Reproductive Isolation and Speciation

Biodiversity and Conservation

What is a species?

A group of organisms that can interbreed to produce fertile offspring.

What is speciation?

The formation of a new species.

What is reproductive isolation?

When populations become unable to interbreed successfully and therefore cannot produce fertile offspring together.

Why is reproductive isolation important in speciation?

It prevents gene flow between populations, allowing their gene pools to become increasingly different.

What is gene flow?

The movement of alleles between populations through interbreeding or migration.

What happens to gene flow when populations become reproductively isolated?

Gene flow between the populations is greatly reduced or stops completely.

How does reproductive isolation lead to speciation?

Gene flow stops, different mutations and selection pressures affect each population, allele frequencies change independently, and genetic differences accumulate until the populations can no longer interbreed successfully.

Why may isolated populations experience different selection pressures?

They may live in different environments with different conditions, resources, predators and competitors.

How does natural selection contribute to speciation?

Different advantageous alleles are selected for in different populations, causing their allele frequencies and gene pools to diverge.

When are two populations considered separate species?

When they can no longer interbreed successfully to produce fertile offspring.

What is hybridisation?

The production of offspring as a result of sexual reproduction between individuals from two different species.

How can hybridisation lead to speciation in plants?

Closely related species may produce fertile hybrids that are reproductively isolated from their parent species and form a new species.

What is an isolating mechanism?

A factor that prevents or reduces mating and gene flow between populations.

What is geographical isolation?

Physical separation of populations by a geographical barrier.

Give examples of geographical barriers.

Rivers, mountain ranges, oceans or other physical barriers.

How can geographical isolation lead to speciation?

It prevents gene flow, allowing the separated populations to evolve independently under different selection pressures.

What is ecological isolation?

When populations live in the same geographical region but occupy different habitats or ecological niches.

How can ecological isolation reduce gene flow?

Individuals from the different populations rarely encounter one another to mate.

What is seasonal isolation?

When populations reproduce at different times of the year.

What is another name for seasonal isolation?

Temporal isolation.

How can seasonal isolation cause reproductive isolation?

Different populations become reproductively active at different times, so they do not mate with one another.

What is behavioural isolation?

When differences in courtship behaviour or mating signals prevent individuals from recognising each other as suitable mates.

Give examples of changes that can cause behavioural isolation.

Changes in courtship rituals, displays, calls, colour or markings.

What is mechanical isolation?

When anatomical differences prevent successful mating or fertilisation.

Give an example of mechanical isolation in animals.

Changes in genitalia may mean individuals from different populations can no longer mate successfully.

Give an example of mechanical isolation in plants.

Changes in the relationship between the stigma and stamens may prevent successful pollination between populations.

What is the most important factor in speciation?

Reproductive isolation.

What is allopatric speciation?

Speciation that occurs when populations are physically or geographically separated, preventing interbreeding and gene flow.

What causes allopatric speciation?

A geographical barrier separates a population and prevents gene flow between the groups.

What natural events can cause allopatric isolation?

Island formation, rising sea levels, changing river courses or lakes drying up or forming.

What human activities can cause allopatric isolation?

The construction of dams, roads and cities can physically separate populations.

What happens to isolated populations during allopatric speciation?

They experience different mutations and selection pressures, so allele frequencies change independently.

What is likely to happen to the gene pools of geographically isolated populations over time?

They become increasingly different.

What is an endemic species?

A species that has evolved in geographical isolation and is found naturally in only one place.

What is adaptive radiation?

The rapid evolution of one ancestral species into several different species that occupy different ecological niches.

What often occurs after allopatric speciation?

Adaptive radiation.

Why does adaptive radiation occur?

Different populations experience different selection pressures in different ecological niches.

What happens to populations during adaptive radiation?

They become adapted to different niches and eventually form separate species.

What is a classic example of adaptive radiation?

Darwin's finches on the Galápagos Islands.

What did Darwin observe about finches on the Galápagos Islands?

Different islands contained finches with different beak shapes adapted to different food sources.

Why was there genetic variation in the original finch population?

Individuals carried different alleles and therefore showed variation in characteristics such as beak shape.

How did different food sources act as selection pressures on Darwin's finches?

Finches with beaks best suited to the available food were more likely to survive and reproduce.

How would natural selection favour a finch with a beak suited to its food source?

It could obtain food more successfully, survive and reproduce, passing its advantageous alleles to offspring.

How can different beak shapes reduce competition between finch species?

Different beaks allow the birds to exploit different food sources and ecological niches.

How did reproductive isolation develop among Darwin's finches?

Populations adapted to different niches and mating preferences developed, reducing interbreeding between them.

Why can mate choice contribute to speciation in Darwin's finches?

Finches may preferentially mate with individuals with similar characteristics, increasing reproductive isolation.

Approximately how many finch species evolved from the common ancestral species described in the textbook?

At least 14 species.

What is sympatric speciation?

Speciation between populations living in the same geographical area that become reproductively isolated.

What does the prefix "sym" in sympatric mean?

Same.

How does sympatric speciation differ from allopatric speciation?

Sympatric speciation occurs without geographical separation, whereas allopatric speciation involves geographical separation.

What mechanisms can cause sympatric speciation?

Mechanical, behavioural, ecological or seasonal isolation.

Does gene flow necessarily stop immediately during sympatric speciation?

No. Some gene flow may continue while reproductive isolation develops.

How can populations in the same area become separate species?

They may occupy different niches or develop differences in mating behaviour, reproductive timing or reproductive structures, reducing gene flow.

What is a population bottleneck?

A large reduction in population size that causes a severe decrease in the gene pool, changes in allele frequencies and reduced genetic diversity.

What can cause a population bottleneck?

Environmental disasters, disease, predation, habitat destruction or human activity.

What happens to the gene pool during a population bottleneck?

Many alleles are lost, so the gene pool becomes much smaller.

What happens to genetic diversity after a population bottleneck?

It decreases dramatically.

Why do allele frequencies change during a population bottleneck?

The small group of survivors is unlikely to contain alleles in the same frequencies as the original population.

Why can rare alleles disappear during a population bottleneck?

The few individuals carrying them may be lost when the population suddenly decreases.

Why can some rare alleles become more common after a population bottleneck?

By chance, they may be present at a relatively high frequency among the survivors.

Why are populations that have experienced a bottleneck vulnerable?

Their low genetic diversity reduces their ability to adapt to environmental change or resist new diseases.

Why can a population remain genetically vulnerable even after its numbers recover?

Population size may recover without restoring the alleles that were lost during the bottleneck.

What happened to the genetic diversity of cheetahs after a historical population bottleneck?

Their genetic diversity became extremely low.

Why are cheetahs vulnerable to environmental change and disease?

They have very low genetic diversity, so fewer individuals may possess alleles that provide resistance or adaptation.

Why can low genetic diversity reduce fertility?

Inbreeding becomes more likely, increasing the chance that harmful recessive alleles are expressed.

Give an example of a human-caused population bottleneck.

Intensive hunting can drastically reduce a population, as occurred in northern elephant seals.

What is the founder effect?

The loss of genetic variation when a small number of individuals become isolated from a larger population and establish a new population.

Why does the founder effect change allele frequencies?

The founders carry only a small, non-representative sample of the alleles in the original population.

Will a founder population normally have the same allele frequencies as the original population?

No. Its allele frequencies may be very different simply by chance.

What can happen to an unusual allele carried by a founder?

It can become much more common as the new population grows.

Why is genetic diversity usually lower in a founder population?

Only a small number of individuals contribute alleles to the new gene pool.

How can the founder effect contribute to speciation?

The isolated founder population has different allele frequencies and can evolve independently until reproductive isolation develops.

What is the key difference between a population bottleneck and the founder effect?

A bottleneck occurs when most members of an existing population are lost, whereas the founder effect occurs when a small group separates from the original population to establish a new one.

What do population bottlenecks and the founder effect have in common?

Both reduce genetic diversity and can produce allele frequencies different from those of the original population.

What type of process causes the changes in allele frequency during bottlenecks and founder effects?

Genetic drift.

Why is genetic drift particularly important in small populations?

Chance events have a much greater effect on allele frequencies when there are few individuals.

What happened to northern elephant seals as a result of intensive hunting?

Their population fell to around 20 individuals before later recovering.

Why do northern elephant seals still have low genetic diversity despite their population recovering?

The modern population descended from the small number of survivors, so much of the original gene pool was permanently lost.

How does a population bottleneck affect the ability of a species to adapt?

Reduced genetic variation means there is less variation for natural selection to act upon.

Why is genetic variation important for survival during environmental change?

Some individuals may possess advantageous alleles that allow them to survive and reproduce under the new conditions.

What is the sequence of events in allopatric speciation?

Geographical isolation → gene flow stops → different mutations and selection pressures → different alleles are selected → allele frequencies diverge → reproductive isolation develops → new species form.

What is the sequence of events in sympatric speciation?

Populations occupy the same area → an isolating mechanism develops → gene flow decreases → allele frequencies diverge → reproductive isolation develops → new species form.

What is the sequence of events in adaptive radiation?

An ancestral species enters new habitats or niches → populations experience different selection pressures → different adaptations are favoured → populations diverge → reproductive isolation develops → several new species form.

Define speciation.

The formation of a new species.

Define hybridisation.

The production of offspring through sexual reproduction between individuals from two different species.

Define allopatric speciation.

Speciation occurring when populations are geographically separated and gene flow between them stops.

Define adaptive radiation.

The rapid development of one species into several different species occupying different ecological niches.

Define sympatric speciation.

Speciation occurring between populations living in the same place as reproductive isolation develops without geographical separation.

Define population bottleneck.

A dramatic reduction in population size that reduces the gene pool, changes allele frequencies and decreases genetic diversity.

Define founder effect.

The loss of genetic variation when a small group becomes isolated from a larger population and establishes a new population with different allele frequencies.