20.4 Speciation and Punctuated Equilibrium
Key Takeaways
The biological species concept groups populations that actually or potentially interbreed and are reproductively isolated from other such groups, and it fits fossils and asexual organisms poorly.
Allopatric speciation begins with geographic separation, while sympatric speciation does not, and polyploidy in plants is the clearest sympatric example.
Prezygotic barriers are habitat, temporal, behavioral, mechanical, and gametic isolation.
Postzygotic barriers are hybrid inviability, hybrid sterility, and hybrid breakdown; a mule shows hybrid sterility, not fusion of the parent species.
Punctuated equilibrium, associated with Eldredge and Gould, is long fossil stasis interrupted by geologically rapid change, in contrast to phyletic gradualism, and it is not a new body plan arising in one individual.
20.4 Speciation and Punctuated Equilibrium
Speciation is the split of one population into groups that no longer exchange genes. CLEP Biology asks for the biological species concept, the geographic modes, the reproductive barriers, and the tempo of change in the fossil record. A mule, extra chromosomes, and a gap in the rocks are different ideas.
The biological species concept
The biological species concept defines a species as groups whose members actually or potentially interbreed and are reproductively isolated from other such groups. Actually covers populations that do meet and mate. Potentially covers populations that could interbreed if they met, as long as reproductive isolation has not already formed. Reproductive isolation means gene exchange is not mixing the gene pools. Looking similar is not the definition.
The concept fits poorly for fossils and for asexual organisms. Fossils do not mate, so interbreeding cannot be tested. Asexual organisms do not interbreed, so a definition built on mating does not sort a lineage of clones. The concept is a strong tool for living, sexual organisms. A stem that hands you a fossil or a clone is usually asking you to notice that poor fit.
Where the split begins
Allopatric speciation begins with geographic separation. A river, a mountain range, a glacier, or an island colonization splits groups that no longer meet, so gene flow stops. Selection can favor different traits if the places differ, and drift can move allele frequencies in the smaller groups. If reproductive isolation follows, the groups are separate species even if they later meet again. Choose allopatry when the first event is a geographic split.
Sympatric speciation occurs without geographic separation. Polyploidy in plants is the clearest example. A polyploid plant carries extra sets of chromosomes. A plant with four sets, living beside the two-set parent population, often produces offspring with an unbalanced chromosome number when the two cross, and those offspring fail to make working gametes. Four-set plants can still reproduce with other four-set plants. Gene exchange stops in the same place. No mountain has to rise first. Polyploidy is a common plant route and not the usual animal route.
Barriers before and after a zygote
Reproductive isolation is enforced by barriers. Prezygotic barriers act before a zygote forms, by preventing mating or fertilization. Postzygotic barriers act after a zygote forms. Fertilization can succeed, and the hybrid still fails to carry genes forward.
Learn the prezygotic list in a fixed order.
- Habitat isolation: they live in different places in one region and do not meet.
- Temporal isolation: they breed in different seasons or at different times of day.
- Behavioral isolation: courtship signals, such as a song or a dance, do not match.
- Mechanical isolation: reproductive structures do not fit, so gametes are not transferred.
- Gametic isolation: gametes meet and do not fuse.
The postzygotic list is shorter, and the exam uses it to catch a famous animal.
- Hybrid inviability means the hybrid embryo fails, or the hybrid dies before it can reproduce.
- Hybrid sterility means the hybrid lives and does not produce offspring. A mule, from a horse and a donkey, is the standard example. It does not found a line that would mix the parent species.
- Hybrid breakdown means first-generation hybrids are viable and fertile, and later generations lose viability or fertility.
| Barrier | Timing | What fails |
|---|---|---|
| Habitat, temporal, behavioral | Prezygotic | The parents do not mate |
| Mechanical, gametic | Prezygotic | Mating or gamete fusion fails |
| Hybrid inviability | Postzygotic | The hybrid does not survive to reproduce |
| Hybrid sterility | Postzygotic | The hybrid lives and does not produce offspring; a mule |
| Hybrid breakdown | Postzygotic | Later generations fail after a viable first cross |
Tempo in the fossil record
Punctuated equilibrium, associated with Niles Eldredge and Stephen Jay Gould, describes long periods of stasis in the fossil record, interrupted by relatively rapid change. The rapid intervals are often discussed with speciation in small populations, where drift and local selection can shift traits faster than in a large stable population. Rapid means rapid on a geological scale. Thousands of generations can look sudden between rock layers and still be slow beside one lifetime. It is not a claim that one mutation builds a whole new body plan in a single individual.
Phyletic gradualism is the contrasting picture: slow, steady change, with intermediates if the rocks are complete. The contrast is about tempo. Both pictures use mutation, selection, drift, gene flow, and speciation. They disagree about whether change is spread evenly through time or concentrated between long stretches of stasis. Punctuated equilibrium is not a rival to natural selection. It describes the pace at which those processes show up as fossils.
Classify the stem in two steps
Decide the geography first, then the barrier, and use the tempo labels only when the stem is about fossils.
- A mountain, island, river, or other geographic split at the start is allopatric speciation.
- Extra chromosome sets in plants that share a range is sympatric speciation by polyploidy.
- A failed courtship, a different breeding season, or gametes that do not fuse is prezygotic.
- A living, sterile hybrid is postzygotic hybrid sterility. A mule is that case.
- Long fossil stasis, then a geologically fast shift, is punctuated equilibrium. Slow steady change is phyletic gradualism.
- A fossil, or an asexual lineage, is a poor direct application of the biological species concept.
Warning
A mule is evidence of a postzygotic barrier, hybrid sterility, not evidence that horses and donkeys have fused into one species. Punctuated equilibrium is not a claim that mutations are sudden jumps of whole new body plans in a single individual. Rapid, in that description, is a geological comparison with long periods of stasis.
Which description matches the biological species concept and the clearest example of sympatric speciation?
Species are any populations that look alike, and polyploidy can occur only after a geographic barrier forms.
Species are groups that actually or potentially interbreed and are reproductively isolated from other such groups, and polyploidy in plants is the clearest sympatric example.
The concept works best for asexual organisms, and allopatric speciation occurs with no geographic separation.
Species are defined by fossil shape alone, and sympatric speciation starts when a mountain range splits a population.
A horse and a donkey produce a mule. The mule lives and does not produce offspring. What does that outcome show?
Punctuated equilibrium, because a new body plan appeared in that single mule.
A prezygotic gametic barrier, because a zygote never formed.
Successful fusion of horses and donkeys into one biological species.
A postzygotic barrier, hybrid sterility, rather than fusion of the two parent species into one species.
What does punctuated equilibrium, associated with Eldredge and Gould, claim about the fossil record?
Speciation occurs only when two populations share a habitat and continue to interbreed freely.
Lineages change slowly and steadily, which is the pattern called phyletic gradualism.
Long periods of stasis are interrupted by change that is rapid on a geological scale, not by a new body plan arising in one individual.
A single mutation builds a whole new body plan in one organism during one generation.
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