Evolution: Macroevolution
n Microevolution: changes on the small scale
n Changes in gene frequencies in a population
n Macroevolution: changes on the large scale
Species Formation
n Earth is home to numerous species
n Estimates range between 10 and 25 million species
n 4 million species is lowest estimate
n Present species are survivors or newcomers
n 99% of all species that have ever lived on Earth are now extinct
What Is a Species?
n All of the species on Earth share a common ancestor ~3.8 billion years ago
n Initial type of organisms branched into two types of organisms
n New “species”
Species Formation
n Process continued, producing all of the species that have ever lived on the planet
n These species branched further
n New species are formed after populations of a single species stop interbreeding
What Is a Species?
Biological species concept
n Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups
What Is a Species?
n Some separate species may be able to interbreed in captivity, but do not do so in nature
n e.g., Lion ♂ + tiger ♀ à liger
n e.g., Lion ♀ + tiger ♂ à tigon
n Since this interbreeding does not occur in nature, lions and tigers are separate species
n Mythical powers
What Is a Species?
n The biological species concept is not always sufficient in defining species
n Many bacteria reproduce asexually, not sexually
n There is sometimes limited very limited gene flow between two species
How Do New Species Arise?
n “Speciation” is the development of new species through evolution
n Branches from parent species, while parent species continues to exist
n Speciation results from the same processes operating in microevolution (changes in allaele frequencies in the population)
How Do New Species Arise?
n Evolution within a population involves a change in the population’s allele frequencies
n Two interbreeding populations will share any changes in allele frequencies
n These populations will evolve together and remain a single species
How Do New Species Arise?
n Two populations that do not interbreed will not share changes in allele frequencies
n Changes will add up over time
n Ultimately, a new species could be formed
How Do New Species Arise?
Allopatric speciation
n Geographic separation can restrict gene flow between populations
n Glaciers can move into an area
n A river can change course
n Ponds can dry up
n Part of a population may migrate into a remote area (e.g., Galápagos Islands, Hawaiian Islands, etc.)
How Do New Species Arise?
Allopatric speciation
n Restricted gene flow between two populations can ultimately result in the formation of a new species
n “Allopatric speciation”
How Do New Species Arise?
n During their geographic separation, allele frequencies of two populations will change differently
n These populations will evolve differently
n Physical or behavioral changes may result
How Do New Species Arise?
n When two geographically separated populations are reunited, they may or may not be able to interbreed
n If not, then speciation has occurred
How Do New Species Arise?
n Mechanisms preventing interbreeding are central to speciation
n Mountains and rivers are extrinsic isolating mechanisms
n Characteristics of the organisms are intrinsic isolating mechanisms
How Do New Species Arise?
n Intrinsic reproductive isolating mechanisms
n Any factor that prevents interbreeding of individuals of the same or closely related species
n Allopatric speciation involves extrinsic isolation (geographic separation) followed by the development of intrinsic isolating mechanisms
How Do New Species Arise?
n Intrinsic reproductive isolating mechanisms
n Ecological isolation
n Temporal isolation
n Behavioral isolation
n Mechanical isolation
n Gametic isolation
n Hybrid inviability or infertility
How Do New Species Arise?
n Ecological isolation
n Two species may feed, mate, and grow in different habitats within a common area
n e.g., Ranges of lions and tigers overlapped
n Lions preferred the open grasslands
n Tigers preferred the deep forests
n No interbreeding occurred
How Do New Species Arise?
n Ecological isolation
n Two species may feed, mate, and grow in different habitats within a common area
n e.g., Ranges of lions and tigers overlapped
n Lions preferred the open grasslands
n Tigers preferred the deep forests
n No interbreeding occurred
How Do New Species Arise?
n Behavioral isolation
n Individuals choose their mating partners
n This choice is often dependent upon courtship rituals
n Closely related species may have incompatible courtship rituals
n e.g., Songs of birds and crickets, fiddler crab claw waving, etc.
How Do New Species Arise?
n Mechanical isolation
n Reproductive organs of two closely related species may have incompatible sizes or shapes
n e.g., Different butterfly species have genitalia that differ in shape
How Do New Species Arise?
n Gametic isolation
n Mating may occur, but the sperm is incompatible with either the egg or the female reproductive system
How Do New Species Arise?
n Gametic isolation
n e.g., Sperm in pollen of one plant species cannot reach egg of related species
n e.g., Sperm of one animal species is killed in reproductive system of related species
n e.g., Sperm of one species cannot bind to receptors on egg of related species
How Do New Species Arise?
n Hybrid inviability or infertility
n Offspring resulting from a mating between closely related species may be unhealthy
n Offspring resulting from a mating between closely related species may be infertile
n e.g., Horse + donkey à mule
n Mules are healthy, but infertile hybrids
Sympatric Speciation
n The fruit fly Rhagoletis pomonella provides one of the best-studied examples of sympatric speciation
Sympatric Speciation
n R. pomonella
n Originally existed solely on hawthorn trees
n “Haw flies”
n Some moved to apple trees newly introduced from Europe
n Flies colonizing apple trees are becoming a new species
n “Apple flies”
Sympatric Speciation
n Haw fly life cycle
n These flies winter underground as larva
n Adult flies emerge in the summer
n Flies fly to their host trees, mate, and lay their eggs in the fruit
n Adult flies live for approximately one month
Sympatric Speciation
n A mutation or new combination of existing rare alleles arose in the ancestral haw flies
Sympatric Speciation
n Mutant flies emerged earlier in the summer
n These flies were attracted to apples as well as hawthorns
n Apples mature slightly earlier than hawthorn fruit
n These early emerging flies interbred amongst themselves to a high degree
n Limited gene flow between these populations
Sympatric Speciation
n Mating periods of “haw flies” and “apple flies” do not fully overlap
n Temporal isolation
n These two types of flies occupy different habitats in the same area
n Ecological isolation
n These two intrinsic reproductive isolating mechanisms have occurred without geographical separation
When Does Speciation Occur?
n Some species remain relatively unchanged for long periods of time
n e.g., Horseshoe crabs have changed little in 300 million years
n Other species change dramatically over relatively short periods of time
n e.g., The 13 species of Darwin’s finches arose from an ancestral species within the past 100,000 years
When Does Speciation Occur?
n Horseshoe crabs are generalists
n Extremely diverse diet
n Eat plants, animals, scavenged debris
n Shifts from one food source to another depending on availability
n Do not adapt in response to changes in food source
When Does Speciation Occur?
n Plants were established on the Galápagos Islands prior to arrival of finches
n No similar birds preceded the finches
n There was very little competition for the resources the islands offered
n Many niches were unoccupied
n Populations could specialize to fill one of many available niches
When Does Speciation Occur?
n Finches could fly between the 25 islands
n Water between the islands did represent a geographical barrier
n Reduced gene flow between populations on different islands
n These populations evolved into multiple species
When Does Speciation Occur?
n Darwin’s finches exemplify an “adaptive radiation”
n Rapid emergence of many species from a single species introduced into a new environment with unfilled ecological niches
n Two conditions conducive to speciation
n Specialization
n Migration to a new environment
Categorization ofEarth’s Living Things
n A taxonomic system is used to classify every known species on Earth
n Organisms are classified into various groups based on their evolutionary relationships
n Further classified according to physical characteristics
n Currently undergoing revision
n May rely solely on DNA analysis
Categorization ofEarth’s Living Things
n Eight basic categories are used
n Species, genus, family, order, class, phylum, kingdom, and domain
n Species is the most specific grouping
n Domain is the broadest grouping
Categorization
n Taxonomy gives a specific (Latin) scientific name to every species
n e.g., Homo sapiens, modern humans
n e.g., Rhagoletis pomonella, a fruit fly species
n e.g., Drosophila melanogaster, another species of fruit fly
n Specific scientific names allow scientists to know which type of fruit fly (for example) they are talking about
Categorization
n Closely related species are combined in a larger group called a “genus”
n The first word in a scientific name is actually the name of the genus
n e.g., Canis lupus, the gray wolf
n e.g. Canis familiaris, the domestic dog
n Both of these species belong in the same genus (Canis)
Categorization
n A variety of techniques are used to construct evolutionary histories
n Comparative morphology, etc.
n Comparisons of DNA, RNA, and protein sequences provide the bulk of this information today
n Evolutionary trees can be constructed
n “Phylogenetic” trees
Categorization
n Homologous structures provide evidence for the occurrence of evolution
n Similar structure due to common descent
n Structures may arise independently in multiple evolutionary lineages
n “Analogous structures” arise through “convergent evolution”
n Similar environmental pressures lead to similar adaptations
Categorization
n Analogous structures can be misinterpreted as homologous structures
n One would conclude that organisms share evolutionary ancestry when in fact they do not
Taxonomy and Relatedness
n There is a well-established system for classifying organisms
n Evolutionary relatedness is the most important factor used in placement of organisms
n Taxonomy sometimes recognizes other factors when placing organisms
Taxonomy and Relatedness
n Class Reptilia includes organisms such as snakes, lizards, crocodiles, and dinosaurs
n Class Aves includes all birds
n Dinosaurs and birds are more closely related than dinosaurs and lizards
n Birds split off of the dinosaur lineage long after dinosaurs split from other reptiles
n Birds are arguably different enough from modern reptiles to have their own class
Polyploidy
n Diploid species possess paired homologous chromosomes
n e.g., 23 pairs in humans, 4 pairs in Drosophila (fruit fly), etc.
n These homologous chromosomes are separated during meiosis
n Gametes receive only one copy of each chromosome
Polyploidy
n Many plants and some animals can produce hybrids
n Products of fertilization between two different species
n Most hybrids are sterile
n Lack pairs of homologous chromosomes
n Homologues cannot pair in meiosis
n Generally cannot produce functional gametes
Polyploidy
n A hybrid zygote may double its DNA in preparation for mitosis, but fail to divide
n The chromosome number has doubled
n This zygote now possesses pairs of homologous chromosomes
n “Polyploid”
n Mitosis will produce a multicellular individual whose cells all possess this doubled number of chromosomes
Polyploidy
n In a polyploid individual
n Sperm and egg can be produced through meiosis
n Self-fertilization is possible
n Fertile offspring are produced
n Fertilization of either parent species will produce infertile offspring
n This individual is reproductively isolated from both parent species
Polyploidy
n Polyploidy produces a new species
n Reproductively isolated from parent species
n Able to perpetuate itself through self-fertilization
Maize
Maize
Wheat
Polyploidy
n Triploid crops: banana, apple, ginger, citrus
n Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek, tobacco, peanut, kinnow, Pelargonium
n Hexaploid crops: chrysanthemum, bread wheat, triticale, oat
n Octaploid crops: strawberry, dahlia, pansies, sugar cane
Polyploidy
n Humans can make use of polyploidy
n Polyploidy can be chemically induced in watermelons
n The polyploid individual is then crossed to a normal diploid watermelon plant
Polyploidy
n Humans can make use of polyploidy
n Offspring from this cross have three copies of each chromosome
n They cannot form functional gametes or functional embryos
n True seeds cannot be produced
n “Seedless” watermelons