Evolution, Natural Selection & Selective Pressures

Evolution, Natural Selection

& Selective & Selective Pressures

Page 1: Concept MapEvolution!Definition:

Natural Natural SelectionDefinition:

Genetic Variation:

Selective pressure:

Many Generations:

Page 2: Categories of Natural Selection

Directional selection:

Stabilizing selection:

Disruptive selection:

What is it?

Example graph with the change

What is it?

Example graph with the change

What is it?

Example graph with the change

What is Evolution?

Evolution = Change in allele frequency in a population over time (generations)over time (generations)

Ex: If any inherited trait is increasing (or decreasing) in frequency, the population is evolving

How does Evolution Occur?

• Evolution occurs through a process called natural selectionselection

• Natural selection occurs when certain individuals are better able to survive and reproduce due to their phenotypes (traits)

Theory of Evolution by Natural

Selection

• Charles Darwin was the first person to propose the idea of evolution viato propose the idea of evolution vianatural selection in 1859

• Many people before him had hypothesized about evolution, but he was the first person to propose a valid mechanism for how it works

What is Needed for Nat. Selection to Occur?

1. A population containing genetic variation

2. Selective pressures in the 2. Selective pressures in the environment that cause certain individuals to have greater fitness(survival and reproduction)

3. MANY reproductive generations (time)

What is a Selective Pressure?

• Selective pressures are any factors in the environmentthat affect the survival &/or that affect the survival &/or reproduction of individuals in a population

• Can be biotic (living) or abiotic (non-living)

What is a Selective Pressure?

• Examples of bioticselective pressures:– Predators, – Predators,

competitors (organisms that compete for the same resources), pathogens….

Predation is

a biotic

selective

pressure

What is a Selective Pressure?

• Examples of abioticselective pressures:– Temperature, – Temperature,

sunlight, humidity, rainfall, snow, wind, soil minerals, salinity, fire… Fire is an

abiotic

selective

pressure

Monogenic Traits

• Some traits are monogenic –coded for by only one gene

• Ex: Widow’s peak, blood type, • Ex: Widow’s peak, blood type, earlobe attachment, many genetic disorders

• Monogenic traits appear as distinct categories, e.g. blood type A, B, AB or O

Monogenic Traits

• Phenotype frequencies for monogenic traits are shown on simple bar graphsare shown on simple bar graphs

• If evolution is occurring, the phenotype frequencies will change over generations

3 phenotypes: brown OR black

OR red

In itia l P opulation G eneration 10

G eneration 20 G eneration 30

Fre

quen

c y o

f

phenoty

pe

What do you notice in this population of lizards with three skin color

phenotypes?

Initial Population Generation 10

Generation 20 Generation 30

Fre

que n

cy o

f ph

e not y

pe

• Evolution is occurring!occurring!

• What selective pressure could cause this selection for black lizards over red and brown?

Polygenic Traits

• Most traits are polygenic – coded for by more than one gene

• Polygenic traits show a wide range of phenotypesphenotypes

• Polygenic traits are continuous,and NOT separate categories

• Ex: Height, eye color, skin color

Polygenic Traits

2 5

3 0

3 5The fact that there is such a wide range of lengths indicates this trait is polygenic

0

5

1 0

1 5

2 0

2 5

2 0 m m 3 0 m m 4 0 m m

Length

Fre

qu

ency

lengths indicates this trait is polygenic

Polygenic traits generally have frequencies that follow a bell-shaped curve

• What do you notice about the wingspan phenotypesafter 250

S e a G u l l P o p u la t io n - G e n e r a t io n 1

0

5

1 0

1 5

2 0

8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5

W i n g s p a n ( c m )

Fre

qu

en

cy

phenotypesafter 250 generations of sea gulls?

W i n g s p a n ( c m )

• There has been selection for longerwingspans

S e a G u l l P o p u la t io n - G e n e r a t io n 1

0

5

1 0

1 5

2 0

8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5

W i n g s p a n ( c m )

Fre

qu

en

cy

W i n g s p a n ( c m )

0

5

10

15

20

25

30

81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97

Fre

qu

en

cy

Wingspan (cm)

Sea Gull Population - Generation 250

What selective pressure could cause this?

Three Categories Of Natural Selection

• Directional (common)

• Stabilizing (common)

• Disruptive (uncommon)

Directional Selection

•

Hawk vision ratings in population

before selection

Vision ratings after many generations –selection for better vision has occurred

Stabilizing Selection

• When phenotypes in the middlegive the organism an advantage

• Ex: Human birth weight• Ex: Human birth weight• Low birth weight = infant mortality

• High birth weight = delivery complications

Stabilizing Selection

Peak weight does NOT change over time since weights at the ends are selected against

Disruptive Selection

• Selection in favor of phenotypes on both ends of the curve

• Ex: Short and long beaks when two sources of food are present

• Ex: Short and long beaks when two sources of food are present

• Short-beaked birds can eat the seeds more easily, long beaked birds can eat the insects more easily

Disruptive Selection

Over many generations, birds with short and long beaks become more common, while

those in the middle die out