Genetic variation and Evolution
Chapter 20
Why are organisms well adapted to their habitats?
Creationist explanation (see section 21.7)
Is a religious view
Divine role
“intelligent design” not scientifically testable
Scientific explanation
“evolution from common ancestor”
‘Natural Selection”
-- is testable
20.2 -- should know H-W equation and why it’s important
-- we may come back to this
20.4 – understand the meaning of fitness, but we’ll not cover the mathematical aspects
20.5 – skip
20.6 – read on your own
20.8 – read on your own
Read or listen to the
The Firing Line 1997 Creation-Evolution Debate http://www.bringyou.to/apologetics/p45.htm
Question
Genetic Variation & Evolution 1

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 2

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 3

Agents of change, con’t
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 – population isolation bottleneck effect – population catastrophe
-- gene pool reduction
Gene Flow
Founder effect Bottleneck effect
Genetic Variation & Evolution 4

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.
Genetic Variation & Evolution 5

How does Darwin’s ‘Natural Selection’ explain giraffe evolution?
“Selective Pressures”
“Adaptation” -- a noun and verb
-- not an ecological term
Genetic Variation & Evolution 6

What is nature of genetic variation? (20.1)
Genetic mutation
DNA Polymorphisms
Different ‘alleles’
Hardy-Weinberg equation (20.2) p 2 + 2pq + q 2
-- frequency of Alleles  frequency of traits
-- can provide evidence for selection
Question
Genetic Variation & Evolution
Genetic variation in
Asiatic lady beetle
7

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 leaflike 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 8

Do organisms always produce more offspring than can survive?
Cichlides:
250,000 eggs
Carmagnola Grey rabbit:
5-8 litters x 7 offspring
Humans:
2- 7 children/family
Genetic Variation & Evolution 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 10

What is ‘Artificial
Selection’?
All these crops are varieties of the species Brassica napa
Are humans still evolving?
Group project
Genetic Variation & Evolution
Colors represent different genetic types. Map shows genetic variation in Africa.
11

Why does ‘in breeding’ cause genetic problems?
species
Zebra lived died
Eld’s deer lived died
Giraffe lived died non-inbred
20
7
13
4
11
3 inbred
3
2
0
7
2
3
Oryx lived died
35
2
0
5
Dorcas lived gazelle died
36
14
17
25
From Klug & Cumminings Genetics 5 th ed
Genetic Variation & Evolution 12

What are the effects of Natural Selection on a population? (20.7)
“Microevolution” vs
“Macorevolution”
Genetic Variation & Evolution 13

What is the effect of
Directional Selection on a population?
Progressive change in a trait within a species
Genetic Variation & Evolution 14

How does Stabilizing Selection affect a population?
Human birth weight & Eurosta gall size
Stabilizing selection & ‘living fossils’
Genetic Variation & Evolution 15

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
Genetic Variation & Evolution 16