Transcript Ch. 16: Evolution of Populations

2/1/ 2011 Warm up
 Summary of Darwin’s Theory
1._______________________differ.
2. competition for _______________
3. best _________to environment survive.
4. Species alive today are descended with
_________________
Ch. 16: Outline
 16-1: Genes and Variation
 How common is Genetic Variation?
 Variation and Gene Pools
 Sources of Genetic Variation
 Single-Gene and Polygenic Traits
16-1: Genes and Variation
 When Darwin developed his theory of
evolution, he stated that organisms pass on
their traits to their offspring (inherited
variation).
 Darwin did all of his work on
natural selection and
evolution, but he did not
even know how heredity
worked! Scientists had not
discovered genes!!!!
 Biologists connected the work of Gregor Mendel to
Darwin in the 1930’s. Our inherited traits are
determined by our genes. Genes pass on our
inherited traits to the next generation.
 It is the differences in the genetic makeup of
individuals within the same species that leads to
natural selection.
 These differences are called genetic (inherited)
variation.
Variation and Gene Pools
 Scientists study genetic variations by studying populations
of species.
 Population: A group of species in a specific area
 Gene pool: All genes (including different alleles) that are
in population.
 Relative frequency: The number of times that an allele
for a specific gene occurs in a gene pool, compared to
the number of times other alleles for the same gene
occur.
 In genetic terms, evolution is any change in the relative
frequency of alleles in a population.
What is genetic (inherited) variation?
 Differences that exist in the genetic
makeup of individuals within the same
species
Sources of Genetic Variation
 How does genetic variation occur? The two
main sources of genetic variation are
mutation and gene shuffling.
 1) Mutation: any change in a sequence of
DNA
 Mutations are caused by a mistake in replication,
radiation, or chemicals in the environment
 Some mutations are harmless, but others can
affect an organism’s fitness:
 Fitness: The ability to survive and reproduce
Sources of Genetic variation
 2) Gene shuffling:
 Genes on chromosomes are
shuffled during MEIOSIS when
gametes (sex cells) are formed!
 Review of Meiosis
 Segregation
 Independent Assortment
 Crossing-over
 Thus, sexual reproduction is a
major source of genetic variation
 Which is a greater source of genetic
(inherited) variation – mutations or
sexual reproduction? Justify your
answer.
 Sexual Reproduction
Single-Gene and Polygenic Traits
 Some inherited traits are controlled by one
(single) gene or many genes (polygenic)
 The number of phenotypes produced for a
given trait depends on how many genes
control the trait.
Single-Gene and Polygenic Traits
 Single-gene traits are
any trait that is
controlled by one
gene. This trait will
have only two alleles
 Widow’s Peak
(dominant) is a singlegene trait
 Only two phenotypes
are possible
 If Widow’s Peak is represented by the
letter W and a normal hair line
represented by w, what are the possible
hairline genotypes in the population ?
 ww (normal hair line)
 Ww (Widow’s Peak, heterozygous)
 WW (Widow’s Peak, homozygous)
Single-Gene and Polygenic Traits
 Polygenic Traits: are
traits that are controlled
by two or more genes.
 These traits have more
than two alleles
 This means that there are
many possible
phenotypes and
genotypes.
 Skin color and height are
examples of polygenic
traits
Ch. 16: Outline
 16-2: Evolution of Genetic Change
 Genetic Drift
 16-3: The Process of Speciation
 Speciation
16-2 Evolution as Genetic Change
 Evolution causes changes over time in the
relative frequencies of alleles (inherited
traits) in a population. Thus, populations
evolve over time.
Genetic Drift
 In small populations, chance alone can
cause a change in the allele frequency in a
population. This is called genetic drift.
 Example of Genetic Drift:
 Founder effect: When allele frequencies
change because of the migration of a small
number of individuals out of a population.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
16-3 The Process of Speciation
 How do the changes in allele frequencies
(evolution) lead to a new species?
 Speciation: the formation of a new species.
 Speciation occurs when two populations
become reproductively isolated from
each other
Isolating Mechanisms
 There are three ways for reproductive
isolation to occur:
 Behavioral Isolation
 Geographic Isolation
 Temporal Isolation
Behavioral isolation:
Two populations have
differences in courtship
rituals or other
reproductive strategies
that involve behavior.
Species live in the
same geographical
area.
Isolating Mechanisms
Geographic isolation: Two populations are
separated by geographic barriers such as
rivers and mountains. (Squirrels)
A. harrisi
A. leucurus
 Geographical
isolation
caused the
speciation of
the finches in
the Galapagos
Islands
Isolating Mechanisms
Temporal isolation: Reproduction occurs at
different times of the year. Species live in the
same geographical area.
The American Toad (Bufo americanus) The Bufo Fowelri mates in late summer
mates in early summer
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
Chapter 17
 17-4: Patterns of Evolution
Extinction
Gradualisms vs. Puntuated
Equilibrium
 The fossil record provides evidence
about the history of the Earth.
 The fossil record indicates that species
have change over time (evolution).
 The fossil record shows that some
species are no longer alive
 Certain fossils can only be found in older
rocks, for example (and are not found in any
recent rocks). When a species is no longer
living, it is extinct.
 Extinct: a species that has died out.
 Reasons for extinction of species
include
 Changes in climate
 Disruption of ecosystems
 Inability to compete for resources
Gradualisms vs. Punctuated Equilibrium
 Modern scientists disagree with Darwin
about the speed of evolution.
 Darwin believed that evolution is a
slow, gradual process. This process is
termed gradualism.
Gradualisms vs. Punctuated Equilibrium
 The fossil record indicates that this is true for some
species.
 But other species are produced by short periods of
rapid change (also supported in the fossil record).
 Puntuated equilibrium: is the pattern of long,
stable periods of change that is interrupted by
brief periods of more rapid change
Time
Gradualism model. Species
descended from a common
ancestor gradually diverge
more and more in their
morphology as they acquire
unique adaptations.
Punctuated equilibrium
model. A new species
changes most as it buds
from a parent species and
then changes little for the
rest of its existence.
Gradualisms vs.
Punctuated Equilibrium
 What causes punctuated equilibrium?
 Migration
 Isolation
 Mass Extinctions