Evolutionary Processes

This is Unit Eight of Biology, Part Two: isolation and population genetics. Part One ended with allopatric speciation, where geography does the isolating. This part opens with the harder case: a population splitting while everyone stays in the same place. Without geography, the isolation has to come from somewhere else — a chromosome change that makes a lineage instantly incompatible, or a shift in habitat or behaviour that narrows who breeds with whom. Then the barriers themselves, sorted by whether they act before or after a zygote forms. That sorting has one trap in it: gametic isolation acts after mating has already happened, and it is still prezygotic, because no zygote ever formed. Then hybrid zones, where two recently diverged species keep meeting. A hybrid zone existing does not mean speciation failed — what happens next depends on how fit the hybrids are. The part closes on population genetics and the Hardy-Weinberg model, whose five assumptions no real population meets. That is the point of it: the model is a baseline to measure departures against, not a description of anything. By the end you will be able to do five things. One: say how sympatric speciation isolates. Two: sort a barrier by when it acts. Three: say what decides a hybrid zone. Four: say why the assumptions are never met. Five: get genotype frequencies from a phenotype count.

Six entries. One overview and two routes — a chromosomal route in four entries and a behavioural one in a single entry. The chromosomal route is the more dramatic: it can isolate a lineage in a single generation.

Same place.

Bio 1697 Sympatric Speciation, Overview

Sympatric speciation is the formation of a new species within a population that keeps living in the same location as its ancestral population, with no geographic separation — cichlid fish diverging within one lake. Because geography does no work here, the isolating mechanisms have to arise from somewhere else entirely.

Extra sets.

Bio 1699 Polyploidy

Polyploidy is a condition where an organism carries extra chromosome sets beyond its species' normal number, often from nondisjunction producing a gamete with a full extra set. It adds whole sets, unlike aneuploidy, which adds or loses only single chromosomes.

From one species.

Bio 1701 Autopolyploidy

Autopolyploidy occurs when an individual carries multiple chromosome sets from its own species alone — a plant producing diploid rather than haploid gametes can self-pollinate or mate with other autopolyploids to found a new lineage. It can no longer produce fertile offspring with the ancestral population, so isolation takes one generation.

From two species.

Bio 1703 Alloploidy

Alloploidy occurs when gametes from two species combine and the hybrid's chromosome number then doubles into a fertile, stable lineage — cultivated wheat combines sets from more than one wild ancestor. It needs two reproductive steps to reach stability, since a first-generation hybrid alone is typically sterile.

The source.

Bio 1705 Nondisjunction as a Route to Polyploid Speciation

Nondisjunction — chromosomes failing to separate during meiosis — produces the unreduced gametes underlying both autopolyploidy and alloploidy, so a gamete carrying the full diploid number traces to one. Polyploidy arises as a byproduct of that meiotic error rather than being a separate error of its own.

No chromosome change.

Bio 1707 Habitat and Behavioral Sympatric Speciation

Sympatric speciation can also proceed without any chromosomal change, as when a subgroup shifts to a new resource or habitat and breeds increasingly among itself — cichlids diverging in diet and depth within one lake. Habitat divergence within a single location can isolate breeding on its own.

Six entries. An overview and five mechanisms, each blocking reproduction at a different stage. The last one is the trap — it acts after mating, and it is still prezygotic, because the test is whether a zygote formed.

Before a zygote.

Bio 1709 Prezygotic Reproductive Isolation, Overview

A prezygotic barrier prevents fertilisation from occurring at all between two populations, blocking reproduction before any zygote can form — two populations whose breeding seasons no longer overlap. A postzygotic barrier, by contrast, acts only after a zygote has already formed.

Different times.

Bio 1711 Temporal Isolation

Temporal isolation is a prezygotic barrier in which two populations breed at different times, so individuals never encounter each other's fertile period — two related frog species sharing an area but breeding in different months. It blocks reproduction by timing alone; the populations might otherwise interbreed fully.

Different places.

Bio 1713 Habitat Isolation

Habitat isolation is a prezygotic barrier in which two populations occupy different habitats within the same broader range, rarely encountering each other — two cricket species preferring different soil types. It needs no geographic distance; populations can be habitat-isolated while living metres apart.

Signals unread.

Bio 1715 Behavioral Isolation

Behavioural isolation is a prezygotic barrier where a species-specific courtship behaviour fails to attract a mate from a related species — a male firefly's light pattern going unrecognised by a female of another species. It is a barrier of courtship communication rather than physical incompatibility.

Structures mismatched.

Bio 1717 Mechanical Isolation

Mechanical isolation is a prezygotic barrier in which two related species' reproductive structures do not physically fit together, preventing mating — as with differently shaped damselfly reproductive organs. It blocks mating itself, distinct from gametic isolation, which blocks fertilisation after mating occurs.

Cells incompatible.

Bio 1719 Gametic Isolation

Gametic isolation is a prezygotic barrier in which two species' egg and sperm cells are incompatible, so fertilisation fails even after mating occurs — sperm from one species failing to bind or fuse with another's eggs. Mating without fertilisation still counts as prezygotic failure.

Three entries. A zygote has formed, and something goes wrong at one of three points — before maturity, at reproduction, or a full generation later in the hybrid's own offspring.

After a zygote.

Bio 1721 Postzygotic Reproductive Isolation, Overview

A postzygotic barrier acts after a zygote has already formed, preventing a hybrid from developing, surviving, or reproducing successfully — a hybrid embryo failing to develop past an early stage. A failed mating attempt is sometimes misfiled here; it blocks fertilisation itself, making it prezygotic.

Two failures.

Bio 1723 Hybrid Inviability and Hybrid Sterility

Hybrid inviability is a postzygotic barrier where a hybrid fails to survive to maturity; hybrid sterility is where a surviving hybrid cannot reproduce — an embryo dying before birth against a mule, viable but sterile. One blocks survival entirely, the other permits survival and blocks reproduction.

One generation on.

Bio 1725 Hybrid Breakdown

Hybrid breakdown is a postzygotic barrier where first-generation hybrids are viable and fertile, but their own offspring show reduced fitness. The failure surfaces only in the hybrid's descendants — the first-generation hybrid can appear entirely healthy and reproduce successfully.

Three entries. A region where recently diverged species keep interbreeding, the three ways that can resolve, and then the question of how fast speciation runs at all.

The overlap.

Bio 1727 Hybrid Zones

A hybrid zone is a geographic area where two closely related, recently diverged species continue to interact and interbreed, producing hybrids — a narrow band where two ranges overlap. A hybrid zone's existence alone does not mean speciation failed; the long-term outcome depends on hybrid fitness.

Three outcomes.

Bio 1729 Reinforcement, Fusion, and Stability

A hybrid zone's outcome depends on hybrid fitness: less-fit hybrids trigger reinforcement of the barriers, while fit or fitter hybrids may fuse the two species back into one. A zone where barriers hold steady and hybrids appear at a low unchanging rate shows stability. Proximity alone predicts none of this.

How fast.

Bio 1731 Gradual Speciation and Punctuated Equilibrium

The gradual model holds that lineages diverge in small steady steps; punctuated equilibrium holds that change occurs in rapid bursts followed by long stasis — two different fossil patterns. Punctuated equilibrium describes a faster tempo, not a different mechanism; selection drives change under either model.

One walkthrough taking an allopatric event from start to confirmation, and its warning is the most useful thing in the topic: divergence on its own does not establish that speciation happened.

River to barrier.

Bio 1733 Tracing One Allopatric Speciation Event

One interbreeding insect population is divided by a new river — vicariance. With gene flow blocked, each side accumulates its own mutations and faces its own selection pressures, so allele frequencies drift apart across many generations. Researchers then reintroduce individuals and watch. Failed mating attempts mean a prezygotic barrier arose; successful mating with inviable or sterile offspring means a postzygotic one. Either outcome confirms speciation. Divergent traits without a demonstrated reproductive barrier confirm nothing.

Three entries. Natural selection and Mendelian genetics were developed independently and reconciled decades later, and that reconciliation is what made the quantitative study of populations possible.

The reconciliation.

Bio 1735 The Modern Synthesis

The modern synthesis, taking shape by the nineteen forties, unified natural selection with Mendelian genetics — rediscovering Mendel's particulate inheritance resolved the difficulty blending inheritance had posed for selection acting on discrete units. Darwin and Wallace worked without knowing Mendelian genetics; the synthesis joined the fields decades later.

The discipline.

Bio 1737 Population Genetics Defined

Population genetics studies how selective forces change a population over generations by tracking shifts in allele and genotype frequencies — measuring an allele's frequency now and again several generations on. It tracks a population's frequencies over time, not any individual's genotype changing.

The measure.

Bio 1739 Allele Frequency

Allele frequency is the proportion of all copies of a gene in a population's gene pool that one specific allele accounts for — count that allele's copies, divide by total gene copies across the population. It describes the gene pool, not the fraction of individuals who are heterozygous or homozygous.

Four entries. A model of a population that is not evolving, five conditions no real population meets, the equations, and a worked problem. The model's value is entirely in the comparison it enables.

Nothing changing.

Bio 1741 Hardy-Weinberg Equilibrium

Hardy-Weinberg equilibrium describes a population whose allele and genotype frequencies stay constant across generations because no evolutionary force acts on it — matching the predicted values generation after generation means no evolution at that locus. It is a theoretical baseline for comparison, not a claim that any real population is non-evolving.

Five conditions.

Bio 1743 Hardy-Weinberg's Five Assumptions

The model assumes an infinite population, no mutation, no migration, random mating and no selection — conditions no real population fully meets. A small, migrating, selectively pressured population violates several at once, which is exactly what makes it worth comparing against the model. Violation is the norm.

Two equations.

Bio 1745 The Hardy-Weinberg Equations

For a two-allele locus, the allele frequencies p and q sum to one, and the genotype frequencies p squared, two p q, and q squared also sum to one — so a dominant allele at zero point eight leaves the other at zero point two. The equations require exactly two alleles; three or more needs a modified form.

A worked population.

Bio 1747 Solving a Hardy-Weinberg Calculation

Five hundred plants, twenty percent white-flowered and recessive. Only the homozygous recessive genotype shows the recessive phenotype, so that frequency is zero point two — which is q squared. Its square root gives q at about zero point four five, so p is about zero point five five. Then p squared gives about thirty-one percent homozygous dominant and two p q about forty-nine percent heterozygous — roughly a hundred fifty-three and two hundred forty-seven plants. Phenotype reveals only the recessive count directly; the other two are estimates the equations supply.

Five things to carry out of Part Two. Sympatric speciation isolates without geography, by two routes. Chromosomally, a polyploid individual carrying extra whole sets can become reproductively incompatible with its ancestral population in a single generation. Behaviourally, a subgroup shifting to a new habitat or resource and breeding among itself narrows gene flow with no chromosome change at all. Barriers sort by whether a zygote formed. Prezygotic barriers stop fertilisation — timing, habitat, courtship signals, structural fit, and gamete incompatibility. That last one acts after mating and is still prezygotic, because no zygote resulted. Postzygotic barriers act on a hybrid that already exists. A hybrid zone's outcome turns on hybrid fitness. Less-fit hybrids push barriers to strengthen, which is reinforcement. Fit or fitter hybrids can fuse the two species back into one. And barriers can hold as they are, giving a stable zone with a low steady hybrid rate. The zone existing tells you none of this. Hardy-Weinberg's five assumptions — infinite population, no mutation, no migration, random mating, no selection — are never all met, and that is what the model is for. It gives a baseline for a population that is not evolving, so measuring how far a real population departs from it measures the forces acting. And to get genotype frequencies from a phenotype count: the recessive phenotype appears only in homozygous recessives, so its frequency is q squared. Take the square root for q, subtract from one for p, then p squared and two p q give the other two genotypes — as estimates the equations supply, not observations. Part Three turns to the forces that move allele frequencies, and to how selection actually acts on a population.