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