Genetic Variation & Evolution 1

Genetic variation and EvolutionChapter 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

Question

Read or listen to the The Firing Line 1997 Creation-Evolution Debate http://www.bringyou.to/apologetics/p45.htm

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 aspects20.5 – skip

20.6 – read on your own20.8 – read on your own

Genetic Variation & Evolution 2

Lamarck’s theory of “Inheritance of Acquired Traits”

Question

Darwin was not first to describe evolution

George Cuvier (1790) – Fossils & ‘Catastrophism’

Jean Lamarck (1809) -- Environmental influence

Genetic Variation & Evolution 3

Modern explanations of why traitschange 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 4

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

Founder effect Bottleneck effect

Gene Flow

Genetic Variation & Evolution 5

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 6

How does Darwin’s ‘Natural Selection’ explain giraffe evolution?

“Selective Pressures”

“Adaptation” -- a noun and verb-- not an ecological term

Genetic Variation & Evolution 7

What is nature of genetic variation? (20.1)

Genetic mutation

DNA Polymorphisms

Different ‘alleles’

Hardy-Weinberg equation (20.2)

p2 + 2pq + q2

-- frequency of Alleles frequency of traits-- can provide evidence for selection

Genetic variation in Asiatic lady beetle

Question

Genetic Variation & Evolution 8

What are examples of environmental selective pressures?

Predator-prey interactionsCompetition for resourcesHuman impacts

Natural selection acts upon existing genetic variation-- theory is not ‘teleological”

Wrong:“In order to hide from predators, the dead-leafbutterfly 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 9

Do organisms always produce more offspring than can survive?

Cichlides: Carmagnola Grey rabbit: Humans: 250,000 eggs 5-8 litters x 7 offspring 2- 7 children/family

Genetic Variation & Evolution 10

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 11

What is ‘Artificial Selection’?

All these crops are varieties of the species Brassica napa

Colors represent different genetic types. Map shows genetic variation in Africa.

Group project

Are humans still evolving?

Genetic Variation & Evolution 12

Why does ‘in breeding’ cause genetic problems?

species non-inbred inbred

Zebra lived 20 3died 7 2

Eld’s deer lived 13 0died 4 7

Giraffe lived 11 2died 3 3

Oryx lived 35 0died 2 5

Dorcas lived 36 17 gazelle died 14 25

From Klug & Cumminings Genetics 5th ed

Genetic Variation & Evolution 13

What are the effects of Natural Selectionon a population? (20.7)

“Microevolution” vs“Macorevolution”

Genetic Variation & Evolution 14

What is the effect of Directional Selection on a population?

Progressive change in a traitwithin a species

Genetic Variation & Evolution 15

How does Stabilizing Selection affect a population?

Human birth weight & Eurosta gall size

Stabilizing selection & ‘living fossils’

Genetic Variation & Evolution 16

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