Evolution & Speciation (Continued)

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Evolution and Speciation
Continued
BIOL 1113
September 23, 2016
Forces that Drive Evolution
• Recall: Evolution  change in allele frequencies in a population over
time
• Alleles that confer higher fitness increase in frequency in a
population
• Alleles that confer lower fitness decrease in frequency in a population
• Several forces drive this process of evolution
1. Natural Selection
• How the distribution of a particular phenotypic trait changes over
time depends on the mode of natural selection:
• Stabilizing selection
e.g. human baby weight
• Disruptive selection
e.g. finch beaks
• Directional selection
e.g. antibiotic resistance
Natural selection leads to changes in gene
frequencies through time
• How the distribution of a particular –phenotypic
trait changes over
Before selection
time depends on the mode of natural
selection:
– After
selection
• Stabilizing selection
e.g. human baby weight
• Disruptive selection
e.g. finch beaks
• Directional selection
e.g. antibiotic resistance
https://upload.wikimedia.org/wikipedia/commons/8/84/Selectiontypes-n0_images.png
Sexual Selection
• Traits may appear to be costly (energetically wasteful; increased
visibility to predators) but provide an advantage (preferred by mates)
• Examples:
• Elaborate antlers of many types of deer
• Showy feathers of male peacocks
http://vignette4.wikia.nocookie.net/dragonflyissuesinevolution13/images/c/cf/Peacock_display.jpg/revision/latest?cb=20131005203553
• Darwin  sexual selection maintains and favors these seemingly
maladaptive traits if the individuals with more elaborate traits are
also more attractive to females and thus achieve higher fitness
2. Genetic Drift
• Another mechanism of evolution that can drive variation in allele
frequencies
• The process in which allele frequencies fluctuate as a result of chance
alone
• Fluctuations due to events during sexual reproduction, including
allele segregation and recombination
Review of Mendel’s Laws
• Law of Segregation
• Law of Independent Assortment
• https://www.youtube.com/watch?v=QDAM1-QTcKM
http://guestblog.scientopia.org/wp-content/uploads/sites/35/2012/07/10-04.gif
Genetic Drift may be More Pronounced if:
• The population size is small
Genetic Drift  Changes in allele frequencies
across generations due to chance events
•
Random disturbances
• Small populations
Genetic Drift More Likely in a Small Population
• Leads to fixation of an allele and loss of others
• Many alleles are
lost (go to zero)
• One has become
fixed (at 1, meaning
all individuals carry
it)
Each colored line represents a different allele for given gene.
Trend in the line shows the change in that allele frequency over multiple generations.
Genetic Drift Less Likely in a Large Population
• Some alleles increase over time, and some decrease
• Few become lost or fixed
Each colored line represents a different allele for given gene.
Trend in the line shows the change in that allele frequency over multiple generations.
Effects More Pronounced In Small Populations
Genetic drift is more
likely to occur in a
small population,
leading to fixation of an
allele and loss of others
Genetic drift is less
likely in a large
population, where
random events are
buffered
Genetic Drift may be More Pronounced if:
• The population size is small
• There are weak selective forces
?
Genetic Drift may be More Pronounced if:
• The population size is small
• There are weak selective forces  if the differences between the
alleles have little/no effect on fitness
Genetic Drift > Selective Forces
• Under weak selective forces, random processes can play an overriding
role in determining the frequency of alleles in the population
• If the forces of drift are particularly strong, they can sometimes
override natural selection
• Result  less fit individuals may increase in frequency relative to
more fit individuals
Genetic Drift may be More Pronounced if:
• The population size is small
• There are weak selective forces
• There is a founder effect
Founder Effect
• Influences gene frequencies in isolated subpopulations
• May occur when a small subpopulation becomes isolated from the
larger population
• This occurs often on isolated islands, such as the Galápagos Islands
https://s3.amazonaws.com/classconnection/343/flashcards/8968343/jpg/founder-14F962AEA3908A3DE72.jpg
Founder Effect
• Founder effect occurs if the allele frequency of the subgroup is very
different from that of the parent population
• So named because the alleles present in the "founders" of the
subpopulation are disproportionately represented in subsequent
generations relative to the alleles that were present in the parent
population
https://s3.amazonaws.com/classconnection/343/flashcards/8968343/jpg/founder-14F962AEA3908A3DE72.jpg
Genetic Drift  Changes in allele frequencies
across generations due to chance events
•
Random disturbances
• Small populations
• Also: Founder effect
Founder Effect Causes Differences in Species
• Genetic drift in a founding population can lead to new populations of
the same species that are different from the parent population
• Example: the Afrikaner population of South Africa
• Traces to a small founder group of Dutch immigrants that sailed to
South Africa in 17th century
• Among these original immigrants was a man who happened to
carry the allele for Huntington's disease
• As a result, Huntington's disease is disproportionately
represented among Afrikaners today
Founder Effect May Lead to Speciation
• Over time, genetic drift in the subpopulation (in combination with
selection pressures) can cause a subpopulation to become so
different from the original parent population that a new species forms
• This occurs often on isolated islands, such as the Galápagos Islands
Selective pressure  type of food available
http://www.biology.iupui.edu/biocourses/N100/images/17adapradiation.gif
Bottleneck Effect of Genetic Drift
• Occurs when a population undergoes a
rapid decrease in size
• Results from rapid elimination of alleles
from a population
• Bottlenecks often result from a natural
disaster that decimates a single generation
in a non-selective manner
• Volcanic eruption
• Flood
Bottleneck Effect of Genetic Drift
• Decreases the population's overall
genetic variation
• Allelic distribution in subsequent
generations thus differs from that
of the ancestral, pre-bottleneck
population
Disadvantage of Founder and Bottleneck Events
• Natural selection (and subsequent adaptation) depends on genetic
variation
• Small populations with decreased variability (due to founder effect
or bottleneck event) are particularly susceptible to environmental
challenges and changes
• Animal populations that are overexploited by humans are especially
at risk
• Hunting for food, fur, etc.
• Loss of habitat for industry and expansion
Disadvantage of Founder and Bottleneck Events
• Conservation efforts help to offset these problems, but lack of
genetic variation complicates conservation efforts
• Hawaiian monk seals
• Koalas
• Cheetahs
• California Channel Islands fox
• Least genetic diversity known to
science – 99.9% alike
• Measures are being taken to
ensure their survival (removing
predators, controlled breeding
programs, etc.)
https://upload.wikimedia.org/wikipedia/commons/f/f3/Californian_Channel_Islands_map_en.png
https://psmag.com/the-least-genetically-diverse-animal-known-to-science-is-an-adorable-fox-bfc03ad3e464#.uz1byy3pc
3. Gene Flow
• A third mechanism of evolution that can drive variation in allele
frequencies
• Besides those on remote islands, few populations exist in total
reproductive isolation
• Individuals are constantly migrating into and out of a population
(accidentally or deliberately)
Gene Flow May Be Beneficial
• A mechanism of evolution whereby alleles move between
populations as a result of migration
• Often increases genetic variation (in both populations)
• Often reduces the effects of genetic drift
• Often leads to an increase in fitness
Rates of Gene Flow Vary
• Mobile organisms (e.g. birds, fish) tend to hand higher rates of gene
flow than sedentary organisms
• Sessile organism (e.g. plants and coral) can also have high rates of
gene flow
• Pollen and seed dispersal via wind, water, and/or animals
• Alleles are thus dispersed
http://www.desktopwallpaperhd.net/pollen-wallpaper-flying-desktop-insects-gallery-8926.html
Coral spawning
Bees collecting pollen
http://3.bp.blogspot.com/-pA6VmsLGLSQ/TyhsErQOL2I/AAAAAAAADYw/JxWNhJhluLY/s1600/Covered.jpg
http://3.bp.blogspot.com/-pA6VmsLGLSQ/TyhsErQOL2I/AAAAAAAADYw/JxWNhJhluLY/s1600/Covered.jpg
Disadvantage of High Gene Flow
• Effects of gene flow are not always beneficial
• Can lead to homogenization across populations
• Can dilute the effects of localized natural selection and adaptation
• Leads to a population that is not as adapted to its environment as it
could be
Summary: Mechanisms of Evolution
• Evolution  change in allele frequencies in a population over time
• Natural selection  (3 types of) preferential selection of alleles that
confer the greatest fitness to individuals in a population
• Genetic drift  random change in allele frequencies resulting from
chance events in a population
• Gene flow  change in allele frequencies arising from the migration
of individuals into and out of a population
• Changes in allele frequency in the same generation can lead to
evolution in the next generation
Interactive Example of Evolution in Textbook
Genetic Variation in Populations
• Phenotype: Trait of an individual that can vary in a population
• Diversity can result from genetic variation  individuals within a
population vary in the alleles that they have for certain traits
• Examples: flower color, human blood type, and bacterial
sensitivity to antibiotics
• In order for the frequencies of alleles to change through time, there
must be variation in alleles in the first place
Genetic Traits are Heritable
• Pass from one generation to the next
• Complicated for complex traits (encoded by more than one gen)
• Example: eye color
http://colekcolek.com/wp-content/uploads/2012/03/heredity-2-allele.jpg
Environment Can Also Influence Traits
• However, most traits have both a genetic and an environmental
component
• Example: plant height
• Partially determined by genetics  explains why a tomato plant
cannot grow as tall as an apple tree
• Partially determined by environment  size a plant also depends
on soil, water, temperature, light, etc.
• Traits determined by the environment are not usually passed on to
subsequent generations
Non-Heritable Traits
• Variations in traits that do not pass to the next generation
• Example: An Olympic athlete with a physique that reflects a
lifetime of training will not pass this same athletic build to his/her
child
• If the trait is not heritable, it will not evolve
• Nongenetic variation cannot be passed on
Epigenetic Traits
• Recently, researchers have discovered that some environmental
factors that can alter DNA, is passed on to the next generation
• This is known as epigenetics
• “Epi”  “above or on top of” (e.g. epicenter, epilogue)
• Epigenetic tags can be passed on (epigenome is heritable)
• Example: Methylation of DNA
• Mice fed different diets
• Some mice had a lot of DNA methylation  specific
sequences tagged with methyl groups (-CH3)
• Does not change the gene sequence
• Does regulate which genes are available for use
• Mechanism still being investigated
DNA Methylation Alters Phenotype Expression
• Methyl groups added to cytosine (and adenine in prokaryotes)
• Typically, repress gene expression
• Genes not expressed  RNA not
transcribed  Proteins not translated
 Phenotype not expressed
http://helicase.pbworks.com/f/DNAmeth.jpg
Evolution Depends on Genetic Variation
• Recall: Evolution  change in allele frequencies through time
• Genetic variation determines the range of traits upon which natural
selection or genetic drift can act
• Low genetic diversity can be catastrophic if a species cannot adapt
to environmental changes
• E.g. viruses and pests that wipe out genetically identical plants
Artificial Selection
• Loss of genetic variation often occurs when humans deliberately
select organisms with particular traits and breed them
• This is known as artificial selection
• Purpose is to enhance a particular trait
• Milk production in cows
• Sweetness in oranges
• Hardiness during transportation in bananas
• Typically, artificially bred populations have low genetic diversity
across the genome  particularly susceptible to external stressors
(e.g. new pathogens)
Irish Potato Famine of the 1840s
• Irish fed their growing population with the "lumper" potato variety
• All lumpers were clones, genetically identical to one another
• All susceptible to a rot/blight caused by a fungus-like eukaryote
• 1 in 8 Irish people died of starvation within 3 years in the 1840s
• Should be a
cautionary tale…
https://upload.wikimedia.org/wikipedia/commons/3/37/Irish_potato_famine_Bridget_O'Donnel.jpg
http://evolution.berkeley.edu/evolibrary/article/agriculture_02
Measuring Genetic Variation
• Measure average heterozygosity  proportion of gene loci that are
heterozygous
• Each genes on the homologous chromosomes in a diploid organism
will be either:
• Homozygous  same allele on both chromosomes
• Heterozygous  different alleles on both chromosomes
• Populations with low genetic diversity  many of the loci will be
homozygous
• Populations with high genetic diversity  many of the loci will be
heterozygous
Higher Heterozygosity  Higher Fitness
• Populations with high genetic diversity
• Higher average heterozygosity
• Often have higher fitness
• Three plants and the fruit they
produced are lined up
• Middle plant is a hybrid of the two on
either side  yield is much greater
Hybrid vigor in tomato plants
Average Heterozygosity Is Informative
• Average heterozygosity is useful for determining genetic variation
• Also provides other information about a population:
• Rarity of population?
• Previous bottleneck event?
• Likeliness of gene flow?
• Likeliness of disruptive selective force?
• Estimates of mutation rate?
Geographic Location Affects Genetic Variation
• Geographic features often separate different populations of a species
• Geographic variation is genetic variation maintained across broad
geographic ranges
• Due to different selective pressures or genetic drift and little gene
flow
• Example: Oldfield mice
Geographic Variation
• Example: Oldfield mice from geographically isolated areas have
different traits as a result of differences in genotypes
• Pie charts show the
proportion of the light
fur alleles and dark fur
alleles of a single gene
present in each
population
• Coat color arises from
multiple genes 
contributes to variety
Gene Flow Reduces Geographic Variation
• High rate of interbreeding among populations would reduce variation
as the populations became more similar
• If gene flow between these populations were more common  the
genotypes would be less distinctly different for each population
Example: Fur color in oldfield mice
Gene Flow Across Environmental Clines
• Gene flow can cause gradual change in gene frequencies across
environmental clines
• Cline  a continuum with several gradations from one extreme to
the other
Example: Hemoglobin in deer mice
http://calphotos.berkeley.edu/imgs/512x768/0000_0000/0105/2831.jpeg
Allelic frequencies in Deer Mouse Hemoglobin
• Example: Deer mice along an elevational gradient
• Different populations at 3 different elevations
• Pie charts  allele frequencies for each of 5 amino acid positions
• Allele frequencies change
along an elevational cline
(with varying oxygen
availability)
• Middle population having
allele frequencies between
those of the high and low
altitude populations
3D struc. of hemoglobin
Factors that Affect Genetic Variation
• Mutations
• Changes in the DNA nucleotide sequence caused by errors in DNA
replication, mutagens
• Can occur in coding or non-coding regions of DNA
• Can be caused by mutagens (e.g., UV light, chemicals)
• Can be caused by errors in DNA replication
Factors that Affect Genetic Variation
• Sexual reproduction
• Reshuffling of alleles between individuals
• Variety in allelic combinations leads to new phenotypes
• During meiosis, homologous chromosomes are separated and
recombined
• DNA recombination during chromosome alignment in meiosis
• Independent assortment of chromosomes during meiosis
• Random combination of gametes during fertilization
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