Speciation and Evolution:Speciation and Evolution:

The ProcessesThe Processes

Defining Species: ReviewDefining Species: Review

• Evolution involves populations, not Individuals

• Species is a population of organisms whose members can interbreed under natural circumstances and reproduce fertile (viable) offspring

How Do New Species Arise?How Do New Species Arise?

• Two fundamental processes give rise to new species:• Cladogenesis: The splitting off of one species into two

clades, usually because of geographical isolation, but also because of reproductive isolation.

• Two kinds of species develop by cladogenesis: • Sympatric Species: Those whose speciation is the

product of geographical isolation• Allopatric Species: Those whose speciation is the

product of reproductive isolation of population in the same region.

• Anagenesis: The replacement of an ancestral species by a daughter species over time; the ancestral species become extinct.

Speciation: Evolution Through Speciation: Evolution Through CladogenesisCladogenesis

• Cladogenesis: • Time I: Genes flow freely in region• Time II: Barrier separates two

populations• Time III: Mutations change

genotype and phenotype of 2 populations

• Time IV: Two populations cannot interbreed even with removal of barrier

• Definition: Branching of one species into two

• From clade (“branch”) or group with common evolutionary ancestry.

Speciation: Allopatric SpeciesSpeciation: Allopatric Species

• Allopatric speciation occurs when two populations are separated by a geographical barrier (river, mountain range)

• In this example, three species of fish have evolved in separate zones

Speciation: Sympatric SpeciesSpeciation: Sympatric Species

• Sympatric species are those that are separated by a reproductively isolation mechanism

• Speciation occurs among three populations of fish even though the different species occupy the same region

• There are several ways for subspecies to become reproductively isolated

Modes of Reproductive Isolation In Modes of Reproductive Isolation In SympatrySympatry

• Ecological Isolation: Different populations are separated by occupy a slightly different niche

• Seasonal Isolation: The breeding season of two closely related populations do not match.

• Sexual Isolation: One or both sexes of a species initiate mating behavior that does not act a stimulus to the opposite sex of a closely related population

• Mechanical isolation: Populations do not mate because of an incompatibility of the male and female sex organs of the individuals (extreme example: wolves and Chihuahuas)

• Gamete Isolation: Incompatibility of sex cell with bodily environment

• Hybrid Infertility or Sterility: Hybrids do not survive or reproduce (mules)

Speciation: Role of Speciation: Role of MicromutationMicromutation

• Micromutation: Mutations with• extensive or important phenotypic

results• Example: Axolotl (species of

salamander)• This salamander starts life as

tadpole-like larvae, as do other salamanders

• Axolotl, however, never grows up—doesn’t sprout mature legs, keeps its gills, remains aquatic existence.

• Injection of a hormone enables maturity and to live on land, so that one mutation can and does create major change

Adaptive Radiation DefinedAdaptive Radiation Defined

• Definition: Evolution and spreading out of related species into new niches

• Niche: An environment in which an organism is found and its adaptive response to that environment

• Generalized Adaptive Radiation: The adaptation of a species to a wide range of niches. Homo sapiens is an example.

• Specialized Adaptive Radiation: The adaptation of a species to a narrow range of niches.

Adaptive Radiation: Favorable Adaptive Radiation: Favorable ConditionsConditions

• Absence of similar and therefore competing species

• Occurrence of extensive extinction, thereby emptying an environment of competitors

• Adaptive generalization of new group of related species which enable it to occupy several niches and displace species already there.

Adaptive Radiation: Absence of Adaptive Radiation: Absence of Competing SpeciesCompeting Species

• Example: Darwin’s finches on Galápagos Islands who were blown there by winds from mainland Ecuador

• Niches opened up for 13 varieties with different bills, including those that feed on cactus or eat specific insects in trees

• Others use twig or cactus spine to probe for insects

• A vampire finch sucks blood from larger birds

Finches and Their Adaptive Finches and Their Adaptive RadiationRadiation

• Ground finches (Geospiza) who are seed and cactus eaters;• Tree finches (Camarhynchus), who are insect and bud eaters • Warbler finches (Certhidea) who vary by color.

Adaptive Radiation: SpecializationAdaptive Radiation: Specialization

• Definition: Adaptation of a species to a narrow range of environmental niches

• Example: Again, some species of Darwin’s finches on Galápagos Islands are examples.

• Medium ground finch was nearly wiped out in the 1977 drought

• Sudden change could eliminate this or others of these genera and species of finches

• Example: prosimians adapt on in habitats afforded by Madagascar and are close to extinction.

Adaptive Radiation: GeneralizationAdaptive Radiation: Generalization

• Definition: Adaptation of a species to a wide range of environmental niches

• Examples:• Mammals spread after the disappearance of dinosaurs

65 m.y.a. and occupied innumerable niches, from grassland (ungulates) to trees (bats)

• Monkeys with a mixed diet occupied diverse arboreal (tree) habitats; they displaced the prosimians

• Humans: from frozen north to tropical rainforest or desert—thanks to culture—are the most generalized primate

Types of Evolution: Darwinian or Types of Evolution: Darwinian or Phyletic GradualismPhyletic Gradualism

• Definition:

• Slow, step-by-step changes over time

• Intermediate forms assume “missing links”

• Darwin postulated this model

• Examples: From monkeys to apes; apes to hominins (e.g. Lucy); and from early hominins to modern Homo sapiens

Problems with GradualismProblems with Gradualism

• Fossil record does not reveal fine gradations from one lifeform to a descendant life form: no “missing links.”

• Bipedalism occurred quickly as the fragmentary fossil record shows.

• Reproductive advantage: do slight changes bestow this advantage?

• Continuum question: at which point does a population become two species?

• Sometimes, change can take place rapidly, either through oscillating selection or punctuated equilibrium

Types of Evolution: Oscillating Types of Evolution: Oscillating SelectionSelection

• Definition: Adaptive variation around a norm rather than direction in response to environmental variation

• Example: Medium and small ground finch lacked a bill strong enough to crack tough seeds

• Occurrence of drought selected plants whose seeds had a tough exterior

• Survival of large, longer-billed finches• Smaller, shorter-billed finches returned after the climate

returned to normal,• Shifting bill size and lengths reflected the oscillation of

the environmental conditions.

Types of Evolution: Punctuated Types of Evolution: Punctuated EquilibriumEquilibrium

• Definition: Species tend to remain stable over time, then, evolutionary changes occur suddenly (in terms of centuries or millennia)

• Causation: Populations may become fragmented and isolated, and from there new forms arise

• Small, new populations may invade a region, and through the founder effect and better adaptation, create and spread a new species

• Example: Archaeopteryx (ancient bird), a dinosaur with feathers: suddenly appears and may have created a new class known as Aves (birds)

Phyletic Gradualism and Phyletic Gradualism and Punctuated EquilibriumPunctuated Equilibrium

• A summary of gradualism and punctuated equilibrium

Pseudoscience and CreationismPseudoscience and Creationism

• Pseudoscience consists of scientifically testable ideas in form that are taken on faith even after they are proven as false

• (Scientific) Creationism is the belief in a literal biblical interpretation of the creation of earth in six days 6,000 to 10,000 years ago

• The claim is testable, has been tested, and has been demonstrated to be false.

Counterevidence to CreationismCounterevidence to Creationism

• Existence of strata, such as the Grand Canyon, accumulated over 2 billion years falsifies the claim that the earth is only a few thousand years old

• Presence of extinct lifeforms, from fossil fish to dinosaurs, demonstrate that other forms existed at one time but are now extinct

• Presence of ancient hominins establish extinct humanlike creatures that look like us but are not us.

• Both kinds of evidence are abundant

ConclusionConclusion

• Species is unit of evolution• Evolutionary change is more random than

progressive• Speciation is the basic process of evolutionary

change• Changes may be gradual or rapid• Scientific rule: follow the evidence• Evidence for evolution is overwhelming in the

form of geological strata and fossil lifeforms