natural selection is the mechanism by which organisms change over time as they adapt to their changing enviroments
As a population increases in size more induviduals from that population will die or fail to reproduce due to competition for resources such as food and space.
this striving for survival is known as 'the struglle for existence'
survival of the fittst
in the stuggle for existence there are winners and losers. the winners are the ones who by chance proccess some characteristsic whihc gives them an advantage over others.
this advantage over others gives the induvidual a better chance of survival therefore it is more likely that it will be able to reproduce and pass the characteristic onto its offspring.
such change in form over time is known as evolution
evolution by natural selection
evolution is more presicely defined as ' a change in allele frequency in a population over time '
an allele may be selectively neural up until condistion change giving the characteristic an advanatge over others.
the ability of a population to adapt to new conditions will depend on:
- the strength of the selection presure
- the size of the gene pool
- the reproductive rate of the organism
for exaple mosquitoes have rapidly bcome resistent to insecticides such as DDT. this is because the selection pressure is very strong ( DDT killed 99% of all mosquitoes )
A gene pool consists of all the alleles of all the gens present in a polpuation
populations with a bigger gene pool ( more different alleles of each gene ) are said to have greater genetic diverisity. .
new alleles are produced all the time from mutations in existing alleles.
when the population of a species declines to far, some alleles are lost and the genetic diverity (biodiveristy) of the species decreases.