processes of evolution chapter 16. overwhelming evidence for evolution a) from biogeography b) from...

Processes of Evolution

Chapter 16

Overwhelming Evidence for Evolution

• A) From Biogeography

• B) From Comparative anatomy

• C) From Geologic discoveries

A) Biogeography - is the science which

deals with geographic patterns of species distribution and the processes that result in such patterns.

• Size of the known world expanded

enormously in the 15th century

• Discovery of new organisms in previously

unknown places could not be explained by

accepted beliefs

– How did species get from center of creation

to all these places?

B) Comparative Morphology

• Study of similarities and differences in body plans of major groups

• Puzzling patterns:– Animals as different as whales and bats

have similar bones in forelimbs

– Some parts seem to have no function

C) Geological Discoveries

• Similar rock layers throughout world

• Certain layers contain fossils

• Deeper layers contain simpler fossils than

shallow layers

• Some fossils seem to be related to known

species

Darwin’s Voyage

• At age 22, Charles Darwin began a five-year, round-the-world voyage aboard the Beagle

• In his role as ship’s naturalist he collected and examined the species that inhabited the regions the ship visited

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Voyage of the Beagle

GalapagosIslands

Isabela

Darwin

Wolf

Pinta

Marchena Genovesa

Fernandia

SantiagoBartolomé

RabidaPinzon

SeymourBaltra

Santa Cruz

Santa Fe

Tortuga

Española

San Cristobal

Floreana

• Volcanic islands far off coast of Ecuador

• All inhabitants are descended from species that arrived on islands from elsewhere

Lyell Provided the foundation for Darwin to draw his

conclusions• Lyell’s Principles of Geology

• Subtle, repetitive processes of change, had shaped Earth

• Challenged the view that Earth was only 6,000 years old, to millions of years

Malthus - Struggle to Survive - this too shaped Darwins’

views• Thomas Malthus, a clergyman and

economist, wrote essay that Darwin read on his return to England

• Argued that as population size increases, resources dwindle, the struggle to live intensifies and conflict increases

Darwin put together the short term struggle for existence (Malthus) with the great geological timescale (Lyell)

Galapagos Finches

• Darwin observed finches with a variety of lifestyles and body forms

• On his return he learned that there were 13 species

• He attempted to correlate variations in their traits with environmental challenges

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Darwin’s Theory

Survival of the fittest

On the Origin of Species

• Darwin’s book

• Published in 1859

• Laid out in great detail his evidence

in support of the theory of evolution

by natural selection

Populations Evolve

• Biological evolution does not change individuals

• It changes a population

• Traits (features) in a population vary among individuals

• Evolution is change in frequency Evolution is change in frequency of traitsof traits

When does evolution not occur?

• No mutation

• Random mating

• Gene doesn’t affect survival or

reproduction

• Large population

• No immigration/emigration

When 5 conditions are met

Hardy-Weinberg Rule

At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation:

p2 AA + 2pq Aa + q2 aa = 1

Frequency of allele A = p

Frequency of allele a = q

No Change through Generations

STARTING POPULATION

490 AA butterfliesdark-blue wings

420 Aa butterfliesmedium-blue wings

90 aa butterflieswhite wings


THE NEXT GENERATION

420 Aa butterfliesmedium-blue wings


THE NEXT GENERATION


420 Aa butterfliesMedium-blue wings


Gene Mutations

• Infrequent but inevitable

• Every gene is subject to mutation to some degree

• Changes the frequency of alleles in a population

= EVOLUTION

Directional Selection

• Allele frequencies shift in one direction

Range of values for the trait at time 1

Number of individuals

in population



in population



in population

Giraffes

• Baby giraffes in Africa

• Some have inherited long necks others shorter ones

• When food is limited only the ones with longer necks can feed

• The others die of starvation

• Now the population has individuals with longer neck alleles on average

Natural Selection

• A difference in the survival and

reproductive success of different

phenotypes

• Acts directly on phenotypes and indirectly

on genotypes

Stabilizing Selection

• Intermediate forms are favored and extremes are eliminated

Range of values for wing-color trait at time 1


in population



in population



in population


Stabilizing Selection: Another Example

• Weight distribution for 13,370 human newborns (yellow curve) correlated with death rate (white curve)

Disruptive Selection

• Forms at both ends of the range of variation are favored

• Intermediate forms are selected against


in population



in population



in population


Balanced Polymorphism

• Polymorphism - “having many forms”

• Occurs when two or more alleles are maintained at frequencies greater than 1 percent

Sickle-Cell Trait: Heterozygote Advantage

• Allele HbS causes sickle-cell anemia when heterozygous

• Heterozygotes are more resistant to malaria than homozygotes

less than 1 in 1,600

1 in 400-1,600

1 in 180-400

1 in 100-180

1 in 64-100

more than 1 in 64

Malaria case

Sickle cell trait

Genetic Drift

• Random change in allele frequencies brought about by chance

• Effect is most pronounced in small populations

• Sampling error - Fewer times an event occurs, greater the variance in outcome

Bottleneck

• A severe reduction in population size

• Causes pronounced drift

• Example – Elephant seal population hunted down to

just 20 individuals – Population rebounded to 30,000– Electrophoresis revealed there is now no

allele variation at 24 genes

Founder Effect

• Effect of drift when a small number of individuals start a new population

• By chance, allele frequencies of founders may not be same as those in original population

• Effect is pronounced on isolated islands

Inbreeding

• Nonrandom mating between related individuals

• Leads to increased homozygosity

• Can lower fitness when deleterious recessive alleles are expressed

• Amish, cheetahs

Gene Flow

• Physical flow of alleles into a population• Tends to keep the gene pools of

populations similar• Counters the differences that result from

mutation, natural selection, and genetic drift

• Prevents the two populations forming new species!!!

processes of evolution chapter 16. overwhelming evidence for evolution a) from biogeography b) from...

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