11.4 Speciation

The supercontinent Gondwana broke up before the Proteacea family of plants evolved.

The plant family is found throughout the southern hemisphere.

The diversity of the animals in Australia is a reflection of the island's long isolation.

Australia has an abundance of endemic species which are found nowhere else in the world.
These species evolve into new species that look very different from their ancestors, which may exist on the mainland.
The animals of Australia, the finches on the Galapagos, and many species on the Hawaiian Islands are all found nowhere else but on their island.

Evidence of a common ancestor for all of life is reflected in the near universality of the genetic code and the machinery of DNA replication and expression.

The components of ribosomes and the structures of membranes are examples of the structural differences between the three domains.
The relatedness of groups of organisms is reflected in their similarity to one another.

Some of the mechanisms of evolution have been shown to be possible through the use of DNA.

It is clear that the evolution of new functions for proteins occurs after gene duplication events.
An extra copy of a gene is added to the genome as a result of these duplications.
The free modification of one copy by selection, drift, and selection is possible with these duplications.

The original function can be kept, while evolutionary forces modify the copy until it functions in a new way.

The biological definition of a species is a group of actually or potentially interbreeding individuals.

According to this definition, one species is different from another due to the possibility of breeding between individuals from each species to produce fertile offspring.
There are exceptions to the rule.

The Evolution and Its Processes rule generally holds.

The presence of hybrid species between similar species suggests that they may have descended from a single interbreeding species.

It becomes impossible for individuals from the two new populations to interbreed if two new populations are formed from one original population.

There are two broad categories in which this could occur.

The splitting of one ancestral species into two descendant species is thought to be a speciation event.

There is no reason why there isn't more than two species formed at the same time except that it is less likely and such multiple events can also be considered as single splits occurring close in time.

The diagram shows similarities to the charts that are drawn today.

A population that is continuously growing has a homogeneity in its genes.

Gene flow is relatively free because individuals can move and then mate with each other in their new location.
The allele at one end of the distribution will have the same frequencies as the allele at the other end.
The free-flow of alleles is prevented when populations become geographically discontinuous.
The two populations can evolve in different ways when the separation lasts for a period of time.
As new alleles arise by their own in each population, the frequencies of their alleles gradually become more and more different.
Natural selection favors adaptation in each group when the environment is different for the two populations.
Different histories of genetic drift will lead to divergence because the populations are smaller than the parent population.

If there is a genetic and phenotypic divergence between populations, it is likely that characters that influence reproduction will be less likely, or offspring will be infertile.

The reproductive isolation of the two populations may be affected by diverging characters.
Prezygotic mechanisms operate before fertilization and postzygotic mechanisms operate after fertilization.
Prezygotic mechanisms include things that allow people to find each other, such as timing of mating, sensitivity to pheromones, or choice of mating sites.
If individuals are able to meet each other, character divergence may prevent them from getting married or having a baby.
If it is possible to have sex, it may interfere with successful fertilization.

The mule, the infertile offspring of a female horse and a male donkey are examples of postzygotic mechanisms that prevent proper development of the offspring.

If the two isolated populations are brought back together and the hybrid offspring that formed from matings between individuals of the two populations have lower survivorship or reduced fertility, then selection will favor individuals that are able to discriminate between potential mates of their own population and the other population.

The reproductive isolation will be enhanced by this selection.

A river forming a new branch, erosion forming a new valley, or a group of organisms traveling to a new location without the ability to return are some of the ways in which Isolation of populations leading to allopatric speciation can occur.

The separation of populations depends on the biology of the organisms and their dispersal potential.
If two flying insect populations took up residence in separate valleys, chances are that individuals from each population would fly back and forth.

If two populations became separated by the formation of a new lake, continued gene flow wouldn't be possible.

There are two categories for allopatric processes.

There are many cases of allopatric speciation documented by scientists.

The Mexican spotted owl and the northern spotted owl live in different locations.

An example of incipient speciation is the owl.

Scientists have found that the distance between two groups that used to be the same species is more likely to lead to speciation.

As the distance increases, the various environmental factors are likely to be different than in close proximity.
In the north, the climate is cooler than in the south, the other types of organisms are different, and the hunting habits and prey choices of the owls in the south are different from those in the north.
If gene flow between the populations is not restored, over time speciation will likely occur and the owls will likely have evolved differences.

In some cases, a population of one species can spread throughout an area and find a different niche or isolated habitat.

From one point in time, the species has evolved into many new ones.
Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island, which leads to geographical isolation for many organisms.
The birds show adaptive radiation.
Multiple bird species have their own distinctive characteristics.

Change in the genetic variation for beaks in response to natural selection led to evolution of a different beak suited to the specific food source.

The fruit and seed-eating birds have strong beaks that are good at breaking hard nuts.
The birds dip their beaks into flowers to get their food.
The insect-eating birds have beaks that are similar to swords.
Darwin's finches are an example of adaptive radiation.

Click here to see how island birds evolved from five million years ago to today.

There are a number of mechanisms that have been proposed.

A hybrid individual with too many chromosomes can be the beginning of a form of sympatric speciation.

Polyploidy is a condition in which a cell has more than one set of chromosomes.
There are two main types of polyploidy that can lead to reproductive isolation of an individual.
In some cases a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy.
Polyploidy is caused by an error in meiosis in which all of the chromosomes move into one cell.

When it is not followed by cytokinesis, autopolyploidy results.

The gametes have more chromosomes than they should.

The normal gametes produced by this plant species will be incompatible with the new gametes.
They could either self-pollinate or reproduce with other autopolyploid plants with the same diploid number.
These individuals would only be able to reproduce with those of the new kind and not those of the ancestral species.
Polyploidy occurs when individuals of two different species reproduce to create a viable offspring called an allopolyploid.
An allopolyploid occurs when gametes from two different species combine.
It takes two generations, or two reproductive acts, before a viable fertile hybrid results.

Two species mate to produce viable offspring.

A normal gamete from one species is fused with a polyploid gamete from another.
viable offspring are produced by two matings.

Allopolyploids are found in the cultivated forms of wheat, cotton, and tobacco.

Most chromosomal abnormality in animals are lethal, and it takes place most commonly in plants.
More than half of the plant species studied relate back to a species that evolved through polyploidy.

Polyploidy may not be the only thing that takes place in ways other than ssymmetry.

Imagine a fish that lived in a lake.
Competition for food grew as the population grew.
Under pressure to find food, suppose that a group of fish had the genetic flexibility to find another resource that was not being used by the other fish.
The fish that were feeding on the second food source would interact more with each other than the fish that were feeding on the first food source.
Offspring of these fish would likely live in the same area as their parents, keeping them separate from the original population.
sympatric speciation might occur if this group of fish continued to be separate from the first population.

In nature, this scenario and others that lead to reproductive isolation play out.

Lake Victoria in Africa is famous for its diversity of fish.
Over a short period of time, researchers have found hundreds of sympatric speciation events in these fish.
In this locale, there are two types of cichlids that live in the same geographic location, but they have different morphologies that allow them to eat different food sources.

There is evidence of speciation in the fish from Lake Apoyeque.

Lake Apoyeque, a crater lake, is 1800 years old and has a single population of cichlid fish.
Scientists believe that there are two distinct populations in the lake that may be in an early stage of speciation.

The native population of flies only attack hawthorn trees.

The trees are used as a location to meet for sex.
It is thought that flies jump hosts and meet and mate in apple trees, then lay their eggs in apple fruit.
The offspring matured and kept their preference for apple trees, so they were able to divide the original population into two new populations.
The host jump took place in the 19th century, but there are now differences between the two populations of fly.
Host specificity of parasites is a common cause of speciation.

The theory of evolution is not easy to understand.

Some people reject it as an explanation for the diversity of life.

The theory of evolution has some myths associated with it.

Critics of the theory of evolution dismiss it as unimportant because they don't use the word "theory" in the way scientists do.

A theory is a concept that has been tested and supported over time in science.
The theories of the atom, gravity and relativity describe what scientists understand to be facts about the world.
Facts about the living world are described in the theory of evolution.
Scientists are naturally skeptical of a theory in science.
The constantly evolving state of scientific knowledge is reflected in the weight of theories.
In common parlance, a "theory" is a guess or suggested explanation for something.
This meaning is similar to the idea of a "hypothesis" used by scientists, which is a tentative explanation for something that is proposed to either be supported or disproved.
Critics of evolution are implying that there is little evidence supporting it and that it is still being rigorously tested, when they say evolution is just a theory.
This is not a correct description.

The genes that an individual has do not change as it ages.

Natural selection cannot evolve an individual.
The change in genetic composition of a population over time is called evolution.
Development is the process of changing the genes in an individual's body in coordination with their environment.
The change of the average value of the characteristic in the population over time is the best way to think about the evolution of a characteristic.
When natural selection leads to bill-size change in medium ground finches in the Galapagos, this does not mean that individual bills are changing.
If one measures the average bill size among all individuals in the population at one time, and then measures the average bill size in the population several years later after there has been a strong selective pressure, this average value may be different as a result of evolution.
Some individuals will survive from the first time to the second, but they will still have the same bill size.
There may be enough new individuals with different bill sizes to change the average bill size.

An explanation of life's origins is included in evolution.

Critics complain that the theory can't explain the origin of life.
The theory doesn't try to explain the origin of life.
The theory of evolution explains how populations change over time.
It doesn't shed light on the beginnings of life, which is how life is defined.
The origin of life on Earth is a difficult problem because it happened a long time ago and probably happened once.
The presence of life on Earth precludes the possibility that the events that led to life on Earth can be repeated because the intermediate stages would immediately become food for existing living things.
The early stages of life included the formation of organic molecules.
They would be broken down by living things if they were formed today.
The early stages of life may have included more complex aggregations of molecule into enclosed structures with an internal environment and an external environment.
Living organisms would consume or break down such structures if they were formed now.

Once a mechanism of inheritance was in place, the entities would be subject to the principle of natural selection.

The more effective reproducers would have a greater increase in frequencies.
Evolution doesn't explain the origin of life, but it may have something to do with some of the processes that pre-living entities acquired.

It is quite common for a statement such as "organisms evolve in response to a change in an environment" to be made.

There are two easy misunderstandings with this statement.
The statement must not be understood to mean that individual organisms will evolve.
Interpretation of the statement to mean that the evolution is somehow intentional may cause a second misunderstanding.
A changed environment results in some individuals in the population benefiting and therefore producing more offspring than others.
If the characters are genetically determined, the population will change.

The variation that natural selection works on is already in a population and does not arise in response to an environmental change.

Over time, antibiotics will be applied to a population ofbacteria that are resistant to antibiotics.
The application of the antibiotic did not cause the resistance.
The genes for resistance were present in the pool of thebacteria.
The antibiotic selects individuals that are resistant since they are the only ones that survived and divided.
Experiments have shown that antibiotic resistance doesn't arise as a result of antibiotic application.

Evolution is not goal directed.

The species do not become better over time, they just track their changing environment and adapt to maximize their reproduction in a particular environment at a particular time.

Evolution doesn't have a goal of making faster, bigger, more complex or smarter species.

In popular literature, this kind of language is common.
Some organisms are described as the "pinnacle" of evolution.
The variation present and the environment affect what characteristics evolve in a species.
At some point in the future, what trait is fit in one environment at one time may be fatal.
This holds equally well for a species of insect as it does the human species.

When the theory of evolution was first proposed in 1859, it was controversial, yet within 20 years almost every working biologist had accepted evolution as the explanation for the diversity of life.

The rate of acceptance was rapid because Darwin had a large body of evidence.
Scientific arguments against the theory were involved in the early controversies.
The arguments of the biologists were resolved after a short time, while the religious leaders continued to argue.

The theory of evolution was the main theory at the time species were created.

Despite the prevalence of this theory, it was becoming more and more clear that it couldn't explain many observations of geology and the living world during the 19th century.
The ability of the theory of evolution to explain phenomena is what makes it so persuasive.
The leaders cannot accept the replacement of special creation by a mechanistic process that excludes the actions of a deity as an explanation for the diversity of life.
Most of the major denominations in the United States support the acceptance of evidence for evolution as compatible with theirologies.

Over time, the nature of the arguments against evolution has changed.

The theory is still controversial among biologists.
This is not true.
The number of scientists who question the validity of the theory of evolution is small.
Signed statements from many scientific societies show the support for the theory, which includes working scientists as members.
Many non-biologists, such as engineers, physicians, and chemists, reject or question the theory of evolution.
Experimental results and research programs don't contradict the theory.

There are no papers that refute the theory published in peer-reviewed journals.

Scientists are skeptics and there is a long history of published reports that challenged scientific orthodoxy in unpopular ways.
The theory of group selection and the cause of stomach ulcers are examples.

The theory of plate tectonics and the asteroid-impact theory of the Cretaceous extinction can be found in this OpenStax book.

The scientific community considers research with evidence and ideas.
The research that doesn't meet the standards is rejected.

Some religious leaders believe that alternative theories to evolution should be taught in public schools.

Critics of evolution use this strategy to create uncertainty about the validity of the theory.

There are no viable alternatives to evolution.

Lamarck proposed the theory of natural selection in the 19th century.
The Soviet Union's agricultural research program was based on Lamarck's theory in the early 20th century.
Special creation is not a scientific theory since it relies on an untestable explanation.
Despite the claims of its proponents, intelligent design is not a scientific explanation.
The existence of an unknown designer of living organisms and their systems is created by intelligent design.
It is not a scientific explanation if the designer is unknown or supernatural.
There are two reasons not to teach nonscientific theories.
The explanations for the diversity of life do not give rise to research programs that promote our understanding of the natural world.

Natural phenomena cannot be tested in experiments.

It is not in the public interest to teach these explanations as science in public schools.
The United States Supreme Court and lower courts have ruled that the teaching of religious belief, such as special creation or intelligent design, violates the First Amendment of the U.S. Constitution.

The theory of evolution and science is silent on the existence of the spiritual world.

Science can only study the material world.
There are many biologists who are deeply religious.
Biology has nothing to say about the existence of a god.
As they see fit, the individual biologists can reconcile their personal and scientific knowledge.
The National Center for Science Education developed the Voices for Evolution project to gather the diversity of perspectives on evolution to advocate it being taught in public schools.

Natural selection arises from three conditions: individuals within a species vary, some of those variations are heritable, and organisms have more offspring than resources can support.

The consequence is that people with relatively good variations will be more likely to survive and have higher reproductive rates than people with bad variations.
The offspring will be given more of the beneficial traits.
The trait will lead to genetic change in the population by having higher representation in the next and subsequent generations.

Reconciliation of Darwin's, Wallace's, and Mendel's thoughts on evolution and heredity led to the creation of the modern synthesis of evolutionary theory.

Population genetics is a framework for describing evolutionary change in populations.
Population genetics defines evolution as a change in allele frequencies over generations.
In the absence of evolutionary forces allele frequencies will not change in a population.
Natural selection, genetic drift, and migration act to change allele frequencies in all populations.

There are four factors that can change the frequencies of a population.

Natural selection works by selecting for all genes that are beneficial while also selecting against those that are bad.
New alleles are introduced into a population.
Genetic drift is caused by the chance occurrence that some individuals have more offspring than others.
Gene flow can change frequencies when individuals leave or join the population.

Evidence for evolution can be found in living things and extinct species through fossils.

Evidence for the evolution through extinct forms that led to modern species can be found in fossils.
There is a rich fossil record that shows the evolutionary transitions from horse ancestors to modern horses that document intermediate forms and a gradual adaptation.
Common structures in different lineages have been modified over time by evolution.
The history of continental movements is reflected in the geographical distribution of living species.
The patterns of similarity expected from descent with modification are reflected in the structures of molecules.

There are two main pathways for speciation: geographic separation and mechanisms that occur within a shared habitat.

Both pathways cause reproductive isolation.

Errors in meiosis can lead to gametes with extra chromosomes called polyploidy.

The difference between autopolyploidy and allopolyploidy is that autopolyploidy occurs within a single species.

If the two populations come together again, evolutionary divergence can take place leading to the evolution of reproductive isolating traits that prevent interbreeding.
After a period of isolation, the viability of hybrid offspring is expected to decline.

The theory of evolution is not easy to understand.

The factual nature of evolution is often challenged by wrongly equating the scientific meaning of a theory with the vernacular meaning.
Evolution can be wrongly interpreted to mean that individuals evolve, when in fact only populations can evolve as their genes change over time.
Evolution does not speak to the origin of life.
It is often said that organisms change through intention and that selection operates on a population that has not arisen in response to a particular environmental stress.
Evolution is accepted by the vast majority of scientists, even though it is sometimes characterized as controversial.
There are no viable alternative scientific theories to evolution that should be taught in public schools.
In the United States, the alternative religious beliefs are unconstitutional because they can't be proven.
While scientists reconcile religious belief and scientific knowledge, science is silent on the question of a god.

The arm of a human is similar to the wing of a bird.

The seeds of two plants are near each other.

Two types of fish eat the same type of food.

The situation would most likely lead to allopatric organisms, like people and elephants.

A lake is formed by a flood.

A storm causes trees to fall.

dispersal and time are different.

One leads to allopatric speciation while the other leads to sympatric speciation.

One involves the movement of organisms.

Population genetics is the study of people.

The Cruz and San Cristobal islands are separated by 100 km of ocean.

Occasionally, people from different islands go to the other island to stay.

The theory of evolution uses the word "theory" instead of "gene flow".

Two species of fish have recently undergone sympatric species in an area, how would natural selection work in speciation.

The ideal conditions for evolved are provided by island chains.