IB Biology SL 2025 Exam - Unit 7 Study Guide

Covering topics from Unit 7 that could be on the 2025 test!

Why are organisms so varied?

SNPs cause multiple variations of genes in the gene pool

Mutations cause different alleles to exist

Every single person’s DNA is different

Carl Linnaeus

Studied with other biologists on the outer and inner structure of organisms, known as morphology

Morphological Species Concept

Group of organisms that share a particular outer form and inner structure

Binomial Nomenclature

Genus and species. Genus has capital, while species has lowercase

Biological Species Concept

Group of organisms that can successfully interbreed and produce successful offspring

Difficulties distinguishing between populations and species due to divergence of non-interbreeding populations during speciation

Since populations can exist in different areas, they may diverge when they reproduce, resulting in the species eventually becoming two separate species. Because this happens gradually, it’s hard for scientists to tell when the change happens exactly

Diversity in chromosome numbers of plant and animal species

Since organisms are all different in their biological processes, as well as their complexity varying from organism to organism, result in different amounts of DNA

Karyograms

Visual profile of stained chromosomes

Unity and diversity of genomes within species

Typically, the members of a species have the same genes, in the same sequence, along each of their chromosomes

However, diversity is largely due to variation because of alleles, which causes genetic variation across multiple organisms within a species

Diversity of eukaryote genomes

The variation in genome size (measured in base pairs)

The variation in base sequences

These two are factors that lead to variety within eukaryotes

Current and potential future uses of whole genome sequencing

Whole genome sequencing is the entire base sequence of an organism’s DNA

The Earth BioGenome Project aims to use base sequencing to determine the genomes of all known species

Sequencing human genomes could lead to the development of personalized medicine

Evolution

The change in the frequency of an inherited trait in a population over time

Evidence for Evolution in DNA

Since the DNA & RNA sequences change due to SNPs, it’s hard to say that evolution does not come from the DNA changes, as the organisms with the favorable traits will survive and pass on the changed DNA

The more closely related two species are, the fewer differences in their base sequences

Selective Breeding

Form of artificial selection, whereby humans intervene in the breeding of species to produce desired traits in offspring

Homologous Structures

Features with similar anatomical position and structure despite differences in function

Pentadactyl limbs in humans, moles, horses, porpoises and bats

Convergent Evolution

A certain trait is favored in a certain environment

Two or more distinct develop the trait despite not having a common ancestor

Analogous Structures

Similarities in usage but construction is different

Tail fins of fishes and tail fins of whales, as well as wings of birds and insects

Speciation by splitting of pre-existing species

Speciation is the natural process by which species diverge to form new species

Once species diverge due to the inability to be able to successfully interbreed, they are considered new species

Roles of reproductive isolation and differential selection in speciation

Reproductive Isolation - When two species are unable to interbreed due to certain factors

Differential Selection - When there are significant differences in natural selection

These are two factors that could cause speciation

Natural Selection

Organisms produce more offspring than the environment can support. Since these organisms are genetically varied, the ones that are better suited to the environment will live longer and reproduce more, resulting in the favorable trait becoming more common in a species

Roles of mutation and sexual reproduction in generating the variation on which natural selection acts

Mutations help cause genetic variation

During meiosis, new combinations of alleles are created due to crossing over

The random orientation of bivalents further causes genetic variation between haploid cells

When two animals sexually reproduce, half of the genes come from each parent, adding on to the genetic variation

Overproduction of offspring and competition for resources as factors that promote natural selection

Since organisms tend to overproduce, which leads to greater competition for resources, only the best ones will be able to survive the longest, helping to promote natural selection

Abiotic factors as selection pressures

Natural disasters, such as flooding, earthquakes, fire, pollution, rapid temperature changes

Differences between individuals in adaptation, survival, and reproduction as the basis for natural selection

Some individuals will be better fit for an environment, which influences whether or not an individual survives for long enough to be able to reproduce

The ones that are best fit for the environment will typically survive the longest, produce the most offspring that will live, and pass on the adaptation through the generations

Requirement that traits are heritable for evolutionary change to occur

The traits must be randomly caused by mutations, they cannot be acquired throughout one’s lifetime

Sexual selection as a selection pressure in animal species

Mates need to be fit in order to survive and reproduce effectively, otherwise the animal’s genes will not persist in the longer term

Sexual selection can be found with bird plumages or pure strength in other animals

Biodiversity

The variety or multiformity of life

Ecosystem Diversity

Variety in the combinations of species living together in communities. Due to the varied environments on Earth as well as the geographical ranges of organisms

Species Diversity

The many different species on the evolutionary tree of life

Genetic Diversity

Variety in the gene pool of each species

Comparisons between current number of species on Earth and past levels of biodiversity

The past levels of biodiversity can be seen through fossil records, which show that there have been 5 mass extinctions

The current number of species is growing, but a 6th mass extinction could be lowering this amount

North Island giant moas (Dinornis novaezealandiae) extinction

They became extinct during the last 20,000 years

Mainly native in New Zealand, they went extinct due to overhunting

Caribbean monk seals (Neomonachus tropicalis) extinction

They became extinct only recently, around the mid 20th century

Mainly native in the Caribbean Sea, Gulf of Mexico and western Atlantic Ocean. Went extinct due to overhunting for blubber

Causes of ecosystem loss

Land-Use Change, Urbanization, Overexploitation, Mining, Building of Dams, Drainage or diversion of rivers, Leaching, Climate Change

Evidence for a biodiversity crisis

Decreasing population sizes, decreasing range of a species, decreasing diversity of a species, richness and evenness of biodiversity in an ecosystem, extent of degradation, number of threatened species

Causes of the current biodiversity crisis

Hunting and other forms of over-exploitation

Urbanization, with towns and cities growing larger

Deforestation and clearance of land for agriculture, leading to loss of natural habitats

Pollution of land and sea

Spread of invasive species due to global transport or deliberate introductions

Selection of evolutionarily distinct and globally endangered species for conservation prioritization in the EDGE of Existence program

EDGE asks two questions:

Does the species have few or no close relatives, so it is a member of a very small clade?

Is the species in danger of extinction, because all of its remaining populations are threatened?

Using these questions, EDGE prioritizes certain species to protect them from extinction

Population

Group of organisms of the same species, living in the same area, at the same time

Genome

All of the genetic information of one individual organism or group of organisms

Lamarckism

The initial leading theory of evolution based on inheritance of acquired characteristics

Proposed by Jean-Baptiste Lamarck

In Situ Method

Conserving species in their natural habitats

In Situ Advantages

Ensures that a species lives in the abiotic environment to which it is adapted

Allows the species to interact with other wild species

Animal behavior patterns can remain normal

In Situ Disadvantages

Requires active management to ensure that an appropriate and sustainable ecological balance is maintained

Interventions may be required to prevent habitat degradation or competition from invasive species

Legislation often necessary to ensure adequate funding for policing and education

Ex Situ Method

Preservation of species outside their natural habitats

Ex Situ Advantages

Greater control of essential conditions

Improves the chances of successful breeding

Ex Situ Disadvantages

Does not prevent the potential destruction of their natural habitats

Species less likely to be successfully reintroduced into the wild

Increases inbreeding by restricting the gene pool and restricts the evolution of species