How Populations Evolve

1
How Populations Evolve
2
Population Genetics

• turning point for evolutionary theory
• emphasizes extensive genetic variation within
  populations
• recognizes importance of quantitative characters
• reconciled Darwinism and Mendelism

3
neo-Darwinism

• Integrates ideas from many different fields
• paleontology
• taxonomy
• biogeography
• population genetics
• Architects of neo-Darwinism
• Theodosius Dobzhansky
• Ernst Mayr
• George Gaylord Simpson
• G. Ledyard Stebbins

4
POPULATIONS

• a localized group of individuals belong to the
  same species

5
Adapted from Lederer R.J. 1984. Ecology and
field biology. Menlo Park, CA.
Benjamin/Cummings. P. 200.
population
population
population
population
population
population
population
community
6
Population Fluctuations
() natality
Population of organisms
(-) emigration
() immigration
(-) mortality
Adapted from Lederer RJ. 1984. Ecology and
field biology. Menlo Park, CA.
Benjamin/Cummiongs. P. 207
7
A large mammals
Number of survivors
B humans, fruitflies, hydras, birds tree seedlings
Adapted from Lederer R.J. 1984. Ecology and
field biology. Menlo Park, CA.
Benjamin/Cummings. P. 210.
C oysters, salmon most invertebrates
Age
8
B birth D death
Equilibrium phase
Carrying capacity of environment
B approx D
Number of organisms
Exponential Growth
BgtD
Adapted from Lederer R.J. 1984. Ecology and
field biology. Menlo Park, CA.
Benjamin/Cummings. P. 211.
Lag phase
B approx D
Time
9
Adapted from Miller, Jr. GT. 1990. Living in
the environment, 6th ed. Belmont, CA. Wadsorth.
P. 4.
16
Exponential Growth of World Population
Billions of people
8
Black death (bubonic plague)
0
8000bc 7000bc 6000bc 5000bc 4000bc 3000bc
2000bc 1000bc 1ad 1000ad 2000ad
21000ad
10
SPECIES

• a group of populations that have the potential
  to interbreed in nature
• each species has a geographical range where
  individuals are concentrated in several localized
  populations
• these populations may be isolated, rarely
  exchanging genetic material

11
Territory - area of home range defended
Adapted from Lederer R.J. 1984. Ecology and
field biology. Menlo Park, CA.
Benjamin/Cummings. P. 237-238.
territory
territory
Personal Space
territory
territory
territory
Private Space
territory
territory
Home Range
Home Range - any location an organism may go
12
(No Transcript)
13
(No Transcript)
14
Functions of Territories

• Familiarity with area
• Distribution of resources
• Reduces aggression with competitors
• Less Disease because less density
• Environment for rearing young
• Food supply

15
(No Transcript)
16
Types of Territory

• Intraspecific
• Interspecific

17
Niche

• Role an organism plays in a community

18
GENE POOL

• the total aggregate of genes in a population at
  any one time

19
Total genetic availabiltiy within a population
individual
individual
individual
individual
individual
individual
individual
individual
Assumes panmixis (random mating)
Gene Pool
20
Gene Pool

• consists of all alleles at all gene loci in all
  individuals of a population
• for diploid species, each locus is represented
  twice in the genome of an individual (either
  homo- or heterozygous for those homologous loci

21
MICROEVOLUTION

• when the relative frequencies of alleles in a
  population change over a succession of generations

22
HARDY-WEINBERG THEOREM

• frequencies of alleles in a populations gene
  pool remain constant over the generations unless
  acted upon by agents other than sexual
  recombination

23
HARDY-WEINBERG THEOREM

• VERY LARGE POPULATIONS
• ISOLATION FROM OTHER SPECIES
• NO NET MUTATIONS
• RANDOM MATING
• NO NATURAL SELECTION

24
Hardy-Weinberg Equation

• p2 2pq q2 1
• Frequency Frequency Frequency
• of AA of Aa of aa

25
Pink and White Wildflowers

• A represents an allele for pink
• a represents an allele for white
• diploid organism
• AA homozygous pink
• Aa heterozygous pink
• aa homozygous white
• Assume 500 flowers

26
(No Transcript)
27

• Assume 20 of the flowers are white, 480 must be
  pink
• Assume of 480 pink, 320 are AA and 160 are Aa
• 500 plants 1000 genes for flower color
• How many express A
• 320 x 2 640 (AA)
• 160 x 1 160 (Aa)
• 800 express A
• Gene Frequency 80 or .8

28

• Since there are only 2 forms of the gene, A and
  a and 80 are A, then 20 or 0.2 must be a
• How will genetic recombination (sex) affect the
  gene frequencies we just calculated?
• Assume sperm and egg unite completely randomly
• a sperm will have 0.8 chance to contain an A
  allele and 0.2 chance to contain an a allele
• The same for each egg

29
Product Law of Probability

• Probability of picking two A alleles is
  0.8x0.8.64 (64)
• Thus 64 of the plants in the next generation
  will have genotype AA
• Probability picking two a alleles is
  0.2x0.20.04 (4)
• Therefore, 32 (0.32) will be Aa

30
Second Generation

• Second generation genetic make-up will be 0.64
  Aa, 0.32 Aa, and 0.04 aa
• If we repeat the process, another mating would
  result in the same gene frequencies
• Results in equilibrium of the population,
  referred to as Hardy-Weinberg Equilibrium

31
Hardy-Weinberg Equation

• allows you to calculate frequencies of alleles in
  gene pools if you know frequencies of genotypes
• allows you to calculate frequencies of genotypes
  if you know the frequencies of alleles in gene
  pools

32
Example

• What percentage of the population carries the
  allele for PKU (phenylketonuria)? (recessive
  allele)
• 1 in 10,000 babies in US are born with PKU
• frequency of PKU babies in US q2 in H-W
  equation
• q2 0.0001
• q 0.01
• Frequency of dominate allele p 1-q or 0.99

33

• Carriers (pq) 2pq or
• 2x0.99x0.010.0198
• Approximately 2 carry the PKU gene

34
Microevolution

• Potential Agents of Microevolution
• genetic drift
• gene flow
• mutation
• nonradom mating
• natural selection

35
Genetic Drift

• changes in the gene pool of a small population
  due to chance
• small populations
• wildfire
• floods
• other types of destruction of alleles due to
  chance

36
(No Transcript)
37
Bottleneck Effect and Genetic Drift

• disasters that reduce the size of a population by
  killing individuals unselectively
• surviving population unlikely to be
  representative of the original population
• examples in Florida are the Florida panther that
  has been reduced to too few breeding individuals
  for any type of genetic variability

38
(No Transcript)
39
Founder Effect

• When a few individuals colonize an isolated area,
  the small sample size will probably not represent
  the original population from whence it came
• Darwins finches
• retinitis pigmentosa on Tristan da
  Cunha(progressive loss of retinal response,
  atrophy, clumping of pigment)

40
Peterson Roger Tory. 1967 Apr. The
Galapagoseerie cradle of new species. National
Geographic. p. 544-545.
41
(No Transcript)
42
Gene Flow

• The migration of fertile individuals or the
  transfer of gametes between populations
• pollination from other species from another area
• breeding panthers from west to those from east

43
Mutation

• spontaneous
• induced

44
Nonrandom Mating

• inbreeding
• assortative mating - individuals select partners
  like themselves in certain phenotypic traits

45
These beetles, Lytta magister, are found in the
Sonoran Desert of Arizona. They pair off,
posterior to posterior, according to size. They
may continue to feed on brittlebush flowers
during copulation. Copulation may last several
hours.
46
Natural Selection

• some organisms produce more offspring than others
• salt and pepper moths

47
From Starr Cecie. 1997. Biology concepts and
applications, 3rd ed. Belmont, CA Worth.
Microevolution p. 229.
48
The Genetic Basis of Variation

• In what ways do members of a population vary?
• How extensive is that variation?
• What mechanisms generate and maintain variations
  in a population?
• Do all variations function as raw material for
  selection?

49
From Starr Cecie. 1997. Biology concepts and
applications, 3rd ed. Belmont, CA Worth.
Microevolution p. 224.
50
The Nature and Extent of Genetic Variation Within
and Between Populations

• people reflect individual variations in
  appearance and temperament
• not all variation observed is heritable (some is
  environmental)
• Discrete and quantitative contribute to variation

51
QUANTITATIVE AND DISCRETE

• QUANTITATIVE
• - plant height in wild flowers (vary
  continuously) tall, short, and everything in
  between
• DISCRETE
• - flower color, i.e. red versus white (vary
  categorically) determined by a single gene locus
  with different alleles to produce distinct
  phenotypes

52
Morphs

• two or more forms of a Mendelian character
  represented in a population, i.e. red versus
  white flowers
• population is said to be polymorphic if two or
  more morphs are represented in high enough
  frequencies to be readily noticeable, i.e.
  A,B,AB, O blood types

53
Geographical Variation

• differences between populations in their
  frequencies of alleles
• environmental factors may be different from one
  place to another
• natural selection contributes, i.e. what if more
  insects are attracted to white flowers instead of
  red flowers in a certain area?

54
Clines

• a graded change in some trait along a geographic
  axis
• For example, average body size of North
  American mammals increases with increasing
  latitude. The reduced ratio of surface area to
  volume with larger size helps animals in cold
  climates conserve body heat

55
Types of Clines

• Based on
• color
• shape
• size
• metabolism
• appendage length
• others

56
Bergmans Rule

• farther away from the equator one goes in a
  species range, the larger the body size becomes.
• Volume increases relative to the surface area to
  lessen heat loss
• people in colder habitats are taller and heavier

57
Allens Rule

• homeotherms farther from the equator have
  shorter appendages than do those animals in
  warmer areas.
• Lessens heat loss

58
Glogers Rule

• Organisms in warmer, more humid regions are
  darker in color than those in colder, drier
  regions
• temperature regulation?
• Protective coloration?

59
Sources of Genetic Variation

• mutation (gene and chromosomal)
• recombination (from the gene pool)

60
(No Transcript)
61
Preserving Genetic Variation

• diploidy
• balanced polymorphism

62
Balanced polymorphism is a finch population. Note
differences in bill sizes of the black-bellied
seedcrackers, a species of finch found in
Cameroon, West Africa. No finches with
intermediate sized bills occur. The small billed
finch on the left feeds mainly on soft seeds,
while the one on the right feeds on hard seeds.
63
Balanced Polymorphism

• The ability of natural selection to maintain
  diversity in a population
• heterozygote advantage, i.e. sickle cell anemia
• hybrid vigor, i.e. hybrid corn
• frequency-dependent selection (when reproductive
  success of one morph declines if that phenotypic
  form becomes too common

64
(No Transcript)
65
Modes of Natural Selection

• stabilizing selection (acts against extreme
  phenotypes and favors the intermediate)
• directional selection (shifts the frequency curve
  for variations in some phenotypic trait in one
  direction or the other by favoring rare
  individuals)
• diversifying selection (disruptive) (when
  environmental conditions favor individuals of
  both extremes)

66
(No Transcript)