Natural Selection

  • Darwin 
  • Useful / heritable traits are passed on; traits of less successful individuals are lost
  • Those with useful traits produce more offspring
  • Therefore there will be more with the useful trait in later generations
  • Diversity allows species to adapt to different environments, e.g. the peppered moth
  • E.g. owl butterfly - not all adaptations based on camoflage, it can sometimes be useful to stand out
  • E.g. changes in domestic animals due to selective brreding

Development of the Human Brain

  • Has increased in size during evolution - mainly in cerebrum
  • Allows for more complex cognitions but bigger brain doesn't = greater intelligence
  • Brain stops growing at around 20 years but always gain new neurons
  • Humans have the largest brain size to body weight ratio in mammals (apart from tree shrew)
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  • Bred sweet pea plants
  • Dominant v recessive alleles
  • Two dominant or two recessive alleles = homozygous 
  • One dominant + one recessive = heterozygous
  • One brown plant + one white plant = four brown plants as brwon is dominant
  • Two brown plants = three brown plants + one white plant as each parents had one white-seed gene
  • Dichotomous traits - two different types of traits
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Transition of Traits

  • DNA code / helix identified in 1962
  • Genes synthesise proteins
  • Banisters = sugars and phosphates
  • Rungs = nucleotide bases: adrenine, thymine, guanine and cytosine which are the blueprint for making protein
  • A and T pair up; G and C pair up
  • Set of genes controlling a trait are found on the same part of the chromosome - called an allele
  • DNA pulls apart slightly in the nucleus
  • mRNA copies the bases in the nucleus and takes code into cytoplasm - cytoplasm is needed as that's where amino acids are
  • Ribosomes read code 3 bases at a time which represents an amino acid
  • Amino acids are linked to create a protein
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Process of Genetic Transmission


  • Sperm and egg cells created in this way - half of each genetic material 
  • Sperm - chromosomes duplicate; cell divides to create two cells, each with 23 duplicated chromosomes; each cell divides again to create four sperm cells, each with 23 chromosomes
  • Egg - chromosomes duplicate; cell divides to create two cells, one of which dies; egg becomes fertilised then divides to create two cels; the cell without the sperm dies; then the sperm and egg combine to form a zygote with 23 pairs of chromosomes


  • Zygote grows in this way
  • The 23 pairs of chromosomes duplicate by dividing over and over again
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Linkage & Human Genome Project


  • Tendency of traits encoded on the same chromosome to be inherited together
  • Use this to find the location of a gene on a chromosome
  • If two genes are inherited together more than 50% of the time, this suggests they are linked on the same chromosome
  • Disrupted by crossing over 

Human Genome Project

  • 1990 - 2003
  • Humans have about 20,000 genes
  • Led to discovery of 1,800+ disease genes and 2,000 genetic tests so people can learn of their genetic risks for disease
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Influence of Gender & Single Genes


  • Females have 23 matching pairs of chromosomes; men have 22 matching and one pair that is different
  • Colour blindness and haemophilia is more common in males than females as they are carried on the X chromosome, not the Y
  • Men don't have another X chromosome to counteract the recessive gene so express the trait

Single Genes

  • Down syndrome = extra chromosome
  • PKU is transmitted by a recessive gene mutation
  • Cannot break down phenyalanine (amino acid found in certain foods) so builds up in bloodstream
  • Results in brain damage and mental retardation
  • Screening injection for babies to test for this
  • Women with PKU should go on phenyalanine diet before conceiving
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Genetic Control of Behaviour

  • Maze bright / maze dull experiment to see if intelligence can be influenced by genetics - selectice breeding
  • There is some kind of genetic control over intelligence
  • Difference between maze bright and maze dull rats was only evident if both were in an impoverished environment
  • Much smaller difference for maze dull rats if they were in an enriched environment - suggests environment is a more important factor
  • Epigenetics - interaction between genetics and environment
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Natural Selection

  • May be beneficial for older males to be attracted to younger females
  • Younger females = healthier = more likely to have healthy offspring / more offspring
  • Male mammals are rarely responsible for raising offspring 
  • May be due to different energy costs - females put in considerably more than males
  • However this isn't always the case, e.g. male seahorses carry their offspring
  • Adopting may not appear beneficial
  • Ducks and seagulls often adopt - cost of taking on an extra chick is relatively small
  • Adoptintg another one outweighs the cost of mistakenly rejecting your own and it's beneficial to have more healthy chicks for the group
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Kin Selection & Direct & Indirect Fitness

Hamilton - R x b - c >0  R = relatedness to recipient  B = benefit to gene pool of act  C = cost of act

  • Rule of altruistic behaviour
  • Only works if you can recognise your kin
  • E.g. bees have a haplodiploid genetic makeup - 75% related to their siblings and 50% related to their offspring, so don't raise their own offspring
  • Females pass on more genes by helping to raise sisters rather than their daughters
  • Eusocial vertebrates, e.g. naking mole rats are inbred due to the harsh conditions they live in
  • Altruism is evident in non-relations, e.g. food sharing in bats
  • A set pair share food - beneficial due to harsh environment
  • Remebered if a bat didn't share food - cheater

Direct & Indirect Fitness

  • Direct - ability to survive and contribute large numbers of fertile offspring to next generation
  • Indirect - ability to help kin to survive so they can go on to contribute large numbers of offspring to the next generation
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