AP Biology 7.7-7.8: Speciation and Variation

Biological species concept

A species is a group of organisms whose members can interbreed in nature and produce viable, fertile offspring. Populations are considered different species when gene flow between them is prevented by reproductive incompatibility.

What is speciation?

Speciation is the evolutionary process by which one ancestral population splits into two or more populations that become reproductively isolated and develop into distinct species.

Why is the ability to produce fertile offspring more useful than appearance for defining a species?

Organisms can look very different yet interbreed successfully, as in many dog breeds, while similar-looking organisms may produce no offspring or sterile hybrids. Reproductive compatibility therefore provides a more informative criterion than morphology alone.

Gene pool

The gene pool is the complete collection of alleles present in a population or species. Evolutionary change occurs when the frequencies of alleles in the gene pool change across generations.

Mutation as a source of population variation

Mutation is a change in DNA that can create a new allele. Mutations provide the original source of new genetic variation, although their effects may be beneficial, harmful, or neutral.

Why must a genetic change occur in a gamete or reproductive lineage to affect evolution in a sexually reproducing population?

Only genetic changes passed through sperm or eggs can enter offspring and become part of the next generation's gene pool. A mutation confined to a nonreproductive somatic cell is generally not inherited.

How does sexual reproduction generate genetic variation?

Meiosis generates genetically different gametes through crossing over and independent assortment of chromosomes. Random fertilization then combines alleles from two parents in new combinations.

Gene flow

Gene flow is the movement of alleles between populations through migration and subsequent reproduction. Geographic separation reduces gene flow, allowing populations to accumulate genetic differences.

What conditions must be met for two populations to become separate species?

The populations must diverge genetically and phenotypically until individuals can no longer successfully exchange genes. This usually requires reproductive isolation that persists over time.

Allopatric speciation

Allopatric speciation occurs when a physical geographic barrier separates populations, after which mutation, natural selection, and genetic drift cause them to diverge. Reproductive isolation may eventually evolve.

How does geographic isolation promote allopatric speciation?

A barrier prevents or greatly reduces migration and mating between populations. Their allele frequencies then change independently in response to different environments, mutations, and random genetic drift.

Dispersal versus vicariance in allopatric speciation

Dispersal occurs when some individuals move to and establish a population in a new area. Vicariance occurs when a newly formed geographic feature, such as a river or valley, divides an existing population.

Why does the effectiveness of a geographic barrier depend on the organism?

A barrier must prevent gene flow for that particular organism. A narrow valley may isolate a flightless rodent but not a flying insect, whose individuals could continue moving between populations.

How do natural selection and genetic drift contribute to divergence after geographic isolation?

Different environments can favor different adaptations through natural selection, while random changes in allele frequencies—especially in small populations—occur through genetic drift. Together, these processes can increase reproductive differences.

Peripatric speciation

Peripatric speciation is a form of geographic speciation in which a small peripheral population becomes isolated from a larger ancestral population. Founder effects and genetic drift can be especially influential because the isolated population is small.

Parapatric speciation

Parapatric speciation occurs when neighboring populations occupy partially different environments and have limited gene flow rather than complete geographic separation. Divergent selection can produce reproductive isolation across the environmental boundary.

Sympatric speciation

Sympatric speciation occurs without a physical geographic barrier. Reproductive isolation can arise through mechanisms such as polyploidy, ecological specialization, or assortative mating within the same area.

Adaptive radiation

Adaptive radiation is the relatively rapid diversification of one ancestral species into multiple species adapted to different ecological niches. It is especially likely when populations colonize an area with many available habitats and limited gene flow, such as an island chain.

How can island environments promote adaptive radiation?

Islands can isolate populations from one another while offering distinct resources and habitats. Natural selection then favors different traits in each isolated population, producing multiple descendant species.

How does resource specialization lead to sympatric speciation?

If individuals in one population begin using different resources or habitats, they may interact and mate more often within their ecological subgroup. Reduced interbreeding allows genetic differences to accumulate even though the groups occupy the same geographic region.

Prezygotic reproductive barrier

A prezygotic barrier prevents mating or fertilization before a zygote forms. Examples include temporal, habitat, behavioral, mechanical, and gametic isolation.

Postzygotic reproductive barrier

A postzygotic barrier acts after fertilization has occurred. The resulting hybrid may fail to develop, have low viability, or survive but be sterile.

Temporal isolation

Temporal isolation occurs when populations reproduce at different times of day, seasons, or years. Even if they occupy the same area, their breeding periods do not overlap sufficiently for mating.

Habitat isolation

Habitat isolation occurs when populations live in different habitats within the same general region and therefore rarely encounter one another for mating. Different soil preferences in closely related insects are an example.

Behavioral isolation

Behavioral isolation results when courtship signals or mating behaviors differ between populations. Individuals may fail to recognize, attract, or accept members of another population as mates.

Mechanical isolation

Mechanical isolation occurs when reproductive structures are incompatible, preventing successful mating or pollen transfer. The structures may not fit together or may not place gametes where fertilization can occur.

Gametic isolation

Gametic isolation is a prezygotic barrier in which sperm and egg, or pollen and ovule, cannot successfully recognize, reach, or fuse with one another.

Hybrid inviability

Hybrid inviability is a postzygotic barrier in which a fertilized hybrid fails to develop normally or dies before reaching reproductive maturity.

Hybrid sterility

Hybrid sterility occurs when hybrid offspring survive and mature but cannot produce functional gametes or otherwise cannot reproduce. The classic example is a sterile hybrid produced from two related species.

Polyploidy

Polyploidy is the condition of possessing three or more complete sets of chromosomes. It can create rapid reproductive isolation because polyploid individuals may be unable to produce fertile offspring with normal diploid individuals.

How can chromosome-number errors cause sympatric speciation?

If chromosome sets fail to separate properly during cell division, an organism may gain an entire extra chromosome set. Its altered chromosome number can prevent successful meiosis with the ancestral population, isolating it reproductively without geographic separation.

Aneuploidy

Aneuploidy is the gain or loss of one or more individual chromosomes rather than complete chromosome sets. Nondisjunction can produce gametes with $n+1$ or $n-1$ chromosomes; in a diploid organism, fertilization can produce $2n+1$ or $2n-1$ offspring.

Why is aneuploidy involving chromosome loss often more harmful than chromosome gain?

Loss of a chromosome removes many genes and is frequently lethal. An organism with an extra chromosome, such as $2n+1$, may be more likely to survive than one missing a chromosome, such as $2n-1$, although both can cause serious developmental problems.

Autopolyploidy

Autopolyploidy occurs when an organism has multiple complete chromosome sets derived from the same species. For example, chromosome doubling can produce a tetraploid with $4n$ chromosomes from a diploid ancestor with $2n$.

Allopolyploidy

Allopolyploidy results when chromosome sets from two different species combine, often through hybridization followed by chromosome doubling. The doubled hybrid can be fertile with other allopolyploids even if the original hybrid was sterile.

Why can chromosome doubling restore fertility in a hybrid?

A hybrid with chromosome sets from different species may lack homologous partners during meiosis, producing abnormal gametes. Chromosome doubling provides matching copies for pairing, allowing more regular meiosis and potentially restoring fertility.

How do natural selection and sexual selection influence speciation?

Natural selection can favor different adaptations in populations using different environments or resources. Sexual selection can produce divergence in mate preferences or courtship traits, reducing interbreeding and strengthening reproductive isolation.

Reinforcement

Reinforcement is the strengthening of prezygotic reproductive barriers when hybrid offspring have low fitness. Natural selection favors individuals that avoid mating with members of the other population, reducing costly hybridization.

How can hybridization contribute to the formation of a new species?

Hybridization can combine genomes from two species, and chromosome doubling or selection can make the hybrid fertile. If the fertile hybrid is reproductively isolated from both parent species, it can form a new species.

What is the difference between anagenesis and cladogenesis?

Anagenesis is evolutionary change within a single lineage without branching into multiple species. Cladogenesis is the splitting of one lineage into two or more descendant lineages and is the process associated with speciation.

Why does the existence of hybrids not necessarily disprove that two organisms are different species?

Different species may occasionally mate, but their hybrids can be inviable or sterile and therefore fail to transfer genes effectively. Species boundaries depend on overall reproductive isolation, not an absolute prohibition of every mating event.

How would you distinguish allopatric from sympatric speciation in a population-genetics scenario?

Allopatric speciation begins with geographic separation that sharply reduces gene flow. Sympatric speciation occurs in the same geographic area and requires reproductive isolation to arise through mechanisms such as polyploidy, ecological divergence, or mate choice.