How are Variants Created

1
How are Variants Created

• Gene unknown to Darwin
• Work of Mendel not known until 1900
• Inheritance believed to blend
• Actual inheritance is particulate

2
Blending Inheritance

• Would lead to dilution of unique traits
• Would lead to uniformity of individuals

3
Theory of Acquired Characteristics (Lamarck)

• Changes in environment create changes in needs of
  organisms
• Changes in needs occasion changes in behavior to
  satisfy the needs
• Changes in behavior result in increased use and
  development of certain parts
• Changes are passed on to offspring

4
Weismann discovers chromosomes

• Discovered chromosomes in nucleus
• Realized genetic material found in nucleus

5
Human chromosomes
6
Rejection of Acquired Characteristics

• Genes not influenced by adult behavior
• Inheritance of acquired characteristics
  impossible

7
How does variation occur?

• Sexual reproduction creates new combinations of
  genes
• Creates individuals with new combinations of
  traits
• Individuals subject to natural selection

8
Rediscovery of Mendel

• Mutations solely emphasized
• Mendelians downplayed natural selection
• Controversy over source of evolutionary change
  (mutations or natural selection)

9
Modern Synthesis

• 1930s work fused genetics and evolutionary
  biology
• Population genetics-combines Mendelian genetics
  with natural selection

10
Microevolution

• Is the study of of how variation is generated and
  passed on within a population
• uses mathematical models to describe gene
  frequencies
• changes in the frequencies of alleles of genes
  within a population
• distinguished from macroevolution-the formation
  of new species

11
What is the source of variation?

• Homologous chromosomes
• Alleles
• Same allele on each chromosome-homozygous
• Different allele on each chromosome-heterozygous

12
Homologous chromosomes
AA Homozygous dominant aa Homozygous
recessive Aa Heterozygous
13
Mutations

• Mutations result in changes in genes
• Mutations create new alleles
• A gene may be polymorphic

14
Polygenic traits

• Many traits (eye color) are influenced by many
  genes (polygenic)
• degree of polymorphism influences distribution of
  phenotype

15
Bell Curve of Variation
16
What determines genetic variability of a
population?

• How fast mutations accumulate in DNA
• How fast mutations spread in a population
• How fast selection eliminates mutations from a
  population

17
Rate of mutation

• Mutations are random
• Most are negative, some can be neutral, others
  positive
• Occurs constantly

18
Rate of spread in a population

• Sexual reproduction main way mutations spread

19
Changes in Allele Frequencies

• Determined by Hardy-Weinberg Principle

20
Hardy-Weinberg Principle

• Two alleles for a gene (Aa), A p and a q
• p q 1 ie., p 0.6 and q 0.4
• frequencies of genotypes are p 2 2pq q2
• ie, 0.36 for AA, 0.16 for aa, and 0.48 for Aa
• same frequency from generation to generation
• no change no microevolution

21
Assumptions of HW

• Random mating
• Large Population Size
• No Mutations
• No Interbreeding with other populations
• No selection

22
Violation of assumptions

• Natural selection produces adaptive change
• Violations of assumptions may produce
  nonadaptive change

23
Mutations

• Mutations provide new alleles
• But mutations do not change allele frequencies
  much by themselves due to low rate of mutations

24
Random Drift

• Chance errors may lead to loss of an allele from
  a population
• Most pronounced in small populations
• Founder effect-A new population founded from a
  few individuals
• Bottleneck effect-A large population diminishes
  to a few individuals
• Can spread new mutations but not from natural
  selection

25
Random Drift due to Small Population Size
26
Random Drift due to Bottleneck
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Nonrandom mating

• Individuals may choose like (positive
  assortative) to mate
• This increases inbreeding
• Leads to higher than expected homozgyotes
  (greater chance of recessive traits being
  expressed)
• Most species are outbreeders (negative
  assortative) to avoid homozygosity
• Higher than expected heterozygosity

28
Interbreeding between Populations

• Gene flow between populations can introduce new
  alleles
• Particularly when populations live in distinct
  microhabitats

29
How much do populations vary genetically?

• Estimated using various techniques
• Electrophoresis of proteins one common method
• 53 of genes in a fruit fly are polymorphic
• 7 of genes in humans are heterozygous

30
Is all genetic variation subject to natural
selection?

• Only variation that affects the phenotype is
  exposed to selection

31
Why is genetic variation essential to evolution?

• Evolution depends on new variants being created

32
Natural selection -reducing allele frequency

• Operates on the phenotype
• Harmful alleles can be reduced but only to a
  point
• Harmful alleles can be hidden in heterozygotes

33
Natural selection-increasing allele frequency

• Examples include roaches and chemicals
• Roaches resistant to spray after many generations
• Peppered moth example (below)

34
Directionof Selection
35
Stabilizing selection on human birth weight
36
Natural selection and Genetic Variation

• Balanced polymorphism-balance of alleles
• Due to different environments affecting natural
  selection
• Examples from sickle-cell anemia and malaria
• In Africa, carriers (heterozygotes) for
  sickle-cell anemia are benefited where malaria is
  common explains higher rate of sickle-cell
  anemia in African-Americans in US

37
Sexual selection

• The differential ability of individuals with
  different genotypes to acquire mates
• May result in sex differences (sexual dimorphism)
• Female choice-females judge males to choose best
  mate
• Male competition-males compete for resources
  needed to mate (territory, food)