The Patterns of Selection

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The Patterns of Selection

• By Roger Ng

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Types of Selection

• Stabilizing Selection
• Directional Selection
• Disruptive Selection
• Sexual Selection

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Stabilizing Selection

• The population of a certain species focuses
  mainly on a particular common variation of a
  trait
• Decreases genetic diversity as more members of
  the population become traits that carry
  variations of form and functions are prevented
  from being passed on

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• Ex. The birth of babies
• Most babies are born around 3kg as opposed to any
  other weights because
• babies who weighs below 3kg causes the baby to
  lose body heat and be more vulnerable to diseases
  
• babies who weighs over 3kg causes difficulties
  for delievery and endangers the life of the
  mother as well
• This shows that the variations of the trait which
  causes the baby to be either underweight/overweigh
  t are not selected, meaning that the variation of
  the trait which causes the baby to be around 3 kg
  is selected instead

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• Ex. Animal Shapes
• Hummingbirds who live in environments with
  flowers of middle length will have beaks of
  middle length while beaks of short/long lengths
  will be selected against because they are
  ineffective and unnecessary

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Directional Selection

• When the environment changes, the population must
  adapt to it by choosing new variations of the
  trait when necessary
• A different variation of the common trait is
  chosen and that becomes the new "common" variation

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• Ex. Animal Shapes
• If those same hummingbirds from the previous
  example, were to mirgrate into an environment
  with long flowers, the hummingbirds with medium
  beaks would be ineffective in collecting as much
  pollen. This causes future generations to
  develop longer beaks and the variation of the
  trait that is responsible for the longer beaks is
  selected more while the previous common trait is
  less selected.

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• This also applies if the hummingbirds were to
  migrated into an environment with short flowers
  except with the variation of the trait
  responsible for short beaks being selected instead

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Disruptive Selection

• The opposite of stabilizing selection where the
  opposite extreme variations of a trait is chosen
  instead
• This promotes genetic diversity within a
  population

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• Ex. Animal Shapes
• If the hummingbirds were to migrate to a third
  environment for whatever reason, and that
  environment had both long flowers and short
  flowers, then the hummingbirds would adapt again
  so that it's population would contain both long
  beaks and short beaks.

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Sexual Selection

• This type of selection deals with choosing which
  traits would help further reproduction
• These traits are known as sexual dimorphism which
  is responsible for the differences in appearances
  for males and females
• And traits which are responsible for behaviour
• Unlike the selections above, this is not
  influenced by the environment

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• Ex. Plants
• Plants, since they cannot move and find a mate of
  their own through insects who help exchange
  pollen, as such they pick traits that make them
  more attractive and captivating to the insects.

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• Although this is beneficial in terms of
  evolution, this may not be beneficial
• This makes them more vulnerable as prey

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• Ex. Animal Plants
• Plants make themselves alluring are often picked
  by humans for decoration resulting in its death
  and prevention from further reproduction
• Animals go through the same things because as
  they make themselves seem more acceptable as a
  possible mate, some also make themselves easy
  targets, such as peacocks, the more beautiful the
  feathers are, the more they are hunted

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Runaway Selection

• This is a part of sexual selection in which case
  the variation of the trait that strongly promotes
  sexual reproduction is selected and exaggerates

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• Ex. Animal Shapes
• Male peacocks choose the variations of the trait
  that is responsible for long and colourful tail
  feathers, which results in a better chance for
  sexual reproduction
• However if this were to continue, the offspring
  would have long tail feathers, which would be
  then considered the new average length tail
  feather
• Eventually the length of the tail feather would
  continue to be bigger until it becomes a
  vulnerability for predators

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STSE

• Natural selection is seen in our daily lives
• In the weight of babies, as shown in Stabilizing
  Selection5
• In the appearance of people
• This theory has been changed through the past 100
  years, from belief in God, to Darwins Theory of
  Natural Selection, to Mendels attribution to
  Darwins Theory
• Darwins Theory of Natural Selection had not
  taken in the new variations that were developed
  with each new generation which would eventually
  result in future generations being identical to
  one another
• Mendels attribution helped show the new
  variations of the new generations which were
  then added to Darwins Theory

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• The End