Selective Breeding (VCE SSCE Biology): Revision Notes

Selective Breeding

Introduction

Selective breeding is a powerful tool that humans have used for thousands of years to shape the characteristics of plants and animals. From the vegetables we eat to the pets we keep, many organisms around us have been modified through this process. Understanding selective breeding helps us appreciate both the benefits and potential risks of human intervention in natural populations.

What is selective breeding?

Selective breeding (also called artificial selection) is the changing of a population's gene pool due to humans altering the breeding behaviour of animals and plants to develop a selected trait.

This process differs from natural selection, which is a mechanism through which organisms that are better adapted to their environment have an increased chance of surviving and passing on their alleles. In selective breeding, humans take control of which organisms reproduce, rather than leaving this to environmental pressures.

A desirable trait is a heritable phenotype that humans select for during selective breeding. This could be anything from wool density in sheep to disease resistance in crops.

Requirements for selective breeding

Selective breeding shares many similarities with natural selection. Both processes require three key conditions to work:

Requirement	Description
Variation	Individuals in a population vary genetically, which leads to phenotypic differences. Without variation, there would be no different traits to select from.
Selection pressure	Direct human intervention places an artificial selection pressure upon a population of individuals, only allowing certain individuals with desirable traits to breed together. This is where humans make deliberate choices about which organisms reproduce.
Heritability	The trait selected must be heritable, allowing it to be passed on from the parents to their offspring. Therefore, after the breeding population reproduces, the frequency of the selected allele will increase. If a trait cannot be inherited, selecting for it would have no lasting effect.

Selective breeding versus natural selection

While selective breeding and natural selection share the same three requirements, they differ fundamentally in the source of the selection pressure:

Mechanism	Selection pressure	Description
Selective breeding	Artificial	Involves human-induced selection pressures in the form of humans directly selecting desirable traits or removing particular traits from a population.
Natural selection	Environmental	Involves naturally occurring environmental selection pressures such as predation, disease, and climate change, which select individuals with a selective advantage within their environment.

The key difference is control. In natural selection, the environment determines which traits are advantageous. In selective breeding, humans decide which traits are desirable, regardless of whether those traits would help the organism survive in the wild.

How selective breeding works

Selective breeding follows a systematic process that gradually changes the genetic makeup of a population. Here's how it works, using sheep wool density as an example:

This process can also work in reverse. Instead of selecting for a desirable trait, humans can select against an unwanted trait to remove it from the population. For example, selectively removing large-bodied fish from a population (through overfishing) will eventually result in a population dominated by small-bodied fish.

Examples of selective breeding

Russian fox domestication experiment

Modern dogs descended from wild wolves, but the exact process of domestication was not well understood until recently. Since the 1960s, Russian zoologist Dmitry Belyaev has conducted an ongoing experiment that demonstrates how selective breeding can domesticate wild animals.

Belyaev began with a population of wild silver foxes. He ranked individuals based on traits associated with tameness, such as low aggression and affection towards people. The tamest individuals were bred together, and this process was repeated over many generations.

The results were remarkable. After just ten generations, almost 20% of bred foxes could be categorised as 'domesticated elite', displaying behaviour similar to modern dogs. Interestingly, selecting for tameness also changed other characteristics that weren't directly selected for. Some foxes developed changes in coat colour, developmental patterns, tail shape, floppy ears, and jaw structure. These changes mirrored some traits that distinguish modern dogs from wolves, suggesting that selecting for behavioural traits can inadvertently affect physical characteristics.

Maize development from teosinte

Maize (also known as corn) has been a staple crop in agriculture for thousands of years. For a long time, biologists were puzzled about the origin of maize. It was eventually discovered that maize descended from the wild grass teosinte.

Teosinte is generally a poor crop species with small seed cases and few kernels. However, through selective breeding over millennia, humans selected for suitable farming characteristics such as soft kernels, large cobs, many kernels, and kernel permanence (kernels that don't fall off easily). This gradual process transformed teosinte into the modern maize we recognise today.

Effects on genetic diversity

While selective breeding can produce desirable traits, it can have significant negative effects on genetic diversity. When breeding practices are poorly implemented, selective breeding can cause a human-induced genetic bottleneck.

In large populations, only a small percentage of individuals typically express the traits that humans desire. By restricting breeding to only these individuals, the process reduces the variety of alleles in the population. As the frequency of the selected allele increases over generations, genetic diversity decreases because the population's phenotypes are driven towards a specific allele combination.

The diagram above shows how selecting for dark-coloured beetles (AA or Aa genotypes) increases their frequency in the next generation, whilst the frequency of light-coloured beetles (aa genotype) decreases. This demonstrates how artificial selection pressure changes the gene pool over time.

This simplified diagram illustrates the same concept. Starting with a genetically diverse population (Population 1) containing both orange and green individuals, humans select only the orange individuals. After breeding, Population 2 has much less genetic diversity, containing almost exclusively orange individuals.

Negative consequences of selective breeding

Reduced genetic diversity from selective breeding creates two major problems that can threaten a population's survival:

Increased inbreeding

When genetic diversity is low, individuals in a population are more closely related to each other. This increases the likelihood of inbreeding (breeding between closely related individuals). Inbreeding raises the frequency of homozygous individuals (those having identical alleles for the same gene on homologous chromosomes), which can reveal harmful recessive traits.

Expression of deleterious alleles

A deleterious allele is an allele that has an overall negative effect on individual fitness when expressed. Many deleterious alleles are recessive, meaning they can remain hidden in a population when individuals carry only one copy.

A recessive allele is a trait that can be masked by a dominant allele on a homologous chromosome. When inbreeding increases homozygosity, previously hidden deleterious recessive alleles become expressed in the phenotype, causing health problems.

Reduced adaptive potential

Adaptive potential is the ability for a population to adjust to new environmental selection pressures. When genetic diversity is low, a population has fewer genetic variations available to respond to environmental changes. This makes the population more vulnerable to diseases, climate change, and other environmental challenges.

These effects are detrimental to the survival of a population and demonstrate why careful management of breeding programmes is essential.

Case study: English bulldogs

English bulldogs provide a striking example of the negative consequences of selective breeding. Over 100 years of selective breeding has dramatically changed the skull shape of English bulldogs.

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