Microevolution

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Microevolution

• How populations evolve

Campbell 13.6 13.22 Lecture 10 Friday 23
January 2004
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Variation is extensive in most populations

• Phenotypic variation may be environmental and/or
  genetic in origin
• But only genetic changes result in evolutionary
  adaptation

The phenotype describes the physical attributes
of an individual (for example, shell banding
pattern in the example given here)
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Evolutionary fitness

• An individuals fitness is measured by the
  contribution it makes to the gene pool of the
  next generation, relative to the contribution
  made by other individuals
• Production of fertile offspring is the only score
  that counts in natural selection

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1. Key Concept

• Individuals of a population have the same genes

Two copies in diploids
Gene
Paired Chromosomes
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Key Concept

• In a population, a gene may exist in different
  forms known as alleles

Homozygote
Allele A
Allele B
Paired Chromosomes
Heterozygote
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Key Concept

• Microevolution follows changes in a populations
  allele frequencies

Allele A
Allele B
Paired Chromosomes
Population 1 Frequency of A 0.2
Population 2 Frequency of A 0.8
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Individuals Dont Evolve, Populations Do

• Population a local and geographically defined
  group of organisms sharing a common gene pool
• Gene Pool - pool of genetic resources that is
  shared by all members of a population
• Polymorphism - traits or genes that come in two
  or more distinct forms

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Neutral genetic variation

• Some variations may be neutral, providing no
  apparent advantage or disadvantage
• Example human fingerprints

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2. Genetic Equilibrium

• Genetic equilibrium is a theoretical state in
  which a population is not evolving
• Genetic equilibrium occurs only if
• There is no mutation
• The population is very large
• There is no emigration or immigration
• Mating is random
• There is no selection
• Known as the Hardy-Weinberg rule
• See section 13.8 for a full derivation!

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3. How populations evolve

• Populations evolve when allele frequencies change
• Allele frequencies can change through disturbing
  the factors that support genetic equilibrium
• Allele frequencies can change though
• 1. mutations
• 2. gene flow
• 3. natural selection
• 4. genetic drift
• We will deal with each of these in turn

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3.1 Mutations generate variation

• But Mutations are very rare
• (In a DNA sequence in vertebrates, a mutation
  occurs 1 in 100 000 000 times per gene per
  generation)
• and.
• Very few are
• beneficial.

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3.1 Recombination generates variation in sexual
species
A1
A1
A2
A3
Parents
MEIOSIS
A2
A1
A3
Gametes
FERTILIZATION
and
A1
A3
A1
A2
Offspring, with newcombinations of alleles
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3.2 Gene Flow

• Gene Flow
• Flow of alleles through the emigration and
  immigration of individuals

Immigration
Population 1
Population 2
Population 2
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3.3 Natural selection
Original population
Frequency ofindividuals
Phenotypes (fur color)
Evolvedpopulation
Original population
Stabilizing selection
Directional selection
Diversifying selection
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3.3 Directional Selection

• Shift in allele frequency in a consistent
  direction
• For example Phenotypic Variation in a population
  of butterflies

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Example Peppered Moths

• Industrial revolution
• Pollution darkened tree trunks
• Camouflage of moths increases survival from
  predators
• Directional selection caused a shift away from
  light-gray towards dark-gray moths

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Example Antibiotic Resistance

• Excessive use of antibiotics is leading to the
  evolution of antibiotic-resistant bacteria
• Example Mycobacterium tuberculosis

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3.3 Stabilizing selection

• Intermediate forms of a trait are favored
• Alleles that specify extreme forms are eliminated
  from a population

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Example Gall size in Eurosta solidaginis
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3.3 Disruptive Selection

• Both forms at extreme ends of a distribution are
  favored
• Intermediate forms are eliminated

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Example Bill Size in African Finches
60
50
40
Number of individuals
30
20
10
10
1.12
15.7
18.5
Widest part of lower bill (millimeters)
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3.3 Types of Selection Sexual selection

• Leads to the evolution of secondary sexual
  characteristics
• These may give individuals an advantage in mating
• Sexual dimorphism

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3.4 Genetic Drift

• Genetic Drift
• Random change in allele frequencies over
  generations brought about by chance alone
• In the absence of other forces, drift leads to
  loss of genetic diversity

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3.4 Genetic Drift

• Magnitude of drift is greatest in small
  populations

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Genetic drift Founder effects
phenotypes of original population
A founder event in this case, a seed carried to
a new place by a bird.
phenotype of island population
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Genetic Drift Bottlenecks
Originalpopulation
Bottleneckingevent
Survivingpopulation
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4. Relevance Endangered species

• Low genetic variability may reduce the capacity
  of endangered species to adapt to a changing
  environment
• Studies have shown that cheetah populations
  exhibit extreme genetic uniformity
• Thus they may have a reduced capacity to adapt
  to environmental challenges

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4. Genetic Drift and Inbred Populations

• Inbreeding
• Genes become homozygous
• Elimination of variations in alleles
• Increase in susceptibility to environmental
  changes and disasters

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Study Guide lecture structure

• Intro
• individuals within most populations are very
  variable
• Key Concepts
• individuals dont evolve, populations do.
• Genetic equilibrium
• theoretical conditions for equilibrium
• What happens if we disturb equilibrium?

• Factors that cause evolution
• mutation
• migration
• selection (directional, stabilizing, disruptive)
• genetic drift
• Relevance
• endangered species and genetic drift
• endangered species and inbreeding

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Study guide

• Individuals of a population have the same genes,
  but forms of these genes (alleles) may vary
  between individuals. What is meant by
  heterozygosity? Homozygosity?
• A population is evolving when the frequency of an
  allele or a genetic based trait in a population
  changes. What unit evolves a population or an
  individual?
• Deviations from the Hardy Weinberg principles are
  used to detect microevolutionary change. What are
  the Hardy Weinberg principles?

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Study Guide

• Gene mutations are the only source of new
  alleles. Are mutations frequent or rare? Can we
  rely on mutations to regenerate diversity in
  small endangered populations?
• Gene flow is the exchange of genes between
  different populations and may result in changes
  in gene frequencies in the recipient populations.
• Natural selection is an outcome of differences in
  survival and reproduction among individuals. How
  does natural selection act? Think of Darwins
  theory.

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Study guide

• Selection pressures may change the range of
  variation for a trait in three main ways
• Stabilizing
• Directional
• Disruptive
• What is the difference between the three types of
  selection? Give examples of each
• Genetic drift is a change in allele frequencies
  over the generations due to chance events
• Genetic drift is pronounced in small populations.
  Why? How is this fact relevant to endangered
  species?