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)