from a dpiloid parent cell
• genetically different daughter cells result from the independent segregation of homologous chromosomes
• crossing over between homologous chromosomes results in further genetic variation among daughter cells
• growth (mitosis) where a diploid zygote grows into a multi cellular adult, all daughter cells have the same number of chromosomes as the parent cell
• sexual reproduction (meiosis) the number of chromosomes is halved so that the gametes are haploid
• meiosis involves the reduction division of a diploid cell into four genetically distinct haploid nuclei
• two nuclear divisions results usually in the formation of four haploid daughter cells from a single diploid parent cell
• genetically different daughter cells result from the independent segregation of homologous chromosomes
• two equally probable arrangement at metaphase 1 give rise to different chromosome combinations
• Genetic diversity


  The number of different alleles of genes in a population
• Genetic diversity is a factor enabling natural selection to occur
• Principles of natural selection in the evolution of populations

  1. Random mutation can result in new alleles of a gene

  2. Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success

  3. The advantageous allele is inherited by members of the next generation

  4. As a result, over many generations, the new allele increases in frequency in the population
• Directional selection


  • Antibiotic resistance in bacteria
• Stabilising selection


  • Human birth weights
• Natural selection results in species that are better adapted to their environment
• Adaptations


  • Anatomical
  • Physiological
  • Behavioural
• Allele


  One of a number of alternative forms of a gene
• Antibiotic resistance


  The development in microorganisms of mechanisms that prevent antibiotics from killing them
• Directional selection


  When selection pressures on a population cause more individuals at one extreme of the phenotype to survive and reproduce
• Founder effect


  The loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population
• Gene mutation


  A change to one or more nucleotide bases in DNA resulting in a change in genotype which may be inherited
• Genetic bottleneck


  A sharp reduction in the size of a population due to environmental events or human activities often resulting in a reduction in genetic diversity
• Genetic diversity


  The number of different alleles present in a population
• Mutation


  A sudden change in the amount or the arrangement of the genetic material in the cell
• Plasmid


  A small circular piece of DNA found in bacterial cells
• Population


  A group of individuals of the same species that occupy the same habitat at the same time