D4.1 Natural Selection

Define natural selection

When individuals with traits better suited to their environment have higher survival and reproductive success

Define evolution

A change in the heritable characteristics of a population over time

What are the types of traits species can have?

• Characteristics of species can either be heritable or acquired

  • Heritable traits - Encoded in DNA; is subject to mutation, recombination, & natural selection  

  • Acquired traits - Results from environmental influence or learning in a lifetime

Describe Lamarckism (3)

• Jean-Baptiste Lamarck proposed that acquired traits could be inherited

  • Example: Giraffes were believed to have long necks since ancestors stretched to reach high leaves

• However, this was later disproved by molecular biology & replaced by Darwinian Evolution

Explain Darwinian Evolution (6)

• Charles Darwin’s theory explains only genetically inherited traits change populations

• The principles of evolution by natural selection include:

1. Overproduction of offspring → competition for resources (food, territory, mates)

2. Genetic variation - Variance in traits due to random mutations, crossing over, independent assortment etc. 

3. Differential survival & reproduction - Some variants are better suited to environment & have higher chance of survival & reproduction → pass advantageous traits to offspring

4. Evolution - Over many generations (billions of years), traits become more common → heritable change so population evolves

Describe sources of variation (5)

1) Mutation creates new alleles → introduced to gene pool

2) Sexual reproduction shuffles alleles through meiosis & fertilisation → creates new genetic combinations 

• Independent assortment 

• Crossing over between homologous chromosomes 

• Random fertilisation of gametes 

Explain how overproduction of offspring promote natural selection (4)

1. Many species produce more offspring than can survive 

2. Not all offspring receive enough resources to live & reproduce → struggle for existence

3. Hence, organisms must compete for limited resources (e.g. food, water, space, mates) 

4. Those best adapted are most likely to pass on alleles to offspring → natural selection 

What is carrying capacity & how is it determined? (3)

Definition: The maximum number of individuals of a species that an environment can support

• The carrying capacity of an environment is determined by limiting factors (resources in shortest supply) 

• These factors vary depending on ecosystem (e.g. water is a limiting factor in deserts)

What are abiotic factors, & their role in natural selection?

• Abiotic factors are non-living environmental factors that influence living organisms 

  • They can act as selection pressures by limiting population size & affecting which individuals survive 

  • Unlike biotic factors, they are often density-independent (affecting populations regardless of size) 

Describe examples of abiotic factors & their impact (4)

Abiotic factor	Effect on population
Temperature extremes	Can kill or stress organisms not adapted to survive high or low temps
Salinity	Affects water balance in aquatic organisms
Drought/flooding	Affects water availability & ecological health
Soil	Soil composition & acidity (pH) impacts plant survival

Define fitness, & how is it determined? (4)

Definition: How well an organism is adapted to its environment

• Fitness depends on survival value & reproductive potential

  • Survival value - How likely an individual is to survive to reproductive age

  • Reproductive potential - How many offspring an individual can produce

What is intraspecific competition, and its role in natural selection? (2)

• Intraspecific competition refers to competition within the same species for limited resources 

• This leads to differences in survival & reproduction → natural selection

What type of traits are required for evolution? (3)

• For evolution to occur, traits must be heritable (passed from parents to offspring)

• Only genetic changes (e.g. mutations, gene combinations) are inherited

• Acquired traits do not alter DNA sequence in reproductive cells → cannot be passed onto the next generation 

What is sexual selection, & its features? (3)

A type of natural selection where individuals w/ certain traits are more successful at attracting mates → increasing reproductive success & evolution

• Traits can be physical or behavioural

• They often signal overall health or genetic fitness

Describe an example of sexual selection as a selection pressure (5)

Birds of Paradise

• Male birds of paradise have bright, elaborate plumage & perform complex courtship dances

• Females prefer males w/ most impressive displays

• Over generations → evolution of extravagant plumage & behaviours

• These traits improve male reproductive success despite survival risks

[Endler’s Guppies] What were the aims? (2)

1) To study how sexual & natural selection affected guppy populations 

2) To see how predation pressure & mate choice influence guppy colouration & survival

[Endler’s Guppies] What was the method? (3)

1. Guppies were transferred to streams w/ different predator densities:

a. No predation

b. High predation (w/ strong predator of Pike-cichild) 

c. Low predation (w/ weak predator of Killifish)

2. Observed changes in male guppy color patterns and survival

3. Controlled for other variables to isolate effects of selection pressures

[Endler’s Guppies] What were the findings? (3)

Type of selection pressure	Result
Natural selection (predation)	Favoured duller, less conspicuous males to avoid predators
Sexual selection (mate choice)	Favoured brightly coloured males

—————————————————————————————————

Conditions:

1. No predation - 

• Average no. of spots increased over time 

• W/o predators, sexual selection was primary force (females preferred more spots) → increase in trait

2. Weak predation -

• No. of spots remained stable

• Killifish posed moderate threat, balancing natural selection & sexual selection

3. High predation -

• No. of spots decreased significantly 

Pike-cichilds drove natural selection (strongly favoured males w/ less spots)

Define gene pool

All the genes and their different alleles in a population

Compare large and small gene pools (3)

• Larger gene pools = more genetic variation

  • This is the source of heritable traits natural selection acts upon 

• Small gene pools are at risk of inbreeding & reduced adaptability 

What is allele frequency? (2)

Definition: How common a specific allele is in a population’s gene pool

• Allele frequency is expressed as a percentage or proportion of all alleles for a gene

Explain how geographical isolation change allele frequencies (2)

• When populations are geographically isolated, gene flow is limited 

• This leads to differences in allele frequencies due to mutation, natural selection, genetic drift, & sexual selection

Explain how natural selection change allele frequencies (2)

• Natural selection changes allele frequencies in a population’s gene pool 

• Individuals w/ advantageous heritable traits are more likely to survive & reproduce → their alleles increase in frequency over generations

Describe Neo-Darwinism (3)

• Darwin proposed natural selection but didn’t consider genes 

• Modern evolutionary theory integrates genetic inheritance w/ Darwin’s theory → neo-Darwinism 

• This theory explains evolution through changes in allele frequencies 

Describe the features of types of natural selection (4×3)

• There are three types of natural selection that causes allele frequency change

• They differ in which traits are favoured & how population’s traits shift over time

	Directional selection	Stabilising selection	Disruptive selection
Traits favoured	One extreme phenotype	Intermediate phenotypes	Both extreme phenotypes > intermediates
Direction of shift	Mean trait shifts toward one extreme	Reduces variation but mean stays similar	Splits towards two or more distinct forms (bimodal distribution)
Effect on population	Better adaptation to environment	Maintains genetic stability	Divergence & possibly speciation long-term
Change in allele frequency	Increases AF for one extreme trait	Increases AF for intermediate traits	Increases AF for both extremes

What is the Hardy-Weinberg equation? (3)

• A mathematical model that predicts allele & genotype frequencies in a population

• Conditions that must be followed - 

  • It assumes no evolution occurs, none of the following:

    • Mutation

    • Selection

    • Migration 

  • It also assumes that: 

    • Mating is random

    • Population is large 

What are the variables & basic equations of Hardy-Weinberg equation? (7)

Variables: 

• p = frequency of one allele (typically dominant allele) 

• q = frequency of the other allele (typically recessive allele) 

Basic equations: 

1) p+ q = 1 

2) p² + 2pq + q² = 1

• p² = frequency of homozygous dominant genotype

• q² = frequency of homozygous recessive 

• 2pq = frequency of heterozygous 

What is required for the Hardy-Weinberg principle? (3)

• The Hardy-Weinberg principle predicts allele & genotype frequencies will remain constant across generations but only if specific conditions are met  

• Any violation = evolutionary change

  • For example, if mating is non-random or survival rate across individuals vary

What is artificial selection and its features? (4)

Definition: The process where humans deliberately choose specific animals or plants to breed based on desirable traits

Features:

• Favours traits that meet human needs, often at expense of organism’s fitness 

• This is carried out in crop plants & domesticated animals 

⭐️ Unexpected consequences from human actions are NOT due to artificial selection but natural selection (e.g. antibiotics driving evolution of resistance in bacteria)