Microevolutionary Changes and Speciation (HSC SSCE Biology): Revision Notes

Microevolutionary Changes and Speciation

Understanding evolution and the environment

What drives evolutionary change?

Environmental change acts as a major driving force behind changes in living organisms. The environment consists of two main components:

When the environment changes, resources often become limited. This creates competition among organisms for essential resources:

• In plants: light, soil nutrients, and water
• In animals: food, water, shelter, mates, and breeding territory

Selection pressures

Environmental changes create selection pressures that influence which organisms survive and reproduce. The main selection pressures include:

• Environmental change
• Competition
• Predation
• Disease

Organisms with variations that make them better suited to the changed environment have a survival advantage. A population with diverse individuals is more likely to survive sudden environmental changes because some individuals will possess beneficial traits.

Natural selection and adaptation

When resources become limited, organisms compete for survival. Those with advantageous variations compete more successfully, survive longer, and pass their genes to the next generation. This process is called natural selection - organisms without helpful variations are less likely to survive and reproduce.

The diagram above shows how environmental change leads to adaptation through natural selection. The cycle begins with environmental change, creating limited resources and selection pressure, which leads to competition. The organisms best suited to survive ("survival of the fittest") pass their genes to the next generation, resulting in adaptation to the environment.

Microevolution versus macroevolution

Evolution operates on different timescales and produces different outcomes.

Microevolution

Microevolution involves:

• Changes occurring over shorter time periods
• Changes within populations (groups of organisms sharing the same gene pool and able to interbreed)
• Development of new varieties or races within a species
• Does not generally produce entirely new species

Macroevolution

Macroevolution involves:

• Changes occurring over millions of years (geological time)
• Development of new species and even larger groups (families, orders)
• Major evolutionary transitions

Mechanisms of evolutionary change

Three main mechanisms drive major evolutionary changes:

1. Mutations - changes in genetic material that create heritable characteristics
2. Migration - movement of organisms between populations
3. Natural selection - differential survival and reproduction based on advantageous traits

Evolution of the horse

Why study horse evolution?

The horse provides an excellent case study for evolution because:

• It has one of the most complete fossil records of any organism
• The fossil evidence shows a branching pattern rather than simple linear progression
• We can trace clear changes in anatomy over $50$ million years

Overview of horse evolution

The horse evolved from a small, dog-sized, forest-dwelling animal called Hyracotherium to the modern horse Equus over approximately $50$ million years.

The diagram shows the evolutionary timeline of horses from the Eocene epoch ($60$ million years ago) to the present. Note the branching nature of evolution - multiple horse species existed at the same time, and evolution did not follow a simple straight line.

Small horses of the Eocene epoch ($55.8-33.9$ mya)

The first horse, Hyracotherium, looked more like a dog than a modern horse:

Characteristics of Hyracotherium:

• Height: approximately $25-50$ cm
• Had a long tail
• Short legs, snout, and back
• Four toes on the forefoot (numbered $2$, $3$, $4$, $5$)
• Diet: fruits and soft plant materials
• Short face with eye sockets positioned in the middle
• Low-crowned teeth with ridges
• Short diastema (space between front teeth and cheek teeth)

Medium-sized horses of the late Eocene and Oligocene ($34$ mya)

During this period, the climate became drier, forests shrank, and grasses became more prevalent. This created selection pressure for:

• Eating tougher plant materials
• Increased body size
• Faster movement through grasslands

Key species from this period include Miohippus and Mesohippus.

Horses of the Miocene moving onto the plains ($23-5$ mya)

About $18$ mya, horses underwent several important changes:

Horses of the late Miocene, Pliocene and Pleistocene ($5 mya onwards)

Pliohippus was the "grandfather" of modern horses and part of the merychippine line. This group led to the "true equines" - two groups that independently lost their side toes.

Important development: Side ligaments developed around the fetlock (ankle joint), stabilising the central toe for running.

Key species from this period:

• Pliohippus
• Astrohippus
• Dinohippus
• Equus (modern horses, asses, and zebras)

About $2.6$ mya, some species crossed from the Americas to Africa, Asia, and the Middle East via land bridges. Dinohippus is believed to be the closest relative to Equus.

This cave painting provides additional evidence of horse evolution, supporting the fossil record.

How microevolution explains horse evolution

Modern theories explain horse evolution through microevolutionary changes in genes that drove speciation over time.

Genetic variation arises from:

• Mutations (changes to genetic material)
• Natural selection
• Genetic drift (variation in gene frequencies in small populations)
• Speciation (formation of new distinct species)

The process works as follows:

1. Mutations occur: Random changes in genetic material create new variations
2. Gene frequency changes: The mutation changes the frequency of specific genes in the population
3. Genetic drift: In small populations, chance events can significantly change gene frequencies
4. Population isolation: If a population becomes separated from the main group, it develops independently
5. Selection pressure: Different environments select for different favourable genes
6. Increased frequency of beneficial genes: Organisms with advantageous mutations breed and pass on these genes
7. Speciation: The isolated population becomes so different that it can no longer interbreed with the original population, forming a new species

Evolution of the platypus

Australian mammals - a unique story

Australia is unusual because it has:

• An abundance of marsupials (pouched mammals like kangaroos and koalas)
• Monotremes (egg-laying mammals like the platypus and echidna)
• Very few placental mammals (mammals that nourish young via a placenta inside the mother's body)

The puzzling platypus

The platypus is a monotreme native to eastern Australian rivers. It displays an unusual combination of features:

• Bird-like features: bill and webbed feet (similar to a duck)
• Reptile-like features: venom glands, egg-laying
• Mammal features: hair on body, suckle their young

This combination initially confused scientists. When the first platypus specimen arrived at the British Museum in $1798$, scientists examined it for stitching, thinking someone had sewn a duck's bill onto a mammal's body! Eventually, careful examination confirmed it was genuine.

Evolutionary relationships

The platypus fossil record is very poor compared to the horse. However, genetic and fossil evidence provides important clues about its evolution.

Ancient reptiles called cynodonts are believed to be the earliest ancestors of all mammals.

This cladogram shows the evolutionary relationships between major vertebrate groups. Key points to understand from the diagram:

Major divergence points:

• Synapsids (mammal line) and Sauropsids (reptile/bird line) split $315$ million years ago
• Monotremes split off first around $166$ mya
• Marsupials and placentals diverged around $148$ mya

The platypus and echidna share a common ancestor that lived approximately $19-48$ mya, based on DNA analysis and fossil evidence.

Genetic evidence

Comparing the genomes of platypus, marsupial, and placental mammals reveals that approximately $82\%$ of genes are common to all groups.

Key genetic findings:

Yolk production: The platypus has a gene for yolk protein production (absent in humans) because it lays eggs with yolk.

Lactation: All three groups (monotremes, marsupials, placentals) have genes related to tooth production and milk protein production. This tells us that milk production ability arose before the Jurassic period, before these groups diverged.

Why monotremes survived in Australia

Fossil evidence shows monotremes once lived in South America (a fossilised tooth found in Argentina, dated $63-61$ mya), and placental mammals once lived in Australia (fossilised tooth from $55$ mya).

Why did monotremes survive in Australia but not South America, while placental mammals disappeared from Australia?

Fossil evidence for platypus evolution

Four extinct platypus-related species have been found in Australia:

Modern platypus compared to ancestors:

• No longer has teeth (has horny pads instead)
• Highly evolved electroreception system in the bill for detecting prey in murky water
• More restricted distribution (now only found in eastern Australian river systems)

Other fossil species:

Steropodon galmani:

• Found in New South Wales
• One of the oldest Australian mammals ($110$ mya, Cretaceous period)
• Jaw fragments show significant differences from modern platypus

Macroevolution in the platypus

The evolution of the platypus represents macroevolution because it:

• Occurred over a very long time period (more than $100$ million years)
• Resulted in the evolution of new species
• Shows major anatomical and physiological changes

Central question: Is macroevolution simply an accumulation of many microevolutionary changes over a long period? Scientists continue to study this question using genetic and fossil evidence.

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