Selective Breeding and Its Impact
This topic contrasts selective breeding, where humans direct the evolution of a species for desired traits, with natural selection, where the environment is the driving force. Understanding the mechanisms and consequences, particularly the impact on genetic diversity, is key for the ESAT.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The primary similarity between selective and natural selection is the mechanism: individuals with advantageous traits are more likely to reproduce and pass those traits to the next generation.
- The key difference is the 'selecting agent'. In natural selection, it is the environment (e.g., predators, climate). In selective breeding, it is humans choosing organisms based on desirable characteristics (e.g., milk yield, crop size).
- Selective breeding typically operates much faster than natural selection, as the selection pressure is intense and deliberately applied by humans.
- A major impact of selective breeding is a significant reduction in the gene pool (genetic diversity) of a population, as only individuals with specific alleles are chosen to breed.
- This reduction in genetic diversity leads to inbreeding, which increases the frequency of homozygous recessive genotypes and can cause harmful genetic disorders to become more common.
- Selecting for one extreme trait can inadvertently lead to other, often undesirable, health problems, as seen in many pedigree dog breeds.
Definitions
- Selective Breeding
- The process by which humans intentionally cross-breed individual organisms to develop or maintain desired characteristics in the offspring. Also known as artificial selection.
- Natural Selection
- The process where organisms with heritable traits better suited to their environment tend to survive, reproduce, and pass on those traits at a higher rate than other individuals.
- Gene Pool
- The total collection of different genes and their alleles within a given population of a particular species.
- Inbreeding
- The breeding of closely related individuals, which increases the probability that offspring will inherit two copies of the same recessive allele.
Worked example
Wolves in a cold climate have evolved thicker fur over thousands of years. In parallel, a sheep farmer has bred a flock for increasingly thick wool over 50 years. Which statement correctly identifies a primary difference between these two processes?
- 1
Step 1:
Identify the two processes.
The wolves evolving thicker fur is an example of natural selection.
The sheep being bred for thicker wool is an example of selective breeding.
- 2
Step 2:
Analyse the selective pressure in each case.
For the wolves, the pressure is the cold environment; wolves with thinner fur are less likely to survive and reproduce.
The environment is the 'selecting agent'.
- 3
Step 3:
Analyse the selective pressure for the sheep.
The pressure is the farmer's choice; the farmer intentionally selects only the sheep with the thickest wool to breed from.
The human is the 'selecting agent'.
- 4
Step 4:
Compare the outcomes and drivers.
While both result in a population with thicker coats, the driving force is different.
One is an unconscious environmental pressure for survival, the other is a conscious human choice for economic benefit.
- 5
Step 5:
Conclude the primary difference.
The fundamental distinction lies in the agent of selection:
the environment for the wolves versus the human farmer for the sheep.
Answer: The primary difference is the agent of selection. For the wolves, the selection agent is the cold environment, whereas for the sheep, the agent is the human farmer.
Common mistakes
- ×Forgetting the main drawback of selective breeding, which is the drastic reduction in genetic diversity, leading to inbreeding and increased susceptibility to disease.
- ×Confusing the agent with the mechanism. The underlying mechanism of inheritance is the same in both; the key difference is what or who is doing the selecting.
- ×Assuming selective breeding creates new traits. It only acts upon existing genetic variation within a population, which originally arises from random mutation.
No-calculator tips
- ✓To distinguish between natural and selective selection, ask 'Who or what decides which individuals reproduce?'. If the answer is 'humans for a specific goal', it is selective breeding. If it's 'environmental pressures and survival', it is natural selection.
- ✓Always link 'selective breeding' with 'reduced gene pool' and 'inbreeding' in your mind. They are the most important consequences to remember for this topic.
- ✓Use familiar examples to frame your thinking. Contrast a wild wolf (natural selection) with a domestic pug dog (selective breeding) to quickly recall the different pressures and potential health outcomes.