Download Chapter 23 Evolution of Populations

Chapter 23 Evolution of
Populations
Population Genetics
• Microevolution – change in genetic make up of a
population from generation to generation
-modern view that refined Darwin’s ideas
-Darwin’s mechanism was for change in
species over time
- did not account of how inheritable
traits appeared in population or how
they were passed to offspring
- based on old blending theory rather
than Mendel’s particulate theory which
appeared years after Origin of Species was
published
Modern Synthesis
• Original thought was that Mendel’s work
contradicted Darwin’s theories
• Result of decades of practice and debate led to
the blended ideas from both
*Population genetics – study of how
populations change genetically over time
• Modern synthesis – comprehensive theory of
evolution that integrated ideas from many other
fields such as statistics, Mendelian genetics,
botany, natural selection
Mutation and Sexual
Recombination
• Mutations allow for variation in populations
• Only mutations that occur in gametes can be passed on
to offspring – small fraction
• Point mutations mainly harmless and unnoticeable
• Chromosomal mutations can delete, disrupt, and
rearrange and are considered harmful
- duplication is the main source of variation
• Mutation rate is about 1 in 100,000 genes in humans,
much higher rate in viruses and microbes
• Sexual recombination far more important in producing
variation from generation to generation
Hardy Weinberg Theorem
• Gene pool – aggregate of genes in a population at
any one time
• Hardy Weinberg Theorem states that the
frequencies of alleles and genotypes in a
population’s gene pool remain constant from
generation to generation – not evolving
• Hardy Weinberg Equilibrium – results from
random donation of gametes and random mating
yielding same allele frequencies in each
generation and predictable genotypes
Hardy-Weinberg Equilibrium
HW = p2 + 2pq + q2 = 1
p = dominant allele
Pq = heterozygous
q = recessive allele
Conditions of HW Equilibrium
• 5 criteria for non-evolving populations:
1. Extremely large population size
2. No gene flow
3. No mutations
4. Random mating
5. No natural selection
*conditions are rarely met for long in nature
*departure from these conditions leads to
evolution
Factors of Evolution
1. Natural selection – variation in heritable traits
make individuals better suited and produce more
offspring that will survive in environment
2. Genetic drift – unpredictable fluctuations in allele
frequencies from one generation to the next
a. Bottleneck effect – sudden change in
environment that drastically reduces size of
population and results in restrictive gene pool
*can be severely altered by humans
Factors of Evolution
Factors of Evolution
• Genetic drift
b. Founder effect – occurs when individuals
become isolated from a larger population and
gene pool bottlenecks enough to establish a new
population that does not reflect source
population
• Gene Flow – transfer of alleles between
populations – may gain or lose alleles, reduces
differences between populations
Adaptive Evolution
• Of all factors that influence evolution natural
selection will adapt a population to an
environment- accumulation and maintenance of
favorable genes
• Not all phenotypic variation is heritable – product
combines inherited genotype and multiple
environmental influences
• Variation can be from discrete or quantitative
characters
- discrete – single character; single gene
- quantitative – single character; 2 or
more genes
Polymorphism
• Genetic polymorphism – more than one
possibility for a character influence by several
loci
Ex: height in humans varies but
follows a continuum.
• Phenotypic polymorphisms – occurs if a
population have 2 or more distinct forms of a
character represented in high enough
frequencies to be noticeable
Genetic Variation
• Average heterozygosity- measurement of the
percent of loci for a genome being
heterozygous
-used to measure polymorphisms at
molecular and whole gene levels
Ex: fruit fly has average
heterozygosity of 14% or 1,920 of
13,700 genes are heterozygous while
rest are homozygous
Evolutionary Fitness
• Fitness – refers to the contribution an individual makes
to gene pool relative to contribution of others
Ex: barnacle producing more eggs because
of efficient food collecting; moths having
more offspring than others because body
coloring conceals more from predators
Relative fitness – involves genotype contribution
compared to alternative genotypes for the same
locus
Ex: Zero relative fitness for sterile plant even
if it outlives other members of population
Modes of Selection
• 3 modes of selection that can alter the frequency distribution
of heritable traits:
1. Directional selection – shifts the frequency curve for
a phenotypic character one direction or the other by
favoring those that deviate from the average
Ex: darker mice favored to light colored mice
because be concealed from predators
2. Disruptive selection – favors those whose characters
are at extreme ends
Ex: Large billed finches can crack hard seed
shells, small billed can feed on soft seeds, but
medium billed have trouble with both
3. Stabilizing selection – favors individual in the medium
or intermediate variants and acts against extremes
Modes of Selection
• Balanced selection – in order to preserve variation, occurs
when natural selection maintains stable frequencies of two
or more phenotypic forms in a population, or polymorphs
Heterozygote Advantage – heterzygotes at a gene
locus have greater fitness than homozygotes
Ex: Hemoglobin coding in humans
-homozygous recessive form sickle cell
disease
-homozygous dominant have normal
hemoglobin, but are susceptible to
malaria
-heterozygous are resistant to malaria and
have normal hemoglobin
Modes of Selection
• Frequency dependent selection – fitness of a
morph declines if it becomes too common
• Neutral variation – occurs when genetic variation
has little or no advantage to reproductive success
• Sexual selection has implications for mating
success
1. Intrasexual – males competing for dominance and
maintaining status with group of females mates
2. Intersexual – mate choice in which females choose
the healthier, showy male
Enigma of Sexual Reproduction
• Sexually reproduction organisms are at a
handicap for rapid population expansion
Why Natural Selection Cannot
Make Perfect Organisms
1. Evolution is limited by historical constraints
2. Adaptations are often compromises
3. Chance and natural selection interact
4. Selection can edit only existing variations