Speciation

Essential Question

How does one species split into two?

Earth has millions of species, and they all came from earlier species. A new species forms when two parts of a population stop exchanging genes and become reproductively isolated. In this lesson, you’ll learn what a species is, how barriers to reproduction arise, and how geographic separation or a change in chromosome number can start the split.

What Is a Species?

• Species: organisms that can interbreed and produce fertile offspring
• Horse × donkey: a mule
• Mules are sterile, so horses and donkeys are separate species
• Speciation: one species splits into two

The most common definition of a species is a group of organisms that can interbreed in nature and produce fertile offspring. Horses and donkeys can mate, but their offspring, mules, are sterile. A horse egg or sperm has 32 chromosomes and a donkey’s has 31, so a mule has 63. Its chromosomes cannot pair up properly, so it cannot make normal eggs or sperm. Because their genes do not mix, horses and donkeys are separate species. Speciation is the process by which one species splits into two or more species.

Reproductive Isolation

• Reproductive isolation: two groups cannot produce fertile offspring together
• Barriers can act before mating or after it
• Isolation lets each group evolve separately

Barrier	How it works	Example
Habitat	Groups live in different places	One garter snake lives in water, another on land
Timing	Groups breed at different times	Two pine species release pollen in different months
Behavior	Groups do not respond to each other’s courtship	Eastern and western meadowlarks sing different songs
Mechanical	Reproductive structures do not fit together	Snails whose shells coil in opposite directions often cannot mate
Hybrid sterility	Hybrid offspring cannot reproduce	Horse × donkey = mule
Hybrid failure	Hybrid embryos do not develop	Sheep × goat embryos usually die

Two groups are reproductively isolated when they cannot produce fertile offspring together. Some barriers act before mating: the groups live in different places, breed at different times, or do not attract each other. In some species, the reproductive structures of the two groups do not fit together, so mating cannot succeed (a mechanical barrier). Others act after mating: hybrid offspring may fail to develop or may be sterile. A geographic barrier, such as a canyon or a river, can also keep two groups apart, as described under Allopatric Speciation below. Once two groups are reproductively isolated, genes no longer flow between them, and each group can evolve on its own.

Allopatric Speciation

• Allopatric: a geographic barrier splits a population
• Each side faces different conditions and chance events
• Over time, the groups can no longer interbreed
• Example: squirrels on the two rims of the Grand Canyon

In allopatric speciation, a geographic barrier, such as a canyon, a river, or a mountain range, splits a population into two groups. The groups no longer exchange genes. Each group experiences different selective pressures and different mutations and chance events, so they slowly become different. If the barrier disappears later, the two groups may have changed so much that they can no longer interbreed, and they are now separate species. Two closely related squirrel species on opposite rims of the Grand Canyon are a likely example.

Sympatric Speciation

• Sympatric: speciation without a geographic barrier
• Polyploidy: an organism gets extra sets of chromosomes
• Common in plants
• A new species can form in one generation

In sympatric speciation, a new species arises in the same area as its ancestor with no geographic barrier. In plants, this can happen in one generation through polyploidy. A mistake in meiosis can produce a gamete with all the chromosomes instead of half. When two of these gametes combine, the offspring has twice the original number of chromosomes (36 instead of 18). The new plant can reproduce with itself, but a cross with the original plant gives an odd number (18 + 9 = 27) and sterile offspring, so the new plant is instantly reproductively isolated.

Adaptive Radiation

• Adaptive radiation: one ancestor gives rise to many species
• Each species fills a different niche
• Occurs on new islands and after mass extinctions
• Example: Galápagos finches

When a species enters a new environment with many open niches, it can diversify quickly into many species, each adapted to a different way of life. This is called adaptive radiation. Darwin’s finches are a classic example. A small group of finches from the mainland reached the Galápagos Islands, and their descendants evolved into species with different beaks suited to different foods. Adaptive radiation also follows mass extinctions, when many niches become open at once.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Allopatric or Sympatric?

Decide whether each example is allopatric or sympatric speciation, and name the cause. (a) A river changes course and splits a population of mice. Thousands of years later, the two groups cannot interbreed. (b) A plant has a meiosis error that doubles its chromosome number, and the new plants cannot breed with the original plants. (c) In one field, some insects feed on apple trees and others on hawthorn trees, and the two groups gradually stop mating with each other.

Show the solution

1. (a) Allopatric: the river is a geographic barrier that separated the mice.
2. (b) Sympatric: polyploidy creates a new species in the same place as the original, with no geographic barrier.
3. (c) Sympatric: the insects live in the same field, and the difference in host plants reduces mating between the groups.
4. Answer: (a) Allopatric (river barrier). (b) Sympatric (polyploidy). (c) Sympatric (different host plants).

Example 2: Do Flies Prefer Their Own Kind?

Sample data modeled on an experiment: fruit flies raised on starch or on maltose were placed together. The table shows how many matings occurred between each pair. (a) How many matings were there in all? (b) What percent were between flies raised on the same food? (c) If flies mated at random, what percent would be between flies raised on the same food? What do the results suggest?

Male food × female food	Number of matings
Starch × starch	18
Maltose × maltose	22
Starch × maltose	7
Maltose × starch	3

Show the solution

1. (a) Total: 18 + 22 + 7 + 3 = 50 matings.
2. (b) Same food: 18 + 22 = 40. 40 ÷ 50 × 100 = 80%.
3. (c) Random: with two equal groups, half of the matings (50%) would be between flies from the same food by chance. The observed 80% is much higher, so the flies prefer mates raised on the same food. This is the beginning of reproductive isolation, a step toward speciation.
4. Answer: (a) 50. (b) 80%. (c) 50% expected at random, so the flies prefer similar mates.

Example 3: Counting Chromosomes

A plant species has 2n = 18 chromosomes. A meiosis error produces a tetraploid plant. (a) How many chromosomes are in a normal gamete? (b) How many chromosomes does the tetraploid plant have? How many are in its gametes? (c) How many chromosomes does the offspring of the tetraploid and a normal plant have? Is it likely to be fertile?

Show the solution

1. (a) Normal gamete: 18 ÷ 2 = 9 chromosomes.
2. (b) Tetraploid: 2 × 18 = 36 chromosomes. Its gametes have 36 ÷ 2 = 18.
3. (c) Cross: 18 (tetraploid gamete) + 9 (normal gamete) = 27 chromosomes. The odd number cannot pair evenly in meiosis, so the plant is sterile.
4. Answer: (a) 9. (b) 36 and 18. (c) 27, so it is sterile.

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Practice

Analyze a diagram of allopatric speciation, classify barriers to reproduction, work out chromosome numbers in a mule and a polyploid plant, and explain sterility. Try it on your own first, then check your work with the answer key.

• Speciation Practice Worksheet (PDF)
• Speciation Practice Answer Key (PDF)

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Lesson Slides

• 4.8 Speciation Slides (PDF)

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Big Ideas

• A species can interbreed and produce fertile offspring
• Reproductive isolation stops gene flow
• Allopatric: a geographic barrier; sympatric: no barrier
• Polyploidy can make a new plant species in one generation
• Adaptive radiation: one ancestor gives many species

Back to the essential question: a new species forms when two groups of a population become reproductively isolated and evolve separately. A geographic barrier starts the split in allopatric speciation, and a change such as polyploidy can start it in sympatric speciation. When many niches are open, one species can radiate into many. Next, you’ll look at how the rates of speciation and extinction have changed over Earth’s history.

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