Natural Selection

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Chapter 48

• Natural Selection

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What is Natural Selection?

• Darwin stated that all species will eventually
  exceed their resources
  
• Therefore only a fraction of individuals in a
  species of animals might survive
  
• Darwin called these the favored
• This process of survival of the favored, Darwin
  termed Natural Selection

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What is Natural Selection?(Continued)

• In terms of Population Genetics, nowadays Natural
  Selection is defined as differential rate of
  reproduction of different genotypes in a
  population

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Natural Selection and the Maintenance of
Variability

• Darwins ideas led to the formulation of the
  synthetic theory
  
• Synthetic theory natural selection would serve
  only to eliminate the less fit.
  
• Consequently, it would reduce genetic variation
  in a population
  
• This would act as an anti-evolutionary force

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Natural Selection and the Maintenance of
Variability(Continued)

• However, an anti-evolutionary force does not
  occur as natural selection has been found to help
  promote the variability of a population

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on/finches.png
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Balanced Polymorphism Color and Banding in Snail
Shells

• Polymorphism Coexistence within a population of
  2 or more phenotypically distinct forms
  
• One of the best examples is the land snail
  Cepaea. They are banded (with as many as 5
  colors) or un-banded.

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How the bands affect them

• These snails are eaten by birds, one type is
  song thrushes
  
• Studies have shown that snails with better
  camouflage for their habitat allows them to live
  longer and avoid being preyed on by song thrushes

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url
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What is selected?

• Phenotype is not only the physical appearance
• The phenotype also includes optimum temp. at
  which an enzyme works, or the speed of response
  to stimulus
  
• The phenotype includes all observable attributes
  of an organism

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What is selected? (Continued)

• It is very rare that a single allele can
  determine a winning phenotype
  
• Co adaptive gene complexes- groups of genes that
  collectively produce coordinated phenotypic
  characteristics
  
• When these groups are linked together on one gene
  they are called Super genes

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Types of Selection

• Selection processes can be categorized into 5
  main types.
  
• Effect on distribution of characteristics within
  a population stabilizing, disruptive, or
  directional.
  
• Frequency-dependent selection - influenced by
  relative proportions of different phenotypes
  within a population

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Stabilizing Selection

• Stabilizing Selection- exists in all populations
  elimination of extreme individuals
  
• Mutant forms are immediately weeded out this way
  (often in zygote or embryo)
  

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ecture10/img023.jpg
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Disruptive Selection

• Disruptive Selection- increases the two extremes
  of a population while decreasing the intermediate
  forms
  
• Example plants that have evolved in
  uncontaminated soils vs. plants that have evolved
  to grow in contaminated soils can not compete in
  each others soils
• This phenomena also occurs in salmon

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ecture10/img027.jpg
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Directional selection

• Directional Selection- increase in individuals
  with an extreme phenotypic characteristic
  
• Results in replacement of one group of alleles by
  another in a gene pool
  

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ectstabil.gif
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Frequency-Dependent Selection

• Frequency-dependent selection- acts to decrease
  the frequency of more common phenotypes and to
  increase the frequency of less common ones.
  
• Predator-prey interaction led to this
• Can also be a factor in maintaining polymorphisms

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heetah20hunting20an20impala.jpg
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Sexual Selection

• Sexual selection- struggle between members of
  one sex, generally the males, for the possession
  of the other sex.
  
• Results in many conspicuous adaptations
• Sexual Adaptation generally takes 1 of 2 forms
  intrasexual selection, intersexual selection

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ar0110.jpg
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phant_seals_fighting.jpg
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Sexual Selection (Continued)

• intrasexual selection- competition between
  members of one sex for mating with the opposite
  sex
  
• intersexual selection- members of either sex
  exert strong selective pressures on the
  characteristics of the opposite sex through their
  choices of mates.

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urismus_kultur_20_p5.jpg
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Sexual Selection(Continued)

• Polygyny- a few males sire most of the young,
  females care for young
  
• Monogamy- both sexes contribute substantially to
  mating and care
  
• Polyandry- females have several mating partners,
  males have less or only one. Males care for
  offspring more

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The Result of Natural Selection Adaptation

• Natural selection results in adaptations
• Examples bushy squirrel tale, a dogs devotion
  to humans, and certain adapted bird beaks

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mals/squirrel.jpg
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ro20man20with20dog.jpg
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Adaptations to the Physical Environment Clines
and Ecotypes

• Phenotypic variations can often be correlated
  with changes in surrounding climate
  
• Such a graded variation or a complex of
  variations is known as a cline
  
• Ecotype- a group of distinct phenotypes

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Adaptations to the Biological Environment
Coevolution

• Coevolution- populations of two or more species
  interact so closely that each is a strong
  selective force on the other.
  
• Some plants use bad taste (Milkweeds) as a
  deterrent to keep herbivores from eating them
  
• Animals that taste bad or are poisonous have
  distinct colors to deter predators (Caterpillars)

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7.jpg
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Evolution and the Idea of Progress

• Evolution progress (normally)
• This is reinforced by the fact that over time
  organisms have become larger, more complex, more
  sophisticated (and even much more sophisticated
  humans)
• Natural Selection deals with the here and now.

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Evolution and the Idea of Progress (Continued)

• Capacity to keep up depends on existence of
  sufficient genetic variability in the population.

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Development and Structural Constraints

• Evolution can only evolve preexisting body
  structures
  
• Example- if you had a land animal and you wanted
  to turn it into a flying animal it would be
  easier just to scrap the land animal and start
  over. But evolution must act on organisms that
  are present

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Eyeless Arthropods and Other Degenerates

• Natural Selection may lead to the loss of
  capacities instead of increasing complexity
  
• Example- Cave animals living in perpetual
  darkness lack eyes
  
• Such organisms are highly evolved to live a
  particular way of life even though they may
  appear primitive and simple

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Patterns of Evolution

• There are many patterns for evolution
• There is coevolution (already covered)
• Other patterns may produce remarkably similar
  phenotypes of distantly related organisms (and
  visa versa)
  

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Convergent Evolution

• Organisms that occupy similar environments often
  come to resemble one another even though they are
  very different phylogenetically.
  
• Examples whales and sharks, cacti and euphorbs

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y/tmolge.jpg
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Divergent Evolution

• Divergent Evolution- population becomes isolated
  from the rest of the species and due to
  particular selection pressures follows a
  different evolutionary course
• Example Brown bears were isolated in glaciations
  during the Pleistocene period and evolved into
  Polar Bears

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wn-Bear-5.jpg
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ar.jpg
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Bibliography

• 1.) Barnes, N. Sue, and Helena Curtis . 5th ed.
  New York City Worth, INC., 1989. 991-1009.
  
• 2.) Leff, David . About Darwin . 1 Mar. 2004.
  Royal Society, London, England. 13 Nov. 2004
  .html.
• 3.) Storey, Malcolm . The Virtual Field-Guide
  (UK). 25 Aug. 2004. 13 Nov. 2004
  .