Download Microevolution and Speciation

Microevolution and Speciation
(14.4, 15.1)
Microevolution

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Evolution on the smallest scale- a
generation to generation change
Comes from a change in a population’s
gene pool

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Gene pool- consists of all the alleles in all of
the individuals that make up a population
Remember a population is a local group of
individuals of the same species

A population is the smallest level at which
evolution can occur
Gene Pools
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A gene pool is like a reservoir from
which the next generation of
individuals gets their genes
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It is the raw material for evolution
Gene pools reflect the variation
among individuals that is largely a
result of sexual recombination

Meiosis and fertilization shuffle alleles
and deal out fresh combinations to the
offspring
The many different colors of
wild mustangs are a
reflection of genetic
variation in their gene pool
Humans have a variety of
possible genetic combinations
available in the gene pool
Changes in the Gene Pool
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Processes that lead to genetic variation
are random, but natural selection is not)
The environment favors genetic
combinations that increase the chance of
survival and reproductive success of an
individual

Some alleles become more common than
others; frequency of alleles- how often certain
alleles pop up in the gene pool
Hardy-Weinberg
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In contrast to microevolution
populations that do not

undergo a change in their
gene pool are at HardyWeinberg equilibrium

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This means the frequency of
alleles in the gene pool are
constant over time
This rarely happens in
nature but is useful
because it allows a “no
change” baseline for
comparison when looking

at a population that is
changing

A controlled variable
Hardy-Weinberg fails
to apply when there
is:
 Mutation
 Gene flow
 Genetic drift
 Nonrandom mating
 Natural selection
All of which occur
naturally in
populations
Genetic Drift

A change in the gene pool of a
population due to chance

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Only the alleles of organisms that
successfully reproduce in one
generation appear in the gene pool of
the next generation
All populations are subject to drift,
the smaller the population, the
more impact genetic drift will have
The genes of the next generation will be the
genes of the “lucky” individuals, not necessarily
the healthier or “better” individuals. That, in a
nutshell, is genetic drift. It happens to ALL
populations—there’s no avoiding the erratic
nature of chance.
Genetic drift affects the genetic makeup of the
population but, unlike natural selection, through
an entirely random process. So although genetic
drift is a mechanism of evolution, it doesn’t work
to produce adaptations.
Only the alleles of organisms that
successfully reproduce in on
generation appear in the gene pool of
the next. In this population of ten plants,
the frequency of white-flower alleles was
reduced to zero due to genetic drift.
Bottleneck Effect

Reducing the size of a population also
reduces the size of its gene pool


Due to natural disaster, predation or habitat
reduction
By chance, certain alleles may occur more
frequently than others among survivors,
some alleles may be eliminated all
together- this is the bottleneck effect
Some endangered species, like the cheetah, are believed to
have undergone a bottleneck effect due to the fact they have
very little genetic variation in the remaining populations
http://www.youtube.com/watch?v=AcuQbaid0SY
Founder Effect
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Genetic drift also occurs when a few
individuals colonize a new habitat

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Isolated island, lake etc
The smaller colony has very little genetic
representation of the larger population it
came from
Known as founder effect because the
change relates to the genetic makeup of
the founders of the colony

Finches on Galapagos Islands
Gene Flow
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Genetic drift and natural selection are the
main causes of changes in gene pools,
gene flow and mutation also have a role
Gene flow- the exchange of genes with
another population

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Ex: neighboring wildflower populations
Gene flow tends to reduce the genetic
difference between populations

If it is extensive, it can eventually mix
neighboring populations into one large
population with a single gene pool
Close populations can have gene flow between them.
Eventually may lead to a single population
Mutation

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A change in an
organism’s DNA
Natural selection
and/or genetic drift
can influence the
frequency of a new
mutation

Ex- albino deer- light
colored fur would
prove hazardous but
gene gets passed
down in
heterozygous
individuals

Mutations play a key
role in evolution as
raw material for
natural selection

Important in
asexually producing
organisms such as
bacteria
Albino deer
and normal
deer
Why would this color fur be
hazardous for the deer?
Natural Selection and Fitness
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Genetic drift, gene flow and
mutation can all lead to
microevolution but do not
necessarily lead to adaptation
Only natural selection usually leads
to adaptation

Chance comes from mutations and
sexual recombination of alleles which
produce random genetic variation in a
population
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Survival of the fittest
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Fitness- the contribution that an
individual makes to the gene pool of the
next generation
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Misconception that there are direct contests
between individuals
Who can reproduce
Production of healthy, fertile offspring is
all that counts in natural selection
Check:
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What is a gene pool?
What are the two main forces of
evolutionary change in gene pools?
Speciation
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How do biologists identify species?

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Biological species concept- a population
or group of populations whose member
can breed and produce viable (fertile)
offspring
This concept can not be applied to
all life


Asexual reproducers, fossils
But is still useful in classifying and
identifying new organisms
Micro to Macroevolution
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Microevolution and adaptation
explain how populations evolve
Macroevolution looks at the major
biological changes in the fossil
record

These changes occur with the origin of
new species, extinction and evolution
of major features of living things such
as wings or flowers

Speciation- the origin of new
species

Leads to biological diversity
Speciation can lead to
an increase in the
number of species

Barriers to Speciation
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Reproductive isolation A condition in which a
reproductive barrier
keeps species from
breeding
Some barriers that
contribute to this are
timing, behavior and
habitat
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Timing- 2 closely related
species have different
breeding seasons
Behavior- 2 similar
species have different
courtship or mating
behaviors
Habitat- some species
are adapted to different
habitats
Other barriers include
infertility, reproductive
structures are
incompatible
The eastern and
western spotted
skunk are very
closely related
but are kept in
reproductive
isolation due to
different
breeding
seasons
The eastern and western
meadowlark remain
separate because their
courtship rituals differ
Geographic Isolation
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Separation of populations as a
result of geographic change or
dispersal to geographically isolated
locations
Can occur when mountain ranges
form, glaciers move, small groups
of population colonizing an island
These two species
of antelope
squirrels live on
opposite sides of
the Grand
Canyon- this
geographic barrier
keeps these
populations
separated
For each small, isolated population that becomes a
new species, many more fail to survive in their new
environment
Adaptive Radiation
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Evolution from a common ancestor
that results in diverse species
adapted to different environments
Studied on islands which are used
as living laboratories for speciation
Separated populations have
different gene pools and frequency
of alleles.
Darwin’s Finches
adapted different
beaks on different
parts of the
Galapagos Islands
Species A comes from the
mainland and undergo
genetic changes to become
Species B. Species B can
migrate to a neighboring
island and become Species
C – could probably coexist
with B but geographic
isolation keeps that from
occurring
Punctuated Equilibrium
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A model that suggests species often
diverge (separate) in spurts of
relatively rapid change

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Seen in the fossil record
In just a few hundred to a few
thousand generations, genetic drift
and natural selection can cause
significant changes in a population