Evolution in a Genetic Context
EVOLUTION
IS A PROCESS OF CHANGE
OVER TIME
Microevolution
Microevolution involves the evolutionary
changes within a population.
• A population is defined as all the members of a
single species occupying a particular area and
reproducing with one another.
• While variation within a population is important
to evolution, it is not the only factor.
Evolution in a Genetic Context
• Evolution at the population level can be
studied using population genetics.
• In population genetics, the various alleles
at all the gene loci in all the individuals
make up the gene pool of that population.
• The gene pool of a population can be
described in terms of gene frequencies.
The members of a population vary
from one another.
Variation is the raw material
for evolutionary change.
What Causes Variations?
SOURCES OF VARIATIONS
•
•
•
•
MUTATIONS
CROSSING OVER
INDEPENDENT ASSORTMENT
RANDOM FERTILIZATION
• GENETIC VARIATION CAN
BE DETERMINED
• EVOLUTION IS THE
CHANGE OF GENE
FREQUENCY WITHIN A
POPULATION
HARDY-WEINBERG LAW
• This law states an equilibrium of allele
frequencies in a gene pool remains in
effect in each succeeding generation of a
sexually reproducing population if five
conditions are met.
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s4/assets/real/evm2s4_2.rm
Evolution in a Genetic Context
(cont.)
• Hardy-Weinberg equilibrium is expressed
as a simple binomial equation.
p2 + 2 pq + q2
• The letters p and q are used to represent
the frequency of the two alleles in the
population.
Evolution in a Genetic Context
(cont.)
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s4/assets/real/evm2s4_2.rm
Evolution in a Genetic Context
(cont.)
Evolution in a Genetic Context
(cont.)
• Hardy-Weinberg equilibrium is maintained
in a population of sexual reproducing
individuals if five conditions are met.
– No net change in frequency due to mutations
– No gene flow (migration of alleles in or out of
the population)
– Random mating must occur
– No genetic drift
– No natural selection
Evolution in a Genetic Context
(cont.)
• These conditions are rarely if ever met in
the real world.
• Thus allele frequencies continually change
and microevolution occurs.
• The value of the Hardy-Weinberg principle
is that it describes the factors that cause
evolution.
Evolution in a Genetic Context
(cont.)
• In order for natural selection to act on
allele frequencies, the change must affect
the phenotype associated with the gene.
• A classic example of microevolution is
industrial melanism.
Evolution in a Genetic Context
(cont.)
Evolution in a Genetic Context
(cont.)
Causes of Microevolution
• Deviations from the conditions of HardyWeinberg equilibrium cause the allelic
changes associated with microevolution.
– Mutations
– Gene flow
– Genetic drift
– Nonrandom mating
– Natural selection
1. Genetic Mutations
• Mutations are the raw material of evolutionary change.
(alteration in the DNA nucleotide sequence of an allele)
• Mutation introduces new variation into a population.
• Gene mutations provide new alleles, and therefore are
the ultimate source of variation.
• This variation is adaptive if it helps members of a
population adjust to specific environmental conditions.
2. Gene Flow
• Gene flow, or gene migration, occurs when
breeding members of a population leave a
population or new members enter.
• Gene migration can introduce new alleles to
populations.
• However continual gene flow between
populations decreases differences in allele
frequencies, preventing speciation.
3. Genetic Drift
• Chance events that cause the allele
frequency to change is called genetic drift.
• The effect of genetic drift becomes
increasingly important as the size of the
population decreases.
Genetic Drift (cont.)
Genetic Drift (cont.)
• An example of genetic drift is the bottleneck
effect.
• A bottleneck occurs when an event or a
catastrophe drastically reduces the number of
organisms in a population.
• The variation in that population may also be
reduced, changing the allele frequencies within
the population.
Genetic Drift (cont.)
•
Bottleneck effect is caused by a severe
reduction in population size due to
natural disaster, predation, or habitat
reduction.
•
Bottleneck effect causes severe
reduction in total genetic diversity of the
original gene pool.
•
The cheetah bottleneck causes relative
infertility because of the intense
interbreeding when populations were
reduced in earlier times.
Genetic Drift (cont.)
• The founder effect is another example of
genetic drift.
• The founder effect occurs when
combinations of alleles occur at a higher
frequency in a population that has been
isolated from a larger population.
Genetic Drift (cont.)
Genetically inherited diseases like
Ellis-van Creveld are more
concentrated among the Amish
because they marry within their
own community, which prevents new
genetic variation from entering the
population.
Genetic Drift (cont.)
•
This is due to founding individuals
containing a fraction of total genetic
diversity of original population.
•
Which particular alleles are carried by
the founders is dictated by chance alone.
4. Nonrandom Mating
• When males and females reproduce together
strictly by chance it is called random mating.
• Any behavioral activity that fosters the selection
of specific mates is nonrandom mating.
– Assortive mating occurs when organisms select mates
with a similar phenotype.
– Sexual selection favors traits that increase the
likelihood of securing a mate.
5. Natural Selection
• Natural selection is the process that
adapts populations to the environment.
• Operates to select certain fit phenotypes,
which add more genes to successive
generations
5. Natural selection
• The differential reproductive
success of different genotypes.
Requires:
• Variation (members differ from one another)
• Inheritance( differences between individuals
are heritable genetic differences)
• Differential adaptedness (affect how well an
organism is adapted to its environment)
• Differential reproduction ( better adapted
individuals are more likely to reproduce)
15.2 Natural Selection (cont.)
Types of Selection
• The variation within a population creates
different phenotypes for a given trait.
• The distribution of those phenotypes
typically forms a normal distribution.
• The effect of the three types of natural
selection have different effects on this
normal distribution.
TYPES OF SELECTION
• DIRECTIONAL SELECTION
• STABILIZING SELECTION
• DISRUPTIVE SELECTION
Directional Selection
• When one extreme phenotype is favored
by natural selection, the distribution of the
phenotype shifts in that direction.
• This type of selection is therefore called
directional selection.
Directional Selection (cont.)
Examples
–A shift of dark-colored
peppered moths from lightcolored correlated with
increasing pollution.
–Increases in insecticideresistant mosquitoes and
resistance of malaria agents to
medications
Stabilizing Selection
• Stabilizing selection occurs when the
intermediate, or most common, phenotype is
favored.
• This type of selection tends to narrow the
variation in the phenotype over time.
• This is the most common type of selection
because it is associated with the adaptation of
an organism to the environment.
Stabilizing Selection (cont.)
Disruptive Selection
• In disruptive selection, natural selection
acts upon both extremes of the phenotype.
• This creates a increasing division within
the population which may ultimately lead
to two different phenotypes.
• Disruptive selection is the process that
leads to speciation.
British snails
Vary because a wide range causes natural
selection to vary
In forest areas, thrushes feed on snails with
light bands.
In low-vegetation areas, thrushes feed on
snails with dark shells that lack dark bands
Disruptive Selection (cont.)
Disruptive Selection (cont.)
DIRECTIONAL
DISRUPTIVE
STABILIZING
Maintenance of Variations
• The preservation of variation in a population is
important because it provides a foundation on
which natural selection can act.
• Variation is preserved by a variety of processes.
–
–
–
–
–
Mutations and genetic recombination
Gene flow
Natural selection
Polymorphisms (differences in form)
Diploidy and the heterozygotes
Diploidy and the Heterozygote
• Natural selection can only cause evolution if the
different alleles produce different phenotypes.
• Because many organisms are diploid,
heterozygotes are carriers of recessive alleles,
preserving them in the population.
• This also creates another phenotype that may
contribute to ratio of balanced polymorphisms.
Diploidy and the Heterozygote
(cont.)
• Surprisingly, the recessive allele occurs at
a higher than expected frequency in
regions where malaria is present, such as
in Africa.
• This occurs because the heterozygote
phenotype is favored under these
conditions and homozygotes are selected
against.
Macroevolution
• Microevolution involves changes on the
small scale at the level of gene pool
alleles.
• In contrast, macroevolution involves
evolution at the large scale as species
originate, adapt to their environment, and
possibly become extinct.
Defining Species
• Speciation is an evolutionary event that gives
rise to new species.
• The biological species concept provides one
definition of a species.
– A group of organisms that interbreed with each other
and share the same gene pool.
– A group of organisms that produce fertile offspring.
• Each species is reproductively isolated from
every other species.
• Speciation is the final result of changes in gene
pool allele and genotypic frequencies.
• For two species to separate, gene flow must not
occur between them.
Reproductive Barriers
• In order for species to be reproductively
isolated, they must be separated by
barriers which prevent gene flow.
• Reproductive barriers are also called
isolating mechanisms.
Reproductive Barriers (cont.)
Reproductive Barriers (cont.)
• Prezygotic isolating mechanisms prevent
reproduction and make fertilization unlikely.
• Habitat isolation occurs when organisms cannot
reproduce because they are in different habitats.
• Temporal isolation occurs if the reproductive
cycles of organisms occurs at different times.
Reproductive Barriers (cont.)
• The unique courtship patterns displayed
by organisms can create behavioral
isolation.
• Mechanical isolation occurs when the
genitalia are structurally incompatible.
• Genetic isolation occurs when the
fertilization does not occur, even when
sperm and egg are brought together.
Reproductive Barriers (cont.)
• Postzygotic isolating mechanisms prevent
hybrid organisms from developing (zygote
mortality) or reproducing (hybrid sterility).
• In the case of F2 fitness, even a hybrid
organism develops and reproduces, but
the offspring of the hybrid are sterile.
Reproductive Barriers (cont.)
Models of Speciation
• There are different ways in which the
process of speciation can occur.
• In allopatric speciation, an ancestral
population is geographically isolated,
resulting in the evolution of separate
species.
Allopatric speciation
Populations begin to diverge when
gene flow between them is restricted
and variations accumulate until the
populations are reproductively
isolated.
http://www.mhhe.com/biosci/esp/2001_gbio/default.htm
Allopatric speciation
occurs when new species result
from populations being separated
by a geographical barrier that
prevents their members from
reproducing with each other.
Models of Speciation (cont.)
Models of Speciation (cont.)
• Sympatric speciation involves speciation
without a geographic barrier.
• One example of sympatric speciation is
polyploidy, found more often in plants.
• Polyploidy occurs when the number of
chromosome sets increase to 3n or more.
Models of Speciation (cont.)
Adaptive Radiation
• Adaptive radiation involves the evolution of
several new species from an ancestral
species.
• Adaptive radiation occurs as natural
selection drives members of the ancestral
species to adapt to several different
environments.
Adaptive Radiation (cont.)
Natural Selection – Five Key Components:
1. There is genetic variation in populations.
2. Some of these are considered Adaptations (they act to
increase the chances of survival)
3. More offspring are produced each generation than
can possibly survive
Population limiting factors – competition, predation, disease etc
4. Those that survive reproduce and may pass their
adaptations that happened help them survive on to
offspring.
5. Over time, small changes accumulate leading to
changes in population characteristics that are noticeable.