The Origin of Species Define: Species Speciation Role in Speciation Biological Species Concept Ecological Species Concept Speciation Sympatric Speciation Allopatric Speciation Reproductive Barriers Instantaneous Speciation - Polyploidy Role in Speciation Genetic Drift Adaptive Radiation Pace of Evolution Gradualism Punctuated Equilibrium

Concept of species must account for two phenomena: I. Define: Species What is a species? One of the basic units of biological classification and taxonomic rank A group of organisms capable of interbreeding and producing fertile offspring Concept of species must account for two phenomena: Distinctiveness of species that occur together in a single location or population The connection that exists among different populations within the same species

Speciation is the process by which new species arise, either by: Transformation of one species into another Splitting of one ancestral species into 2 descendant species Vermilion Cardinal (Cardinalis phoeniceus) Northern Cardinal,Cardinalis cardinalis Northern Cardinal female

Bengal tiger, (Panthera tigris tigris) Leopard (Panthera Pardus) Melanistic morph Siberian tiger (Panthera tigris altaica)

Population - any group of individuals, single species, occupying a given area at the same time. E.g. herd of buffalo, school of fish, flock of birds Subspecies - a geographically defined population or group of populations within a single species that has distinctive characteristics E.g. wolves, coyotes, and dogs

Populations

Biological Species Concept Developed by Ernest Mayr in 1942 which defines a species as: “…groups of actually or potentially interbreeding natural populations which are reproductive isolated from other such groups” Therefore: Members of a population mate with each other AND produce fertile offspring

Biological species concept defines species as members of populations that actually/potentially interbreed in nature not according to similarity of appearance Appearance is helpful in identifying species, it does not define species. Organisms may look different and yet be same species Concept relies on reproductive isolation between species in nature. Problems arise due to hybridization.

Western & Eastern Meadowlarks look the same, & ranges overlap, but they do not interbreed due to differences in their distinct songs one might think they are distantly related species. In fact, they are sisters — of species Pheidole barbata . They are fulfilling different roles in same colony

Ecological Species Concept An alternative hypothesis that proposes the distinctions among species are maintained by Natural Selection. With this hypothesis, hybridization has little effect because alleles introduced into one species’ gene pool from other species are quickly eliminated by natural selection.   This concept also has some exceptions. In reality some species follow the biological species concept while others follow the ecological species concept.

Species that occur together: II. Speciation Sympatric Speciation – differentiation of populations within a common geographic area into new species; some sort of reproductive barrier has to develop Species that occur together: Are distinctive entities Phenotypically different Utilize different parts of the habitat Separate behaviors – reproduction, nesting, migration  

Allopatric Speciation – differentiation of populations that are geographically isolated; also known as geographic speciation; physically isolation creates a reproductive barrier

Reproductive Barriers In nature barriers have developed between species that cause species to be reproductively isolated, which means that population members do not mate with each or cannot produce fertile offspring. These barriers are called reproductive isolating mechanisms because they prevent genetic exchange between species.

There are two broad categories of isolating mechanisms: Pre-zygotic – barrier that prevents the formation of the zygote (before-zygote) Post-zygotic – barrier that prevents the proper functioning of a zygote after it forms (after-zygote)

Pre-Zygotic Geographic Isolation Description – species occur in different areas, which are often separated by a physical barrier, such as a river or mountain range E.g. Grand Canyon – Antelope Squirrels and Tassel-eared Squirrels

Ecological Isolation Description – Species occur in the same area but occupy different habitats and rarely encounter each other E.g. Two species of garter snakes in the genus Theamnophis occur in the same are but one lives main in water and the other is primarily terrestrial

Behavioral Isolation Description – Species differ in their mating rituals Unique visual signals and cues Unique call or sounds Unique chemical signals – pheromones E.g. Birds of Paradise, Blue- Footed Boobies, insect calls, frog croaks, etc. Birds of Paradise

Temporal Isolation Description – Species reproduce in different seasons or at different times of the day E.g. Field Crickets – mate different time of year Spotted Skunk – mate different times

Mechanical Isolation Description – structural differences between species prevent mating E.g. Different species of snail, Bradybaena, have shells that spiral in opposite directions which interferes with the alignment of their sex organs. Also seen in many plant species.

Prevention of Gamete Formation Description – gametes of one species function poorly with the gametes of another species or within the reproductive tract of another species. E.g. Egg and sperm are not compatible – Seen in lots of aquatic animals that release sperm and egg into the water in order to become fertilized.

Post Zygotic Hybrid Inviability Description – hybrid embryos do not develop properly and/or hybrid adults do not survive in nature E.g. hybrid embryos between sheep and goats die early in development. Hybrid seeds between different species of plants never germinate into a new plant.

E.g. mule, liger, zorse, beefalo, cama Hybrid Infertility Description – Hybrid adults are sterile or have reduced fertility E.g. mule, liger, zorse, beefalo, cama

How do ancestral species become reproductively isolated? Speciation is a continuous process 2 populations present but only have partially reproductive isolation with no isolating mechanism yet. These populations will interbreed. Hypothetically - hybrids between these two populations have decreased fertility and are not well adapted. What would happen to these hybrids? Selection is favoring the parental populations and begins to prevent hybridization. The 2 populations eventually becomes reproductively isolated so that no genetic exchange occurs creating two different species Natural selection reinforces incomplete isolating mechanism unit it is complete

Instantaneous Speciation This type of speciation can occur by polyploidy which creates an individual that is now reproductively isolated from the other member of the species. Polyploidy – individuals with more than 2 sets of chromosomes. Seen in plants which have 4 sets of chromosomes (4n = tetraploids) that survive but cannot be fertilized by the original diploid (2n) individuals.

Tetraploids can interbreed creating a new species. Allopolyploid – when two species hybridize which results in an offspring with 1 chromosome from each species and cannot be fertilized by the original 2 species (infertile). Can reproduce asexually Fertile if chromosomes double (become polyploidy)   Tetraploids can interbreed creating a new species. Frequently seen in Plants Sometimes in insects, fish and salamanders

III. Role of Speciation Genetic Drift Random fluctuations in allele frequencies over time by chance Random changes may cause reproductive isolation, which results in new species. Genetic drift in small populations can occur due to:

Founder Effect Loss of genetic variation that occurs when a new population is established by a small number of individuals E.g. Common in island populations - Finches Isolated religious communities - Amish

Bottleneck Effect Population’s number vastly decreases Due to natural disaster E.g. Populations that survives after a forest fire or earthquake

Adaptive Radiation The evolution of several divergent forms from a primitive and unspecialized ancestor Can also result when a new trait, called a key innovation, evolves within a species allowing it to use resources or other aspects of the environment that were previously inaccessible. Related species formed that adapted to different parts of a new environment often with decreased competition and many resources. Requires both speciation and adaptation to a new environment. E.g. Finches

IV. Pace of Evolution Gradualism – accumulation of small changes over a long period of time. Evolution is a long gradual process. (older hypothesis) Punctuated Equilibrium – long period of no change, followed by rapid change. More recent hypothesis – “bursts of speciation” * NOTE: Evolution may include BOTH models of change.