BIOLOGY O LEVEL(FORM FOUR) NOTES – ORGANIC EVOLUTION

ORGANIC EVOLUTION

CONCEPT OF ORGANIC EVOLUTION

Organic evolution is the gradual development of organisms from simple life forms to more complex life forms over time. It explains why organisms exhibit such great diversity and differences. Evolution is also the process of change by which new species are formed from pre-existing species.

The main theory of evolution states that populations of living things undergo changes over generations. According to this theory, some organisms resemble each other because they share a common ancestor, for example, humans and primates, donkeys and zebras.

Importance of Organic Evolution

1. It results in the emergence of new species from pre-existing ones. These new species are able to adapt to changing environmental factors such as climate and food availability.
2. Organic evolution can modify some body structures of organisms to better suit the needs of their environment.
3. It can also modify the immune system to increase an organism’s survival value in changing conditions.

ORIGIN OF LIFE

It is believed that Earth formed around 4.54 billion (4.54×10⁹) years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere, which contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten due to extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have caused the Earth’s tilt and formed the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.

The first life forms appeared between 3.8 and 3.5 billion years ago. The earliest evidence for life on Earth includes graphite found to be biogenic in 3.7 billion-year-old meta-sedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose. During the Cambrian period, it experienced a rapid diversification into most major phyla.

THEORIES OF THE ORIGIN OF LIFE

There are four main theories regarding the origin of life:

1. Special creation
2. Spontaneous generation
3. Steady state
4. Organic evolution (scientific evolution)

1. Theory of Special Creation

This theory states that life did not exist before a particular time, and then a supreme being or supernatural creator created all living things and life on Earth. This belief is found in many holy books.

2. Theory of Spontaneous Generation

Scientists once believed that living matter developed spontaneously from non-living matter. For example, worms and frogs were thought to arise from mud, dust, or rotten food. This theory was accepted until the 19^th century but is no longer supported.

3. Steady State Theory

This theory does not explain the origin of the planet Earth or all organisms. It suggests that life was found without a specific source or origin.

4. Organic Evolution (Scientific Theory of Evolution)

This theory proposes that life probably started through catalytic effects that caused free elements to combine and form molecules. Elements combined due to catalytic effects such as lightning, ultraviolet radiation, or possibly gamma rays. The first four gases to form were ammonia, hydrogen, water vapor, and methane.

Stanley Miller supported this theory by using an electric spark to synthesize amino acids from these four gases.

These molecules further combined to form a stable system capable of releasing energy and replicating itself. These were the first living organisms resembling present-day viruses and bacteria. More complex organisms developed later.

Origin of Species

This is the process by which new species are formed from pre-existing ones.

Causes of Origin of Species

1. Isolation mechanisms: Organisms were not evenly distributed on land; they became isolated due to natural disasters or behavioral changes. These isolations led to the origin of new species.
2. Reproductive isolation: Changes that create barriers to successful mating between individuals of the same species.
3. Ecological isolation: Environmental barriers that keep populations or demes apart, causing them to occupy different habitats from the original type.
4. Geographical isolation: Physical barriers such as oceans, seas, mountains, and ice valleys prevent organisms from exchanging genes.
5. Behavioral isolation: Changes in behavior before mating periods, such as courtship or nesting, prevent interbreeding.

Evidence of Organic Evolution

1. Fossil records
2. Cell biology
3. Comparative anatomy
4. Comparative physiology
5. Comparative embryology

1. Fossil Record

Fossils are remains of organisms that lived in ancient times. The age of a specimen can be determined by the amount of carbon it contains. For example, a fossil containing 5g of carbon indicates the organism was about 5,600 years old.

Archaeological discoveries show that the Hominidae family, to which humans belong, and the Pongidae family, which includes apes like chimpanzees and gorillas, arose from primate stock called Proconsul. According to archaeological evidence, the earliest hominins appeared about four million years ago. They did not have cultures of tool-making or fire-making. From the same family genus Homo, some developments led to Homo habilis.

Homo habilis used tools and became extinct about 1.5 million years ago.

The next species to emerge was Homo erectus, an organism able to stand erect. Homo erectus showed communal life and used fire and tools.

The present-day species are Homo sapiens, or rationalizing man, who exhibit high intellectual capacity, the ability to communicate through language, and many skills.

2. Cell Biology

All cells of higher organisms show basic similarities in structure and function. All cells have DNA as the carrier of genetic information, use roughly the same 20 amino acids to synthesize proteins, and use ATP as an energy carrier. The fact that all cells have cell membranes, ribosomes, and mitochondria performing similar functions indicates that all organisms had a common ancient origin.

There are other structures and chemical substances confined to specific groups of organisms. Organisms sharing the same chemical characteristics are considered more closely related. This principle, known as biochemical homology, has been recently used to confirm phylogenetic relationships.

Examples of Biochemical Homology

• Most plants contain chlorophyll, cellulose, and starch, which are absent in animal tissues.
• Vertebrates are the only animals that possess adrenaline and thyroxine.
• Only algae possess the orange pigment called fucoxanthin.

3. Comparative Embryology

This branch of embryology compares and relates the embryos of different species. It helps to show how all organisms are related. Many living things are compared, such as whether an organism has a notochord or gill arches. Many factors are included in comparative embryology.

Comparative Anatomy

Organisms with basic structural similarities have a common related ancestor. Based on structural similarity and their functions, anatomical studies are divided into:

1. Homologous structures: Structures that perform different functions but have similar ancestral origin.

Examples:

• Beak structures in birds
• Feet structures in birds
• Limb structures in vertebrates

The type of evolution whereby organisms with similar ancestral origin develop structures that form different functions is called divergent evolution.

2. Analogous structures: Structures that perform similar functions but have different ancestral origins.

Examples: Wings in birds and insects; eyes of humans and octopuses.

Convergent evolution: This type of evolution occurs when organisms with different ancestral origins develop structures that appear similar in form and function.

Vestigial structures: These are structures that persist from generation to generation but serve no useful function. Examples include the appendix in humans and wings on flightless birds like the ostrich.

4. Comparative Physiology

Due to continental drifting, organisms with a common ancestral origin became isolated and evolved into different species. For example, monkeys with long tails are found in the Amazon, while monkeys with short tails are found in the African continent.

Mechanism of Evolution

There are several theories that explain possible mechanisms of evolution:

1. Lamarck’s theory
2. Darwin’s theory
3. Neo-Darwin’s theory
4. Punctuated equilibrium theory

1. Lamarck’s Theory

Lamarck proposed the principle of natural use and disuse of structures. He observed that the more an individual used a part of its body, the more developed that part became.

If an individual failed to use a particular part, that part became weak and eventually disappeared. Lamarck concluded that the giraffe developed a long neck due to its use, while flightless birds had their wings reduced and functionless due to disuse.

Lamarck proposed that these changes acquired during the lifetime of an individual were transmitted to their offspring, resulting in the emergence of new species.

Merits of Lamarck’s Theory

According to Lamarck, an individual can develop structures to suit the needs of the environment, increasing the organism’s survival in diverse environments.

Demerits of Lamarck’s Theory

Modern genetics shows that phenotypically acquired characteristics cannot be inherited; therefore, Lamarck’s theory cannot explain the emergence of new species.

2. Darwin’s Theory

Charles Darwin proposed three main concepts:

1. Natural selection: Some individuals possess particular variations in a population, which are key factors in determining survival and adaptation to the environment. These variations provide advantages to some organisms over others.
2. Struggle for existence: As the population increases, competition arises between individuals for environmental resources, creating a struggle for existence.
3. Survival of the fittest: Organisms with favorable variations are selected by the environment, while unfavorable variations (unfit characters) are eliminated.

Merits of Darwin’s Theory

According to Darwin, the number of species tends to remain constant due to regulating factors such as food availability, disease, and predation.

Demerits

• Darwin’s theory did not explain how changes occurred by chance.
• He was unable to explain how transmission of traits occurs.

3. Neo-Darwin Theory

This theory arose after Darwin’s theories. It states that the processes of evolution can be traced to changes that take place due to mutation.

Mutations bring changes that can be passed to the next generation, leading to the emergence of new species.

4. Punctuated Equilibrium Theory

This theory was proposed by Stephen Gould and Niles Eldredge. They suggested that species remain stable for long periods (equilibrium) but often experience sudden and unpredictable changes (punctuations), giving rise to new structures and new species.

EFFECTS OF ORGANIC EVOLUTION

1. Sickle cell anemia: A hereditary disorder in which red blood cells assume a sickle or crescent shape, making them less efficient in oxygen transport. People with sickle cell traits are more resistant to malaria than normal people.
2. Resistance to insecticides and drugs: Some organisms develop resistance to chemicals such as insecticides and antibiotics with continued use. Examples include DDT-resistant mosquitoes and penicillin-resistant bacteria.

Melanin Pigment

Some organisms occur in two or more distinct forms, known as polymorphism. An example is the peppered moth, which occurs in white and melanin (dark) forms.

The moths are normally found on the trunks and branches of trees where they camouflage against predators.

Due to environmental pollution, tree trunks and branches have darkened, making the dark moths less subject to predation, while the population of white moths is decreasing.

Factors That Bring About Evolution

Evolution in living organisms is brought about by various factors such as:

1. Mutation
2. Migration
3. Environmental changes
4. Crossing over
5. Artificial selection