Evolution of populations microevolution

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Evolution of populations(microevolution)

• 23 January 2008
• modern studies of evolution focus on popn
  genetics
• there are several mechanisms for evolution
• new phenotypes do occur
• natural selection is the major mechanism for
  adaptive evolution

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• Background
• --How are traits passed from parent to offspring?
• Darwin didnt know (!)
• Gregor Mendel
• proposed discrete heritable units
• paper on pea studies went nowhere until
  discovered years later

www.jic.ac.uk
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• The Modern Synthesis (mid-20th)
• comprehensive theory of evolution
• Integrates multiple lines of evidence
• from diverse scientific disciplines
• ? Continues to expand with new info

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H/W predicts allele phenotype frequencies (Mend
els rules of inheritance only)
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• How to avoid evolution
• (conditions for Hardy-Weinberg equilibrium)
• large popn
• random mating
• no selection
• no migration (? gene flow)
• no mutations
• (not likely)

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• Mechanisms for evolution
• Natural selection (more later)
• Gene flow (btwn pops)
• Genetic drift (small popn)
• --after genetic bottlenecks
• --due to founder effect

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Gene flow reduces isolation of popns
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Genetic drift

• small popn
• random survival before reproduction (ex?)

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ex?
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founder effect ex island species
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More mechanisms for evolution (where do new
phenotypes come from?) 4) Mutations ? new
traits possible (alleles) 5) Sexual
recombination ? new combinations of traits
(genotypes)
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A genetic mutation can result in a new
allele. A mutation is a random change in the
structure or number of DNA molecules in a
cell. HOW? -- exposure to ionizing radiation
or mutagenic chemicals -- random mistakes when
DNA copied during cell division Most mutations
dont work (lethal to that individual).
However, the rare mutation is beneficial ?
results in improved fitness for that individual
(and its offspring).
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Which mechanism best explains long-term adaptive
change?
horned lizard (uts.cc.utexas.edu)
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• and back to natural selection
• 1) long-term patterns of selection
• --directional, disruptive, stabilizing
• 2) Special types of selection
• a) sexual selection
• ? sexual dimorphism
• b) balancing selection

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Sexual dimorphism
Nyala antelope
male
Elephant seal
male
female
female
www.cathouse-fcc.org
www.indie.ca
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Mate choice (sexual selection) often based on
structural or behavioral diffs
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• Apparent paradox
• Showy structures or behaviors would seem
• to be a disadvantage, yet they persist.
• Importance of mate choice (all envts?)
• Links to other advantageous traits?

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? Adaptations are often compromises ex
balancing selection