Download (Chap. 20 Genes within Populations (Evolution) 2014)

Genes Within
Populations
Chapter 20
1
• “Through time, species accumulate
differences; as a result, descendants differ
from their ancestors. In this way, new
species arise from existing ones.”
– Charles Darwin
2
Evolutionary Theories
• Jean-Baptiste Lamarck
– “inheritance of acquired characteristics”
Evolutionary Theories
• Charles Darwin
– “Descent with
modification”
Evolution Review
• Progress of
evolution:
Gene 
Mutation 
Trait 
Natural selection 
Microevolution.
5
Genetic Variation and Evolution
• Genetic variation:
– “Differences in alleles of genes found within
individuals in a population”
– Raw material for natural selection if it meets three
criteria.
– Evolution results in a change in the genetic
composition of a population.
6
• Natural selection and evolution are not the
same
• Evolution driven by natural selection creates
populations that are better adapted to their
environment
7
Fitness and its measurement
• Fitness
• Fitness has many components
– Survival
– Sexual selection – some individuals more
successful at attracting mates
– Number of offspring per mating
8
4 agents of evolutionary change
9
10
Selection vs Other Processes
• Natural selection can be altered by
other processes:
–Mutation
–Gene flow
–Genetic drift
11
Founder effect
Bottleneck effect
12
Case Studies
Cheetah
Bearded
Vulture
13
Selection
• Many traits affected by more than one gene
• There are 3 types of multiple gene selection
– Disruptive - “it pays to be very different”
– Directional – “there can be only one”
– Stabilizing - “it pays to be average”
14
Diversifying (Disruptive) Natural
Selection
African Black-billed
seedcracker finches
15
Directional Natural Selection
16
Stabilizing Natural Selection
17
Limits of selection
• Selection is the most powerful of all the
forces behind genetic change. But there are
limits to what it can accomplish.
• Lack of genetic variation
• Pleiotrophy
- Multiple phenotypic effects of alleles
• Epistasis
18
Gene Pools and Allele Frequencies
• Gene pool – “all the alleles for all the DNA loci
(locations) for all the individuals in a
population”.
• Each allele has its own frequency in a population.
That frequency can be calculated.
• If you know the frequency of an allele you can
determine how often the trait it controls appears
in a population and its ‘place’ in natural
selection.
19
Hardy–Weinberg principle
• Hardy–Weinberg equilibrium
– “Genotypes frequencies do not change in a
population if:
1. No mutation takes place
2. No genes are transferred to or from other
sources
3. Mates are chosen randomly
4. The population size is very large
5. No natural selection occurs”.
20
Hardy – Weinberg Calculations
• For 2 alleles (p and q)
p2 + 2pq + q2 = 1
p2 = homozgous dominant (BB)
pq = heterozygous dominant (Bb)
q2 = homozygous recessive (bb)
21
Hardy – Weinberg Calculations
22
Allelic Frequency vs. Genotypic Frequency
1. No spots is a recessive trait in toads. The frequency
of the recessive allele is 10%. What is q for this
population? (Allelic frequency)
2. A population shows that 16% have the recessive trait
of no spots. What is q for this population?
(Genotypic frequency)
23
Practice
In a certain population of 1000 fruit flies, 640 have
red eyes while the remainder have sepia eyes. The
sepia eye trait is recessive to red eyes.
How many individuals would you expect to be
homozygous for red eye color?
24
Extra help
• Go to YouTube
• Type in Bozeman Biology Hardy Weinberg
25