Speciation (College Board AP® Biology): Study Guide

The mechanism of speciation

• A species, according to the biological species concept, is defined as:

a group capable of interbreeding and exchanging genetic information to produce viable, fertile offspring

• Speciation refers to the evolution of a new species

• Speciation occurs as follows:

  1. restriction of gene flow between groups of organisms

  2. the separate groups then evolve differently under different selection pressures

  3. over time the genetic differences between the two groups mean they are no longer able to interbreed to produce fertile offspring

     • The populations are said to be reproductively isolated and are no longer considered to be the same species

The rate of speciation and evolution

• The rate of evolution and speciation is influenced by ecological factors:

Gradualism

• Gradualism is the name given to speciation that occurs gradually through the accumulation of small changes over time

• The fossil record shows evidence of slow, long-term evolutionary changes aligned with geological cycles

• Charles Darwin supported gradualism, citing vestigial structures as evidence of evolutionary remnants from ancestral species

  • E.g. the human appendix or wings of flightless birds

Punctuated equilibrium

• In punctuated equilibrium, speciation can occur abruptly, characterized by long periods of stability followed by short bursts of rapid evolution

• This may occur after sudden environmental changes, e.g. through events like volcanic eruptions or meteor strikes

• Some evidence for punctuated equilibrium has come from the fossil record which shows breaks with no intermediate species, challenging the idea of gradualism

Divergent evolution

• Speciation can occur more rapidly if new habitats become available through adaptive radiation

  • Several groups of organisms evolve adaptations that allow them to occupy different ecological niches

  • Different selection pressures result in the accumulation of genetic differences leading to speciation

  • E.g. Darwin's Finches

Types of speciation

• Speciation introduces diversity to life forms

Allopatric speciation

• Allopatric speciation occurs due to geographical isolation

  • A population is split into two or more groups by natural barriers, e.g. mountains pr rivers, or man-made barriers, e.g. roads

  • There is no gene flow between populations

  • Selection pressures and genetic drift cause changes in allele frequencies and phenotypes, leading to divergence

  • Over time populations become physiologically, behaviorally, and morphologically distinct, forming new species

    

Allopatric speciation examples

• Hawaiian Drosophila: two distinct species of fruit fly, of the genus Drosophila, have emerged by allopatric speciation on different islands of the Hawaiian archipelago

  • Ancestral Drosophila colonized the islands, then as new islands formed the populations were physically isolated

  • This isolation prevented gene flow between populations on different islands

  • Over time selection pressures, e.g. differences in climate, food sources, and habitats, and genetic drift caused the populations to diverge

  • These evolutionary changes led to the development of distinct traits in each population

    • Drosophila silvestris now inhabits Hawai’i Island and Drosophila planitibia inhabits the island of Maui 

• Caribbean Anolis: one colonizing species has diverged into around 150 distinct species via geographical isolation across the islands of the Caribbean

  • An ancestral Anolis species colonized different islands in the region

  • Physical separation by water created isolated populations on each island, preventing gene flow

  • Each island presented unique environmental conditions and selection pressures

  • Over time genetic drift and natural selection led to the evolution of populations with distinct traits

Sympatric speciation

• Sympatric speciation takes place with no geographical barrier

• Populations in the same location diverge into distinct groups with no gene flow between them

• Mechanisms for separation include:

  • ecological separation, where populations are separated because they live in different environments within the same area

    • E.g. soil pH can differ greatly in different areas, having a major effect on plant growth and flowering

  • behavioral separation, where populations are separated because they have different behaviors

    • E.g. differences in feeding, communication or social behavior

Sympatric speciation example

• Apple maggot Rhagoletis

  • The apple maggot fly (Rhagoletis pomonella) originally laid eggs on hawthorn fruits

  • A new niche was introduced with some flies laying eggs on apples

  • This gave two ecological niches within the same geographical area: hawthorn and apple

  • Flies developed a preference for either hawthorn or apple, which influenced mating behavior since mating occurs near the host fruit

  • Over time genetic isolation arose due to the lack of gene flow between flies using different fruits

  • Overtime, this led to the evolution of two distinct populations despite no geographical barrier

Maintaining reproductive isolation

• Reproductive isolation occurs when species are unable to interbreed to produce fertile offspring; this indicates that speciation has taken place

• Reproductive isolation can be maintained by mechanisms that prevent gene flow

  • These barriers may occur at different stages in the reproductive life cycle, either before or after zygote formation

Pre-zygotic barriers

• Barriers that prevent fertilization or zygote formation are described as pre-zygotic

• Examples of pre-zygotic barriers include:

  • habitat isolation; organisms in different locations cannot interact to mate

  • temporal isolation; mating is prevented by differences in activity periods or breeding seasons

  • behavioral isolation; mating occurs only with compatible behaviors, e.g. specific mating calls or dances

  • mechanical isolation; incompatible reproductive organs block gamete exchange

  • gametic isolation; gametes cannot fuse, preventing zygote formation

Post-zygotic barriers

• Barriers that prevent the zygote from developing into a viable or fertile adult are described as post-zygotic

• Examples of post zygotic barriers include:

  • reduced hybrid viability; offspring fail to survive to reproductive age, restricting allele mixing to one generation

  • reduced hybrid fertility; hybrids are sterile, preventing further reproduction

  • hybrid breakdown; hybrids may initially be viable and fertile, but their descendants weaken and disappear over generations