Evolution – Chapter 11
Microevolution vs. Macroevolution
 Microevolution ~ Small scale changes in gene frequencies in a population
 Mutations
 Adaptation
 Macroevolution ~ Large scale changes in traits through geological time
 Speciation
 Extinction
Microevolution
 Changes that occur in a population’s allele frequencies over time
 Allele frequencies can change through
 Mutation (with positive and negative results)
 Gene flow
 Genetic drift
 Natural selection
 Only mutation can produce new alleles
Darwin’s Voyage
Darwin’s Voyage
 Dates: 27 December 1831 - 2 October 1836
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Commander: Captain Robert FitzRoy (1805-1865)
Scientist: Charles Darwin
Voyage of the Beagle
Galapagos Islands
 Volcanic islands far off coast of Ecuador
 All inhabitants are descended from species that arrived from elsewhere
Galapagos Finches
 Darwin observed 13 species of finches with a variety of lifestyles and body forms
 He attempted to correlate variations in their traits with success at meeting
environmental challenges
Malthus - Struggle to Survive
Malthus and Darwin
Darwin’s Theory
 A population can change over time when individuals differ in one or more heritable
traits that are responsible for differences in the ability to survive and reproduce
On the Origin of Species
 Darwin’s book
 Published in 1859
 Laid out his evidence in support of the theory of evolution by natural selection
Populations Evolve
 Traits (alleles) in a population vary among individuals
 Evolution is a change in frequency of traits through time (and the alleles that underlie
the traits)
Biological evolution does not change individuals, it changes a population
Microevolutionary Processes
 Small-scale changes in allele frequencies that drive a population away from genetic
equilibrium within a few generations
 Traits (alleles) are selected for or against
Examples of Selection
 Human, Artificial
 Crops
 Domesticated animals
 Human, Accidental
 Evolution of pesticide resistant organisms
 Evolution of antibiotic-resistant diseases
Natural Selection
Brassica oleracea
Natural Selection
 Natural selection occurs when individuals with some traits survive and reproduce
better than do individuals with other traits
 Traits of the survivors passed on to offspring become increasingly more and more
common in populations
The Gene Pool
 All of the genes in the population
 Can be shared (in theory) by all members of population
Variation in Phenotype
 Each kind of gene in gene pool may have two or more alleles
 Individuals inherit different allele combinations
 This leads to variation in phenotype
 Offspring inherit genotypes, NOT phenotypes
Genetic equilibrium
 How do you tell that a population is evolving?
 Genetic Equilibrium: the Hardy-Weinberg Law
 Gene frequencies are inherently stable in a population
 If this law is violated for a population, then it must be changing
Five Assumptions of Hardy-Weinberg Equilibrium
 No mutation
 Random mating
 Gene doesn’t affect survival or reproduction
 Large population
 No immigration/emigration- population is isolated
Variation in Populations
 Individuals in a population have similar genes that specify the same assortment of
traits, but variation in the alleles of those genes produce different phenotypes
 Some phenotypes compete better than others
Conclusion ~ Change Over Time
 Over time, alleles that produce the most successful phenotypes will increase in the
population, while others decrease, and some don’t change
 The most successful phenotypes, have the highest “fitness”
 Natural selection results in adaptation
Gene Flow
 Physical flow of alleles into a population
 Immigration vs. emigration
 Counters changes that result from mutation, natural selection, and genetic drift
Genetic Drift
 Change in allele frequencies brought about by chance over generations
 Drift is most pronounced in small populations, where every chance event has a bigger
impact
Computer Simulation
Computer Simulation
Bottleneck
 A severe reduction in population size that causes pronounced drift in the surviving
population
 Example
 Elephant seal population hunted down to just 20 individuals
 Population rebounded to 30,000
 Electrophoresis revealed there is now no allele variation at 24 genes
Founder Effect
 Effect of drift when a small number of individuals start a new population
 Effect is pronounced on isolated islands
Inbreeding
 Nonrandom mating between related individuals leading to increased homozygosity
and a lack of genetic variation
 Can lower fitness when deleterious recessive alleles are expressed
Ellis-van Creveld Syndrome
Phenotypic variation due to environment
Ecotypes
Biological Species Concept
“Species are groups of interbreeding natural populations that are reproductively isolated
from other such groups.”
Ernst Mayr
Species are created through genetic divergence
 Gene flow reduced in isolated populations
 Gradual accumulation of differences in the gene pools of populations through natural
selection, genetic drift, and mutation
 Reproductive isolation occurs as a by-product of genetic change
Genetic Divergence
What is a Species?
 A species is one or more populations of individuals that …
 Interbreed under natural conditions
 Produce fertile offspring
 Are reproductively isolated from other such populations
Separate species may hybridize
Mechanisms of Speciation
 Allopatric speciation
 Sympatric speciation
 Parapatric speciation
Mechanisms of Speciation
Allopatric Speciation
 Populations are first geographically isolated, then become reproductively isolated
 Species separated by geographic barriers will diverge genetically
 If divergence is great enough it will prevent inbreeding even if the barrier later
disappears
 Effectiveness of barrier varies with species
Geographic Isolation
 With the members (and their gene pools) isolated from one another, they become
TWO UNIQUE populations
Mechanisms of Speciation
Parapatric Speciation
Adjacent populations evolve into distinct species while maintaining contact along a
common border
Mechanisms of Speciation
Sympatric speciation
 Speciation by polyploidy is a common event in plants
 Polyploids have more than two sets of chromosomes
Speciation by Polyploidy
 Change in chromosome number
(3n, 4n, etc.)
 Offspring with altered chromosome number cannot breed with parent population
 In autopolyploids, offspring have a doubling of chromosome number from parents
 Allopolyploids are interspecific hybrids
Evolution of wheat
Sterile hybrids
Rates of speciation
 Gradual model
 Species emerge through many small morphological changes that accumulate over a
long time period
 Punctuation model
 Most changes in morphology are compressed into brief period near onset of
divergence
 Adaptive radiation
Evolutionary Trees