evolution evolution is genetic change in a line of descent through successive generations cumulative...

Evolution

• Evolution is genetic change in a line of descent through successive generations

• Cumulative change in a species over time

Early Scientific Theories

• Scala Naturae – Aristotle: life forms could be arranged on a scale of increasing complexity

• The Great Chain of Being– All species separate link– Each link created at the same time

in the center of creation– All links have remained the same

since creation• New Evidence challenged these

Biogeography

• The world’s distribution of organisms

• 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?

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

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

19th Century - New Theories• Scientists attempt to reconcile evidence of

change with traditional belief in a single creation event– Georges-Louis Leclerc de Buffon – perhaps

species originated in more than one place and perhaps they were modified over time

– Georges Cuvier - multiple catastrophes– Jean Lamarck - inheritance of acquired

characteristics (environmental pressures change individuals and the changes are passed on to their offspring)

• Use and disuse

The Theory of Uniformity

• Lyell’s Principles of Geology (book)

• Subtle, repetitive processes of change had shaped Earth (theory of uniformity)

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

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

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

Figure 17.4dPage 275

Malthus - Struggle to Survive

• Thomas Malthus, a clergyman and economist, wrote an essay that Darwin read on his return to England

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

Darwin Expands• Suspected that any population has the

capacity to produce more individuals than the environment can support

• The environment restricts the number of reproducing individuals

• Variations in traits might affect an individual’s ability to secure resources, survive, and reproduce in particular environments

Darwin’s Theory

• A population can change over time when

individuals differ in one or more heritable

traits that are responsible for differences in

the ability to survive and reproduce in certain

environments. (descent with modification)

• Gradual evolution by natural selection

Darwin’s “Theory”

• In what way is evolution both fact and theory?

Alfred Wallace

• Naturalist who arrived at the same

conclusions Darwin did

• Wrote to Darwin describing his views

• Prompted Darwin to finally present

his ideas in a formal paper

Populations Evolve

• Biological evolution does not change individuals, it changes a population

• Traits in a population vary among individuals

• Evolution is change in frequency of traits

Genetic Equilibrium

• Allele frequencies at a locus are not changing

• Population is not evolving

Hardy-Weinberg theorem: Five Conditions for Genetic Equilibrium

• No mutation

• Random mating

• Gene doesn’t affect survival or reproduction

• Large population

• No immigration/emigration

Microevolutionary Processes

• Drive a population away from genetic equilibrium

• Small-scale changes in allele frequencies brought about by:– Natural selection

– Gene flow

– Genetic drift

Gene Mutations

• Infrequent but inevitable

• Each gene has own mutation rate

• Lethal mutations

• Neutral mutations

• Advantageous mutations

Hardy-Weinberg Rule (read the focus on science section in this chapter (17.5)

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

p2 + 2pq + q2 = 1

– Frequency of dominant allele = p, frequency of

recessive allele = q

• Used to see if allelic frequencies change between generations (can detect evolution)– Can only remain stable if the 5 assumptions of the

rule are met (no mutations, large population, isolated population, all individuals survive and reproduce equally, and random matings occur)

Natural Selection

• A difference in the survival and reproductive

success of different phenotypes

• Acts directly on phenotypes and indirectly on

genotypes

• Eventually the individuals of a population will end up competing for resources

• Some phenotypes compete better than others

Change over Time

• Over time, the alleles that produce the most successful phenotypes will increase in the population

• Less successful alleles will become less common

• Change leads to increased fitness– Increased adaptation to environment

Directional Selection

• Allele frequencies shift in one direction

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Range of values for the trait at time 1

Range of values for the trait at time 2

Range of values for the trait at time 3

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Figure 17.10Page 282

Stabilizing Selection

• Intermediate forms are favored and extremes are eliminated

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Range of values for the trait at time 1

Range of values for the trait at time 2

Range of values for the trait at time 3Figure 17.12Page 284

Disruptive or Diversifying Selection

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

• Intermediate forms are selected against

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Range of values for the trait at time 1

Range of values for the trait at time 2

Range of values for the trait at time 3

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Figure 17.14Page 285

Sexual Selection

• Selection favors certain secondary sexual characteristics

• Through nonrandom mating, alleles for preferred traits increase

• Leads to increased sexual dimorphism

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

Figure 17.17Page 286-287

Gene Flow• Physical flow of alleles into and out of a

population– Through immigration and emigration

• Tends to keep the gene pools of populations similar– Keeps separated populations genetically similar

• Counters the differences that result from mutation, natural selection, and genetic drift

• Could move a unique gene in or out that would push the population away from equilibrium

Genetic Drift

• Random change in allele frequencies brought about by chance

• Effect is most pronounced in small populations• Increases the chance that an allele will become

more or less prevalent when the number of individuals in a population is small

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 starts a new population

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

• Effect is pronounced on isolated islands