Lesson 2: Natural Selection and Artificial Selection

2.1 Populations

• To understand the life of organisms in an ecosystem it is not enough to study one individual; we study populations.
• A population is a group of individuals of the same species, able to reproduce with each other, living in the same place at the same time.

• Populations are not static.
• The number of individuals in some species stays steady for long periods; in others there is a rapid rise or a sudden collapse.
• This shows the interaction between organisms and environmental factors.

2.2 Population growth

Imagine a few rabbits introduced to a new place that has plenty of food and no competitors:

1. At first, the rabbits adjust to the new place.
2. They begin to reproduce.
3. With abundant food, the population rises quickly.
4. As numbers rise, food begins to run out.
5. Competition increases and the growth slows down.
6. The population reaches a relatively stable size balanced with its environment.

The growth curve has four phases: lag phase (slow start), exponential growth phase (rapid increase), slowing-down phase, and stationary phase (at the carrying capacity).

Exponential growth. Under ideal conditions a population can multiply rapidly: 2 → 4 → 8 → 16 → 32 …, i.e. 2¹, 2², 2³, 2⁴, 2⁵ …

Worked example 1

A culture of bacteria doubles every hour starting from 1 cell. After 1 hour: 2 cells (2¹); after 2 hours: 4 (2²); after 5 hours: 2⁵ = 32; after 10 hours: 2¹⁰ = 1024 cells. If it starts from 3 cells, after 4 hours there are 3 × 2⁴ = 48 cells.

Environmental resistance.

• In nature, growth cannot go on for ever because resources (food, water, space) are limited.
• The factors that limit population growth are called environmental resistance:

• depletion of resources,
• increased competition among individuals,
• fast spread of diseases and parasites (and predators).

• Because of environmental resistance, the population stops growing at the carrying capacity: the maximum number of individuals of a species that an environment can support continuously without damaging its resources.
• If a population goes above the carrying capacity, resources are exhausted, so deaths exceed births and the number falls back.

Application. In nature reserves, scientists determine the carrying capacity of each species and intervene if a population grows too much or declines sharply, to maintain the balance of the ecosystem.

2.3 Factors affecting population growth

Two main groups:

• 1.
• Environmental factors. All living (biotic) and non-living (abiotic) components of the environment: food, water, temperature, disease, predators and space.

• When the environment is rich (plenty of resources), survival and reproduction increase and the population rises.
• When it deteriorates (scarcity, danger, pollution), the population decreases due to lower reproduction, higher death rate, or both.

• 2.
• Genetic factors. The inherited traits that affect an organism's ability to adapt, survive and reproduce.
• Individuals in a population are not identical; the genetic variation gives some a better chance in a given environment.
• Examples: fur colour, resistance to disease, tolerance of heat or cold, sharp eyesight, strong sense of smell, strong beaks for cracking seeds.

Classifying cases:

Resource abundance followed by scarcity: the population first increases, then, as resources run short, competition rises and the population falls until a balance with the available resources is reached.

2.4 Natural selection

Natural selection

• explains how populations change (evolve) over time.
• Individuals that have inherited traits suited to their environment are more likely to survive and reproduce than individuals without those traits.
• These useful traits are passed to the offspring and become more common in each generation.

Important points:

• The environment does not give the organism new traits. It selects, among the traits that individuals already have, those that help survival.
• Better-adapted individuals live longer and leave more offspring.
• Less adapted individuals are more likely to die before reproducing or reproduce less.
• Over many generations the proportion of useful traits rises, while the proportion of less useful traits falls.

Example: peppered moths and lichens

• Before the Industrial Revolution in Europe, tree trunks were light-coloured and covered with lichens.
• Light-coloured moths were hard for predators (birds) to see on them, so they survived more often.
• After the Industrial Revolution, air pollution killed the lichens and darkened the trunks.
• Now light moths were easy to see and were eaten, while dark moths were hidden.
• Over time dark moths became common and light moths became rare.

Explanation using natural selection: (1) the moths were already varied in colour (variation); (2) the changed environment made the dark colour an advantage; (3) dark moths survived and reproduced more; (4) they passed the dark-colour allele to their offspring; (5) the proportion of dark moths rose over generations.

• Lichens as indicators. Lichens are formed by a fungus living with an alga or cyanobacterium.
• They are sensitive to air pollution, so their presence or absence shows the quality of the air.

Worked example 2: a mathematical model of selection

A population has 100 rabbits: 60 fast and 40 slow. After predation, 50 fast and 10 slow rabbits remain.

• Fast rabbits surviving: 50/60 = 83.3%. Slow rabbits surviving: 10/40 = 25%.
• Before: fast = 60% of the population. After: total = 60 survivors; fast = 50/60 = 83.3% of the population.
• Conclusion: the fast trait is selected for. If the survivors breed, the next generation will contain more fast rabbits. Repeating this over generations makes the fast trait common: natural selection is a cumulative process.

Worked example 3: adaptation to the cold

In a cold region, animals with thick fur lose less heat. Suppose in a population of 200 animals, 120 have thick fur and 80 thin fur. A very cold winter kills 10% of the thick-fur animals and 50% of the thin-fur animals. Survivors: thick: 120 − 12 = 108; thin: 80 − 40 = 40. Total survivors: 148. Thick fur is now 108/148 = 73% (up from 60%). The trait that helps survival has become more common.

2.5 Artificial selection

Artificial selection is the process in which humans select plants or animals with desirable traits and breed them so that the offspring show these traits more strongly.

• Example: a farmer chooses the cows that give the most milk and breeds them.
• Their offspring are more likely to give a lot of milk.
• Repeating this over generations raises the average milk production.

More examples: plants that produce larger fruit; cattle with more meat; hens that lay more eggs; dogs with chosen shape or behaviour.

2.6 Natural versus artificial selection

Stability and change. Organisms keep some inherited traits for long periods, while others change with the environment or because of selection.

2.7 Techniques used in artificial selection

Humans want to keep useful traits to improve production, protect superior breeds and support food security. Two kinds of techniques:

1. Traditional techniques: crossbreeding good breeds, and choosing the best plants for cultivation.
2. Modern techniques: preserving gametes (sperm, eggs) and tissue culture (growing whole plants from small pieces of plant tissue). Tissue culture produces disease-free plants and supports research and treatment; gene banks store gametes, cells and valuable breeds for research, treatment and the protection of biodiversity.

Discussion:

• artificial selection improves breeds and production, but it can also reduce genetic diversity.
• If all individuals are genetically similar, a single disease or environmental change could affect all of them, and the population might disappear.
• Therefore breeders and gene banks aim to keep a balance between improvement and diversity.

2.8 Summary of Lesson 2

• A population is a group of the same species in the same place and time.
• Growth: lag, exponential, slowing, stationary at carrying capacity; limited by environmental resistance.
• Factors: environmental and genetic.
• Natural selection: the environment selects the favourable inherited traits; slow and cumulative.
• Artificial selection: humans choose the traits; faster; may lower genetic diversity.
• Traditional and modern techniques preserve desirable traits.