Transcript Reception for Darwin`s Theory During His Time

Remainder of Chapter 23
• Read the remaining materials; they
address information specific to
understanding evolution
• Always read the Featured Investigation
and Genomes and Proteomes sections of
each chapter
Gene - specific location of the genetic
information for a given trait
Allele - The actual chemical composition of
a gene. Determines how characteristic/
trait is expressed.
Polymorphism – two or more forms present
Allele Frequency - The frequency of
occurrence of alleles in a population.
Genotypic Frequency - frequency of
occurrence of genotypes in a population.
Population – group of individuals of the
same species that live in the same area
(can interbreed if reproduce sexually).
Gene Pool – All of the genes (more
accurately all of the alleles) present in a
population.
• Genotype - specific chemical composition
of alleles defining a trait.
– AA
– Aa
– aa
Homozygous Dominant
Heterozygous
Homozygous Recessive
• Phenotype - physical expression of a trait
– If the alleles for a trait are simple dominant
and recessive, then:
• For AA and Aa, dominant trait is
physically expressed
• If aa, recessive trait is expressed
Evolution
• Is a genetic change in a population (not an
individual) over time
• Scientists look at phenotypic (physical
changes), in most cases, because that is
how we recognize populations.
• It is, however, changes in the genotype, or
more specifically, the gene pool.
Allele Frequencies
The frequency of occurrence of alleles in a
population.
If we use the simple one dominant and one
recessive allele model, this can be demonstrated
by:
p = frequency of the dominant allele
q = frequency of the recessive allele
Example
AA - 30 individuals
Aa - 20 individuals
aa - 50 individuals
p = 2(# individuals AA) + # individuals Aa
2(Total # individuals in population)
p + q = 1; therefore q = 1 - p
Example
p = 2(30) + 20
2(100)
= 0.4
p + q = 1; therefore q = 1 - 0.4 = 0.6
With these values, we can calculate the
probability of what genotypes would be present
in the next generation if this population were to
mate randomly
Genotypic Frequencies
p2 = probability of AA
q2 = probability of aa
2pq = probability of Aa
p2 + 2pq + q2 = 1
Mechanisms for Evolutionary
Change
 Mutation
 Genetic Drift (small population size)
 Gene Flow (immigration and emigration)
Non-Random Mating
Natural Selection
Hardy-Weinberg Equilibrium
In diploid, sexually reproducing
organisms, phenotypes, genotypes and
genes all tend to come to equilibrium in
populations in certain conditions are met
Hardy-Weinberg Equilibrium
 No Mutation
 Large Population Size
 No immigration or emigration
 Random Mating
 No Selection for Traits
Hardy-Weinberg Equilibrium
Provides a means of experimentally
demonstrating what happens to
populations in the absence of
evolution.
How Natural Selection Works
• Variation occurs in every group of living
organisms. Individuals are not identical in any
population.
• Every population produces an excess of
offspring.
• Competition will occur among these offspring
for the resources they need to live.
How Natural Selection Works
• The most fit offspring will survive.
• If the characteristics of the most fit organisms
are inherited, these traits will be passed on to
the next generation.
Darwinian Fitness
Relative contribution an individual
makes to the gene pool of the next
generation
Natural Selection
• The most fit genotypes will be more
strongly represented in subsequent
generations
• Less fit genotypes will remain in the
population, but at low numbers
• If environmental conditions change,
fitness will change
figure 21-12.jpg
Figure 21.12
Figure 21.12
Stabilizing Selection
Directional
Selection
Figure
21.13
Figure 21.13
Disruptive
Selection
Fig. 24.5a – Disruptive Selection
Maintenance of Variation
• Less fit alleles not completely eliminated
• Still reproduce, but do not produce as
many offspring
• Also interbreed with more fit individuals
• Balancing Selection
– Heterozygous advantage
– Negative frequency-dependent selection
Fig. 24.6
Properties of Fitness
• Fitness is a property of a genotype, not an
individual or population.
• Fitness is specific to a particular environment.
As the environment changes, so does the fitness
of genotypes.
• Fitness is measured over one generation or
more.
Sexual Selection
• Traits that infer
greater fitness
• Sexual dimorphism
– Secondary sex
characteristics
• Intrasexual
• Intersexual
• Featured
Investigation
22.16 The Longer the Tail, the Better the Male (Part 1)
22.16 The Longer the Tail, the Better the Male (Part 2)
Genetic Drift
• Random events – missed opportunity,
disturbance
Fig. 24.10
Gene Flow
• Changes in gene pool resulting from
immigration or emigration
• random
• Founder effect
Founder Effect – colonizers
establish genetic make up of
new population
Mutation
• Changes in chemical composition of a
gene
• Random
• Only way new alleles can be added
• Most mutations are deleterious
• Neutral mutations add variation without
changing phenotype
Nonrandom Mating
• Mating due to some
attribute
– Sexual selection
– Similar phenotype