A Scientific Explanation for Similarities and Differences between Organisms

Definitions
• Evolution is a change in the characteristics of a population over time
• Two characteristics identified by Darwin’s work
– Descent with modification: species diverge from a common ancestor
– Natural selection: individuals with favorable traits accumulate within a population
• Evolution is a change in the genetic composition of a population from generation to generation
– Can include mechanisms other than natural selection such as genetic drift and gene flow
Populations evolve as natural selection acts on its individuals.

Prior to Darwin
• Prevailing view that species were unchanging
– Aristotle proposed a “scale of nature”, that organized fixed species from least to most complex
– Special Creation as the origin of species was reinforced by religious thought and interpretation of scientific evidence, such as the classification scheme of Linnaeus
• Differing views of the history of the earth as seen in the fossil record
– Catastrophism: events in past occurred suddenly and are unlike mechanisms of today
– Uniformitarianism: mechanisms of change are constant over time

Evolutionary Thought Before Darwin
• Lamarck developed ideas for changes among species
– Use and disuse – Individuals retain characteristics that are required for their survival, leads to adaptation
– Inheritance of acquired traits – Individuals inherit traits that were developed over the lifetimes of their ancestors
– Innate drive for species to become more complex

A Flowchart of Evolutionary Reasoning
Variations exist between members of a population
Relatively constant resources and population over time
At least some variation is hereditary
More offspring are born than survive and reproduce observations inferences
Individuals whose inherited traits give a higher probability of survival and reproduction leave a greater number of offspring
Favorable traits will accumulate in the population over generations
2
Identify the inference that shows
1. Evolution
2. Natural Selection
1

Darwin’s
Observations
Islands with saddle shell giant tortoise had tall prickly pear cacti.
Adaptation: characteristic that favors survival and reproduction

Darwin’s Observations
Galapagos finches had beak shapes suited to food sources on specific islands.
Cactuseater
Seed-eater
Insect-eater

Darwin’s Observations
Galapagos finches resembled the grassquit found on the coast of Ecuador.
Recent studies suggest that the finch ancestor may have originated from the
Caribbean islands.

Explaining the long neck of a giraffe
Giraffes stretched their necks to reach food on tall trees and passed the longer necks onto their offspring

Evolution as a Change in Genetic Composition
Natural selection led to an increase in dark colored peppered moths in industrial regions of Britain.

How is this trait inherited?
• Wing color is the result of a single gene with two alternative forms called alleles
• One allele codes for dark color, the other codes for light color
• Each moth inherits two alleles for wing color
• Only one dark allele is needed for the moth to have dark wings since the dark color allele is dominant over the recessive light color allele
• Two light color alleles are required for a moth to have light wings
• If birds remove light winged moths from the population, dark winged moths survive and pass the dominant allele for dark wings to their offspring
• The increase in frequency of the dark wing allele is evidence of evolutionary change

How is this trait inherited?
Phenotype
(trait)
Genotype
(inherited alleles)
Dark moth DD
Dark moth
Light moth
Dd dd
Homozygous
(identical alleles)
Heterozygous
(different alleles) homozygous
The homologous chromosomes carrying alleles of the same gene separate during Meiosis I so that each gamete receives only one of the two alleles.

How is this trait inherited?
Genotype for Dark moth
Dd d d
Genotype for Light moth dd
D Dd Dd d dd dd
50% dark moths
50% light moths
Genotype for Dark moth
Dd
D d
Genotype for Dark moth
Dd
D DD Dd d
Dd dd
75% dark moths
25% light moths

Dd dd dd
Dd dd dd dd dd dd dd
Birds remove half of light wing moths
Dd dd
Dd dd dd dd
Simulating
Natural Selection
Reproduction occurs
Dd x dd  50% Dd + 50% dd
Dd x dd  50% Dd + 50% dd dd x dd  0% D_ + 100% dd
Total Dd = 100/300 = 33.3% dark wing moths
20% dark wing moths
33.3% dark wing moths
Natural selection acts on the phenotype.
Evolution is the resulting change in genotypes within the population .

Evidence for Evolution: Artificial Selection

Evidence for Evolution: Adaptation
Object Mimicry provides camouflage for evading predators
Walking stick
Treehoppers

Evidence for Evolution: Direct Observation
Increase in guppy coloration with predator change

Evidence for Evolution: Direct Observation
Resistance of HIV virus to drug therapy

Evidence for Evolution: Fossil Record
Living organisms resemble extinct fossil forms

Evidence for Evolution: Fossil Record
Progressive changes can be seen from simpler to more complex organisms

Evidence for Evolution:
Comparative Anatomy
• Structures that do not show a common origin
– Analogous structures: superficially similar
• Structures that show a common origin
– Homologous Structures: same evolutionary origin despite differences in function
– Vestigial Structures: serve no purpose but are homologous to structures in related organisms

Analogous and Homologous Structures
Not homologous; analogous
Not homologous; not analogous
Homologous; not analogous
Homologous; analogous

Homologous Structures
Flying Swimming Running Grasping

Vestigial
Structures
Remnants of hindlimb seen in boa and whale
Functional hindlimb in salamander

Vestigial Structures

Evidence for Evolution: Developmental Biology

Evidence for Evolution: Biochemistry
Molecular similarities between different organisms

Evidence for Evolution: Molecular Biology
Similarities in sequence measured by ease of separating hybrid DNA strands by heat

Evidence for Evolution: Genetics
• Mutation generates diversity
• Meiosis and Fertilization generate new combinations due to
– Crossing Over
– Alternate patterns of chromosome segregation
– Unique chromosomes of fertilizing sperm combined with unique chromosomes of egg

Evidence for Evolution: Biogeography
Plants and animals of each continent are distinctive

Evidence for Evolution: Convergent Evolution
Unrelated organisms in similar environments show some of the same adaptations

Evidence for Evolution:
Convergent Evolution
Yellow-Throated
Longclaw of Africa
Cactus of
North America
Euphorb of
Africa
Meadowlark of
North America

Other Patterns: Divergent Evolution
Different phenotypes arise as related species encounter environmental differences

Other Patterns: Coevolution
Interacting species adjust together to maintain a relationship
Predators and their Prey
Flowering plants and their Pollinators

Applying Your Knowledge
1. Adaptation
2. Homologous Structures
3. Analogous Structures
4. Divergent Evolution
5. Convergent Evolution
A. Leads to similar traits in unrelated species
B. Traits that have a common origin
C. A trait that makes a species survival more likely is called a(n)