Speciation: Study Guide (Biology 7.10)

From populations to species

Topic 7.9 showed that every node on a tree is a point where one lineage became two. This topic explains how that split happens. The short answer: two populations stop exchanging genes. Once gene flow stops, mutation, natural selection and genetic drift change each gene pool in its own way. If the populations become so different that they could no longer interbreed successfully even if they met, they are separate species. The process is speciation.

What is a species?

The definition used most often for sexually reproducing organisms is the biological species concept. A species is a group of populations whose members can interbreed in nature and produce viable, fertile offspring. Viable means the offspring survive; fertile means they can reproduce in turn.

The key idea is reproductive isolation: members of different species do not exchange genes, so each species evolves as a separate unit.

The concept has limits (background):

• It cannot be applied to organisms that reproduce asexually, such as bacteria.
• It cannot be tested on fossils, so palaeontologists use body form instead.
• Some separate species occasionally hybridize where their ranges meet.

Reproductive barriers

A reproductive barrier is anything that stops two populations from producing fertile offspring together. Barriers are grouped by when they act, relative to the zygote (the fertilized egg).

Type	Barrier	How it blocks gene flow	Example (fictional unless named)
Pre-zygotic	Habitat	populations live in different habitats in the same area and rarely meet	one snail on rocks, another in mud, in the same bay
	Temporal	they breed at different times of day or year	one frog breeds in March, another in May
	Behavioural	courtship signals are not recognised	different mating songs or displays
	Mechanical	reproductive structures do not fit	flower shape suits only one pollinator
	Gametic	sperm and egg cannot fuse	egg-surface proteins bind only sperm of the same species
Post-zygotic	Reduced hybrid viability	hybrid zygotes develop poorly or die young	hybrid embryos fail to develop
	Reduced hybrid fertility	hybrids survive but are sterile	mule (horse × donkey)
	Hybrid breakdown	first-generation hybrids are fertile, but their offspring are weak or sterile	second-generation hybrids fail

Why is a mule sterile? A horse has 64 chromosomes and a donkey 62. A mule gets 32 from its horse parent and 31 from its donkey parent: 63 in total. In meiosis, chromosomes must pair with a matching partner. The horse and donkey sets do not match, so pairing fails and the mule makes few or no working gametes.

Pre-zygotic barriers “save” effort: no energy is spent making hybrids that will fail. Post-zygotic barriers still block gene flow, because hybrids leave few or no descendants.

Allopatric and sympatric speciation

Speciation is classified by where the diverging populations live.

Allopatric speciation (“other homeland”) happens when populations are geographically isolated. A river changes course, a mountain range rises, sea level rises and cuts off an island, or a few individuals reach a new island. With no gene flow, each population changes independently. Differences build up in mating signals, timing or chromosomes, often as a side effect of adapting to different conditions. If the populations meet again later and cannot interbreed successfully, speciation is complete. Allopatric speciation is thought to be the most common route in animals.

Sympatric speciation (“same homeland”) happens while populations overlap geographically. Something other than distance must cut gene flow:

• Polyploidy. An error in cell division can produce offspring with extra full sets of chromosomes. In plants this happens often, and a polyploid plant can be isolated from its parents in a single generation (Worked example 1). Polyploidy is common in flowering plants.
• Habitat or host shift. The apple maggot fly, Rhagoletis pomonella, originally laid its eggs on hawthorn fruit. After apples were brought to North America, a race that uses apples appeared in the 1800s. The flies mate on their host fruit, and apples ripen earlier than hawthorn, so apple flies and hawthorn flies mostly breed at different times and places. In the laboratory they can still produce offspring, so this is usually described as speciation in progress.
• Mate choice. If females in part of a population come to prefer a different signal, matings between the groups can become rare.

How fast does speciation happen?

Two models describe the pattern of change, especially in the fossil record.

• Gradualism: species change slowly and steadily, over hundreds of thousands or millions of years. Fossils from successive rock layers would show many small, in-between forms.
• Punctuated equilibrium: a species stays almost unchanged (stasis) for a long time, then changes rapidly, often around the time a new species forms. “Rapid” here can still mean thousands of years, which is a very short slice of the fossil record. In-between forms are then rarely preserved.

Both patterns occur. Which one you see depends on the group and its conditions. Stable environments tend to favour stasis; a sudden change in the environment, or a small isolated population, can favour rapid change.

Divergent evolution and adaptive radiation. When related populations adapt to different habitats, their phenotypes spread apart: this is divergent evolution. It is fastest during an adaptive radiation, when one ancestral lineage quickly gives rise to many species, each suited to a different way of life. Radiations happen when many new habitats or niches open up with few competitors, for example on new volcanic islands or after a mass extinction removes other groups. The Hawaiian islands, for instance, hold hundreds of species of Drosophila fruit flies found nowhere else.

Convergent evolution is the opposite pattern. Similar selection pressures produce similar phenotypes in lineages that are not closely related. On each of the four large Caribbean islands (Cuba, Hispaniola, Jamaica and Puerto Rico), Anolis lizards have evolved much the same set of body types, called ecomorphs, matched to where they live: for example, forms suited to tree trunks, to the canopy and to thin twigs. DNA comparisons show that the similar forms on different islands mostly arose independently, each from lizards already on that island. Convergent traits are analogous, which is why they can mislead a tree built only from body form (Topic 7.9).

Worked example 1: speciation by polyploidy

Question. A fictional meadow flower is diploid with 2n = 14. In one plant, an error in cell division doubles the chromosome number in the cells that make flowers. It produces some tetraploid offspring (4 sets).

(a) Give the chromosome number of the tetraploid and of its gametes. (b) A tetraploid is pollinated by a normal diploid. Give the chromosome number of the offspring and explain whether it is fertile. (c) Explain whether the tetraploids form a new species, and whether this is allopatric or sympatric speciation.

(a) One set has n = 14 ÷ 2 = 7 chromosomes. Tetraploid: 4 × 7 = 28 chromosomes. Its gametes carry half: 14 (2 sets).

(b) Tetraploid gamete (14) + diploid gamete (7) = 21 chromosomes, which is 3 sets (triploid). In meiosis, chromosomes pair with a matching partner. With three copies of each chromosome, one copy is always left over, so the gametes get unbalanced numbers of chromosomes. Most gametes fail, so the triploid is largely sterile. This is a post-zygotic barrier (reduced hybrid fertility).

(c) Tetraploid × tetraploid gives 14 + 14 = 28: fertile tetraploids. So tetraploids breed successfully with each other but not with the diploids. Under the biological species concept they are a new species, formed in one generation. They grew in the same meadow, so this is sympatric speciation.

Check. Every count is a whole number of sets of 7: 7, 14, 21, 28. An odd number of sets (here 3) is the warning sign for sterility.

Worked example 2: is speciation complete?

Question. A fictional seed bug feeds on two plants that grow side by side: river bush (R bugs) and hill bush (H bugs). Bugs mate on the plant they feed on. In a large cage holding equal numbers of each type and both plants, 200 matings were observed: R × R 90, H × H 82, R × H 28. Egg hatch rates were R × R 91%, H × H 89%, R × H 85%. Hybrid females lay 40 eggs each, against 120 for either pure type.

(a) Calculate the percentage of same-type matings and compare it with random mating. (b) Identify the barriers that are acting and classify each. (c) Decide whether R and H are separate biological species. (d) Predict what would happen if hill bush died out in an area and all H bugs had to feed on river bush.

(a) Same-type matings = 90 + 82 = 172. Percentage = 172 ÷ 200 × 100 = 86%. With equal numbers and random mating, about 50% would be same-type (100 of 200). So bugs strongly prefer their own type.

(b)

1. Mating on the host plant: habitat isolation (pre-zygotic). There may also be behavioural isolation if bugs choose mates by type.
2. Hatch rates of hybrids (85%) are close to those of pure crosses (89–91%), so hybrid viability is barely reduced.
3. Hybrid females lay 40 eggs, which is 40 ÷ 120 × 100 = 33.3% of the normal number: fertility is reduced by 66.7%. This is reduced hybrid fertility (post-zygotic).

(c) Isolation is strong but not complete: 14% of matings are mixed, hybrids survive, and hybrid females are partly fertile. Some genes still flow between R and H. They are best described as diverging populations, or speciation in progress, not yet fully separate species.

(d) With only one host plant, R and H bugs would meet and mate on the same plant. Habitat isolation would disappear, mixed matings would rise and gene flow would increase. Unless the remaining barriers (mate choice and low hybrid fertility) are strong enough, the two gene pools could merge back into one.

Common misconceptions

• “If two organisms can mate, they are the same species.” The offspring must be viable and fertile, and gene flow must happen in nature. Mules are viable but sterile.
• “Speciation always needs a physical barrier.” Sympatric speciation, for example by polyploidy, happens with no barrier at all.
• “Individuals form new species.” Populations, not individuals, become reproductively isolated over generations. Polyploidy is a special case where isolation arises at once, but a new species still needs a breeding population.
• “Punctuated equilibrium means new species appear overnight.” “Rapid” means quickly in geological terms, which can still be thousands of years.
• “Gradualism and punctuated equilibrium cannot both be right.” They describe different patterns, and both are seen in different groups.
• “Convergent species are closely related.” They look alike because of similar selection pressures, not shared ancestry.
• “Pre-zygotic means before mating.” It means before a zygote forms. Gametic isolation acts after mating but before fertilization.

Where this leads

Speciation needs variation within populations to act on. The next topic, Variations in Populations, looks at why genetic diversity helps populations survive change. Now test yourself with the practice questions, then use the revision notes and the checklist. To see speciation drawn as nodes on a tree, look back at Phylogeny.