Species and Speciation

What is genetic drift?

A change in allele frequencies caused by random events Genetic drift is a random change in allele frequencies over time.

allopatry

Speciation begins when gene flow between populations is reduced or eliminated, causing genetic isolation. Genetic isolation happens routinely when populations become geographically separated. Populations that are geographically separated are said to be allopatry

allopatric speciation

Speciation that begins with geographic isolation

How do autopolyploidy and allopolyploidy differ?

The chromosomes of an autopolyploid individual come from the same species, while an allopolyploid individual has sets of chromosomes from different species. This is the key difference, producing a new combination of chromosomes by merging two different taxa in a way that also doubles the chromosomes.

synapomorphy

a shared, derived trait found in two or more taxa that is present in their most recent common ancestor but is missing in more distant ancestors. Useful for inferring evolutionary relationships. traits that help identify monophyletic groups. homologous traits that can be identifies at genetic, development, or structural level. Researchers use DNA sequence data to identify synapomorphies and estimate phylogenetic trees.

A vicariant event causes

a split between two groups - they become isolated from one another.

speciation

a splitting event that creates two or more distant species from a single ancestral species. When speciation is complete, a new branch has been added to the tree of life. result from genetic isolation and genetic divergence. the study of speciation is critical to understanding how life evolved and has many practical implications for the preservation of biodiversity on Earth.

prezygotic isolation

a type of reproductive isolation. prevents individuals of different species from mating Processes: - Temporal - Populations are isolated because they breed at different times. ex - Bishop pines and Monterey pines release their pollen at different times of the the year. - Habitat - Populations are isolated because they breed in different habitats. ex. Parasites that begin to exploit new host species are isolated from their original population - Behavioral - Population do not interbreed because their courtship displays differ. ex. To attract male fireflies, females fireflies give a species-specific sequences of flashes. - Gametic barrier - Matings fail because eggs and sperm are incompatible. ex - In sea urchins, a protein called binding allows sperm to penetrate eggs. Differences in the amino acid sequence of binding cause matings to fail between closely related populations. - Mechanical - Matings fail because male and female reproductive structures are incompatible. ex. In alpine sky pilots (a flowering plant), the length of the floral tube varies. Bees can pollinate in populations with short tubes, but only hummingbirds can pollinate in populations with long tubes.

postzygotic isolation

a type of reproductive isolation. the offspring of mating between members of different species do not survive or reproduce. Processes: - Hybrid viability - Hybrid offspring do not develop normally and die as embryos. ex. When ring-necked doves mate with rock doves, less than 6% of eggs hatch -Hybrid sterility - Hybrid offspring mature but are sterile as adults - Eastern meadowlarks and western meadowlarks are almost identically morphologically, but their hybrid offspring are usually infertile.

vicariance

a way geographic isolation can occur physical splitting of a habitat

Male frogs give calls that attract female frogs to approach and mate. Researchers examined mating calls of closely related tree frogs in South America. If reinforcement is occurring, what would you expect if you compare the calls of the two species in zones of sympatry versus zones of allopatry? a. Calls would be more different in areas of sympatry. b. Calls would be more similar in areas of sympatry. c. Calls would be about the same in both areas.

a. Calls would be more different in areas of sympatry. so that they have less of a chance of mating since they are living in the same area

How is a hybrid zone most likely to change over time if hybrid offspring have higher fitness than both parental populations? a. It should expand into the ranges of both parental populations. b. It should expand into the range of only one parental population. c. It should disappear. d. It should get smaller but not disappear.

a. It should expand into the ranges of both parental populations. Hybrid offspring should outcompete parental populations at least in the areas just outside the original hybrid zone, resulting in its expansion.

divergence/divergent evolution

accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations allowing differentiated fixation of characteristics through genetic drift and natural selection.

monophyletic group

also called a clade, or a lineage - consists of an ancestral population, all of its descendants, and only this descendants. Identified by synapomorphies

nondisjunction

an error that can occur during meiosis or mitosis, in which one daughter cell receives two copies of a particular chromosome and the other daughter cell receives none.

sympatric speciation by polyploidization

autopolyploidy and allopolyploidy Polyploid is common in plants. Polyploids have higher levels of heterozygosity than their diploid relatives. Poly plods can tolerate higher levels of self-fertilization because they are not as affected by inbreeding depression as their diploid relatives. Genes on duplicated chromosomes can diverge independetly, increasing genetic variation in the population. Compared to the gradual process of speciation by geographic isolation or by disruptive selection in sympatry, speciation by polyplodization is virtually instantaneous. It is fast, sympatric, and common.

Which concept(s) for identifying species CANNOT be applied to asexual or fossil species? a. Morphospecies concept b. Biological species concept c. Phylogenetic species concept d. All of the above.

b. Biological species concept The criterion of reproductive isolation cannot be evaluated in fossil species or species that reproduce asexually.

Which of the following statements about reinforcement is true? a. Reinforcement is also called dispersive selection. b. Reinforcement decreases the morphological difference between two incipient species. c. Reinforcement is a type of natural selection. d. Reinforcement increases the number of intermediate phenotypes.

c. Reinforcement is a type of natural selection. Reinforcement is a type of natural selection called disruptive selection, which favors extreme phenotypes over intermediate phenotypes.

Which description of reinforcement is correct? a. Reinforcement leads to development of a hybrid zone. b. Reinforcement is the loss of genetic differences between two populations due to gene flow. c. Reinforcement is natural selection against interbreeding and production of hybrid offspring. d. Reinforcement is the genetic divergence of populations in different environments.

c. Reinforcement is natural selection against interbreeding and production of hybrid offspring. In this way not only can natural selection operate differently on the two populations, but individuals produced as a result of genetic mixing are eliminated. a. hybrids produced from the two species breeding have low fitness so no hybrid zones form b. gene flow cannot occur between the two species because the hybrids that are produced have low fitness c. can also be in the same environment - symparty

A subset of a population of birds leaves its habitat on the mainland and colonizes a nearby island. The birds, after a period of time, become reproductively isolated. The island sinks and the population of birds that lived on the island returns to its original habitat. Which of the following statements about these bird populations is true? a. The populations will be able to interbreed even though they are different species. b. The birds were separated by a vicariance event. c. The populations will not be able to interbreed because they are different species. d. The populations were sympatric while they diverged.

c. The populations will not be able to interbreed because they are different species. They became reproductively isolated, so they cant mate anymore - different species.

Which mechanism causes postzygotic reproductive isolation? a. mechanical isolation b. temporal isolation c. hybrid sterility d. habitat isolation

c. hybrid sterility Sterile hybrids would isolate the two populations, but zygotes can still be formed.

A population of birds colonizes an area in which the insects upon which they feed live inside trees. Which of the following events accounts for an observed increase in average beak size in the bird population over time? a. Decreased fitness of the insects, allowing the birds to catch them more easily b. Increased fitness of the large-beaked birds, creating a new species c. Decreased fitness of small-beaked birds, creating a new species d. Increased fitness of large-beaked birds, leading to natural selection

d. Increased fitness of large-beaked birds, leading to natural selection Birds with larger beaks are better able to survive in the new environment.

species

evolutionary independent population or group of populations of the many criteria that can be used to identify species, the three most common ones are (1) the biological species concept, (2) the morphospecies concept, and (3) the phylogenetic species concept.

True or false? The last ice age produced many different species mainly because populations dispersed and colonized new habitats.

false The last ice age produced many different species mainly because of vicariance events; the glaciers physically separated populations from each other.

Reinforcement is most likely to occur when _____.

hybrids have lower fitness than either parent population

phylogenetic species concept

identifies species based on the evolutionary history of populations. the reasoning behind this concept began with Darwin's theory that all species are related by common ancestry ( all species form a monophyletic group - tree of life) advantages - it can be applied to any population (fossil, asexual, sexual), and it is logical because different species have different synapomorphies only if they are isolated from gene flow and have evolved independently. disadvantage - carefully estimated phylogenies are available only for a tiny (though growing) subset of populations on the tree of life.

hybrid zones

in some cases hybrids are capable of mating and producing viable offspring that have features that are intermediate between those of the parent populations. a geographic are where interbreeding occurs and hybrid offspring are common. depending on the fitness of the hybrid offspring and the extend of the breeding between parental species, hybrid zones can be narrow or wide, and long or short lived. example. towns end's warblers and hermit warblers live in the coniferous forests of North America's Pacific Northwest. In western Washington state, where their ranges overlap, the two species hybridize extensively. Townsend males invade hermit territories, drive off the hermit males, and mate with the hermit females.

allopolyploid

individuals are created when parents that belong to different species mate and produce offspring with two different sets of chromosomes. Mating between two species with the same number of chromosomes. The hybrid offspring is sterile because the offspring has chromosomes that do not pair normally during meiosis. if there is a mutation during mitosis that doubles the chromosome number in the individual before meiosis, then each chromosome gains a homolog and meiosis can proceed normally. The hybird that is produced that can produce offspring functions as polyploidy because it has two copies each of two sets of chromosomes. This offspring has four copies of genes that are common to both parent species, instead of just two. parents: 2n = 12 and 2n = 12....sterile offspring 2n = 12 and fertile offspring 2n = 24

autopolyploid

individuals are produced when a mutation results in a doubling of chromosome number and the chromosome all come from the same species. Nondisjuction during meiosis leads to autopolyploidy resulting in diploid gametes rather than haploid gametes. In self-fertilization the diploid gametes can come together to form tetraploid offspring. much less common than allopolyploidy tetraploid offspring can self-fertilize or mate with their tetraploid parent or each other, but not with other diploids in the population. Polyploid individuals are reproductively isolated from the original diploid population and thus evolutionary independent, because breeding between diploids and tetraploids generally result in sterile offspring.

dispersal

is the movement or expansion of a species into a new area. species disperses across an existing barrier - eg. islands already exist - some individuals made it to the next island, underwent allopatric speciation.

Biological, morphological, and phylogenetic species concepts allow biologists to identify evolutionarily independent groups because they all rely on criteria indicative of _____.

no gene flow between populations if thers gene flow then the individuals are still breeding. when there no gene flow, there no breeding between the two groups which will eventually lead to independent groups - new species.

genetic divergence

occurs because selection, genetic drift, and mutation proceed independently in the isolated populations.

polyploidy

occurs when an error in meiosis or mitosis results in a doubling of the chromosome number - a massive mutation. two types of polyploidy - autoployploid and allopolyploid

Two species of tree frogs that live sympatrically in the northeastern United States differ in ploidy: Hyla chrysoscelis is diploid, and Hyla versicolor is tetraploid. The frogs are identical in appearance, but their mating calls, which females use to find mates, differ. Which difference most likely evolved first?

polyploidy polyploidy occurred first when nondisjunction occurred during meiosis, and when they couldn't mate and produce offspring, a new mating call was developed so that the two species could locate their memebrs of their own species.

sympatry

populations living in the same geographic area, or at least close enough to one another to make interbreeding possible are living in sympatry

subspecies

populations that live in discrete geographic areas and have distinguishing features, such s coloration or calls, but are not considered dusting enough to be called a separate species

genetic isolation

results from lack of gene flow

sympatric speciation

speciation that occurs even though populations live within the same geographical area Two types of events can initiate the process of sympatric speciation: 1. external events, such as disruptive selection for extreme phenotypes based on ecological niches 2. internal events, such as chromosomal mutations.

Under the phylogenetic species concept, ____ are defined as the smallest monophyletic groups on the tree of life.

species

biological species concept

states that the main criteria for identifying species is reproductive isolation. No gene flow occurs between populations that are reproductively isolated from each other - specifically, if two different populations do not interbreed in nature, or if they fail to produce viable offspring when mating takes place, they are considered distant species. the criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually. The concept is difficult to apply when closely related populations do not happen to overlap with each other geographically

outcompete

surpass (someone) in competition.

fixation

the action of making something firm or stable.

If a hybrid that can not only survive and reproduce, but can also possess a unique combination of traits that happen to be adaptive in their particular environment, then...

the hybrid offspring created can be know as a third, new species that has unique combinations of alleles from each parental species and therefore different characteristics.

niche

the range of ecological resources that a species can use and the range of conditions that it can tolerate.

bigeography

the study of how species and populations are distributed geographically.

True or false? A flood that separates a population of frogs onto opposite sides of a lake is an example of a vicariance event that may result in allopatric speciation.

true Allopatric speciation may occur when populations are separated geographically; this type of speciation can begin with a vicariance event, in which a geographic barrier such as water isolates a population, or a dispersal event, in which a subset of a population migrates to a different habitat.

Sympatric speciation by disruptive selection

Even though sympatric populations are not geographically isolated, they may become reproductively isolate by adapting to different ecological niches via disruptive selection. example: apple maggot flies rely on apples to complete their life cycle - apples are an important part of their niche. Male and female apple maggots usually court and mate on apple fruits. The female then lays a fertilized egg inside the fruit, which will be to food source for the growing larva, After the fruit drops off the tree, the larva burrows into the ground and pupates. The new adult then emerges the following spring, starting the cycle anew. Phylogenetic trees, estimated from synapomorphies in DNA sequence data, indicate that apple maggot flies are very closely related to hawthorn maggot flies, which are native to North America. These data suggest that apple maggot flies originated from hawthorn maggot flies following the introduction of apples (apple trees were introduced to North America 300 years ago from Europe). Hawthorn flies lay their eggs in hawthorn fruits and apple flies lay their eggs in apples, even though hawthorn trees and apple trees of the grow almost side by side. Only 6% of matings observed are between apple and hawthorn maggot flies. -Apple flies respond most strongly to apple scents. Hawthorn flies respond most strongly to hawthorn scents. -In both types of flies, there is no difference in the response to a mix of both scents or no scent at all. -Apple flies avoid hawthorn scent, and hawthorn scent avoid apple scent. -A fly's ability to discriminate scents has a genetic basis - meaning apple flies and hawthorn flies have different alleles associated with attraction to fruit -Specific odor receptor cells are responsible for the difference in scent response -Hybrid individuals do not orient to fruit scents as well as their parents. The upshot is that although apple flies and hawthorn flies live in the same geographical area, pre zygotic reproductive isolation is occurring as a result of natural selection for adaptive to two different niches. Apple flies mate on apples (avoiding hawthorn fruits) and hawthorn flies mate on hawthorn fruits (avoiding apple fruits). Hybrid flies have lower fitness due to their reduced success in finding fruits and thus mates - disruptive selection is occurring.

Summary: Possible Outcomes of Secondary Contact between populations

Fusion of the Populations: the two populations freely interbreed Example - occurs whenever populations of the same species come into contact Reinforcement of Divergence: if hybrid offspring have low fitness, natural selection favors the evolution of traits that prevent interbreeding between the two populations. Example - appears to be common in fruit fly species that occupy the same geographic areas Hybrid Zone Formation: hybridization occurs in a well-defined geographical area. This may move over time or be stable. Example - hybrid zone between hermit and towns end's warblers appear to have moved over time (moving south) Extinction of one Population: If one population or species is a better competitor for shared resources, then the poorer competitor may be driven to extinction. Example - Townsend warblers may be driving hermit warblers to extinction Creation of New Species: If the combination of genes in hybrid offspring allows them to occupy distinct habitats or use novel resources, they may form a new species. Example - Hybridization between sunflowers gave rise to a new species with unique characteristics.

Why are the large finches now living on the Galápagos Islands different from the original source population from a nearby island?

Genetic drift occurred in the two populations. The separation of habitats reduced gene flow between the populations. Natural selection favored individuals that were more fit in the new environment.

allopatric speciation by vicariance

If a new physical barrier such as a mountain range or over splits the geographic range os species, vicariance has taken place. Such change are commonplace on Earth due to continental drift, climate fluctuations, and many other factors. ex. geologists estimate that the Isthmus of Panama closed about 3 million years ago, forming a lang bridge between North and South America. this vicariance event split the caribbean and pacific populations of many marine species, including snapping shrimp. A phylogenetic tree of the snapping shrimp shows that many of the species found on either side of the isthmus are sister species. his is exactly the pattern predicted if the populations of many species were split into two: the populations on with side of the isthmus subsequently diverge to form distant species. Geographic isolation of populations via dispersal or vicariance produces genetic isolation due to the interruption of gene flow - the first requirement of speciation. When genetic isolation is accompanied by genetic divergence due to mutation, selection, and genetic drift, speciation results.

reinforcement

If two populations have diverged extensively and are distinct genetically, it is reasonable to expect that their hybrid offspring will have lower fitness than their parents. When post zygotic isolation occurs, there should be strong selection against interbreeding because hybrid offspring represent a wasted effort on the part of the parents - especially for females, due to their typically higher investment in offspring. Individuals that do not interbreed, because of different courtship ritual or pollination system or other form of pre zygotic isolation, should be favored because they produce more viable offspring Selected traits reinforce differences that evolved while the populations were isolated from one another. Patterns for closely related species: - If closely related species are sympatric - meaning that they live in the same area - individuals from the two species are seldom willing to mate with one another - If the species are allopatric - meaning that they live in different areas - then individuals from the two species are often willing to mate with one another. The pattern is logical because natural selection can act to reduce mating between species only if their ranges overlap. Thus it is reasonable to find that sympatric species exhibit pre zygotic isolation but that allopatric species do not,

allopatric speciation by dispersal

In 1973 the Grants began to observe a yearly migration of large ground finches front heir home island to Daphne Major for a few months in between breeding seasons. Finally in 1982, thirteen colonists stayed on Daphne Major throughout the breeding season, and five of them produced offspring. These colonists represented a new population, allopatric with their migrant source population, because the two populations bred and nested on different islands. Over twelve years. the Grants caught, weighed, and measured most of the parents and offspring on Daphne Major. When they compared the data with the measurements of the large ground finches in the migrant population, they discovered that the average beak size int he colonist population was much larger. Two evolutionary process could be responsible for the chafe in beak size: 1. The founder event produced a colonizing population that happened to have particularly large beaks relative to the migrant population (genetic drift) 2. Natural Selection in the new environment could favor alleles associated with larger beaks. The population of large ground finches is not yet considered a separate species from the migrant population, because some gene flow continues to occur - such as when a new migrant joins the colonist population. Given enough time, however, the populations may continue to diverge. Dispersal and colonization, followed by genetic drift and natural selection, are thought to be responsible for speciation in Galapagos finches and many other island groups.

morphospecies concept

under this concept, researchers identify evolutionary independent lineages by different size, shape, or other morphological features. Distinguishing features are most likely to arise if populations are independent and isolated from gene flow. this concept is widely applicable. it is useful when biologists have no data on the extent of gene flow, and it is equally applicable to sexual, asexual, or fossil species. its disadvantages are that (1) it may lead to the naming of two or more species when their is only one polymorphic species with differing phenotypes, such as the spotted and black morphs of jaguars; (2) it cannot identify cryptic species, which differ in traits other than morphology, such as meadowlarks (differ based on location); and (3) the morphological features used to distinguish species are subjective. Different researchers working on the same populations disagree not he characters that distinguish species, such as when paleontologists disagree on the identity of an extinct human species based on measurements of fossil fragments.