5.2 NATURAL SELECTION

Natural Selection: Trait that improve organism's ability to survive in environment and therefore are more likely to reproduce

Natural Selection depends on variation within species. If all individuals are identical there is no way for some individuals to be favoured over others

There is variation between members of a species as a result of:

• Random Mutation
• Sexual Reproduction in Meiosis 

- Phrophase 1: Crossing Over (Recombination)

- Metaphase 1: Random Assortment of chromosomes

Adaptations: Characteristics that make an individual better suited to its enviroment and way of life

Adaptations develop over time and so the species evolves

Adaptations develop by Natural Selection

There is a close relation to structure and function:

• Structure of a bird's beak and its method of feeding
• Water storage tissues in cactus and infrequent rainfalls

Species produce more offspring than the environment can support which results in:

• Struggle for existance
• Competition for resources

Adaptations develop by natural selection NOT with the direct purpose of making an individual better suited to its environment. Does not develop during the lifetime of one individual

Acquired characteristics: Do develop during a lifetime and cannot be inherited

• There is variation within a species

• Selection pressure happens: Food shortage where the leaves are on high brances

• Less adapted die and fail to reproduce: Short giraffes

• Better adapted survive and reproduce: Tall giraffes

Heritable: Variation between individuals can be passed on to offspring 

• Maasai children inherit the darker skin colour of their parents
• Light skinned European parents will pass on a light skin colour to their children

Variation in behaviour can be heritable:

• Different genes some birds migrate southwards in Winter while others fly north

Features acquired during a lifetime:

• Elephant with broken tusk
• Darker skin colour from the sun

are not inherited and therefore these aquired characteristics are not significant in the evolution of a species

Natural Selection increases frequencies of characteristics that make individual better adapted

Natural Selection decreases the frequencies of other characteristics

This leads to change within a species

Better adapted individuals - survive and reproduce - pass on characteristics to their offspring - over time (generations) the characteristics of the population gradually change - evolution by natural selection

Not likely to observe major evolutionary changes in a lifetime 

• Variation in the shape and size of the finch beaks is mostly due to genes with only some effect coming from the environment
• Beak size and diet are closely related

• Observed many medium-sized finches with only little amounts of small finches
• Both species can feed on small seeds
• The small finches are only small in body and beak size on Daphne Major (island)
• When there is no competition for small seeds the small finches are also able to feed on large seeds

Medium sized finches fed more as better adapted beaks for larger seeds

• During a shortage of small seeds the small beaked finch fed on larger harder seeds but this lead to the highest mortality in finches with short beaks
• 1983 High rainfall then supplied small soft seeds with less hard and large seeds, from this the short beak finches reproduced quickly untill 1987 with less amounts of small seeds

In 1987 the small beak finches had longer and narrower beaks compared to beak sizes in 1983 correlating with the reduction of small seeds

Antibotic resistance is due to genes in bacteria so it can be inherited