Natural Selection and Evolution

In short: Evolution is the change in the inherited features of a population over many generations, driven mainly by natural selection. Individuals vary, those with features that suit the environment survive and reproduce more, and they pass those features on, so the population changes.

Starting point: what you already know

• Offspring inherit features from their parents through DNA.
• Individuals of the same species are not all the same: they show variation.
• Adaptations help organisms survive (see the previous chapter).

Darwin and Wallace

In the 1830s, Charles Darwin sailed around the world on HMS Beagle, including a visit to Sydney. On the Galápagos Islands he noticed that finches on different islands had different beaks suited to their food. In the 1850s, Alfred Russel Wallace came to the same idea in South-East Asia. In 1858 their ideas were presented together, and in 1859 Darwin published On the Origin of Species.

Evolution is the change in the inherited features of a population over many generations.

Natural selection, step by step

"Fittest" does not mean strongest. It means best suited to the environment, with the most offspring.

How new species form

A species is a group of organisms that can breed together to produce fertile offspring. A horse and a donkey can breed, but the mule they produce is infertile, so they are different species.

Allopatric speciation (speciation in separate places):

1. A population is split by a barrier: a mountain range, a river, rising sea.
2. The two groups cannot breed with each other (isolation).
3. Different selection pressures act on each group, and different mutations arise.
4. Over a long time they become so different that they can no longer interbreed even if they meet. They are now two species.

When Bass Strait flooded about 12 000 years ago, populations in Tasmania were cut off from the mainland and began to change separately.

Patterns of evolution

Quick summary

• Evolution is the change in inherited features of a population over generations.
• Natural selection: variation, overproduction, struggle, survival and reproduction of the fittest, inheritance, change over time.
• Populations evolve, not individuals. Features gained during life are not inherited.
• Allopatric speciation: a barrier isolates groups, they change separately and can no longer interbreed.
• Divergent evolution makes many species from one ancestor. Convergent evolution makes unrelated species look alike.

Quiz