Natural Selection. The Key Mechanism of Evolution. Natural Selection (film page) (copy the following at the top of the page). the process by which characteristics become either more or less common in a population
The Key Mechanism of Evolution
1. Distinguish between Genotype & Phenotype
Genotype: the genetic makeup.
Phenotype: the physical expression of the genes; traits (not necessarily a trait which is visable)
So called “FIT” organisms are those with characteristics which allow them to survive and reproduce…..they don’t necessarily have to have the very BEST of the characteristics…just “good enough”…or “fit enough” - not necessarily the“fittest”.
3. Does selection have to cause death?
No – not necessarily. A selection pressure just has to prevent reproduction.
Long ago both short and long necked giraffes existed. Then, food for short necked giraffes declined, but lots of food in tall trees for long necked giraffes. Long necked giraffes reproduced more long necked offspring while short necked giraffes starved and did not reproduce.
Nature selected long necks over short.
Plants can be easily cloned. Because of this, scientists can place genetically identical plants in different environments to prove that evolution in fact occurs by natural selection….that environmental pressures actually force and direct evolution to occur through adaptation to the different environments
They provide the variation upon which Natural Selection can act. No variation means that nature has nothing to choose.
Only somewhat. Many natural selection pressures are gone (ex. Food and water are plentiful and there are no predators), but some new pressures exist that can prevent reproduction (stress, boredom)
8. How has man interfered with Natural Selection?
Cures for disease, habitat destruction, pollution, genetic modification, artificial (human) selection…
of Natural Selection
Population of non-resistant survivors designed to kill themDDT Selection
Wild Type – original population
Low % Survival of low levels of DDT
Population surviving low levels of DDT
Mortality (death rate)
Population surviving high levels of DDT
1. What does it mean that there is no apparent difference between R and NR mosquitos?
You can not tell by looking at mosquitos which ones are R and which are NR
2. What change did DDT bring about?
It brought about evolution through adaptation/Natural Selection
3. If no heterozygositywas present, would R have evolved?
No. The gene for the R trait had to have existed in two heterozygous parents in order for offspring to show the R trait. Heterozygosity provides raw material upon which Natural Selection can act
4. What was the role of DDT?
It was the selective agent. It played the role of Natural Selection, selecting the R mosquitos to live and the NR ones to die
Original Gene Pool consisting of both dark and light moths
Birds are Selective Agents
1. Causative agent of change in phenotypes?
The Industrial Revolution was the cause, with factories emitting high amounts of coal smoke and soot adhering to tree bark, blackening the moths environment
2. Part played by the soot?
It changed/blackened the environment – the soot became the camoflage for the dark colored moths
3. Part played by the birds?
They were the selective agents – in the dark environment, the birds could see and thus chose white moths to eat, leaving more dark moths alive.