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Introduction to evolution

Evolution is the change in the genetic composition of a population over time.

The genetic composition changes primarily through shifts in allele frequencies (e.g. the frequency of purple and white pea flowers) in a population.

The diversity of life on Earth is a result of evolution.

Every single organism that has ever lived, from a bacterium to the blue whale, evolved from a single common ancestor. This common ancestor lived 3.5 to 3.8 billion years ago.

Evolution is ultimately a random process. It has no predetermined direction (for example towards greater complexity).

The evolutionary tree
The evolutionary tree

Charles Darwin (1809-1882) was the founder of evolutionary biology.

Darwin famously studied animals living in different niches and observed subtle differences between them.

Darwin's finches are a well known example of new species arising due to a selection pressure.

In 1859, he published On the Origin of Species by Means of Natural Selection . This is one of the most important scientific works and, is the basis of current evolutionary biology.

According to Darwin's theory of natural selection, advantageous inherited traits will become more common in a population.

Organisms that are better adapted to their environment are more likely to survive and pass on their genes (differential reproductive success).

Natural selection is also known as the survival of the fittest.

The fossil record provides excellent evidence for evolution through the documentation of historic species.

Dead organisms usually decompose rapidly. Under specific conditions, some of the harder parts of an organism can be preserved by petrification (i.e. turning into stone).

These petrified remnants are fossils. It is possible to study the structure of an organism millions of years after it has died by studying the fossil.

Fossils are often shells or bones trapped between layers of sediments that compress into rocks.

From the fossil record, it is clear that most species that were present 100 million years ago are not present today. In many cases, there are close relationships between current and earlier species.

The fossil record is incomplete because fossilisation requires very precise circumstances. As a result, it is not always clear how one species has evolved into another.

Biogeography looks at the distribution of species and populations of organisms now and in the past.

Species that are geographically close tend to resemble each other more than species that live in similar habitats but much further away.

Cave dwelling animals resemble neighbouring non cave-dwelling animals much more than other cave-dwelling animals living further away.

This implies that species in the same region evolved from the same ancestral species. As a result, they share many homologous features despite being adapted for life in different habitats.

Convergent traits are traits with a similar form and function that are shared between species with different evolutionary ancestry.

The fins of fish and whales are a convergent trait.

Many physical characteristics common to different organisms have evolved convergently (separately).

Wings have evolved independently in birds, insects and bats.

Homologous traits are characteristics that are shared between different species descended from a common ancestor with that trait.

A human leg, a bird wing and a whale fin are homologous. They are examples of tetrapod limbs, and all evolved from one ancestor with a tetrapod limb

 Different forms of the tetrapod limb
Different forms of the tetrapod limb

Homology can be seen on anatomical (similar physical features), embryological (similar foetal development), and molecular (similar nucleotide sequence) levels.