Download What is a population? Review of Genetics Terminology

What is a population?

Review of Genetics Terminology
A group of individuals (that
can and do interbreed)
living in a distinct
geographic area
 Gene – sequence of DNA that codes
Microevolution
for a particular trait
 Evolution on a
 Allele – alternative versions of a gene
small scale (within
 Genotype – combination of alleles
a single species or
present in the individual’s DNA
population)
 Phenotype – the physical characteristics
 Change in gene
resulting from the genotype
frequency within a
 Gene pool – all the alleles for a
What is a species? population
gene that exist in a population
group of individuals
 Mutation – random change in DNA o A
that actually or
sequence within an individual
potentially interbreed
in nature
o The biggest gene pool
possible under natural
conditions
Macroevolution
Evolution on a grand scale
(above the species level)
Large-scale history of life
Genes and evolution

Only traits controlled by genes can be affected by natural selection.
Genetic Variation
• Necessary for evolution
• Sources:
• Mutations (random
changes in genes)
• Gene flow (movement of
genes from one population to
another of the same species)
• Sexual Reproduction (creates
new combinations of alleles
within a population)
Gene Frequency
The number of times an allele
occurs in a gene pool compared
with the number of times other
alleles occur
EVOLUTION = CHANGE IN GENE
FREQUENCIES!
What causes a mutation?
 DNA fails to copy accurately during replication
 Damage to DNA from mutagen
Types of gene mutations
o Somatic mutation
o Occurs in a non-reproductive cell
o Not hereditary
o Germ-line mutation
o Occurs in a reproductive cell (gamete)
o Heritable!
o Drives evolution
Potential effects of germ-line
mutations
 No change in phenotype
 Small change in phenotype
 Major change in phenotype
Mutation
o Changes in DNA that affect phenotype
o Example: Some beetles with “green genes”
randomly mutate to “brown genes”
Migration (Gene Flow)

Movement of genes from one
population to another population
of the same species
Genetic Drift
Allele frequencies fluctuate unpredictably from one generation to the next
Causes:
First Generation
Second Generation
Random “shuffling”
of alleles from one
generation to the
next
Beetles example:
1st Generation:
75% Green
25% Brown
2nd Generation:
71% Green
29% Brown
75%
25%
71%
29%
Genetic Drift
Allele frequencies fluctuate unpredictably from one generation to the next
Causes:
Random “shuffling” of alleles from one generation to the next
Bottleneck Effect:
Sudden change in environment 1. Initial population has equal
drastically reduces population
frequencies of red and yellow “alleles”
and alters allele frequencies
2. Chance event greatly reduces the size
of the population
3. Surviving individuals have “allele”
frequencies different from those of the
original population
4. Resulting population has far more red
“alleles” than yellow
1
2
3
4
Genetic Drift
Allele frequencies fluctuate unpredictably from one generation to the next
Causes:
Random “shuffling” of alleles from one generation to the next
Bottleneck Effect: Sudden change in environment drastically reduces
population and alters allele frequencies
 Cheetahs: so close that
 European bison: entire
living population today
descends from only 12
individuals.
skin grafts do not trigger
an immune response
 Elephant seals: only 30
in the 1890s; 10’s of
thousands today
Genetic Drift
Allele frequencies fluctuate unpredictably from one generation to the next
Causes:
Random “shuffling” of alleles from one generation to the next
Bottleneck Effect
Founder Effect:
A few individuals from a population start a new population with a
different allele frequency than the original population
1. Several individuals become isolated
from a larger population and…
2. …establish a new population.
3. The new population’s gene pool is not
reflective of the source population.
1
2
3
Genetic Drift
Allele frequencies fluctuate unpredictably from one generation to the next
Causes:
Random “shuffling” of alleles from one generation to the next
Bottleneck Effect
Founder Effect:
A few individuals from a population start a new population with a
different allele frequency than the original population
Sample of Original
Population
Founding
Population A
Descendants of A
Founding
Population B
Descendants of B
1. Several individuals
become isolated
from a larger
population and…
2. …establish a new
population.
3. The new
population’s gene
pool is not
reflective of the
source population.
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
• Example:
Predators eat
more of the
green ones, so
brownof
is beetles
more
Original population
likely togreen
survive
is predominantly
and reproduce
Remaining
population
Green
beetles are
more is
predominantly
brown
visible
against tree bark
than brown ones.
Fitness
 Ability of an individual to survive and
reproduce in its specific environment
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
• Example:
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
• Example:
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
• Example:
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
• Example:
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
• Example:
1850
1900
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
Sexual Selection:
• Non-random mating within a population
Sexual Dimorphism
 Males and females of a species differ
significantly in appearance
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
Sexual Selection:
Sexual Dimorphism
 Males and females of a species differ
significantly in appearance
• Non-random mating within a population
• Types of sexual selection:
• Intrasexual selection
• Competition within the same sex
for mates of the opposite sex
• Favors traits that deal with conflict
between individuals of the same sex
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
Sexual Selection:
Sexual Dimorphism
 Males and females of a species differ
significantly in appearance
• Non-random mating within a population
• Types of sexual selection:
• Intrasexual selection
• Competition within the same sex
for mates of the opposite sex
• Favors traits that deal with conflict
between individuals of the same sex
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
Sexual Selection:
Sexual Dimorphism
 Males and females of a species differ
significantly in appearance
• Non-random mating within a population
• Types of sexual selection:
• Intrasexual selection
• Competition within the same sex
for mates of the opposite sex
• Favors traits that deal with conflict
between individuals of the same sex
• Intersexual selection
• Individuals of one sex (usually female)
are selective when choosing a mate
• Favors traits that enhance attractiveness
to the opposite sex, such as plumes, wattles,
bright color, songs, symmetry, etc.
Natural Selection
• Changes in the inherited characteristics
of a population, which increase the
species’ fitness in its environment
Sexual Dimorphism
 Males and females of a species differ
significantly in appearance
Sexual Selection:
Cauliflower
• Non-random mating within a population
• Types of sexual selection:
• Intrasexual selection
• Competition within the same sex
for mates of the opposite sex
• Favors traits that deal with conflict
between individuals of the same sex
• Intersexual selection
Wild
• Individuals of one sex (usually female) Mustard
are selective when choosing a mate
• Favors traits that enhance attractiveness
to the opposite sex, such as plumes, wattles,
bright color, songs, symmetry, etc.
Brussels
Sprouts
Flower clusters
Lateral buds
Cabbage
Terminal buds
Kale
Broccoli
Artificial Selection:
• Selective breeding
• Natural variations exist within a species,
but humans select which organisms get to reproduce
Stems
Kohlrabi
Leaves
Stems & flowers
Stabilizing Selection



Trend toward reducing phenotypic variation and maintenance of the status quo
Individuals with the average phenotype (center) are selected
Examples:
 Tuatara reptiles – very little change over long periods of time
 Birth weight of humans – too small or too large tend to have lower survival rates
Selection against both extremes
Very low birth
weight babies
are less likely
to survive
Extremely large
babies may not
survive birth
(may also cause
death of mother
during delivery)
Original
population
~7.5 lbs
Average weight
Population
after selection
Directional Selection
 Trend toward one extreme and away from the other extreme, such that the
population as a whole tends to change in a similar fashion
 Individuals with one phenotype (at one end) are selected
 Examples:
 Peppered moths, insecticide resistance, antibiotic resistance
Selection against one extreme
Original
population
Pre-industrial
moths were
light-colored
Population
after selection
Post-industrial
moths were
dark-colored
Disruptive Selection
 Two or more phenotypes survive and reproduce more successfully than
intermediate forms
 Individuals with one of two phenotypes (both ends) are selected
 Example:
 African Swallowtail Butterfly
Selection against the average
Female mimics
an unpalatable
species
Original
population
Population
after selection
Male or female is palatable
and does not mimic any
unpalatable species
Female mimics
a different
unpalatable
species
Stabilizing selection
Individuals with the average
phenotype (center) are selected
Original
population
of brown
mice
Directional selection
Individuals with one phenotype
(one end) are selected
Disruptive selection
Individuals with one of two
phenotypes (both ends) selected
Original
population
Phenotypes (fur color)
Population
after selection
Wide variation
in fur color
Medium
colors selected
Medium
color selected
Stabilizing Selection
Darker colors
selected
Directional Selection
against
Disruptive Selection
Macroevolution


Evolution above the species level
Overall trends and transformations in evolution
Speciation
• Development of new species.
• Over long periods of time, natural selection
leads to the accumulation of changes that
differentiate groups from one another.
Patterns in
macroevolution
1. Stasis
 Lineages that change very
little over long periods of
time (ex: coelocanths)
2. Character change
 Characteristics of lineages
can change quickly or slowly
over time
 Characteristics may change
in a consistent manner or
may progress and regress
over time
3. Lineage-splitting (speciation)
 Speciation within a lineage
can happen often or
infrequently
4. Extinction
 May occur frequently
or rarely in a lineage
 Simultaneous extinctions
across many lineages are
known as “mass extinctions”
What distinguishes one species from another?
Morphological Species Concept:
 Characterizes a species by its body shape,
size, and other structural features
 Applies to asexual and sexual organisms
 Useful without information on gene flow
 Often used in paleontology, since little
information may be known about mating
abilities of fossilized extinct organisms.
Phylogenetic Species Concept:
 Set of organisms with a unique
genetic history
 Relatedness determined by:
 Comparison of physical
characteristics
 Molecular similarities
(DNA)
Ecological Species Concept:
 Focuses on differences in ecological niche rather than appearance (i.e. food source)
Biological Species Concept:
 Groups of actually or potentially
interbreeding natural populations
 Produce fertile offspring
 Reproductively isolated from other groups
 Commonly accepted as a basic working
definition of species (we’ll use this one!)
Therefore…
…two populations
are separate species
if they cannot (or do
not) successfully
interbreed in nature.
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Geographic Isolation
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Ecological Isolation
(different pollinators)
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Different habitats:
•Open woods
•Beech trees
•Alders
•Conifer woods
•Willowy thickets
Ecological Isolation
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Temporal
Isolation
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
(breeds in
September/October)
(breeds in March/April)
Temporal Isolation
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
4. Behavioral isolation – little or no sexual attraction between males and females
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Behavioral
Isolation
(different mating songs)
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
4. Behavioral isolation – little or no sexual attraction between males and females
5. Mechanical isolation – anatomical differences prevent exchange of gametes
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Mechanical
Isolation
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
4. Behavioral isolation – little or no sexual attraction between males and females
5. Mechanical isolation – anatomical differences prevent exchange of gametes
6. Gametic isolation – female and male gametes fail to unite in fertilization
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Gametic Isolation
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
4. Behavioral isolation – little or no sexual attraction between males and females
5. Mechanical isolation – anatomical differences prevent exchange of gametes
6. Gametic isolation – female and male gametes fail to unite in fertilization
Post-zygotic Barriers
1. Hybrid inviability – hybrid zygotes fail to develop or fail to reach sexual maturity
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Hybrid
Inviability
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
4. Behavioral isolation – little or no sexual attraction between males and females
5. Mechanical isolation – anatomical differences prevent exchange of gametes
6. Gametic isolation – female and male gametes fail to unite in fertilization
Post-zygotic Barriers
1. Hybrid inviability – hybrid zygotes fail to develop or fail to reach sexual maturity
2. Hybrid infertility – hybrids fail to produce functional gametes
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Hybrid
Infertility
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
Pre-zygotic Barriers:
 Factors that may inhibit:
 Mating between individuals
 Fertilization of gametes
Post-zygotic Barriers:
 Factors that may inhibit:
 Survival of embryo or offspring
 Fertility of surviving offspring
Pre-zygotic Barriers
1. Geographic isolation – populations are separated by a physical boundary (i.e. river)
2. Ecological isolation – populations live in the same area, but do not meet due to
differences in ecology (i.e. different pollinators or food sources)
3. Temporal isolation – mating or flowering occurs at different seasons or time of day
4. Behavioral isolation – little or no sexual attraction between males and females
5. Mechanical isolation – anatomical differences prevent exchange of gametes
6. Gametic isolation – female and male gametes fail to unite in fertilization
Post-zygotic Barriers
1. Hybrid inviability – hybrid zygotes fail to develop or fail to reach sexual maturity
2. Hybrid infertility – hybrids fail to produce functional gametes
3. Hybrid breakdown – 1st generation hybrids can reproduce, but 2nd cannot
What factors may prevent successful interbreeding
between populations (aka reproductive isolation)?
F1 hybrids are viable
F2 fails to germinate
Hybrid Breakdown
The hybrid produced by a lion
and a tigress (known as a
"liger") is usually larger and
stronger than either of its
parents, whereas the
reciprocal cross produces a
hybrid (the "tigon") that tends
to be smaller than either of its
parents (Gray 1972,
McCarthy, in prep.). The
former cross, then, is an
example of positive heterosis,
in which the hybrid exceeds
the range of variation
exhibited by its parents, while
the latter is an example of
negative heterosis, in which
the hybrid falls below the
range of parental variation
with respect to a given trait.
Phyletic Gradualism
 Speciation caused by small adaptations
accumulating throughout the history of a
species in response to new environmental and
biological selective pressures
 Changes are smooth, steady, and incremental
rate (on a geological timescale)
 Changes are difficult to notice over short
periods of time
 No clear line of demarcation between an
ancestral species and a descendant species
unless a splitting event occurs or the graduallychanging lineage is divided arbitrarily.
 Problem: transitional fossils?
Punctuated Equilibrium
 Changes come in spurts
 Long periods of stasis followed by very brief
periods of great change that result in abrupt
lineage-splitting (speciation)
 Changes may be due to random mutations or
sudden changes in the environment
 Explains lack of transitional fossils
Punctuated Gradualism
Long periods of gradual
change may be interrupted
by brief periods of rapid
change without leading to
lineage-branching
Punctuated Equilibrium
Phyletic Gradualism
Specialists
 May thrive only in a narrow range
of environmental conditions
 May have a limited diet
 Examples:
 Panda: feed primarily on
bamboo
 Koala: diet consists almost
exclusively of eucalyptus
leaves
 Advantages of specializing:
 More effective at competing
in its specific niche
 Tend to thrive in stable
environments
 Disadvantages of specializing:
 Populations may be limited
due to limited suitable
environments
 More likely to go extinct
when conditions change
Generalists
Often able to adapt to a wide
range of environments
Diet may be varied (omnivores
are often generalists)
Frequently adapt well to urban
environments
Examples:
Opossums
Raccoons
Advantages of being a
generalist:
May occupy much wider
ranges, allowing much
larger populations
More likely to adapt well to
changes in environment
Disadvantage of being a
generalist:
May be less able to
compete with specialists
during periods of stable
environment