Download Chapter 15 - Clayton State University

Chapter 15
Evolution in the Fast Lane
Read about Manatee’s
• Group of mammals called sirenians
• Closest living relative- dugong
• Four living species of sirenians
– Fifth species, Steller’s sea cow is now extinct due
to hunting in 1770’s
• Manatee were in danger but not protected by
federal and state government
• Listed as endangered species
Natural Selection
• Define
• What are the effects on small population?
• What is you have very limited diversity?
The effect of small, isolated
populations on genetic diversity
• Small, isolated populations tend to have less genetic diversity
than larger populations made up of many interbreeding
individuals.
The effect of small, isolated
populations on genetic diversity
• Limited genetic diversity can seriously impair a population’s
evolutionary success in the face of a changing environment.
Populations and Genetic Diversity
• From a genetic perspective, each population of an organism
has its own particular collection of alleles. The total collection
of alleles in a population is the population’s gene pool.
What is the total gene pool in the
class?
• How many students?
• Each student has how many chromosomes?
• What is our gene pool?
Populations and Genetic Diversity
• Within the gene pool, each allele is present in
a certain proportion, or allele frequency,
relative to the total number of alleles.
• Example
– Homozygous dominant allele for brown hair (BB)
– Any brown heads in the class???
– How many total alleles for brown hair?
– What is the allelic frequency in the class?
– What is the genotypic frequency in the class?
Populations and Genetic Diversity
• When alleles change frequency over time, a
population evolves.
• Evolutionary changes in a gene pool can have
good, bad, or neutral lasting consequences in
a population.
• The gene-pool-altering force that results in
adaptation is natural selection.
Populations and Genetic Diversity
• Natural selection is not the only force that
alters allele frequencies: i.e. mutations, etc.
– Random and does not along cause adapation.
• A change in allele frequency that does not by
itself lead a population to become more
adapted to its environment is nonadaptive
evolution.
• Nonadaptive evolution is caused by mutation,
genetic drift, and gene flow.
– All contribute to evolution in some way but alone
do not lead to a population becoming adapted
Genetic drift
• Genetic drift is a change in allele frequencies
between generations that occurs purely by
chance.
– By simple chance, some individuals just happen to
survive and reproduce, whereas others do not.
– Those that pass on their genes were not necessary
more fit or better adapted, just lucky
Genetic drift
• Because only a subset of the population (with
a subset of the total alleles) reproduces, only a
subset of alleles is represented in the next
generation.
– Over time, genetic drive decreases the genetic
diversity of a population.
Genetic drift
• Genetic drift tends to have more dramatic
effects in smaller populations than in larger
ones, because in a population with few
individuals, any single individual that does not
reproduce could spell the loss of alleles from
the population.
Genetic drift: The founder effect
• The founder effect is a type of genetic drift in
which a small number of individuals leave one
population and establish a new population; by
chance, the newly established population may
have lower genetic diversity than the original
population.
Genetic drift: The founder effect
• When the founders begin to reproduce and populate a new
area, the genetic diversity of the new population reflects
the reduced diversity of the founders.
Genetic drift: The bottleneck effect
• The bottleneck effect is a type of genetic drift that
occurs when a population is suddenly reduced to a
small number of individuals and alleles are lost from
the population as a result.
Genetic drift: The bottleneck effect
• When a population is cut down sharply, and
often suddenly, there’s a good chance that the
remaining population will possess a greatly
impoverished gene pool.- become extinct
Genetic drift: The bottleneck effect
• Bottlenecks
can also occur
from natural
causes like an
extremely
cold winter
that causes
half the
population to
die.
Genetic drift: The bottleneck effect
• Populations forced through a genetic
bottleneck contain only a small fraction of the
original starting diversity in the population.
Why is genetic diversity important?
• A diverse gene pool gives a population more
flexibility to survive in a changing environment
• The more genetically diverse a population is,
the more ways it has to adapt.
How do biologists measure the genetic
health of populations?
• Population biologists compare the gene pool
at a given time to a population that is known
not to be evolving.
– If the two populations differ, the population is
evolving, and you can begin to investigate why.
How do biologists measure the genetic
health of populations?
• Allele frequencies in a non-evolving
population behave in a predictable way: they
do not change over time.
• In a non-evolving population, genotype
frequencies remain unchanged from one
generation to the next, a condition known as
Hardy-Weinberg equilibrium.
Hardy-Weinberg equilibrium
• The Hardy-Weinberg equilibrium provides a
baseline from which to judge if a population is
evolving or not.
– For a gene with one dominant and one recessive
allele, p and q, this formula can be written as:
p2 + 2 pq + q2 = 1
o
o
o
p2 is the frequency of homozygous dominants
2 pq is the frequency of heterozygotes
q2 is the frequency of homozygous recessives
Hardy-Weinberg equilibrium
• By definition, a population is not evolving (and
is therefore in Hardy-Weinberg equilibrium)
when it has stable allele frequencies and,
therefore, stable genotype frequencies from
generation to generation.
Hardy-Weinberg equilibrium
Five conditions must be met for a population to
be in Hardy-Weinberg equilibrium:
1. No mutation introducing new alleles into the population
2. No natural selection favoring some alleles over others
3. An infinitely large population size (and therefore no
genetic drift)
4. No influx of alleles from neighboring populations (i.e., no
gene flow)
5. Random mating of individuals
Hardy-Weinberg equilibrium
• In nature, no population can ever be in strict
Hardy-Weinberg equilibrium because it will
never meet all five conditions.
• In other words, all natural populations are
evolving.
– By describing the pattern of genotypes in a nonevolving population, Hardy-Weinberg equilibrium
provides a baseline from which to measure
evolution.
Hardy-Weinberg problems
• Handout in class
Inbreeding
• Inbreeding is mating between closely related
individuals. Inbreeding does not change the
allele frequency within a population, but it
does increase the proportion of homozygous
individuals to heterozygotes.
Inbreeding
• Because closely related individuals are more
likely to share the same alleles, the chance of
two recessive alleles coming together during a
mating is high.
• When that happens, homozygous recessive
genotypes are created, and previously hidden
recessive alleles start to affect phenotypes.
Inbreeding
• The negative reproductive consequences for a
population associated with having a high
frequency of homozygous individuals
possessing harmful recessive alleles is an
inbreeding depression.
Gene flow between populations
increases genetic diversity
• Isolated
populations have
limited genetic
diversity because
they only share
alleles with
themselves, and
the likelihood of
inbreeding goes
up.
Gene flow between populations
increases genetic diversity
• Gene flow is the movement of alleles from
one population to another, which may
increase the genetic diversity of a population.
Gene flow between populations
increases genetic diversity
• Like genetic drift,
gene flow is a type
of evolution that
does not lead to
adaptation. Unlike
genetic drift, gene
flow increases the
genetic diversity of
a local population
by introducing
alleles from its
neighbors.
What is a species?
• The term species comes from the Latin word
for “kind” or “appearance.” Evolutionary
biologists define a species using the biological
species concept, which defines a species as a
population of individuals whose members can
interbreed and produce fertile offspring.
What is a species?
• Members of different species cannot mate
with each other because their populations are
reproductively isolated. Reproduction
isolation occurs as a result of mechanisms
that prevent mating (and therefore gene flow)
between members of different species.
What is a species?
• Reproductive isolation can be caused by a
number of factors:
– Ecological isolation
– Temporal isolation
– Behavioral isolation
– Mechanical isolation
– Gametic isolation
– Hybrid inviability
– Hybrid infertility
What is a species?
• Ecological isolation
occurs when species live
in different
environments and
therefore never
encounter one another.
What is a species?
• Temporal isolation
occurs when species
display mating
behavior or fertility at
different times.
What is a species?
• Behavioral
isolation occurs
when species have
different mating
activities or
behaviors.
What is a species?
• Mechanical
isolation occurs
when mating
organs are
incompatible.
What is a species?
• Gametic isolation
occurs when
gametes cannot
unite.
What is a species?
• Hybrid
inviability
occurs when
gametes unite,
but viable
offspring
cannot form.
What is a species?
• Hybrid infertility occurs when viable hybrid
offspring cannot reproduce.
Speciation
• Speciation is the
genetic divergence
of populations
owing to a barrier
to gene flow
between them,
leading over time
to reproductive
isolation and the
formation of new
species.
Speciation
• To study
ancestry,
researchers
often rely on
sequences of
mitochondrial
DNA.
Speciation
• Speciation that
occurs because
of geographic or
ecological
separation is
known as
allopatry.