0610 · 18.3Selection

Selection and inherited variation

0:00Imagine a population of beetles living on dark soil, where some beetles are brown and others are green. A bird searching for food can spot the green beetles more easily, while the brown beetles are harder to see. Over many generations, brown beetles could become more common, even though no beetle deliberately changes its colour. How can a population change when the individuals are not trying to change? The answer is selection, which depends on some individuals contributing more offspring to the next generation than others.

0:27We will follow that idea through beetles, antibiotic resistant bacteria, and plants and animals bred by humans. By the end, you should understand how natural selection works, how it produces adaptation, and how it differs from artificial selection. The key is to follow what happens to inherited differences over several generations. We need three pieces of background knowledge before we begin. A population is a group of organisms of one species living in the same area at the same time.

0:53A gene is a length of deoxyribonucleic acid that codes for a protein, and an allele is an alternative form of a gene. Different alleles can contribute to differences between individuals, and alleles can be passed from parents to offspring. Variation means differences between individuals of the same species, but not every difference is inherited. A plant might grow taller because it receives more light, rather than because it has different alleles.

1:17For selection to produce an inherited change over generations, the relevant variation must have a genetic basis. Otherwise, selecting a successful individual does not necessarily mean its offspring will inherit the feature that made it successful.

Natural selection in beetles

1:29Natural selection begins with genetic variation within a population. In our beetle example, suppose differences in alleles produce differences in body colour, so the population already contains brown and green individuals. The beetles also produce many offspring, more than the environment can usually support to adulthood. There is not enough food, space or other resources for every offspring to survive and reproduce. That creates competition for resources and a struggle for survival.

1:56Competition does not have to involve animals fighting each other, because two beetles eating the same limited food supply are also competing. The struggle for survival also includes threats such as predators and disease. These conditions mean that individuals do not all have the same chance of surviving long enough to reproduce. On dark soil, brown beetles are better camouflaged than green beetles. Birds are therefore less likely to eat the brown beetles, giving them a greater chance of surviving and reproducing.

2:20When they reproduce, they pass on their alleles, including alleles contributing to brown colour, to their offspring. This means the next generation can contain a greater proportion of brown beetles than the previous generation. Notice that the important change is in the proportion of individuals with an inherited feature. A green beetle does not become brown because it needs better camouflage, and the bird does not create the useful allele.

2:42Instead, an existing inherited difference affects which beetles are more likely to reproduce. Repeated over many generations, this can make alleles associated with a beneficial feature more common in the population. Being better adapted does not guarantee survival, because an individual may still be eaten or fail to reproduce. Natural selection works through differences in chances across a population, rather than a guarantee for every individual. Also, survival alone is not enough to explain the change across generations.

3:10The crucial connection is that surviving individuals reproduce and pass on their alleles. The phrase better adapted always needs an environmental context. Brown colour is helpful in our example because the soil is dark and birds hunt by sight. If the background changed so that green beetles were less visible, green colour could become advantageous instead. There is no single feature that is automatically best in every environment, so selection depends on the conditions organisms face.

3:36Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. That definition describes a change in a population across generations, not an individual changing during its lifetime. An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. Here, camouflage is the feature, while adaptation is the process through which the population becomes better suited to its surroundings.

4:03We can now connect the complete natural selection sequence. There is genetic variation within populations, and organisms produce many offspring. Limited resources lead to competition and a struggle for survival, so individuals that are better adapted are more likely to survive and reproduce. These individuals pass on their alleles to the next generation, and the population can become better suited to its environment over many generations.

Antibiotic resistance

4:26Now let us apply exactly the same reasoning to antibiotic resistant bacteria. An antibiotic is a drug used to treat bacterial infections, and it kills bacteria or prevents their growth. A resistant bacterium can survive exposure to an antibiotic that would affect susceptible bacteria. Here, susceptible simply means that the bacterium is affected by that antibiotic, rather than being resistant to it. Before treatment, a bacterial population may already contain a small number of resistant bacteria.

4:53Random mutations can produce new alleles, and some mutations can give bacteria resistance to a particular antibiotic. The important point is that the mutation is not a deliberate response to what the bacterium needs. For this example, resistance is already present in some bacteria before the antibiotic is applied. When the antibiotic is applied, susceptible bacteria are killed or their growth is prevented. Resistant bacteria are more likely to survive, so they face less competition from susceptible bacteria for resources.

5:22The surviving resistant bacteria reproduce, producing offspring that inherit their resistance alleles. As those descendants multiply, resistant bacteria form a greater proportion of the population. This explains how strains of antibiotic resistant bacteria can develop through natural selection. The antibiotic changes which bacteria are more likely to survive and reproduce, rather than teaching each bacterium to become resistant. Repeated selection can produce a population in which resistance is common, making that antibiotic less effective against it.

5:51It is the bacteria that are resistant, not the patient’s body. The same distinction helps us avoid a common mistake about antibiotics. Saying that the antibiotic causes bacteria to develop the resistance they need suggests that mutations happen for a purpose. Instead, mutations occur randomly, and exposure to the antibiotic selects bacteria with resistance that enables them to survive. Selection changes which inherited variants become common. It does not involve bacteria trying to adapt.

Artificial selection

6:18Now consider a different situation, where humans decide which organisms will produce the next generation. A farmer might want crop plants with larger fruits, or cattle that produce more milk. Humans can choose parents with these desirable inherited features and breed them together. This is selective breeding, also called artificial selection, because humans make the breeding choices. The process begins with humans selecting individuals that show desirable features. These selected individuals are crossed to produce the next generation, meaning they are bred together.

6:47The offspring are examined, and those showing the desirable features are selected as parents for another generation. Repeating this process over many generations can improve crop plants or domesticated animals for human purposes. Why does the selection have to be repeated, rather than stopping after one cross? Offspring inherit combinations of alleles from their parents, so they may differ in how strongly they show the desired feature. Breeders keep choosing offspring that best meet their aim, making relevant alleles more common in the breeding population.

7:15This only works reliably when the feature has an inherited basis, rather than being caused entirely by the environment.

Selective breeding wheat

7:22Let us work through a breeding problem rather than simply naming the steps. Suppose a grower wants wheat plants that produce a high yield and are resistant to a particular disease. High yield means producing a large amount of useful crop, such as grain. The grower has one group with high yield and another with disease resistance, so the aim is to bring both desirable features into the same plants.

7:41First, select high yielding parent plants and disease resistant parent plants. Cross those parents to produce offspring with combinations of inherited characteristics from the two groups. Then assess the offspring for both yield and disease resistance, selecting those that show both desirable features. Cross the selected offspring and repeat the assessment and selection over many generations, rather than assuming that every offspring from the first cross will have both features.

8:06The reasoning matters here because simply choosing the highest yielding offspring could lose the disease resistance the grower also wanted. The selection must match the complete aim of the breeding programme. Similarly, cattle could be selected for high milk yield, bred together, and their offspring assessed as adults for the same feature. In each context, identify the desired inherited feature, select suitable parents, breed them, and select suitable offspring repeatedly.

8:31Natural and artificial selection therefore share an important foundation. Both depend on inherited variation and on some individuals contributing more offspring to later generations than others. In natural selection, environmental conditions affect which individuals are more likely to survive and reproduce. In artificial selection, humans choose which individuals reproduce according to features they consider desirable. The outcomes can also differ because human preferences are not necessarily the same as survival advantages in the wild.

8:58A large crop fruit may be desirable to a grower without making the plant better at surviving without human care. Artificial selection improves organisms for a chosen human purpose, while natural selection can make populations more suited to their environment. Neither process means that organisms consciously decide which alleles their offspring should inherit.

Exam answers and recap

9:17For an exam, the command word describe asks you to give the relevant features or sequence of events. Outline asks you to set out the main points, without needing unnecessary detail. Consider this question. Describe how antibiotic resistant bacteria become more common in a population after treatment with an antibiotic. Start with variation, then explain survival, reproduction and inheritance, rather than jumping straight to the final resistant population. Random mutation produces genetic variation, so some bacteria have alleles that give resistance to the antibiotic.

9:49The antibiotic kills susceptible bacteria or prevents their growth. Resistant bacteria are more likely to survive and reproduce. Their offspring inherit the resistance alleles. Over generations, the proportion of resistant bacteria in the population increases. A second question might ask you to outline how selective breeding could produce sheep with more wool. Humans select parent sheep that produce large amounts of wool. They cross these individuals to produce offspring. Offspring showing the desirable feature are selected for breeding.

10:17This process is repeated over many generations to increase wool production in the breeding population. The biggest mistakes all break a link in the explanation. Saying that individuals change because they need to ignores the inherited variation already present in the population. Saying that only the strongest survive ignores the fact that a useful feature depends on the environment and may be camouflage or resistance rather than strength. Saying that successful individuals survive, but not mentioning reproduction and the passing on of alleles, leaves out how the next generation changes.

10:46Let us finish with the central idea connecting everything. Natural selection acts on genetic variation within populations, with competition and the struggle for survival favouring individuals that are better adapted and more likely to reproduce. They pass on their alleles, so adaptation can occur as populations become more suited to their environment over many generations. Artificial selection uses repeated human choices of parents and offspring to increase desirable inherited features, but both processes change populations across generations rather than making individuals change because they need to.