18: Variation and Selection

• variation definition?

  the differences between individuals of the same species
• phenotypic variation?

  the difference in features between individuals of the same species. Some of these differences result from gene differences, also known as genetic variation.

  
  

  can be caused in two main ways: genetic (controlled entirely by genes)

  or environmental (caused entirely by the environment in which the organism lives):

  
  

  Phenotypic variation can be divided into two types, depending on how you can group the measurements:

  Continuous and Discontinuous variation
• continuous variation

  is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale

  ex: height, mass, finger length, etc. (where there can be many 'in between' groups)

  
  

  continuous variation gives smooth bell curves on a graph(a result of the small degrees of difference)
• discontinuous variation?

  is when there are distinct differences in a characteristic

  ex: people are either blood groups A, B, AB, or O; either male or female; can either roll their tongue or not; there are no 'in-betweens'.

  discontinuous variation gives a 'step-like' shape
• what are examples of genetic variation?

  blood group

  eye color

  gender

  ability to roll tongue

  whether ear lobes are attached (lobeless) or free (lobed)
• what is environmental variation?

  environmental variation is based on factors such as climate, diet, culture, lifestyle and accidents during lifetime. It simply means 'outside of the organism.'
• examples of environmental variation?

  an accident may lead to scarring on the body

  eating too much and not leading an active lifestyle will cause weight gain

  being raised in a certain country will cause you to speak a certain language with a certain accent

  a plant in the shade of a big tree will grow taller to reach more light
• Genetic causes

  Variation caused by genes inherited from parents
• Environmental causes

  Variation caused by factors other than genes, such as diet, climate, and upbringing
• Discontinuous variation is usually caused by genetic variation alone
• Tall parents

  Pass genes to their children for height
• Children inherit genes for height

  Have the genetic potential to also be tall
• Poor diet

  Impacts children's height and prevents them from growing well
• Although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop
• What are mutations?

  random genetic changes to the base sequence of DNA

  • most do not affect the phenotype, as the protein that the mutated gene produces may work just as well as the protein from the non-mutated gene.
  • Rarely, mutations lead to the development of new alleles and so new phenotypes and if they do, most have a small effect on the organism. Occasionally, the new allele gives the individual a survival advantage over other members of the species
• example of survival advantages?

  a bird develops a mutation leading to a change in feather color, making it more attractive to birds of the opposite sex which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation
• Remember...

  • mutations can also lead to harmful changes that can have dramatic effects on the organism (ex: sickle cell anaemia in humans)
  • they happen spontaneously and continuously but their frequency can be increased by exposure to...
  • gamma rays, x-rays and ultraviolet rays - all types of ionising radiation which can damage bonds and cause changes in base sequences
  • certain types of chemicals (ex: tar in tobacco)

  Increased rates of mutation can cause cells to become cancerous which is why the above are linked to increased incidence of different types of cancer
• sources of genetic variation in populations?

  mutations: new alleles form through random changes to DNA

  meiosis: new allele combinations form through segregation

  random mating: which partnerships form for sexual reproduction

  random fertilisation: which sperm and egg combinations occur during sexual reproduction
• what are adaptive features?

  the inherited functional features of an organism that increase its fitness, which is the probability of an organism surviving and reproducing in the environment in which it is found
• what are hydrophytes?

  plants adapted to live in extremely wet conditions

  • large air spaces in their leaves for flotation, to keep the leaves close to the surface of the water where there is more light for photosynthesis
  • small roots as they can also extract nutrients from the surrounding water through their tissues
  • stomata usually open all the time and mainly found on the upper epidermis of the lead where they can exchange gases much more easily with the air
• what are xerophytes?

  Plants adapted to live in extremely dry environments.

  Common adaptions include:

  • thick waxy cuticle
  • sunken stomata: stomata may be sunk in pints in the epidermis; moist air trapped here lengthens the diffusion pathway and reduces the evaporation rate
  • leaf rolled with stomata inside and an inner surface covered in hairs - traps moist air and prevents air movement across stomata which reduces transpiration
  • small leaves: reduces surface area and therefore the evaporating surface
  • extensive shallow roots
  • thickened leaves or stems which contain cells that store water
• how does the cuticle cut down water loss?

  the waxy cuticle does it in two ways:

  1. acts as barrier to evaporation
  2. the shiny surface reflects heat and so lowers the temperature
• what is the function of extensive shallow roots in xerophytes?

  quick absorption of large quantities of water when it rains
• Natural selection


  The result of individuals with the best adaptive features being most likely to survive and reproduce
• Natural selection


  • Individuals in a species show a range of variation caused by differences in genes
  • As individuals reproduce they produce more offspring than the environment is able to support
  • This leads to competition for food and other resources which results in a 'struggle for survival'
  • Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
  • The alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
  • In the next generation, there will be a greater number of individuals with the better-adapted variations in characteristics
• Examples of Natural Selection

  Snail shell color (black or grey)

  evolution of peppered moths

  
  

  While there are thousands of examples of natural selection, they ALL follow the same sequence:

  • based on the idea that within a species there is always variation and chance mutations, some individuals will develop a phenotype (characteristic) that gives them a survival advantage and therefore will live longer, breed more and be more likely to pass their genes on
  • repeated over generations, the 'mutated' phenotype will become the norm
• adaption by natural selection:

  if the environment does not change, selection does not change.

  • this will favour individuals with the same characteristics as their parents
  • if the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
  • so the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring and overtime will bring a change in the characteristics of the species - it will produce evolution
• what is evolution and how does it happen?

  evolution is defined as the change in adaptive features of a population over time as a result of natural selection since natural selection results in a process of adaptation.

  • over generations, those features that are better adapted to the environment become more common (how evolution occurs) and populations of organisms become better suited to their environment
• what is a good example of evolution?

  the development of antibiotic resistance by bacteria

  1. A population of bacteria in the gut, some have antibiotic resistance
  2. when exposed to an antibiotic, bacteria causing illness, as well as healthy gut bacteria are killed
  3. with reduced competition for nutrients, antibiotic-resistant bacteria multiply forming a larger population that is difficult to control

  plasmids with antibiotic-resistant genes can be shared between bacteria of both the same and different species
• what is selective breeding?

  selective breeding means to select individuals with desirable characteristics and breed them together. However the process isn't complete because it's likely that not all of the offspring will show the characteristics you want so offsprings that do show the desired characteristics are selected and bred together.

  • this process has to be repeated for many successive generations before you can definitely say you have a 'new breed' which will reliably show those selected characteristics in all offspring
• example of selective breeding

  dog breeders who select which dogs can mate together to increase the likelihood of puppies displaying desirable characteristics e.g. coat color.

  This has given rise to the many recognisable dog breeds of today, all the same species (Canis familiaris) and all descended from one breed
• differences between natural and artificial selection

  natural selection

  • occurs naturally
  • results in the development of populations with features that are better adapted to their environment and survival
  • usually takes a long time to occur

  artificial selection

  • only occurs when humans intervene
  • results in development of population with features that are useful to humans and not necessarily to survival of the individual
  • takes less time as only individuals with the desired features are allowed to reproduce