Download 16-1 Genes and Variation

16-1 Genes and Variation
16-1 Genes and Variation
Slide
1 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Variation and Gene Pools
Variation and Gene Pools
Genetic variation is studied in populations or group
of individuals of the same species that interbreed. .
A gene pool consists of all genes, including all the
different alleles, that are present in a population.
The relative frequency of an allele is the number
of times the allele occurs in a gene pool, compared
with the number of times other alleles for the same
gene occur (usually expressed as a percent).
Slide
2 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Variation and Gene Pools
How is evolution defined in genetic
terms?
In genetic terms, evolution is any
change in the relative frequency of
alleles in a population.
Slide
3 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Sources of Genetic Variation
Sources of Genetic Variation
What are the main sources of heritable
variation in a population?
The two main sources of genetic
variation are mutations and the genetic
shuffling that results from sexual
reproduction.
Slide
4 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Sources of Genetic Variation
Mutations
A mutation is any change in a sequence of DNA.
Mutations occur because of mistakes in DNA
replication or as a result of radiation or chemicals
in the environment.
Mutations do not always affect the way an
organism looks, or its phenotype.
Slide
5 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Sources of Genetic Variation
Gene Shuffling
Most heritable differences are due to gene
shuffling.
Crossing-over, which occurs during meiosis,
increases the number of genotypes that can
appear in offspring.
Sexual reproduction produces different
phenotypes, but it does not change the amount
of alleles in a population.
Slide
6 of 24
Copyright Pearson Prentice Hall
16-1 Genes
Variation
16-2 Evolution
asand
Genetic
Change
16-2 Evolution as Genetic Change
Slide
7 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
16-2
16-2
Evolution
Evolution
as Genetic
as Genetic
Change
Change
Natural selection affects which individuals
survive and reproduce and which do not.
If an individual dies without reproducing, it
does not contribute its alleles to the
population’s gene pool.
If an individual produces many offspring, its
alleles stay in the gene pool and may increase
in frequency.
Slide
8 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
16-216-2
Evolution
Evolution
as Genetic
as Genetic
Change
Change
Evolution is any change over time in the
relative frequencies of alleles in a
population.
Populations, not individual organisms, can
evolve over time.
Slide
9 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Single-Gene Traits
Natural Selection on Single-Gene Traits
How does natural selection affect single-gene
traits?
Natural selection on single-gene traits can lead
to changes in allele frequencies and thus to
evolution.
Slide
10 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Single-Gene Traits
Organisms of one color may produce fewer
offspring than organisms of other colors.
For example, a lizard population is normally
brown, but has mutations that produce red
and black forms.
Red lizards are more visible to predators, so
they will be less likely to survive and
reproduce. Therefore, the allele for red color
will become rare.
Slide
11 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Single-Gene Traits
Black lizards may warm up faster on cold days. This
may give them energy to avoid predators. In turn,
they may produce more offspring.
The allele for black color will increase in relative
frequency.
Slide
12 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Natural Selection on Polygenic Traits
How does natural selection affect polygenic
traits?
Natural selection can affect the distributions of
phenotypes in any of three ways:
directional selection
stabilizing selection
disruptive selection
Slide
13 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Directional Selection
When individuals at one end of the population
curve have higher fitness than individuals in the
middle or at the other end, directional selection
takes place.
The range of physical characteristics, phenotypes,
shifts as some individuals survive and reproduce
while others do not.
Slide
14 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
In this case, birds with larger beaks have
higher fitness. Therefore, the average beak
size increases.
Slide
15 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Stabilizing Selection
When individuals near the center of the curve
have higher fitness than individuals at either end
of the curve, stabilizing selection takes place.
This keeps the center of the curve at its current
position, but it narrows the overall graph.
Slide
16 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Human babies born at an average mass
are more likely to survive than babies born
either much smaller or much larger than
average.
Slide
17 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Disruptive Selection
When individuals at the upper and lower ends of
the curve have higher fitness than individuals near
the middle, disruptive selection takes place.
If the pressure of natural selection is strong
enough and long enough, the curve will split,
creating two distinct phenotypes.
Slide
18 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Natural Selection on
Polygenic Traits
If average-sized seeds become scarce, a
bird population will split into two groups:
one that eats small seeds and one that eats
large seeds.
Slide
19 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Genetic Drift
Genetic Drift
What is genetic drift?
A random change in allele frequency is called
genetic drift.
In small populations, individuals that carry a
particular allele, or characteristic, may leave more
descendants than other individuals do, just by
chance.
Over time, a series of chance occurrences of this
type can cause an allele to become common in a
population.
Copyright Pearson Prentice Hall
Slide
20 of 24
16-1 Genes and Variation
Genetic Drift
Genetic drift may occur when a small group of
individuals colonizes a new habitat.
Those individuals may carry more of a certain
characteristic than the larger population had.
The new population will be genetically
different from the parent population.
When allele frequencies change due to
migration of a small subgroup of a population
it is known as the founder effect.
Slide
21 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Genetic Drift
Genetic Drift
Slide
22 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Genetic Drift
Slide
23 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Genetic Drift
Slide
24 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Genetic Drift
Descendants
Population A
Population B
Slide
25 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Evolution Versus Genetic Equilibrium
The Hardy-Weinberg principle states that allele
frequencies in a population will remain constant
unless one or more factors cause those
frequencies to change.
When allele frequencies remain constant it is
called genetic equilibrium.
Slide
26 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Five conditions are required to maintain
genetic equilibrium from generation to
generation:
there must be random mating,
the population must be very large,
there can be no movement into or out of the
population,
there can be no mutations, and
there can be no natural selection.
Copyright Pearson Prentice Hall
Slide
27 of 24
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Random Mating
Random mating ensures that each individual has
an equal chance of passing on its alleles to
offspring.
Does this happen in nature?
Are mates selected due to some desirable
characteristic?
Slide
28 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Large Population
Small populations will not reach genetic equilibrium
easily due to genetic drift having a greater impact
on a small population.
Allele frequencies of large populations are less
likely to be changed through the process of genetic
drift.
Slide
29 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
No Movement Into or Out of the Population
Because individuals may bring new alleles into a
population, there must be no movement of
individuals into or out of a population.
The population's gene pool must be kept together
and kept separate from the gene pools of other
populations.
Slide
30 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
No Mutations
If genes mutate, new alleles may be introduced
into the population, and allele frequencies will
change.
Slide
31 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
No Natural Selection
All genotypes in the population must have equal
probabilities of survival and reproduction.
No phenotype can have a selective advantage
over another.
There can be no natural selection operating on the
population.
Slide
32 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
16-3 The Process of Speciation
16-3 The Process of Speciation
Slide
33 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
16-3 The Process of Speciation
Natural selection and chance events can
change the relative frequencies of alleles in a
population and lead to speciation.
Speciation is the formation of new species.
A species is a group of organisms that breed
with one another and produce fertile (viable)
offspring.
Slide
34 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Isolating Mechanisms
What factors are involved in the formation of
new species?
The gene pools of two populations must become
separated for them to become new species.
Slide
35 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Isolating Mechanisms
Isolating Mechanisms
As new species evolve, populations become
reproductively isolated from each other.
When the members of two populations cannot
interbreed and produce fertile offspring,
reproductive isolation has occurred.
Slide
36 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Isolating Mechanisms
Reproductive isolation can develop in a
variety of ways, including:
behavioral isolation
geographic isolation
temporal isolation
Slide
37 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Isolating Mechanisms
Behavioral Isolation
Behavioral isolation occurs when two populations
are capable of breeding with one another but have
different ways of finding mates or other
reproductive strategies that involve behavior.
Slide
38 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Isolating Mechanisms
Geographic Isolation
Geographic isolation occurs when two
populations are separated by geographic barriers
such as rivers or mountains.
Geographic barriers do not guarantee the creation
of a new species.
Temporal Isolation
Temporal isolation occurs when two or more
species reproduce at different times.
Slide
39 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
17-4 Patterns of Evolution
Slide
40 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Extinction
Extinction
More than 99% of all species that have ever lived
are now extinct.
In the past, most researchers looked for a single,
major cause for each mass extinction.
Many paleontologists now think that mass
extinctions were caused by several factors.
Slide
41 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Extinction
What effects have mass extinctions had on the
history of life? Mass extinctions have:
provided ecological opportunities for organisms
that survived
resulted in bursts of evolution that produced many
new species
Slide
42 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Adaptive Radiation
Adaptive Radiation
Adaptive radiation is the process by which a
single species or a small group of species evolves
into several different forms that live in different
ways.
For example, in the adaptive radiation of Darwin's
finches, more than a dozen species evolved from a
single species.
Adaptive radiations can occur on a much larger scale.
The disappearance of dinosaurs then resulted in the
adaptive radiation of mammals.
Copyright Pearson Prentice Hall
Slide
43 of 24
16-1 Genes and Variation
Adaptive Radiation
Adaptive Radiation of Mammals
Artiodactyls Cetaceans
Perissodactyls
Tubulidentates
Hyracoids SireniansProboscideans
Ancestral Mammals
Slide
44 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Convergent Evolution
Convergent Evolution
Different organisms undergo adaptive radiation in
different places or at different times but in similar
environments.
The process by which unrelated organisms come
to resemble one another is called convergent
evolution.
Convergent evolution has resulted in sharks,
dolphins, seals, and penguins.
Slide
45 of 24
Copyright Pearson Prentice Hall
16-1 Genes and Variation
Coevolution
Coevolution
Sometimes organisms that are closely connected
to one another by ecological interactions evolve
together.
The process by which two species evolve in
response to changes in each other over time is
called coevolution.
Slide
46 of 24
Copyright Pearson Prentice Hall