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Processes of Evolution

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Title: Processes of Evolution


1
Processes of Evolution
  • Chapter 16

2
Overwhelming Evidence for Evolution
  • A) From Biogeography
  • B) From Comparative anatomy
  • C) From Geologic discoveries

3
A) Biogeography - is the science which deals with
geographic patterns of species distribution and
the processes that result in such patterns.
  • Size of the known world expanded enormously in
    the 15th century
  • Discovery of new organisms in previously unknown
    places could not be explained by accepted beliefs
  • How did species get from center of creation to
    all these places?

4
B) Comparative Morphology
  • Study of similarities and differences in body
    plans of major groups
  • Puzzling patterns
  • Animals as different as whales and bats have
    similar bones in forelimbs
  • Some parts seem to have no function

5
C) Geological Discoveries
  • Similar rock layers throughout world
  • Certain layers contain fossils
  • Deeper layers contain simpler fossils than
    shallow layers
  • Some fossils seem to be related to known species

6
Consider
7
Darwins Voyage
  • At age 22, Charles Darwin began a five-year,
    round-the-world voyage aboard the Beagle
  • In his role as ships naturalist he collected and
    examined the species that inhabited the regions
    the ship visited

8
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9
Voyage of the Beagle
10
GalapagosIslands
  • Volcanic islands far off coast of Ecuador
  • All inhabitants are descended from species that
    arrived on islands from elsewhere

Isabela
11
Lyell Provided the foundation for Darwin to draw
his conclusions
  • Lyells Principles of Geology
  • Subtle, repetitive processes of change, had
    shaped Earth
  • Challenged the view that Earth was only 6,000
    years old, to millions of years

12
Malthus - Struggle to Survive - this too shaped
Darwins views
  • Thomas Malthus, a clergyman and economist, wrote
    essay that Darwin read on his return to England
  • Argued that as population size increases,
    resources dwindle, the struggle to live
    intensifies and conflict increases

13
Darwin put together the short term struggle for
existence (Malthus) with the great geological
timescale (Lyell)
14
Galapagos Finches
  • Darwin observed finches with a variety of
    lifestyles and body forms
  • On his return he learned that there were 13
    species
  • He attempted to correlate variations in their
    traits with environmental challenges

15
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16
Darwins Theory
  • Survival of the fittest

17
On the Origin of Species
  • Darwins book
  • Published in 1859
  • Laid out in great detail his evidence in support
    of the theory of evolution by natural selection

18
The individual does not matter
  • Its offspring do

19
Populations Evolve
  • Biological evolution does not change individuals
  • It changes a population
  • Traits (features) in a population vary among
    individuals
  • Evolution is change in frequency of traits

20
EVOLUTION IS CHANGE IN GENE FREQUENCIES
  • Remember this fact!

21
When does evolution not occur?
When 5 conditions are met
  • No mutation
  • Random mating
  • Gene doesnt affect survival or reproduction
  • Large population
  • No immigration/emigration

22
Hardy-Weinberg Rule
  • At genetic equilibrium, proportions of genotypes
    at a locus with two alleles are given by the
    equation
  • p2 AA 2pq Aa q2 aa 1
  • Frequency of allele A p
  • Frequency of allele a q

23
No Change through Generations
24
Gene Mutations
  • Infrequent but inevitable
  • Every gene is subject to mutation to some degree
  • Changes the frequency of alleles in a population
  • EVOLUTION

25
Directional Selection
  • Allele frequencies shift in one direction

26
Giraffes
  • Baby giraffes in Africa
  • Some have inherited long necks others shorter
    ones
  • When food is limited only the ones with longer
    necks can feed
  • The others die of starvation
  • Now the population has individuals with longer
    neck alleles on average

27
Natural Selection
  • A difference in the survival and reproductive
    success of different phenotypes
  • Acts directly on phenotypes and indirectly on
    genotypes

28
Stabilizing Selection
  • Intermediate forms are favored and extremes are
    eliminated

29
Stabilizing Selection Another Example
  • Weight distribution for 13,370 human newborns
    (yellow curve) correlated with death rate (white
    curve)

30
Disruptive Selection
  • Forms at both ends of the range of variation are
    favored
  • Intermediate forms are selected against

31
Balanced Polymorphism
  • Polymorphism - having many forms
  • Occurs when two or more alleles are maintained at
    frequencies greater than 1 percent

32
Sickle-Cell Trait Heterozygote Advantage
  • Allele HbS causes sickle-cell anemia when
    heterozygous
  • Heterozygotes are more resistant to malaria than
    homozygotes

Malaria case
Sickle cell trait
less than 1 in 1,600
1 in 400-1,600
1 in 180-400
1 in 100-180
1 in 64-100
more than 1 in 64
33
Genetic Drift
  • Random change in allele frequencies brought about
    by chance
  • Effect is most pronounced in small populations
  • Sampling error - Fewer times an event occurs,
    greater the variance in outcome

34
Bottleneck
  • A severe reduction in population size
  • Causes pronounced drift
  • Example
  • Elephant seal population hunted down to just 20
    individuals
  • Population rebounded to 30,000
  • Electrophoresis revealed there is now no allele
    variation at 24 genes

35
Founder Effect
  • Effect of drift when a small number of
    individuals start a new population
  • By chance, allele frequencies of founders may not
    be same as those in original population
  • Effect is pronounced on isolated islands

36
Inbreeding
  • Nonrandom mating between related individuals
  • Leads to increased homozygosity
  • Can lower fitness when deleterious recessive
    alleles are expressed
  • Amish, cheetahs

37
Gene Flow
  • Physical flow of alleles into a population
  • Tends to keep the gene pools of populations
    similar
  • Counters the differences that result from
    mutation, natural selection, and genetic drift
  • Prevents the two populations forming new
    species!!!
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