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Evolution in the Small Scale
Discussed components of evolution
Natural selection
Common descent
Time
Populations
Variation
Put all together
Evolution at Its Smallest Scale
 A species is a group of organisms able to successfully interbreed in nature
 A species generally consists of several smaller groups called populations
 All the members of a species that live in a defined geographic region at a given time
 Populations, not individuals, are the units that evolve
Evolution at Its Smallest Scale
 Evolution can be defined as a change in the characteristics of a population over time
Populations are often isolated to some degree from other populations
The environments inhabited by each population is different
Each population faces the natural selection pressures of its own environment
Different populations can evolve differently
Evolution at Its Smallest Scale
 e.g., Two different frog populations in two different environments may both evolve coloration
patterns making them less visible to predators
 A lighter coloration may benefit one population in their particular environment, while a
darker coloration pattern may benefit a second population in their (different) environment
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Evolution at Its Smallest Scale
 Natural selection acts upon the phenotypes of individuals
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 Individuals with one phenotype may be more fit than individuals with a different
phenotype
These phenotypes are influenced by genes
 More specifically, different alleles of genes
Evolution as a Change in the Frequency of Alleles
An individual’s phenotype may make it more likely to survive longer
 Surviving individuals may reproduce
 Individuals that do not survive certainly cannot reproduce
Evolution as a Change in the Frequency of Alleles
Some individuals are more successful at breeding
 Their alleles are passed on to the next generation in relatively greater numbers
 Alternative alleles possessed by less successful frogs are passed on to the next generation
in reduced numbers
Evolution as a Change in the Frequency of Alleles
The evolution of a population involves a difference in reproductive output between individuals
 This is often the result of a difference in survival between individuals
 Surviving individuals may reproduce
 Individuals that do not survive certainly cannot reproduce
Evolution as a Change in the Frequency of Alleles
“Microevolution” is a change in allele frequencies in a population over a relatively short
period of time
 This is evolution within a population
“Macroevolution” is evolutionary change that results in the formation of new species
 Larger changes resulting in the same way as microevolution
Five Agents of Microevolution
 There are five “agents” of microevolution that can alter allele frequencies in populations
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Mutation
Gene flow
Genetic drift
Sexual selection
Natural selection
Five Agents of Microevolution
 A “mutation” is any permanent alteration in an organism’s DNA
 Mutations can be small
 “Point mutations”: change in single base pair
 Mutations can be large: addition or deletion of whole or parts of chromosomes
 Some mutations are heritable
 Mutation rates are very low
Five Agents of Microevolution
 Perhaps just one base pair per billion
 Very few of these mutations are beneficial
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 Most are neutral, some are harmful
 Beneficial mutations are very important
Mutations are the source of new genetic variation
 New proteins are produced, which can modify the form or capabilities of the organism
Five Agents of Microevolution
The evolution of eyes, wings, and other structures depends upon many mutations
 Many mutations, followed by rounds of genetic shuffling and natural selection are required
 This process may take millions of years
Five Agents of Microevolution
Allele frequencies can change as a result of migration
 Arrival of members from a different population
 “Immigration”
 Loss of individuals to a different population
 “Emigration”
Five Agents of Microevolution
This movement may involve individuals, or may involve gametes
 e.g., Movement of pollen
Five Agents of Microevolution
 Allele frequencies can change simply as a matter of chance
 Random fluctuations in allele frequencies are termed “genetic drift”
 Alleles can increase or decrease in frequency
 Alleles that are removed can only be replaced through new mutations or through
migration
Five Agents of Microevolution
 These fluctuations have the greatest impact on smaller populations
 The bottleneck effect
 The founder effect
Five Agents of Microevolution
 The “bottleneck effect” is a change in a population’s allele frequencies due to chance
following a sharp reduction in population size
Five Agents of Microevolution
 e.g., Northern elephant seals were hunted very heavily in the 1890s
 Only 50 individuals remained
 Protective measures have increased these numbers somewhat
Five Agents of Microevolution
 All members of the current population of northern elephant seals have descended from this
few individuals that survived the bottleneck
 Many alleles were removed from the population during the bottleneck
 All of the members of today’s population are genetically very similar
Five Agents of Microevolution
 Pull a dozen M&Ms from a two pound bag
 Will all of the colors be represented?
 Will the colors be represented in the same proportions as in the original bag?
 If these dozen M&Ms “reproduce” to fill a new two pound bag, what proportions might you
expect?
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 How do your answers differ if you pull 120 M&Ms from the bag?
Five Agents of Microevolution
The “founder effect” occurs when a small population migrates to a new area to start a new
population
 The allele frequencies in this small population will not precisely mirror those of the larger
population
 The effects of genetic drift can be profound in such a small population
Five Agents of Microevolution
“Sexual selection” involves differential reproductive success based on success in obtaining
mating partners
 A form of natural selection
 This mating is based on phenotype
Five Agents of Microevolution
Birds of paradise on Papua-New Guinea
Five Agents of Microevolution
 Differential mating success among members of one sex is generally based on choices made by
members of the opposite sex
 Females generally do the choosing
 Can also be based upon the combative abilities of males
Five Agents of Microevolution
 “Natural selection” is a process in which the differential adaptation of organisms to their
environment selects those traits that will be passed on with greater frequency from one
generation to the next
 Natural selection is the means of adaptive evolution
 Through natural selection, populations become better adapted to their current
environment
Five Agents of Microevolution
 Through natural selection
 Traits of individuals more successful at reproducing will become more widespread in a
population
 The alleles that bring about these traits will increase in frequency from one generation to
the next
Five Agents of Microevolution
 “Adaptation” is a modification in the structure or behavior of organisms over generations in
response to environmental change
 A particular environment may change
 e.g., Streams drying up, etc.
 A population may migrate to a different environment
Five Agents of Microevolution
 Natural selection is the only agent of microevolution that consistently works to adapt
organisms to their environment
 Mutation simply generates variation
 Genetic drift is random
 Gene flow is not related to environment
 Sexual selection deals with mate choice
Natural Selection and Evolutionary Fitness
 “Darwin’s finches” have been studied since the 1970s by Peter and Rosemary Grant
 In 1977, a tiny Galapagos Island suffered a severe drought
 Daphne Major
 This drought had a major impact on the island’s two finch species
 1,300 finches  fewer than 300 finches
 85% of the Geospiza fortis population died
Natural Selection and Evolutionary Fitness
 The G. fortis population that survived the drought had a larger average beak size than the
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pre-drought population
 6% larger beak size
Their offspring had a larger average beak size
A slightly larger beak enables a finch to get into large, tough seeds
Natural Selection and Evolutionary Fitness
In 1984 and 1985, there was excessive rain on this island
 There was an abundance of small seeds
 Finches with smaller beaks survived this event in disproportionate numbers
 The average beak size in the following generation decreased slightly
Natural Selection and Evolutionary Fitness
 Where is the “fittest” bird in all of this?
 There isn’t any
 Evolution was not marching toward the production of some generally superior bird
 Different traits were favored under different environmental conditions
17.5 Three Modes of
Natural Selection
 Many traits are polygenic
 e.g., Human height
 Display continuous variation
 When natural selection acts upon these traits, it can proceed in any of three ways
 Stabilizing selection
 Directional selection
 Disruptive selection
Three Modes of Natural Selection
 Stabilizing selection
 Intermediate forms are favored over extreme forms
Three Modes of Natural Selection
 e.g., Human birth weights
 Bell curve distribution of human birth weights
 Infant mortality is highest for very small and very large infants
 Infants of intermediate size are most likely to survive
 6 – 7 pound average weight is maintained
Three Modes of Natural Selection
 Directional selection
 Natural selection moves a character toward one extreme
 e.g., Cranial capacity in hominids
 Humans and their closest relatives
Three Modes of Natural Selection
 Disruptive selection
 Natural selection moves a character toward both of its extremes
 Less common than the other two modes
 e.g., Pyrenestes ostrinus, a species of finch from West Africa
Three Modes of Natural Selection
 Pyrenestes ostrinus
 Beaks size is either large or small, without intermediate values
 Finches with large beaks specialize in cracking large seeds
 Finches with small beaks specialize in cracking small seeds
 Finches with intermediate-sized beaks are less efficient in cracking both large and small
beaks