Post on 13-Jan-2016
Processes of Evolution
Chapter 16
Overwhelming Evidence for Evolution
• A) From Biogeography
• B) From Comparative anatomy
• C) From Geologic discoveries
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?
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
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
Darwin’s Voyage
• At age 22, Charles Darwin began a five-year, round-the-world voyage aboard the Beagle
• In his role as ship’s naturalist he collected and examined the species that inhabited the regions the ship visited
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Voyage of the Beagle
GalapagosIslands
Isabela
Darwin
Wolf
Pinta
Marchena Genovesa
Fernandia
SantiagoBartolomé
RabidaPinzon
SeymourBaltra
Santa Cruz
Santa Fe
Tortuga
Española
San Cristobal
Floreana
• Volcanic islands far off coast of Ecuador
• All inhabitants are descended from species that arrived on islands from elsewhere
Lyell Provided the foundation for Darwin to draw his
conclusions• Lyell’s 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
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
Darwin put together the short term struggle for existence (Malthus) with the great geological timescale (Lyell)
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
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Darwin’s Theory
Survival of the fittest
On the Origin of Species
• Darwin’s book
• Published in 1859
• Laid out in great detail his evidence
in support of the theory of evolution
by natural selection
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 Evolution is change in frequency of traitsof traits
When does evolution not occur?
• No mutation
• Random mating
• Gene doesn’t affect survival or
reproduction
• Large population
• No immigration/emigration
When 5 conditions are met
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
No Change through Generations
STARTING POPULATION
490 AA butterfliesdark-blue wings
420 Aa butterfliesmedium-blue wings
90 aa butterflieswhite wings
490 AA butterfliesdark-blue wings
THE NEXT GENERATION
420 Aa butterfliesmedium-blue wings
90 aa butterflieswhite wings
THE NEXT GENERATION
490 AA butterfliesdark-blue wings
420 Aa butterfliesMedium-blue wings
90 aa butterflieswhite wings
Gene Mutations
• Infrequent but inevitable
• Every gene is subject to mutation to some degree
• Changes the frequency of alleles in a population
= EVOLUTION
Directional Selection
• Allele frequencies shift in one direction
Range of values for the trait at time 1
Number of individuals
in population
Range of values for the trait at time 2
Number of individuals
in population
Range of values for the trait at time 3
Number of individuals
in population
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
Natural Selection
• A difference in the survival and
reproductive success of different
phenotypes
• Acts directly on phenotypes and indirectly
on genotypes
Stabilizing Selection
• Intermediate forms are favored and extremes are eliminated
Range of values for wing-color trait at time 1
Number of individuals
in population
Range of values for the trait at time 2
Number of individuals
in population
Range of values for wing-color trait at time 2
Number of individuals
in population
Range of values for wing-color trait at time 3
Stabilizing Selection: Another Example
• Weight distribution for 13,370 human newborns (yellow curve) correlated with death rate (white curve)
Disruptive Selection
• Forms at both ends of the range of variation are favored
• Intermediate forms are selected against
Number of individuals
in population
Range of values for wing-color trait at time 1
Number of individuals
in population
Range of values for wing-color trait at time 2
Number of individuals
in population
Range of values for wing-color trait at time 3
Balanced Polymorphism
• Polymorphism - “having many forms”
• Occurs when two or more alleles are maintained at frequencies greater than 1 percent
Sickle-Cell Trait: Heterozygote Advantage
• Allele HbS causes sickle-cell anemia when heterozygous
• Heterozygotes are more resistant to malaria than homozygotes
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
Malaria case
Sickle cell trait
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
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
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
Inbreeding
• Nonrandom mating between related individuals
• Leads to increased homozygosity
• Can lower fitness when deleterious recessive alleles are expressed
• Amish, cheetahs
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!!!