Speciation and extinction: causes and processes
Inheritance, variation and evolution • The development of understanding of genetics and evolution
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Definition of a species and speciation
A species is a group of organisms that can interbreed and produce fertile offspring. New species arise when populations of the same species become reproductively isolated and accumulate genetic differences until interbreeding is no longer possible. Speciation therefore requires interruption of gene flow followed by divergence through mutation, selection or drift .
Steps that produce speciation
Step 1 - Variation: Genetic differences exist within any population because of mutation and sexual reproduction. Step 2 - Isolation: A barrier prevents gene flow between groups. Barriers include geographic features (rivers, mountains, islands), behavioral changes (mating signals), temporal separation (different breeding seasons) and ecological differences (different microhabitats). Step 3 - Divergence: Different selective pressures, random genetic drift or founder effects change allele frequencies in each isolated group. Step 4 - Reproductive isolation: Accumulated differences prevent successful interbreeding (prezygotic or postzygotic barriers). When reproductive isolation is stable, the groups qualify as distinct species. Historical examples include island populations studied by early naturalists and stickleback fish separated during the last ice age, which diverge in different lakes and habitats .
Types of speciation
Allopatric speciation occurs when geographic separation prevents gene flow and allows independent evolution. Sympatric speciation occurs within the same area when reproductive isolation arises through ecological specialization, polyploidy (in plants) or strong disruptive selection. Parapatric speciation occurs along environmental gradients where partial isolation and limited gene flow lead to divergence. The mechanism determines the initial cause of reproductive isolation and the speed of speciation .
Causes and mechanisms of extinction
Extinction results from factors that reduce survival or reproduction below the rate needed for population persistence. Immediate causes include rapid environmental change (climate shifts, asteroid impacts, volcanic eruptions), habitat loss or fragmentation, overexploitation (hunting, fishing), disease outbreaks, invasive species that outcompete or predate natives, and loss of genetic diversity in small populations. Long-term causes include sustained human activity that alters ecosystems and causes elevated extinction rates (the current Holocene extinction). Major past mass extinctions link to catastrophic events such as asteroid collisions and large volcanic eruptions .
Limiting factors for speciation and persistence
Small population size limits adaptive potential and increases extinction risk through inbreeding and loss of variation. High gene flow between populations prevents divergence and therefore prevents speciation. Stable environments reduce the strength of divergent selection and slow speciation. Rapid, large-scale environmental change can outpace a population’s ability to adapt and cause extinction before speciation occurs. Fossil evidence shows that both gradual change and sudden events shape species’ fates over geological time .
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