Natural Selection

• Depending on what environment & conditions the organism is located, the following types of natural selection can occur:– Disruptive Selection– Stabilizing Selection– Directional Selection

Selection that favors both extremes over intermediates is called ____________ selection.14. disruptive

Selection that favors the intermediate phenotype over the extremes is called _______________ selection.Stabilizing

Types of SelectionSelection that favors individuals of one extreme phenotype usually during environmental change is called ______________ selection.

12. directionalANTIBIOTICS

IV. MECHANISMS OF EVOLUTION

Independent Assortment During Meiosis

• Genetic variations in populations– Contribute to evolution

Figure 23.1

Gene Pools and Allele Frequencies• A population– Is a localized group of individuals that are capable of

interbreeding and producing fertile offspringMAPAREA

ALAS

KA

CAN

A DA

Beaufort Sea

Porcupineherd range

• Fairbanks

• Whitehorse

Fortymileherd range

NORTHWEST

TERRITORIESAL

ASKA

YUKO

N

Figure 23.3

Let’s Go Swimmin’

• The gene pool– Is the total amount of genes in a population at

any one time– Consists of all gene loci in all individuals of the

population

The total amount of genes within a population is called the

________ ________.1. gene pool

Eye color

Blood type

intelligenceDiabetes

Cysic fibrosis

Caucasians

• Two processes:– mutation and sexual recombination

– Produce the variation in gene pools that contributes to differences among individuals

Mutation• Mutations

– Are changes in the nucleotide sequence of DNA– Cause new genes and alleles to arise

Figure 23.6

Sexual Recombination • In sexually reproducing populations, sexual

recombination– Is far more important than mutation in producing

the genetic differences that make adaptation possible

A. How variations provide material for natural selection

• Natural selection– Accumulates and maintains favorable genotypes in

a population

Of all the agents that lead to microevolution the one that best adapts a gene pool to a changing environment is ______________ ______________.11. natural selection

Genetic Variation

• Genetic variation– Occurs in individuals in populations of all species– Is not always heritable

Figure 23.9 A, B

(a) Map butterflies thatemerge in spring:orange and brown

(b) Map butterflies thatemerge in late summer:black and white

• Three major factors that cause genetic variation and bring about most evolutionary change– Natural selection– Genetic drift– Gene flow

Natural Selection“Survival of the

Fittest”

Genetic Drift• Statistically, the smaller a sample– The greater the chance of deviation from a predicted

result– Describes how allele frequencies can fluctuate

unpredictably from one generation to the next– Tends to reduce genetic variation

Figure 23.7

CRCR

CRCW

CRCR

CWCW CRCR

CRCW

CRCW

CRCWCRCR

CRCR

Only 5 of10 plantsleaveoffspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of10 plantsleaveoffspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

If a group of organisms within a large population become isolated, they may not represent the entire gene pool, thus leading to changes or

______________ __________________.15. GENETIC DRIFT

TWO TYPES OF GENETIC DRIFT

Bottleneck

Bottleneck & Founder Effect

This process can lead to a change in allele frequencies within a population?

Bottleneck Effect

The Bottleneck Effect• In the bottleneck effect– A sudden change in the environment may

drastically reduce the size of a population– The gene pool may no longer be reflective of the

original population’s gene pool

Originalpopulation

Bottleneckingevent

Survivingpopulation

Figure 23.8 A

(a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.

• Understanding the bottleneck effect– Can increase understanding of how human

activity affects other species

Figure 23.8 B

(b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

The Founder Effect• The founder effect– Occurs when a few individuals become isolated

from a larger population– Can affect allele frequencies in a population

B. The role of geographic isolation in speciation

SPECIATION Organisms that can interbreed and produce fertile

offspring make up a species.

ISOLATING MECHANISMS

21. BEHAVORIAL ISOLATION

22. Geographic

• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

Figure 23.11

EXPERIMENT Researchers observed that the average sizeof yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of theresulting plants.

RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments.

CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates therole of genetic as well as environmental differences.

Mea

n he

ight

(cm

)Ati

tude

(m)

Heights of yarrow plants grown in common garden

Seed collection sites

Sierra NevadaRange

Great BasinPlateau

Allopatric and Sympatric Speciation: A Summary

• In allopatric speciation– A new species forms while geographically isolated

from its parent population• In sympatric speciation– The emergence of a reproductive barrier isolates a

subset of a population without geographic separation from the parent species

C. The importance of the environment in selecting adaptations

• Organisms must adapt to their environment to survive. This has led toward the following types of evolution:

• Convergent Evolution• Divergent Evolution• Adaptive Radiation

Sugarglider

AUSTRALIA

NORTHAMERICA

Flyingsquirrel

Figure 22.17

• Some similar mammals that have adapted to similar environments– Have evolved independently from different ancestors

1 2.4 3.14 5.18 6 7.15

XX1913.1710.169.128.11

1 2.19 3.8 4.16 5.14 6.7

XX15.1813.1711.129.10Figure 23.10

When different species began to look alike because they develop similar adaptations this is called _______________ ___________________24. CONVERGENT EVOLUTION

When a species moves into new environment and becomes less similar to the original species this is called _______ evolution.

25. DIVERGENT EVOLUTION

Adaptive Radiation• Adaptive radiation– Is the evolution of diversely adapted species from

a common ancestor upon introduction to new environmental opportunities

Figure 24.11

ADAPTIVE RADIATION

• The Hawaiian archipelago– Is one of the world’s great showcases of adaptive

radiation

Figure 24.12

Dubautia laxa

Dubautia waialealae

KAUA'I5.1

millionyears O'AHU

3.7millionyears

LANAI

MOLOKA'I

1.3 million years

MAUI

HAWAI'I0.4

millionyears

Argyroxiphium sandwicense

Dubautia scabra Dubautia linearis

N

V. APPLICATIONS (PESTICIDE AND ANTIBIOTIC RESISTANCE)

Evolution can be seen today :

In Disease Resistance

In Pesticide Resistance

A. Discuss the evolutionary selection of resistance to antibiotics and pesticides in various species.

• The Evolution of Drug-Resistant HIV– In humans, the use of drug selects for pathogens

that through chance mutations are resistant to the drugs’ effects

– Natural selection is a cause of adaptive evolution-the HIV adapts to the drugs.

– Researchers have developed numerous drugs to combat HIV

– But using these medications selects for viruses resistant to the drugs

So What!

• The ability of bacteria and viruses to evolve rapidly– Poses a challenge to our society– Puts us at risk of contracting a disease that can no

longer be cured.

Artificial Selection• In the process of artificial selection– Humans have modified other species over many

generations by selecting and breeding individuals that possess desired traits

Figure 22.10

Terminalbud

Lateralbuds

Brussels sproutsCabbage

Flowercluster

Leaves

Cauliflower

Flowerandstems

Broccoli Wild mustard Kohlrabi

Stem

Kale