Download Evolution - Chapter 20

Genetic variation and Evolution
Chapter 20
Why are organisms well adapted to their habitats?
Creationist explanation (see section 21.7)
20.2 -- should know H-W equation
and why it’s important
-- we may come back to this
Is a religious view
Divine role
20.4 – understand the meaning of
fitness, but we’ll not cover the
mathematical aspects
20.5 – skip
“intelligent design”
not scientifically testable
20.6 – read on your own
20.8 – read on your own
Scientific explanation
“evolution from common ancestor”
‘Natural Selection”
-- is testable
Question
Read or listen to the
The Firing Line 1997 Creation-Evolution Debate
http://www.bringyou.to/apologetics/p45.htm
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Darwin was not first to describe evolution
George Cuvier (1790) – Fossils & ‘Catastrophism’
Jean Lamarck (1809) -- Environmental influence
Lamarck’s theory of “Inheritance of Acquired Traits”
Question
Genetic Variation & Evolution
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Modern explanations of why traits
change over time (20.3)
-- Frequency of genes in the population change
5 Agents (mechanisms) of change:
1. Genetic Mutation – random changes to DNA code
-- creates variation in traits
-- ‘raw material’ of all evolutionary mechanisms
2. Natural selection – mechanism proposed by Darwin
-- environment selects certain traits
Genetic Variation & Evolution
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Agents of change, con’t
Gene Flow
Other agents of change
3. Gene flow
-- movement of individuals between populations
4. Nonrandom mating
-- organisms prefer to mate with certain individuals
5. Genetic drift
-- chance may favor certain traits
in a small population
Founder effect
Bottleneck effect
Founder effect – population isolation
bottleneck effect – population catastrophe
-- gene pool reduction
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What are the fundamental premises of
Darwin’s theory of “Natural Selection”?
Read Section about Evolution in Chapter 1
“On the Origin of Species by Means of Natural Selection”
“Evolution” (a process)
does not equal “Natural Selection” (a process)
Natural selection occurs when:
1) There exists genetically based variation (inheritable)
in the traits of a species.
2) Organisms produce more offspring than can survive.
3) The environment selects for those organisms
with the best adapted traits.
4) Organisms best adapted to the environment will reproduce most
successfully.
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How does Darwin’s ‘Natural Selection’ explain giraffe evolution?
“Selective Pressures”
“Adaptation” -- a noun and verb
-- not an ecological term
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What is nature of genetic
variation? (20.1)
Genetic mutation
DNA Polymorphisms
Genetic variation in
Asiatic lady beetle
Different ‘alleles’
Hardy-Weinberg equation (20.2)
p2 + 2pq + q2
-- frequency of Alleles  frequency of traits
-- can provide evidence for selection
Question
Genetic Variation & Evolution
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What are examples of environmental
selective pressures?
Predator-prey interactions
Competition for resources
Human impacts
Natural selection acts upon existing genetic variation
-- theory is not ‘teleological”
Wrong:
“In order to hide from predators, the dead-leaf
butterfly evolved a leaf-like appearance.”
Better:
“Predation selected for butterflies that most
resembled a dead leaf.”
The environment selects among but
does not create variations!
Question
Genetic Variation & Evolution
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Do organisms always produce
more offspring than can survive?
Cichlides:
250,000 eggs
Carmagnola Grey rabbit:
5-8 litters x 7 offspring
Genetic Variation & Evolution
Humans:
2- 7 children/family
9
Why don’t humans evolve wings?
Limits of Selection
Existence genetic variation
Complex traits involve many genes
Suitable selective pressures
Genes have multiple effects
-- developmental genes
Question
Genetic Variation & Evolution
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What is ‘Artificial
Selection’?
All these crops are varieties of the species Brassica napa
Are humans still evolving?
Colors represent different genetic
types. Map shows genetic variation
in Africa.
Group project
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Why does ‘in breeding’ cause genetic
problems?
species
non-inbred
inbred
Zebra
lived
died
20
7
3
2
Eld’s deer
lived
died
13
4
0
7
Giraffe
lived
died
11
3
2
3
Oryx
lived
died
35
2
0
5
Dorcas
gazelle
lived
died
36
14
17
25
From Klug & Cumminings Genetics 5th ed
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What are the effects of Natural Selection
on a population? (20.7)
“Microevolution”
vs
“Macorevolution”
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What is the effect of
Directional Selection on a
population?
Progressive change in a trait
within a species
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How does Stabilizing Selection affect a population?
Human birth weight & Eurosta gall size
Stabilizing selection & ‘living fossils’
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How does Disruptive
Selection affect a population?
Responsible for divergence of a single
species into two different species
Modern example of process:
blackbellied seedcracker
Question
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