2. Five Evolutionary Agents

Part 1: Non-random mating, mutation, gene flow and genetic drift. 

Part 2: Natural selection

Part 1: Success Criteria

Your learning has been successful if you can do the following:

Part 1: Vocabulary

Learn these so you can communicate this concept well.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Think back to Year 11 and 12 Science and Biology. Brainstorm everything you know about mutation, gene flow and genetic drift. 

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

List the 5 agents of evolution.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Evolution:

What is the cause?

Then, write an answer for it.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Natural Selection:

How does this relate to...?

Then, write an answer for it.

Evolution is Caused by Changes in the Gene Pool

EVOLUTION is the cumulative change in ALLELES within a GENE POOL over time

There are two types of evolution: microevolution (over several generations) and macroevolution (over thousands of years).

Changes in the gene pool result from the actions of one or more of these five evolutionary agents

Infographic: A Summary of Evolutionary Agents

Evolutionary Agent #1: Non-Random Mating


If individuals non-randomly mate with their peers by selecting a certain phenotype for a mate, the result can be a changing population. For example, if a female peahens prefer peacocks with bigger, brighter tails, this may increase the frequency of alleles for bigger, brighter tails. 

One common form of mate choice, called ASSORTATIVE MATING, is an individual’s preference to mate with partners who are phenotypically similar to themselves.


Another cause of NON-RANDOM MATING is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.

Relevant concepts: Reproductive isolating mechanisms, geographic isolation. 

Evolutionary Agent #2: Mutation

A MUTATION is a permanent change in the base sequence of DNA. Mutations are an important driver of diversity in populations because they are the source of all new alleles and traits. These traits or phenotypic changes typically arise due to a mutation of a gene that can affect a metabolic pathway, or simply change the proteins produced

Mutations increase the GENETIC VARIATION in a gene pool. Genetic variation is advantageous to a population because it enables some individuals to adapt to new niches if the environment changes, increasing a population's chance of survival.

Mutations occur randomly in cells, but are only passed on to the next generation if they occur in the DNA of sex cells (called gametic mutations). Those occurring in somatic cells (called somatic mutations) may cause cell death or cancer in an organism, but are not inherited. 

Mutations provide the totally new alleles upon which the mechanisms of NATURAL SELECTION can act. 

Natural selection quickly eliminates or reduces the number of individuals with harmful mutations. Mutations that produce small effects, even if these are slightly harmful, may survive in a population long enough to become advantageous to an organism if environmental conditions (and therefore SELECTION PRESSURES) change. 

Importantly, mutations can occur independently and spontaneously in populations. Species evolve because of the accumulation of mutations that occur over time

Evolutionary Agent #3: Genetic Drift

What is it?

GENETIC DRIFT is the random fluctuations in the frequencies of alleles from generation to generation due to chance events. 

Small populations are significantly more affected by the effects of genetic drift.

When the population increases again, the alleles may be present in quite different frequencies from before. The population is also much more likely to be genetically similar (less GENETIC DIVERSITY). 

Lack of diversity in small populations increases the likelihood of their extinction should there be a change in the environment. 

Large populations, on the other hand, are buffered against the effects of chance. 

For example, if one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring ot the next generation, all of its genes - 1/10 of the population's gene pool - will be suddenly lost. 

In a population of 100, that's only 1% of the overall gene pool; therefore it is much less impactful on the population's genetic structure.

How are populations reduced to a small size?

Bottleneck effect

When a large ancestor population is reduced to very small numbers due to an event, this is called the BOTTLENECK EFFECT. The resulting allele frequencies of the small population will not be representative of the original ancestral population. 

Founder effect

A similar effect is found when a small number of individuals colonise a new area. This is called the FOUNDER EFFECT. The resulting small founding population will not be representative of the original population they came from.

Evolutionary Agent #4: Gene Flow

What is gene flow?

GENE FLOW is when individuals (or gametes) move between populations. 

Gene flow occurs due to reproduction, and the heritable features of a gene pool integrating with all other members in the population.

How does gene flow affect the gene pool?

Immigration of individuals into a population generally increases the variety of alleles in the gene pool. Emigration may decrease the variety of alleles present.

Gene flow allows new mutations or new combinations of alleles to move into each population, thus the species does not diverge. 

What happens if there is no gene flow?

If populations are genetically isolated, there will be little to no gene flow, and the populations will be able to accumulate favourable mutations suited to their habitat. After thousands of years, this could lead to SPECIATION (creation of new species).

While some populations are fairly stable, other populations experience more gene flow. 

Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the gene structure of the population, but it can also introduce new genetic variation to populations in different geological locations and habitats.

Part 2: Success Criteria

Your learning has been successful if you can do the following:

Part 2: Vocabulary

Learn these so you can communicate this concept well.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Think back to the previous lesson. Arrange these parts in the order that air travels through when breathing in:

bronchioles, nose, alveoli, traceha, bronchi

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about the Breathing:

What is the cause?

Then, write an answer for it.

Evolutionary Agent #5: Natural Selection

Adaptation and Fitness

Before we can talk about natural selection, we must first recap ADAPTATION and FITNESS.

Mutations can give rise to new traits. If a new trait is inheritable and equips an organism for its niche, contributing to its survival and successful reproduction (ADAPTIVE ADVANTAGE), we call this trait an adaptation

These new adaptations may mean that the individual is better suited to its environment and is more fit. Fit means that they are successful in finding food, finding mates, looking after young.

These fit individuals are selected for. This means they are better suited to the environment, live longer and produce more young. The alleles they have increase increase in frequency in the gene pool.

Variation in Populations

For each trait, the range of phenotypes in a population tends to be distributed normally, producing a 'bell-shaped' curve. 

What is Natural Selection?

NATURAL SELECTION is the processes whereby organisms better adapted to their environment tend to survive and produce more offspring. Natural selection selects for favourable phenotypes – thus selecting for mutations / genotypes. Some individuals have more reproductive success than others and leave more viable offspring over their lifetime. 

Individuals with the most favourable combinations of alleles and therefore characteristics will be better able to survive and pass on their alleles and characteristics to the next generation.

Selection pressures drive natural selection 

SELECTION PRESSURES are factors that affect fitness / reproductive success and include limits on resources (food, habitat, space and mates) and the existence of threats (predators, disease, climate change). 

VIDEO: Natural Selection, Adaptation and Evolution (10:32) - Comprehensive. Includes: Natural selection, Charles Darwin, Variation, Mutation, Modes of Selection

STABILISING SELECTION occurs in a population when intermediate phenotypes have the highest fitness, rather than those at one or both extremes. 

A stable environment tends to select individuals with 'average' phenotypes, selecting against individuals at the extremes of variation. 

Stabilising selection tends to make the population more uniform. 

When the environment is changing, one end of the range of phenotypes may be selected for, resulting in DIRECTIONAL SELECTION

For example, if the climate became warmer, then mammals with a more slender build might be favoured as they would lose heat more easily.

DISRUPTIVE SELECTION occurs in a population when two or more phenotypes have higher fitness than intermediate phenotypes.

Disruptive selection may be thought of as a special sort of directional selection, where both extremes of the natural range of phenotypes are favoured. 

The average phenotype is selected against. This type of selection may also favour ASSORTATIVE MATING and may result in the formation of two species if it is long lasting

Infographic: The Role of Variation in Populations

Tasks & Education Perfect Homework

Resource - Ms. Skevington's Slides

2. Processes of Evolution

Reading - Five Evolutionary Agents

B3.5 (2) Five Evolutionary Agents.pdf

Worksheet: Types of Selection Pressures

B3.5 (2) Selection Pressures (2).pdf

Scaffolded Past Exam Question: Selection

Scaffolded - Mexican Spadefoot Toad 2016.pdf

sciPad Workbook

Education Perfect HOMEWORK

Work through the Education Perfect task called "B3.5 Concept 2: Five Evolutionary Agents"