Define Species The Origin of Species Biological Species

• Define: Species The Origin of Species – 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

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 Vermilion Cardinal (Cardinalis phoeniceus) descendant species Northern Cardinal, Cardinalis cardinalis Northern Cardinal female

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

• 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.

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 (afterzygote)

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 – mature 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.

• 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 (4 n = tetraploids) that survive but cannot be fertilized by the original diploid (2 n) individuals.

• 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.