Download What Darwin Never Knew

What Darwin Never Knew
How Genetics
Influences
Evolutionary Thought
Video Clip
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Natural Selection acts upon variation in a population
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Darwin got a lot right
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Darwin's idea has stood the test of time
Darwin’s observations and
deductions
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Natural selection
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Natural selection
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The conditions for evolution
There are three factors needed for a population of organisms
to be able to evolve:

Variation – There must be differences between the
individuals in a population.

Heredity – The differences between organisms must
be heritable.

Means of selection – There must be
a mechanism or pressure that
selects some variables for the next
generation at the expense of others.
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The origin of genetic variation
Genetic variation is heritable. It is this variation that natural selection acts upon. The
causes of genetic variation are:
Mutation
Sexual recombination

deletion, addition or substitution of a
nucleotide

independent assortment of
chromosomes in meiosis

deletion or translocation of part of a
chromosome

crossing-over during meiosis

aneuploidy – loss or gain of a single
chromosome

random fertilization.

polyploidy – the addition
of whole chromosome sets.
Sources of Variation
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Mutation
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Sexual Recombination
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Asexual or sexual
reproducing organisms
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Gene shuffling through
sexual reproduction
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Can be beneficial,
negative or neutral
depending on
environmental
conditions the
organism finds itself
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This provides MOST of
the heritable
differences between
generations
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In humans: 8.4 million
different combinations
of genes w/ 23
chromosomes
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Genetic causes of variation
Sexual reproduction is the most important cause of genetic variation because it mixes up
genetic material.
How does it do this?
 During meiosis, homologous chromosomes exchange
genetic material. They then line up and separate in
different ways, producing a large variety of different
gametes.
 At fertilization, any male gamete can combine with any
female gamete.
All these events occur randomly and create new combinations of genetic
material.
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Mutation and genetic variation
Mutation is the change in the type or amount of DNA and is therefore
another cause of genetic variation.
Mutations can arise spontaneously; for example,
through the incorrect copying of base pairs during DNA
replication, or the unequal distribution of chromosomes
during cell division.
Mutations can also be caused by environmental factors, such as radiation and certain chemicals.
These factors are called mutagens.
Some mutations may be beneficial, but many are harmful and increase the risk of diseases such
as cancer.
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Genetic variation
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True or false?
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What is a gene pool?
The gene pool can be defined as:
The total information from all the genes and alleles of the
breeding individuals in a population at a particular time.
The gene pool’s composition changes
from one generation to the next as the
relative proportions of alleles vary.
If there is a consistent change in
allele frequency (the proportion
of organisms in the population
carrying a particular allele) then
a population is evolving.
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Factors affecting the gene pool
Evolution in terms of Genetics
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Species = individuals that
can interbreed with viable
offspring
Share a common group of
genes = gene pool
Gene pool = all genes, all
different alleles that are
present in a population
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Relative frequency =
number of times that allele
occurs in a gene pool
Usually a percentage
NOTHING TO DO WITH
DOMINANCE
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What is a species?
It can seem obvious when two individuals are of different species, e.g. the lion and the tiger.
What about the Bengal tiger and the Siberian tiger – are these different species?
If two populations are geographically isolated, it can be
difficult to tell if they are capable of interbreeding to
produce fertile offspring. This is one aspect of the species
problem.
Similarly, the capacity to interbreed cannot be tested in
animals that are extinct, such as the dinosaurs. To
overcome this problem, other definitions of a species are
needed.
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Defining a species
The biological species concept is the most common definition of a species. It defines a species
as a set of individuals who can reproduce to produce fertile offspring. As well as the problem of
geographical separation, another disadvantage of this definition is that it only applies to
organisms that reproduce sexually.
The phylogenetic species concept defines a species
by its evolutionary lineage. Where two lines diverge
sufficiently they are called separate species. A
problem with this definition is deciding what
constitutes sufficient divergence.
Evolution in Genetic Terms
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Evolution = ANY change in relative frequency of alleles in
a population
Types of Evolution
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Microevolution
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Any change in allele
frequency over a long
period of time
Macroevolution
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Large scale
evolutionary pattern or
process change over a
longer period of time
Genetic Drift & Gene Flow
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Natural Selection is NOT the only source of change
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Alleles can become more or less common by chance
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Remember that genetics is a game of chance &
probability
Smaller populations are more affected by this (smaller
gene pool)
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Genetic drift
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Population bottlenecks
A population bottleneck occurs when a large, genetically
diverse population is drastically reduced by a catastrophic,
non-selective event, such as a volcanic eruption.
The total genetic diversity of the few survivors is likely to be
much lower than that of the original population. As the
population re-establishes itself, this low level of diversity will be
maintained.
The cheetah population has an
exceptionally low genetic
diversity. This is thought to be
due to a very narrow bottleneck,
where only a single family group
survived the last ice age.
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The founder effect
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The Hardy-Weinberg principle
Introducing the Hardy-Weinberg
principle
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The Hardy-Weinberg principle is a mathematical model
used to calculate the allele frequencies of traits with
dominant and recessive alleles.
The model assumes that the population:

is large

has random mating

is experiencing no selection

has no mutation, emigration or immigration.
If these assumptions are met then the allele frequencies
of the population will remain stable over time.
Speciation
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Natural selection CAN change the relative frequencies of
alleles
So can artificial selection, sexual selection and genetic
drift
ANY OF THESE can lead to speciation = changes in a
species' gene pool that can create a reproductive
isolation between the groups; creating a separate
species
Types of Speciation
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Behavioral Isolation
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2 populations can interbreed but have differences in
courtship or other reproductive strategies
Types of Speciation
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Geographic Isolation
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2 populations are separated by geographic barriers
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Mountains, rives, etc.
Types of Speciation
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Temporal Isolation
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2 populations reproduce at different times
Types of selection
Selection causes some traits to survive and spread, while others are lost. A selection pressure
determines which traits are successful.
There are three types of selection: disruptive, stabilizing and directional.
Selection can be represented using graphs showing the distribution of individuals with a particular
trait.
Starting population has a normal
distribution of traits.
number of
individuals
●
mode
trait
Selective Pressure
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Recall that selection
causes some traits to
survive and spread, while
others are lost
A selective pressure
determines which traits
are successful
Types of selection
mode
mode
number of
individuals
number of
individuals
mode
trait
disruptive
Selection pressure toward
the extremes creates two
modal values.
number of
individuals
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trait
stabilizin
Selection pressure
toward the
g
center increases the number of
individuals at the modal value.
Selection can cause the mode and/or distribution to change.
trait
directional
Selection pressure toward
one extreme moves the
mode in this direction.
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Selection pressures
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Inbreeding and artificial selection
In artificial selection humans decide which members of a population will breed.
This allows the alleles for desirable
characteristics to be maintained in the
population and others eliminated. This
technique is used by farmers to produce
animals and plants with a high yield.
Artificial selection can lead to inbreeding, which reduces genetic diversity. This can
increase the risk of a disease affecting the whole population.
Human Activity as Selective
Pressure
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Human activity provides
some of the strongest
selection pressure in the
world today
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Examples:
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Antibiotic Use &
Resistance
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Habitat Destruction
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Over Hunting
Patterns of Macroevolution
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Divergent Evolution
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Convergent Evolution
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Coevolution
Patterns: Divergent Evolution
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Two or more related species become more and more
dissimilar
Patterns: Convergent Evolution
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Unrelated species become more and more similar as they
become adapted to similar environments
Recall analogous structures
Patterns: Coevolution
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2 species evolve in response to changes in each other
over time
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Insects & flowering Plants
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Mammals & Flowering Plants
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Predator & Prey – CLIP
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Parasites & Hosts