Evolution may lead to speciation

In a nutshell

Evolution is simply a change in the allele frequencies in a population over time. It happens because organisms vary, and some variants leave more offspring than others.

This subtopic joins those ideas up: where variation comes from, how natural selection (and chance) shift allele frequencies, and how two populations of one species can drift so far apart that they can no longer interbreed, giving a new species.

Assumed knowledge: Populations (gene pool, allele frequency, Hardy-Weinberg), Genetic diversity can arise from mutation and meiosis.

Core content

Where phenotypic variation comes from

Individuals in a population show a wide range of variation in phenotype, due to genetic factors and environmental factors (and the interaction between them).

The variation that matters for evolution is the genetic part, because only that is passed on. It has three sources:

• Mutation is the primary source of genetic variation. It creates new alleles.
• Meiosis shuffles alleles into new combinations (crossing over and independent assortment).
• Random fertilisation of gametes combines two parents' alleles unpredictably.

Environmental factors (food, light, temperature) change the phenotype an organism shows, but they do not change its alleles, so environmentally caused variation is not inherited and does not drive evolution.

Natural selection

Populations produce more offspring than the environment can support, so not all survive. Predation, disease and competition for the means of survival (food, mates, space) cause differential survival and reproduction. This is natural selection.

The logic runs as one chain, and each link is a separate creditable point:

1. There is genetic variation within the population, arising ultimately from mutation.
2. Predation, disease and competition act as a selection pressure.
3. Organisms whose phenotype gives a selective advantage are more likely to survive and reproduce.
4. These organisms pass on their favourable alleles to the next generation.
5. Over generations the frequency of the favourable allele increases in the gene pool.

The mutation comes first and at random. The environment does not create the useful allele to order, it only selects the organisms that already carry it. Saying an organism "developed" resistance because it needed it is the classic error: the mutation was there first, and selection did the rest.

The three types of selection

Selection acts on a phenotype that varies continuously (like body mass or height), so you can show its effect on the whole population's distribution. The type depends on which part of the range is selected against.

Stabilising selection acts against both extremes, favouring intermediate phenotypes. The mean stays the same and the range narrows. It is common in a stable, unchanging environment (for example, very high and very low human birth mass both have higher infant mortality).

Directional selection acts against one extreme, favouring the phenotype at the other end. The mean shifts in that direction. It is common when the environment changes (for example, antibiotic resistance in bacteria).

Disruptive selection acts against the intermediate phenotype, favouring both extremes. The single peak splits into two. It can be a first step towards sympatric speciation.

Evolution

Evolution is a change in the allele frequencies in a population over time.

Natural selection is one cause of that change. It is worth being precise about what changes: individuals do not evolve, and alleles do not "reproduce". Organisms survive and reproduce, and it is the allele frequency in the population's gene pool that changes.

Genetic drift

Allele frequencies can also change purely by chance, not because of any selective advantage. This is genetic drift: which individuals happen to breed, and which alleles happen to end up in their surviving offspring, is partly luck.

Genetic drift is only important in small populations. In a small population a chance event affects a large fraction of the gene pool, so allele frequencies can swing sharply and an allele can even be lost or become the only one present (fixed). In a large population the chance effects on different individuals cancel out, so frequencies stay close to their starting values.

Still don't get it? · why drift only matters in small populations

Flip a fair coin 10 times and getting 8 heads would not shock you. Flip it 10,000 times and getting 80% heads is effectively impossible: the flips average out. The coin is fair both times, but chance shows up much more strongly when the numbers are small.

Alleles behave the same way. In a population of 10 breeding individuals, if a couple of carriers of one allele happen not to breed this year (bad luck, nothing to do with fitness), the allele's frequency lurches. In a population of a million, the same run of bad luck for a few individuals is a drop in the ocean and barely moves the frequency.

Now the exam version. Genetic drift is a chance change in allele frequency, independent of selective advantage. In a small population each individual is a large share of the gene pool, so random variation causes big changes in allele frequency (an allele may even be lost or fixed). In a large population the random effects cancel out, which is why drift is important only in small populations, exactly the phrase the question wants.

You can model this data-handling idea (spec skill): drawing small random samples of "alleles" from a population and tracking how much the sampled frequency jumps compared with large samples mimics drift, and links directly to the mathematics of sampling.

What makes a species, and how a new one forms

Members of one species can interbreed to produce fertile offspring. That is the test a new species must eventually fail against its parent population.

Speciation happens when two populations become reproductively separated (no gene flow between them). Once separated, their gene pools accumulate differences independently, through:

• different mutations arising in each population,
• different selection pressures (different biotic and abiotic conditions) favouring different alleles,
• genetic drift (especially if a population is small).

A new species arises when these genetic differences become so great that members of the two populations can no longer interbreed to produce fertile offspring. In this way, new species arise from existing species.

Still don't get it? · how isolation turns into a new species

Picture two friendship groups in one big group chat. Split them into two chats that never talk to each other. Each chat invents its own in-jokes, nicknames and slang, drifting a little at random and a lot because the two groups care about different things. Leave it long enough and a message from one chat is gibberish to the other. They can no longer communicate, even though they started identical.

Now the exam version. The two chats are two reproductively isolated populations, with no gene flow between them. The drifting slang is their gene pools accumulating differences through different mutations, different selection pressures and genetic drift. When the differences are big enough that the two populations cannot interbreed to produce fertile offspring, a new species has formed. The isolation is the cause of speciation, not a consequence of it.

Allopatric and sympatric speciation

The two types differ only in what causes the reproductive separation.

Both end the same way: enough genetic difference that the populations can no longer interbreed to produce fertile offspring.

From speciation to the diversity of life

Repeated over very long timescales, this process (variation, selection, isolation, divergence) has produced the enormous diversity of species alive today. Because new species arise from existing ones, related species share a common ancestor, which is why classification reflects evolutionary relationships.

Worked examples

Model answer: "Explain how a population of bacteria can become resistant to an antibiotic."

This is a natural-selection answer. Examiners want the chain in order, each numbered point a separate idea:

1. There is genetic variation in the population; by chance a random mutation produces an allele giving antibiotic resistance.
2. The antibiotic is a selection pressure: it kills non-resistant bacteria.
3. Resistant bacteria have a selective advantage, so they are more likely to survive and reproduce.
4. They pass on the resistance allele to their offspring.
5. Over many generations the frequency of the resistance allele increases in the population.

Two things drop marks here: writing "gene" where the point needs allele, and implying the bacteria "became immune because they needed to" (the mutation is random and comes first).

Model answer: "Two groups of one plant species live in the same area. One flowers in early spring, the other in late summer. Explain how these could become separate species by sympatric speciation."

The clue "same area" means this must be answered as sympatric, with no geographical barrier:

1. The two groups live in the same area / habitat (they are not geographically isolated).
2. A mutation caused the difference in flowering time.
3. Because they flower at different times, the groups are reproductively separated: there is no gene flow and their gene pools stay separate.
4. Disruptive selection and different selection pressures favour different alleles in each group, so allele frequencies change differently in the two gene pools.
5. Eventually the two groups are so genetically different that they cannot interbreed to produce fertile offspring, so they are separate species.

The commonest error on a sympatric question is to slip into describing a geographical barrier, which is allopatric and caps the marks. If the stem says "same area", never mention a physical barrier.

Common exam mistakes

• Writing "gene" where the point needs "allele". Evolution and selection change allele frequencies; "gene frequency" and "genotype frequency" are not accepted.
• Saying "alleles survive and reproduce" or "the allele reproduces". Organisms survive and reproduce; it is the frequency of the allele in the gene pool that changes.
• Answering a sympatric question with allopatric content, that is, describing a geographical barrier when the stem says the populations share an area. For sympatric, keep it to the same place and a biological cause of isolation.
• Saying a "species" becomes isolated. It is a population (or group) of one species that becomes isolated; two species already existing at the point of separation is wrong.
• Lamarckism: implying organisms change because they "need to", or that the environment or a chemical causes the useful mutation to appear. Mutations are random and come first; the environment only selects.
• Defining a species by "infertile offspring" or "cannot produce offspring". The wording examiners require is "cannot interbreed to produce fertile offspring".
• Treating "no interbreeding" as something that happens after the new species exists. To earn the isolation point it must be described during the process of speciation, as the cause.
• Leaving out variation or mutation from a selection or speciation answer; both are needed as the raw material.
• Naming the wrong type of selection, or naming a type without explaining the effect on survival and reproduction. For directional selection you must say which extreme is favoured and why it is advantageous.
• Claiming genetic drift matters in large populations, or that "any small population has a small gene pool" without linking to chance changes in allele frequency. Drift is significant only in small populations.
• Saying the number of an allele increases. The mark needs frequency / proportion, not number.

Key definitions

• Evolution - a change in the allele frequencies in a population (over time).
• Gene pool - all the alleles present in a population.
• Allele frequency - how often an allele occurs in a population, as a proportion of all the alleles of that gene.
• Natural selection - the process by which organisms with phenotypes that give a selective advantage are more likely to survive and reproduce, passing on their favourable alleles to the next generation.
• Stabilising selection - selection that acts against both phenotypic extremes, so intermediate phenotypes are favoured and the mean stays the same.
• Directional selection - selection that favours one phenotypic extreme, so the mean of the population shifts in that direction.
• Disruptive selection - selection that acts against the intermediate phenotype, so both extremes are favoured.
• Genetic drift - a change in the allele frequency of a population caused by chance rather than by selective advantage, significant only in small populations.
• Reproductive isolation - when two populations can no longer interbreed, so there is no gene flow between them.
• Species - a group of organisms that can interbreed to produce fertile offspring; a new species has formed when populations can no longer interbreed to produce fertile offspring.
• Allopatric speciation - the formation of a new species from populations separated by a geographical barrier.
• Sympatric speciation - the formation of a new species from populations living in the same area, without geographical isolation.

Specification

• I can explain why individuals within a population show a wide range of variation in phenotype (genetic and environmental factors, with mutation the primary source of genetic variation, and meiosis and random fertilisation adding more).
• I can explain how predation, disease and competition cause differential survival and reproduction (natural selection).
• I can describe the effect of differential reproductive success on the allele frequencies within a gene pool.
• I can describe and distinguish the effects of stabilising, directional and disruptive selection.
• I can state that evolution is a change in the allele frequencies in a population.
• I can explain how natural selection and isolation may change allele and phenotype frequencies and lead to the formation of a new species.
• I can distinguish allopatric and sympatric speciation.
• I can explain why genetic drift is important only in small populations.
• I can explain how evolutionary change over a long period of time has resulted in a great diversity of species.

Related notes

• Inheritance
• Populations (gene pool, allele frequency, Hardy-Weinberg)
• Genetic diversity can arise from mutation and meiosis
• Genetic diversity and adaptation
• Populations in ecosystems