👋 Welcome to Selection and Evolution!
Hello A-Level Biologist! This chapter, Selection and Evolution, is one of the most fascinating parts of the A-Level curriculum. It explains the diversity of life on Earth, from tiny bacteria gaining drug resistance to the development of complex mammals.
Don't worry if concepts like genetic drift or Hardy-Weinberg seem daunting. We will break them down step-by-step using clear language and helpful examples. Understanding this chapter is essential for connecting genetics, ecology, and biochemistry! Let’s dive into the ultimate story of survival and change.
17.1 Variation
Evolution cannot happen without differences between individuals. This difference is called variation.
Causes of Phenotypic Variation
Phenotype (what an organism looks like or how it functions) is determined by two main factors:
1. Genetic Factors: These are determined by the alleles an organism inherits. This variation is inheritable and arises primarily from:
- Mutations: Changes in the DNA sequence.
- Meiosis: Crossing over and random orientation of chromosomes.
- Random fertilisation: Which specific sperm meets which specific egg.
2. Environmental Factors: These include external influences like diet, climate, light intensity, and lifestyle. This variation is not inherited.
Example: A plant's height (phenotype) is affected by its genes (genetic factors) AND the amount of sunlight and nutrients available (environmental factors).
Discontinuous vs. Continuous Variation
Discontinuous Variation
This variation results in distinct, non-overlapping categories. You either have the trait or you don't.
- Appearance: Data can be plotted on a bar chart.
- Genetic Basis: Controlled by alleles of one or a few genes (monogenic). The environment has little or no effect.
- Examples: Human blood group (A, B, AB, or O), specific flower colour (red or white).
Continuous Variation
This variation exists across a range, with intermediate values possible. There are no distinct classes.
- Appearance: Data can be plotted on a histogram, typically showing a normal distribution curve.
- Genetic Basis: Controlled by alleles of many genes (polygenic) and is significantly influenced by environmental factors.
- Examples: Human height, mass, leaf length, or milk yield in cattle.
Quick Review: Statistical Testing (t-test)
The syllabus requires you to know that the t-test is a statistical tool used to compare the means of two different samples.
For example: You could use a t-test to determine if the mean height of plants grown in fertilizer A is significantly different from the mean height of plants grown in fertilizer B.
*** Key Takeaway: Variation is the raw material for evolution. It can be categorical (discontinuous, few genes) or range-based (continuous, many genes + environment). ***
17.2 Natural and Artificial Selection
The Theory of Natural Selection
Natural selection is the core mechanism of evolution, first proposed by Darwin and Wallace. It explains how populations become better adapted to their environment over generations.
The process works in four key steps (the 'struggle for existence'):
1. Overproduction: Populations produce more offspring than the environment can support, leading to competition for resources (food, space, mates).
2. Variation: Individuals in the population show variation in their phenotypes.
3. Selection: Due to environmental pressures, individuals with the traits (alleles) that make them best adapted to the prevailing conditions are more likely to survive.
4. Inheritance: These better-adapted individuals survive long enough to reproduce, passing their advantageous alleles on to the next generation. Over time, the frequency of these favourable alleles increases in the gene pool.
Analogy: Imagine a race. Everyone runs (reproduction), but only those with the fastest genes and best training (best adapted traits) win the prize (survival and reproduction).
Forces of Natural Selection (Types of Selection)
The environment acts as a selection pressure, shaping the population structure in different ways:
1. Stabilising Selection:
- Effect: Favours the average phenotype and selects against both extremes.
- Result: The range of variation decreases, and the mean stays the same.
- Example: Human birth weight. Babies that are too small or too large have lower survival rates; average weight babies are favoured.
2. Directional Selection:
- Effect: Favours one extreme phenotype over the mean or the other extreme.
- Result: The mean phenotype shifts in one direction over time.
- Example: Bacteria resistant to an antibiotic. The drug kills the non-resistant bacteria, shifting the population mean towards resistance.
3. Disruptive Selection:
- Effect: Favours both extreme phenotypes and selects against the average phenotype.
- Result: The population splits into two distinct phenotypic groups (a bimodal distribution).
- Example: African finches, where birds with either very small beaks (for small seeds) or very large beaks (for large seeds) thrive, but medium-beaked birds struggle to efficiently crack either type.
Population Genetics: Genetic Drift and Isolation
Allele frequencies can change not just through selection, but also randomly, especially in small populations.
Genetic Drift:
This is the random fluctuation of allele frequencies from generation to generation due to chance events. It has a much stronger effect in small populations, where the loss of a few individuals can significantly change the frequency of rare alleles.
Founder Effect:
Occurs when a new population is established by a very small number of individuals (the 'founders') from a larger population. The gene pool of the new population will only contain the alleles present in these founders, often resulting in a reduced genetic diversity and a non-representative sample of the original population's alleles.
Bottleneck Effect:
Occurs when a population size is drastically reduced, usually by a catastrophic event (e.g., natural disaster or rapid hunting). Many alleles are lost from the gene pool simply by chance, regardless of their selective advantage. The resulting population has very low genetic variation.
Real-World Example: Antibiotic Resistance (Directional Selection)
This is a key example of natural selection in action today:
- Variation exists in a bacterial population; some individuals possess alleles that confer resistance to an antibiotic, while most are susceptible.
- When the antibiotic (the selection pressure) is applied, most susceptible bacteria die.
- The resistant bacteria survive (selection) and continue to multiply (reproduce), quickly passing on their resistant alleles.
- Over generations, the frequency of resistance alleles increases dramatically in the bacterial gene pool.
The Hardy-Weinberg Principle
The Hardy-Weinberg (H-W) principle provides a mathematical model for population genetics. It describes a hypothetical situation where allele and genotype frequencies in a population remain constant from generation to generation (i.e., no evolution is occurring).
H-W Equations:
1. Allele Frequencies:
\(p + q = 1\)
Where:
\(p\) = frequency of the dominant allele
\(q\) = frequency of the recessive allele
2. Genotype Frequencies:
\(p^2 + 2pq + q^2 = 1\)
Where:
\(p^2\) = frequency of homozygous dominant individuals (AA)
\(2pq\) = frequency of heterozygous individuals (Aa)
\(q^2\) = frequency of homozygous recessive individuals (aa)
Conditions for Hardy-Weinberg Equilibrium:
The principle only applies when the following five conditions are met (which rarely happens in real life, making it a null model):
- No natural selection (all genotypes must have equal survival/reproductive chances).
- No mutation (no new alleles introduced).
- The population is very large (to avoid genetic drift).
- Mating is random.
- No gene flow (no immigration or emigration).
Memory Trick: To remember the H-W conditions, think of the acronym NO MIGRANTS (No selection, Only random mating, No mutation, Genetic drift avoided [large population], No flow).
*** Key Takeaway: Natural selection acts on existing variation, filtering out the less adapted. Genetic drift is random change, while H-W is a baseline model for a non-evolving population. ***
Artificial Selection (Selective Breeding)
Artificial selection (or selective breeding) is the process where humans choose the desirable traits and breed organisms over many generations to enhance those traits. It is essentially forced directional selection.
Principles of Selective Breeding:
- Identify individuals in the population that possess the desired characteristic (e.g., high milk yield or disease resistance).
- Breed those chosen individuals together.
- Select the offspring that show the most enhanced version of the desired characteristic.
- Repeat this process over many generations, leading to increased frequency of the desired alleles.
Examples of Selective Breeding
1. Disease Resistance in Crops (Wheat and Rice)
Farmers breed varieties that naturally possess resistance alleles to common fungal, viral, or bacterial diseases. This reduces crop loss and decreases the need for chemical pesticides, improving yield and sustainability.
2. Vigorous, Uniform Varieties of Maize
Maize breeders use two techniques:
- Inbreeding: Self-pollinating or crossing closely related plants to produce pure-bred (homozygous) strains. This often reduces vigour (inbreeding depression) but produces uniform traits.
- Hybridisation: Crossing two different pure-bred, inbred strains. The resulting F1 generation hybrid often exhibits hybrid vigour—it is taller, stronger, and gives a higher yield than either parent strain. However, the seeds must be purchased anew each year as the vigour is lost in subsequent generations.
3. Improving Milk Yield in Dairy Cattle
Cows are selected based on traits like:
- Total volume of milk produced per lactation.
- Quality of milk (e.g., high butterfat content).
- Farmers use records, selective mating, and sometimes artificial insemination to ensure only cows with the highest yield records and bulls known to produce high-yielding daughters are allowed to reproduce.
*** Key Takeaway: Artificial selection is fast, directed by human needs, and focuses on increasing the frequency of desired alleles, often resulting in pure-bred or hybrid strains. ***
17.3 Evolution and Speciation
Evolution: The Big Picture
Evolution is defined as the process leading to the formation of new species from pre-existing species over time, resulting from changes to the gene pools from generation to generation.
Evidence for Evolutionary Relationships (DNA Sequence Data)
How do scientists determine if two species are closely related?
DNA sequence data (comparing the base sequence of a specific gene or entire genomes) is crucial:
If two species have very similar DNA base sequences for the same gene, it indicates that they have recently diverged from a common ancestor. Conversely, greater differences in DNA sequence suggest the species diverged further back in evolutionary time. This allows scientists to establish detailed evolutionary relationships between species.
Speciation: Forming New Species
Speciation is the process by which one original species splits into two or more new species. This requires genetic isolation, meaning the groups can no longer interbreed, leading to reproductive isolation.
1. Allopatric Speciation (Geographical Separation)
The most common form of speciation:
- A large population is physically separated by a geographical barrier (e.g., a mountain range, a major river, or an ocean).
- The now isolated populations experience different selection pressures and genetic drift.
- The gene pools diverge significantly over time.
- Eventually, even if the barrier is removed, the two groups are reproductively isolated (they can no longer interbreed to produce fertile offspring). They are now separate species.
2. Sympatric Speciation (Ecological and Behavioural Separation)
Speciation occurs within the same geographical area. Genetic isolation is achieved through non-physical barriers:
- Ecological separation: Groups use different niches or habitats within the same environment (e.g., one group feeds high on a tree, another feeds low).
- Behavioural separation: Differences in courtship rituals, mating calls, or mating seasons prevent interbreeding (e.g., one group mates in spring, another in autumn).
In both cases, this separation restricts gene flow, allowing genetic divergence and eventually leading to reproductive isolation.
*** Key Takeaway: Evolution is the gradual change in gene pools. Speciation results from genetic isolation, either due to geography (allopatric) or differences in lifestyle/behaviour (sympatric). ***