Evolution and Natural Selection

Darwin's theory was based on four observations:

•  Individuals within a species differ from each other– there is variation.
•  Offspring resemble their parents – characteristics are inherited.
•  Far more offspring are generally produced than survive to maturity – most organisms die young from predation, disease and competition.
•  Populations are usually fairly constant in size.

Darwin realised that the organisms that die young were not random, but were selected by their characteristics.

He concluded that individuals that were better adapted to their environment compete
better than the others, survive longer and reproduce more, so passing on more of their successful genes to the next generation.

• Darwin used the analogy of selective breeding(or artificial selection) to explain natural selection.
• In selective breeding, desirable characteristics are chosen by humans, and only those individuals with the best characteristics are used for breeding.
•  In this way  species can be changed over a period of time.
•  All domesticated species of animal and plant have been selectively bred like this, often for thousands of  years, so that most of the animals and plants we are most familiar with are not really natural and are nothing like their wild relatives (if any exist).

1.  There is genetic variation in a characteristics within a population

2.  Individuals with characteristics that make them less well adapted to their environment will die young from predation, disease or competition.

3.  Individuals with characteristics that make them well adapted to their environment will survive and reproduce.

4.  The allele frequency will change in each generation.

• Populations change over time as their environment changes.
• These changes can be recorded as changing histograms of a particular phenotype (which of course is due to changes in the underlying alleles).

Directional Selection

• Occurs when one extreme phenotype (e.g. tallest) is favoured over the other
  extreme (e.g. shortest).
•  This happens when the environment changes in a particular way.
• "Environment" includes biotic as well as abiotic factors, so organisms evolve in response to each other. e.g. if predators run faster there is selective pressure for prey to  run faster, or if one tree species grows taller, there is selective pressure for other to grow tall.
•  Most environments do change (e.g. due to migration of new species, or natural catastrophes, or climate change, or to sea level change, or continental drift, etc.), so
  directional selection is common.

Disruptive (or Diverging) Selection.

• This occurs when both extremes of phenotype are selected over intermediate types.
• For example in a population of finches, birds with large and small beaks feed on large
  and small seeds respectively and both do well, but birds with intermediate beaks have no advantage, and are selected against.

Stabilising (or Normalising) Selection.

• This occurs when the intermediate phenotype is selected over extreme phenotypes, and tends to occur when the environment doesn't change much.
•  For example birds’ eggs and human babies of intermediate birth  weight are most likely to survive.
•  Natural selection doesn't have to cause a directional change, and if  an environment doesn't change there is no pressure for a well-adapted species to change.
• Fossils suggest that many species remain unchanged for long periods of geological time.

New species arise when one existing species splits into two reproductively isolated populations that go their separate ways. This most commonly happens when the two populations become physically separated from each other (allopatric speciation):

1. Start with an interbreeding population of one species.

2. The population becomes divided by a physical barrier such as water, mountains, desert, or just a large distance. This can happen when some of the population migrates or is dispersed, or when the geography changes catastrophically (e.g. earthquakes, volcanoes, floods) or gradually (erosion, continental drift). The populations must be reproductively isolated, so that there is no gene flow between the groups.

3. If the environments (abiotic or biotic) are different in the two places (and they almost certainly will be), then different characteristics will be selected by natural selection and the two populations will evolve differently. Even if the environments are similar, the populations may stil lchange by random genetic drift, especially if the population is small. The allele frequencies in the two populations will become different.

4. Much later, if the barrier is now removed and the two populations meet again, they are now so different that they can no longer interbreed. They therefore remain reproductively isolated and are two distinct species. They may both be different from the original species, if it still exists elsewhere.