U4 Adaptations, Natural Selection & Speciation

Gene pools

The sum of the alleles of all the members of a population (members that can readily interbreed)

Described by frequency of alleles of each of the genes present

Allele frequency

Calculated as the presence of a certain allele within a population

• large gene pools have a higher allele variation and are able to cope with environmental changes → HIGH ADAPTIVE POTENTIAL

• small gene pools have low allele variation and little ability to cope with change → LOW ADAPTIVE POTENTIAL

Calculation of total # alleles & alleles frequency

Total Alleles = 2 x Number of Individuals

Allele frequency = (# specific allele / total alleles) * 100

Describe characteristics of large gene pools and small gene pools

Large gene pools have higher allele variation and are able to cope with environmental changes → high adaptive potential

Small gene pools have low allele variation have little ability to cope with change → low adaptive potential

List 4 factors that changes allele frequencies within gene pools

1. Mutation

2. Gene flow

3. Genetic drift

4. Natural Selection

Define mutation and list the three types

= Change in the nucleotide sequence of a section of DNA encoding for a specific trait (responsible for introducing new alleles into a population via changes to DNA)

Mutations may be advantageous or deleterious depending on their reproductive success in their specific environment. 

3 main types of mutations: point, frameshift or block mutations.

Point Mutations

Point mutations involve the substitution of a single base of DNA, which may alter the phenotype by changing the protein sequence

• Silent mutations: DNA change does not alter the amino acid sequence (due to degeneracy of the genetic code)

• Missense mutations: DNA change alters a single amino acid in the polypeptide chain (can create new alleles)

• Nonsense mutations: DNA change creates a premature STOP codon which truncates the polypeptide
  

Frameshift Mutations

= involve either the addition (insertion) or removal (deletion) of a single base of DNA, changing the reading frame

• This change will affect every codon beyond the point of mutation and thus may dramatically change the amino acid sequence

Block Mutations

Changes to segments of a chromosome, resulting in large scale changes in the DNA of an organism

• Block mutations are commonly caused by transposons (mobile genetic elements that can change positions within the genome)

• Duplications – a part of the chromosome is copied, resulting in duplicate segments (potentially increasing gene expression)

• Deletions – a portion of a chromosome is lost (along with any genes contained within this segment)

• Inversions – a segment of a chromosome is removed and then replaced in reverse order

• Translocations – segments of two chromosomes are exchanged (may interrupt gene sequences)

Gene Flow

The movement of alleles between interbreeding populations (as a result of migration and sexual reproduction) → influences allele frequency for given traits

• Gene flow maintains the genetic compatibility between two separate populations and hence functions to prevent speciation

What does genetic variation do?

Genetic variation creates differences in the characteristics an organism possesses that enable it to interact with the environment 

Define adaptation

Characteristic of an organism that can improve their fitness in its specific environment - these arise due to mutations

Organisms evolve to better cope w/ abiotic and biotic factors in their environment

Outline the types of adaptations

• Structural:  Physical differences in biological structure (e.g. neck length of a giraffe)

• Behavioural:  Differences in patterns of activity (e.g. opossums feigning death when threatened)

  • innate: inherited from ancestors (low variation within pop.)

  • learned: developed through experiences (high levels of variation within pop.)

• Physiological:  modification to internal functioning or metabolic processes (e.g. homeothermy, colour perception)

• Biochemical:  Differences in molecular composition of cells and enzyme functions (e.g. blood groups, lactose tolerance)

• Developmental:  Variable changes that occur across the life span of an organism (e.g. patterns of ageing / senescence)

Outline selection pressures

External factors which affect the ability of an organism to survive and reproduce

• reduces variation within a population’s gene pool to create a change in allele frequency (I.e. evolution)

• selection is driven by random processes (genetic drift), the environment (natural selection) or human intervention (artificial selection)

Outline genetic drift and its impact on different population sizes.

Change in composition of a gene pool as a result of random or chance events

• It will occur faster and be more significant in smaller populations, where chance events have a bigger impact on a gene pool

• Larger populations will be less affected by random events and maintain more stable allele frequencies with low genetic drift

• Allele frequencies will change significantly when a large population is reduced to a small population

Two mechanisms causing population change = bottleneck event & the founder effect

Bottleneck event

Rapid reduction (~>50%) in population size due to environmental changes that are either natural (fires, floods) / human-made (overhunting)

The surviving population has less genetic variability than before and will be subject to a higher level of genetic drift

As surviving members begin to repopulate, the newly developing gene pool will be different to the original

ex) northern elephant seals have reduced genetic diversity compared to southern seals due to overhunting

Founder Effect

Occurs when a small group breaks away from a larger population to colonise a new territory

• This population subset does not have the same degree of diversity as a larger population → it is subject to more genetic drift

• As this new colony increases in size, its gene pool will no longer be representative of the original gene pool

ex) Certain Amish communities have higher incidence of polydactyly because of inter-marriage within the community

Compare and contrast the founder effect to population bottleneck

Both results in:

• inbreeding → greater chance of inheriting deleterious alleles

• lower adaptive potential → pop. becomes vulnerable to new selection pressures that could challenge & potentially wipe out the entire population ∵ absense of advantageous alleles

Contrast:

• Founder effect, the original population remains largely intact

Define natural selection

Change in the composition of a gene pool as a result of the presence of selective environmental agents

Survival of the fittest → the process where organisms better adapted to their environment tend to survive, reproduce more, and pass on their advantageous traits to offspring

The four conditions of natural selection are:

1. Variation: there are genetic and phenotypic differences within a population.

2. Selection Pressure: a factor in the environment impacts the survivability of organisms.

3. Selective Advantage: Fitter (more advantageous) phenotypes are able to survive and reproduce more successfully.

4. Inheritance: surviving individuals reproduce and pass on advantageous traits

Effects of natural selection on population diversity

Genetic diversity in a gene pool decreases with natural selection as the alleles for advantageous phenotype is passed onto the next generation more successfully

Therefore, the population becomes better suited for their specific environment.

But, if their environment changes, they may be at higher risk for extinction since population has low genetic diversity

Speciation

Definition: Process in which populations genetically diverge until they become distinct from each other

• distinct = they cannot interbreed to produce viable and fertile offspring

Why it occurs: when reproductive barriers prevent two populations from interbreeding – there is no gene flow between gene pools

• Consequently, the two pops. begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection

• Eventually the two pops. reach a degree of genetic divergence whereby they can no longer interbreed (speciation)

= isolating mechanisms

Outline types of reproductive isolating mechanisms

Pre-zygotic/reproductive barriers

• Temporal: different breeding seasons/feeding times

• Behavioural: different courtship displays / different niches /habitats / feeding areas

• Mechanical: mismatch in reproductive parts (damselfly) 

Post-zygotic/reproductive barriers

• Zygote mortality: fertilisation may occur however the zygote will not survive

• Hybrid sterility: a viable offspring may form but it will not be fertile

Key difference between the two types of speciation

Allopatric speciation involves the geographic separation of a population, while sympatric speciation does not involve physical separation

Allopatric Speciation

Occurs when a geographical barrier physically isolates pops. of an ancestral species → pops. will be exposed to different selection pressures & evolve along divergent pathways

Keypoints:

1. Geographic barrier: Populations are isolated from one another by a physical barrier, restricting gene flow.

2. Selection pressure: Populations are subjected to different selection pressures, resulting in the accumulation of genetic differences.

3. Speciation: Populations can no longer interbreed to form viable and fertile offspring due to genetic differences.

Examples of geographical barriers

• Large expanse of ocean (relative to organism)

• Large expanse of land (relative to organism)

• Mountain range/canyon

Example of allopatric speciation

Example: Galapagos Finch (separated by distance and have different food source)

• These finches have specialised beak shapes depending on their primary source of nutrition (e.g. seeds, insects, nuts, nectar)

• The finches occupied a variety of distinct ecological niches (different islands) with no gene flow between populations

• Over time, this leads to the rapid evolutionary diversification of a single ancestral line (adaptive radiation)

Markscheme Answer:

1. Allopatric speciation was caused by the geographical separation of groups of finches

2. This prevented gene flow between the finches

3. Due to the difference in diet/habitat (selection pressure) on different islands, some finches beak or body type produced a selective advantage

4. This allowed them to survive, reproduce, and pass on favourable alleles to offspring

5. Overtime, genetic mutations also accumulated in each population

Explain = cause & effect → x leads to y

Sympatric Speciation

Sympatric speciation occurs when a population becomes reproductively isolated without being physically separated

• In other words, it involves the divergence of species within a shared geographical location (i.e. no physical separation)
  

• The reproductive isolation can be caused by temporal or behavioural factors, or caused by chromosomal abnormalities (polyploidy)

Example of Sympatric Speciation

Different species of Howea palms on Lord Howe Island

• Howea palms are endemic to Lord Howe island (single location), but may be exposed to different soil conditions (volcanic vs calcareous)

• Palms growing in nutrient-rich volcanic soil (more acidic) tend to flower earlier than palms growing in calcareous soil (more basic)

• Because the palms were flowering at different times, reproduction ceased to be random (assortative mating began to occur)

• This temporal isolation between the two populations of palms caused them to evolve along different pathways (i.e. disruptive selection)

• Over time, the gradual accumulation of genetic differences caused the populations of Howea palms to form separate species

Define extinction

Organisms that no longer live or exist on planet earth

• Fossil records indicates that over geological history, many kinds of organism have become extinct

Outline reasons why the rate of extinction has increased

• excessive clearing of native grasslands

• unsustainable loss of old growth forests

• overgrazing

• changed frequency of fire

• degradation of water quality in rivers, lakes and estuarites

• introduction of exotic plants, predators and diseases

• unregulated exploitation of terrestrial and marine wildlife

• population growth of humans and increased urbanization

• increased levels of pollution

Outline polygeny

= study of relationships among different groups of organisms and their evolutionary development

• these relationships are hypothesized by phylogenetic inference methods that evaluate observed heritable traits e.g. DNA sequences

Phylogenetic Trees

Pattern of branching reflects how species or other groups evolved from a series of common ancestors