Biological Diversity and Evolution (HSC SSCE Biology): Revision Notes

Biological Diversity and Evolution

What is biological diversity?

Biological diversity (or biodiversity) refers to the variety of all forms of life on Earth, including the diversity of characteristics that living organisms have and the variety of ecosystems of which they are components.

Living things exist in many different forms, from unicellular organisms like bacteria to multicellular varieties such as emus and kangaroos. Scientists have discovered approximately $8.7$ million different species, with more being found every year. Many more species have existed in the past, as evidenced by fossils, but have now become extinct (completely died out).

Three levels of biodiversity

Biodiversity exists on three different levels:

1. Genetic diversity: The total number of genetic characteristics in the genetic make-up of a species. This is the raw material of evolution.
2. Species diversity: A measure of the diversity of different species in an ecological community.
3. Ecosystem diversity: The variation of different ecosystems found in a region.

Why is biodiversity important?

Biodiversity is what sustains us and enriches our lives. We depend on it for benefits such as:

• Fresh air
• Food and medicines
• Clean water
• Fuel and other resources

The relationship between evolution, biodiversity and ecology

Evolution, biodiversity and ecology are proving to be far more closely interrelated than was previously understood. Recent research is showing that:

• Not only does evolution rely on biodiversity, but evolution also affects biodiversity and can drive or limit it
• Evolutionary change may happen rapidly in response to strong selection pressures, not just gradually over long periods
• Global changes such as climate change can significantly impact biodiversity

Case study: The Baw Baw frog

Genetic diversity and survival

Genetic diversity within a species is crucial for populations to adapt to changes in the environment. Environments are constantly changing, posing selection pressures that enable some organisms with favourable characteristics to survive and reproduce more successfully than others.

Populations with reduced genetic diversity

Consider these examples:

The Darwin-Wallace Theory of Evolution by Natural Selection

Evolution is a change in living organisms over a long period. While the concept that organisms may change over time had been considered as far back as the 4th century BCE, no testable theory or mechanism was proposed until much later.

Historical development

Evolutionary thinking as we know it today had its beginnings in the mid to late 1700s. In the early 19th century, Jean Baptiste Lamarck proposed a mechanism for evolution (later rejected), but his ideas opened the way for new proposals. This led to the currently accepted Theory of Evolution by Natural Selection, proposed by Charles Darwin and Alfred Wallace in the late 19th century.

Common premises of evolutionary theories

All theories of evolution share some basic premises:

• Living organisms arose from common ancestors or a common life form and have changed over time
• Differences that occur among groups of living organisms imply that living things change over time
• Similarities occur in living things and suggest common ancestry – the basic chemistry inherited from a common life form has remained relatively unchanged and has been passed down through generations

Core concepts of natural selection

The Darwin-Wallace Theory of Evolution by Natural Selection is based on the premise that living things arose from a common ancestor and that some populations moved into new habitats where they adapted over time to their environments, leading to the diversity of life.

Speciation – the formation of new species

Darwin and Wallace's idea that populations change by natural selection and become adapted to the environment gave rise to their ideas on speciation – the formation of new species.

Neo-Darwinism or modern synthesis

Scientists have more recently applied concepts of Mendelian genetics to support and explain Darwin and Wallace's ideas on random genetic variation leading to gradualism and the formation of new species.

It was only in the first decade of the 20th century, after Mendel's experimental results were confirmed and accepted, that the Darwinian theory of evolution was extended to include the genetic processes involved in natural selection. The explanation of Darwinian evolution based on modern genetics is termed neo-Darwinism.

Variation and heredity

In any population, although offspring resemble their parents, they are not identical to them. The term variation applies to differences in the characteristics (appearance or genetic make-up) of individuals within a population. Not all humans, dogs, cats or elephants look exactly alike.

Types of variation:

• Environmental variation: Arises from the interaction of an organism with its environment (e.g., access to water and sunlight affecting plant growth). This type affects the individual organism only.
• Heritable variation: Variations that can be passed from one generation to another. These affect evolution and are essential for evolution to occur. Heritable variations are often produced in a population as a result of mutations.

The importance of the gene pool

The variation in the gene pool of a population (all the possible varieties of a gene within a group of interbreeding organisms) is important in determining the chances of survival of that population.

Types of speciation

Allopatric speciation

Allopatric speciation is speciation that occurs when populations become isolated. This process involves several stages:

1. In a parent population with a large range and common gene pool, there is regular gene flow due to mating between individuals
2. Part of the population becomes separated due to physical barriers, preventing gene flow between the parent population and the isolated population
3. The two populations experience different selection pressures that favour individuals with specific genotypes (genetic make-up) over others. This alters the frequency of specific genes. The isolated population becomes a subspecies
4. If separated long enough, the gene pool of each population changes in isolation. Gene flow does not occur as populations are not in breeding contact. They may become so different that they can no longer interbreed if brought together

Related terms

A modern example of evolution

In species that live in fast-changing environments, there is continual selection of individuals with favoured characteristics. This is particularly evident in species with short generation times.

Case study: Antibiotic resistance in Staphylococcus aureus

The origin and diversification of life

The origin of life on Earth

It is believed that the environment on early Earth provided conditions for the formation of the first living cells:

1. Inorganic molecules formed organic molecules
2. Organic molecules reacted to form more complex organic compounds
3. Complex organic compounds became separated from their surroundings when membranes formed around them
4. This separation allowed entities to metabolise more effectively – these are thought to have been the first prokaryotic cells

From prokaryotes to eukaryotes

Further advances occurred when cells developed specialised compartments to carry out different chemical reactions. Evidence suggests this happened through:

• Larger cells ingesting smaller cells
• Resulting in membrane-bound organelles such as chloroplasts and mitochondria occurring inside cells
• These would have been the first eukaryotic cells

With cells now able to photosynthesise and produce oxygen, the scene was set for increasing diversity, complexity and size of organisms.

Major extinction events and diversification

Biologists have been aware of species that changed over longer periods by examining the fossil record. However, the fossil record does not show a uniform pattern of change. Rather, it shows that after a major extinction event (widespread and rapid decrease in biodiversity), new life forms flourish.

The diversification timeline

Life began to diversify further following the major changes in the Precambrian era:

• $600$ mya: Rise of invertebrates
• $425$ mya: Rise of fish
• $345$ mya: Rise of amphibians
• Large extinction of many invertebrates occurred, and amphibians began to decline
• $250$ mya: Rise and dominance of reptiles, marking the start of the Mesozoic era (including Jurassic and Cretaceous periods when dinosaurs roamed Earth)
• $230$ mya: Mammal-like reptiles dominated after extinction of early land reptiles
• Extinction of marine and aerial reptiles followed

Plant evolution:

• Cycads, conifers and ginkgoes were dominant flora during the Jurassic period
• $135$ mya: Flowering plants arose following major extinction of marine and aerial reptiles

The Cenozoic era ($12$ mya):

• Rise of mammals
• During the quaternary period, extinctions of large mammals including megafauna occurred in the northern hemisphere
• Similar extinctions followed on islands including Australia
• These extinctions occurred as humans expanded in range and coincided with dramatic climate changes and ice ages

From unicellular to multicellular organisms

The move from unicellular to multicellular organisms began when cells clustered together:

1. When cooperation between cells occurred, colonial organisms resulted, giving them a selective advantage over unicells
2. Once cells within the group began to specialise to carry out particular functions, this led to higher organisation and the selection of multicellular organisms

Organisms that have changed little

Some organisms appear to have changed very little when examining their fossil record. Examples include:

• Coelacanth (fish)
• Cycads (plants)
• Tuatara (reptile)
• Horseshoe crab (fossils $445$ million years old appear very similar to living species)

The geological timescale and diversity of life

Palaeontology is the scientific study of fossils (the remains of living things). Fossils have provided scientists with evidence of the diversity of living things on Earth.

Development of the geological timescale

The geological timescale was developed to clearly show how the diversity of life changed:

• Nicolaus Steno (1669): Established that all sedimentary rocks were laid down in layers and that bottom layers were older than top layers
• James Hutton (1795): Suggested geological processes were uniform in frequency and magnitude – the principle of uniformitarianism
• William Smith (1815): Developed a geological map of England using fossils to show a logical progression of time – the principle of faunal succession

This enabled scientists to see how living things have changed over millions of years.

Microevolution and macroevolution

It is evident that natural selection may result in:

• Microevolution: Changes within a species (such as antibiotic resistance in bacteria)
• Macroevolution: Populations becoming so different that new species are formed

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