Evidence of Evolution

Voyage of the Beagle
• Charles Darwin’s observations on a voyage around the world led to new ideas about species

Voyage of the Beagle

Darwin, Wallace, and Natural
Selection
• In 1858, Charles Darwin and Alfred
Wallace independently proposed a new theory, that natural selection can bring about evolution

Variations in Traits
• Darwin observed that variations in traits influence an individual’s ability to secure resources – to survive and reproduce

Theory of Natural Selection
• Natural selection
– The differential in survival and reproduction among individuals of a population that vary in details of their shared traits
– Can lead to increased fitness
• Fitness
– An individual’s adaptation to an environment, measured by its relative genetic contribution to future generations

Fossil Evidence
• Fossils
– Physical evidence of life in the distant past
• Found in stacked layers of sedimentary rock
– Younger fossils in more recently deposited layers
– Older fossils underneath, in older layers

Stratification

Fossilization

Interpreting the Fossil Record
• The fossil record is incomplete
• Favors species with hard parts, dense populations with wide distribution, and that persisted a long time

Plate Tectonics Theory
• Movements of Earth’s tectonic plates rafted land masses to new positions
• Pangea : First ancient supercontinent
– Gondwana (later southern supercontinent)
• Movements had profound impacts on the directions of life’s evolution

Biogeographical Evidence

Morphological Divergence
• Homologous structures : Similar body parts that became modified differently in different lineages
• Evidence of descent from a common ancestor

Comparative Morphological
Evidence

Homologous Structures

Analogous Structures

DNA, RNA, and Proteins
• Comparisons of DNA, RNA, and proteins reveal and clarify evolutionary relationships

Populations Evolve
• Population
– Individuals of the same species in the same area
– Generally the same number and kinds of genes for the same traits
• Gene pool
– All the genes of a population

Variation in Alleles
• Individuals who inherit different combinations of alleles vary in details of one or more traits
• Mutations are the original source of new alleles
– Lethal mutations result in death
– Neutral mutations neither help nor hurt

Phenotypic Variation in
Populations

Microevolution
• Changes in allele frequencies of a population
– Mutation
– Natural selection
– Genetic drift
– Gene flow

Natural Selection
• Natural selection
– Differential survival and reproduction among individuals of a population that show variations in details of their shared traits ( alleles )
• Allele frequencies
– Maintained by stabilizing selection
– Shifted by directional or disruptive selection

Modes of Natural Selection

Peppered Moth

Pocket Mice

Stabilizing Selection: Birth
Weight

Sexual Selection

Evolution of sickle cell anemia

Genetic Drift
• Genetic drift
– Random change in a population’s allele frequencies over time, due to chance
– Can lead to loss of genetic diversity
• Most pronounced in small or inbred populations
– Bottleneck: Drastic reduction in population
– Founder effect: Small founding group

Gene Flow
• Gene flow
– Movement of alleles into or out of a population by immigration or emigration
– Helps keep populations of same species similar
• Counters processes that cause populations to diverge (mutation, natural selection, genetic drift)

Gene Flow Between Oak
Populations

Reproductive Isolation
• Individuals of a sexually reproducing species can produce fertile offspring, but are reproductively isolated
• Reproductive isolating mechanisms evolve when gene flow between populations stops
• Divergences may lead to new species

Mechanical Isolation

Behavioral Isolation

Allopatric Speciation
• A geographic barrier stops gene flow between two or more populations of a species
– Example: Isolated continents or archipelagos
• Genetic divergence and reproductive isolation give rise to new species

Allopatric Speciations

An Isolated Archipelago

Patterns of Macroevolution
• Coevolution
– Close ecological interactions cause two species to act as agents of selection upon one another
• Extinction
– Irrevocable loss of species
– Mass extinctions and recoveries have occurred several times in the history of life
– Most species that ever existed are now extinct

Coevolution

Adaptation to What?
• Evolutionary adaptation
– Heritable traits that improve an individual’s chance of surviving and reproducing (under conditions that prevailed when genes evolved)