Darwin’s Theory of Natural Selection

Darwin on the HMS Beagle

Evolution

Darwin’s role on the ship was as naturalist and companion to the captain.

His job was to collect biological and geological specimens during the ship’s travel.

Section 1

The Galápagos Islands

Evolution

Darwin began to collect mockingbirds, finches, and other animals on the four islands.

He noticed that the different islands seemed to have their own, slightly different varieties of animals.

Darwin’s Theory of Natural Selection

Section 1

Almost every specimen that Darwin had collected on the islands was new to European scientists.

Evolution

Populations from the mainland changed after reaching the Galápagos.

Darwin’s Theory of Natural Selection

Section 1

Darwin Continued His Studies

Evolution

Darwin hypothesized that new species could appear gradually through small changes in ancestral species.

Darwin inferred that if humans could change species by artificial selection, then perhaps the same process could work in nature.

Darwin’s Theory of Natural Selection

Section 1

Natural Selection

Evolution

Individuals in a population show variations.

Variations can be inherited.

Organisms have more offspring than can survive on available resources.

Variations that increase reproductive success will have a greater chance of being passed on.

Darwin’s Theory of Natural Selection

Section 1

Darwin’s theory of natural selection is not synonymous with evolution.

The Origin of Species

Evolution

Darwin published On the Origin of Species by Means of Natural Selection in 1859.

It is a means of explaining how evolution works.

Darwin’s Theory of Natural Selection

Section 1

Evidence of Evolution

Support for Evolution

Evolution

The fossil record

Glyptodont

Fossils provide a record of species that lived long ago.

Fossils show that ancient species share similarities with species that now live on Earth.

Section 2

The ancient Glyptodont have been compared to modern day armadillos.

Fossils

Evolution

Derived traits are newly evolved features, such as feathers, that do not appear in the fossils of common ancestors.

Ancestral traits are more primitive features, such as teeth and tails, that do appear in ancestral forms.

Anatomically similar structures inherited from a common ancestor are called homologous structures.

Evidence of Evolution

Section 2

Homologous Structures

Evolution

Show that functionally similar features can evolve independently in similar environments

Analogous structures can be used for the same purpose and can be superficially similar in construction, but are not inherited from a common ancestor.

Evidence of Evolution

Section 2

Analogous Structures

Evolution

Vestigial Structures

Structures that are the reduced forms of functional structures in other organisms.

Evidence of Evolution

Section 2

Evolutionary theorypredicts that features of ancestors that no longer have a function for that species will become smaller over time until they are lost.

Vestigial Structures

Evolution

Vertebrate embryos exhibit homologous structures during certain phases of development but become totally different structures in the adult forms.

Comparative Embryology

Evidence of Evolution

Section 2

Embryology

Evolution

Common ancestry can be seen in the complex metabolic molecules that many different organisms share.

Comparative Biochemistry

Evidence of Evolution

Section 2

Evolution

Comparisons of the similarities in these molecules across species reflect evolutionary patterns seen in comparative anatomy and in the fossil record.

Organisms with closely related morphological features have more closely related molecular features.

Evidence of Evolution

Section 2

Geographic Distribution

Evolution

The distribution of plants and animals that Darwin saw first suggested evolution to Darwin.

Evidence of Evolution

Section 2

Patterns of migration were critical to Darwin when he was developing his theory.

Evolution is intimately linked with climate and geological forces.

Evolution

Types of Adaptation

An adaptation is a trait shaped by natural selection that increases an organism’s reproductive success.

Fitness is a measure of the relative contribution an individual trait makes to the next generation.

Evidence of Evolution

Section 2

Evolution

Camouflage

Allows organisms to become almost invisible to predators

Evidence of Evolution

Section 2

Mimicry

One species evolves to resemble another species.

Mimicry

Camouflage

Non-resistant bacterium

Resistant bacterium

Antibiotic

When the population is exposed to an antibiotic, only the resistant bacteria survive.

The bacteria in a population vary in their ability to resist antibiotics.

The resistant bacteria live and produce more resistant bacteria.

Physiological adaptations can develop rapidlyPhysiological adaptations can develop rapidly

Today, penicillin no longer affects as many species of bacteria because some species have evolved physiological adaptations to prevent being killed by penicillin.

•Population Genetics – study of the behavior of genes in a population•Within an organism’s lifetime, it cannot evolve a new phenotype by natural selection in response to its environment.• Natural selection acts on the range of phenotypes in a population.• Each member has the genes that characterize the traits of the species, and these genes exist as pairs of alleles.• Evolution occurs as a population’s genes and their frequencies change over time.

15.3: Shaping Evolutionary TheoryPopulations evolve, not individuals15.3: Shaping Evolutionary TheoryPopulations evolve, not individuals

How can a population’s genes change over time?• Picture all of the alleles of the population’s genes as being together in a large pool called a gene pool.• The percentage of any specific allele in the gene pool is called the allelic frequency.• They refer to a population in which the frequency of alleles remains the same over generations as being in genetic equilibrium.

Populations, not individuals, evolvePopulations, not individuals, evolve

• A population that is in genetic equilibrium is not evolving.• Any factor that affects the genes in the gene pool can change allelic frequencies, disrupting a population’s genetic equilibrium, which results in the process of evolution.

•Mutation•Genetic Drift•Gene Flow•Nonrandom Mating•Natural Selection

Changes in genetic equilibriumChanges in genetic equilibrium

Evolution

Genetic Drift

A change in the allelic frequencies in a population that is due to chance

In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.

Shaping Evolutionary Theory

Section 3

Evolution

Founder Effect

Occurs when a small sample of a population settles in a location separated from the rest of the population

Alleles that were uncommon in the original population might be common in the new population.

Shaping Evolutionary Theory

Section 3

Evolution

Bottleneck Occurs when a population declines to a very

low number and then rebounds

Shaping Evolutionary Theory

Section 3

Evolution

Gene Flow

Increases genetic variation within a population and reduces differences between populations

Nonrandom Mating

Promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits

Shaping Evolutionary Theory

Section 3

Evolution

Natural Selection

Acts to select the individuals that are best adapted for survival and reproduction

Shaping Evolutionary Theory

Section 3

Evolution

Stabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness.

Shaping Evolutionary Theory

Section 3

Evolution

Directional selection makes an organism more fit.

Shaping Evolutionary Theory

Section 3

Evolution

Disruptive selection is a process that splits a population into two groups.

Shaping Evolutionary Theory

Section 3

Evolution

Sexual selection operates in populations where males and females differ significantly in appearance.

Qualities of sexual attractiveness appear to be the opposite of qualities that might enhance survival.

Shaping Evolutionary Theory

Section 3

• Species - group of organisms that look alike and can interbreed to produce fertile offspring in nature.•Speciation - occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.

•Geographic Isolation (allopatric speciation)•Reproductive Isolation (sympatric speciation)

15.3: Shaping Evolutionary TheoryReproductive Isolation and Speciation15.3: Shaping Evolutionary TheoryReproductive Isolation and Speciation

Evolution

Allopatric Speciation – Geographic Isolation

A physical barrier divides one population into two or more populations.

Shaping Evolutionary Theory

Section 3

The Grand Canyon is a geographic barrier separating the Abert and Kaibab squirrels.

Evolution

Sympatric Speciation – Reproductive Isolation

A species evolves into a new species without a physical barrier.

The ancestor species and the new species live side by side during the speciation process.

Shaping Evolutionary Theory

Section 3

Evolution

Prezygotic isolation prevents reproduction by making fertilization unlikely.

Prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differences

Eastern meadowlark and Western meadowlark

Shaping Evolutionary Theory

Section 3

Postzygotic isolation occurs when fertilization has occurred but a hybrid offspring cannot develop or reproduce.

Evolution

Prevents offspring survival or reproduction

Shaping Evolutionary Theory

Section 3

The offspring of a male lion and a female tiger is a liger. Ligers are sterile.

Evolution

Adaptive Radiation

Follows large-scale extinction events

Can occur in a relatively short time when one speciesgives rise to many different species in response to the creation of new habitat or some other ecological opportunity

Shaping Evolutionary Theory

Section 3

Evolution

Coevolution

The relationship between two species might be so close that the evolution of one species affects the evolution of the other species.

Mutualism

Coevolutionary arms race

Shaping Evolutionary Theory

Section 3

Evolution

Unrelated species evolve similar traits even though they live in different parts of the world.

Divergent Evolution

Unrelated species evolve similar traits even though they live in different parts of the world.

Convergent Evolution

Shaping Evolutionary Theory

Section 3

Evolution

Rate of Speciation

Evolution proceeds in small, gradual steps according to a theory called gradualism.

Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.

Shaping Evolutionary Theory

Section 3

Evolution

Shaping Evolutionary Theory

Section 3