Natural Selection

Natural selection- def

Natural selection- def

Process by which fitter individuals who r better adapted to the environment survive and pass on the advantageous alleles/genes to future generations.

Main driving force of evolution: “survival of the fittest”.

Evolution

A change in the frequency of alleles in a gene pools over time as a result of natural selection. A change in the adaptive features of a pop over time as the result of natural selection.

Adaptation

Process by which pops become more suited to their environment over many gens.

Environmental factors- def

A feature of the environment of an organism that affects its survival.

Environmental factors

Abiotic/ biotic

Can limit pop size by decreasing the rate of pop growth whenever a pop reaches a certain size.

Pop limitation

Comp for food

Comp for reproductive mate

Supply of water

Temp

Comp for food

Limited supply of prey for predators- many carnivores will be hunting the same prey. If an animal cant hunt/feed- dies

Comp for reproductive mate (lions)

Female lions often outnumber male lions in app. Males compete w each other to mate w females.

During the contest- male lion could be injured/ killed. Male loses- cant pass on his alleles to any offspring

Supply of water

Arid/dry areas- water sources can be far from each other. Can die of dehydration if water sources dries up

Temp

Extreme heat- overheat + die. Can prevent animals from hunting during the day for long periods- less likely to get food

Types of selection

Environmental factors that affect the chance of survival of an organism. A selection pressure determines which traits are successful.

Stablising, directional, disruptive

Selection pressure

An environmental factors that affects the chances of survival of an organism. Increase chance of sample alleles being passed on to the next gen

Stabilising

Keeps allele frequencies constant over generations. Changes if there is a change in the environment

Directional

Produces a gradual change in allele frequencies over several gens. This occurs when there is a change in the environment which can lead to certain alleles becoming advantageous.

Disruptive

Maintains high frequencies of 2 diff sets of alleles. It maintains polymorphism.

polymorphism.- continued existence of 2 or more distinct phenotypes in the species.

Occurs in an environment which shows variation.

Changes in allele frequencies

Caused by evolution. Selective pressure decreases the likelihood that certain individuals w specific alleles survive to reproductive age- prevents alleles being passed on.

Genetic drift

Directional selection

Founder + bottleneck effect

Process of natural selection

Some individuals in a pop- hv a phenotype- aids their survival in presence of selection pressure- r best adapted to survive and breed.

Those that survive- reproduce + pass on advantageous alleles to their offspring- process repeats over many gens- frequency of advantageous allele increases. Those who don’t hv the advantageous allele r less likely to survive + pass on their alleles

Genetic drift- def

A change in allele frequencies due to random sampling/ chance

Alleles can be passed on/ lost purely by chance

Not due to natural selection

Genetic drift- explanation

Small pop- chance can affect what alleles r passed on.

Meiosis- haploid gametes- half of the alleles passed on- results from RANDOM FERTILISATION.

Mos t likely to have when a small no. Of individuals r separated from the rest of a large population.

Founder effect- def

Reduction in a gene pool compared w the main pops of a species, resulting in only 2/3 individuals who start of a new pop.

Founder effect- explain

A few individuals from a pop start of a new pop w a diff allele frequency that the OG pop.

Decrease in genetic diversity

Only some alleles from parent pop will be present. More susceptible to effects of genetic drift as pop is v small. Only part of the gene pool is present in this small pop

Bottleneck effect- def

No. Of species fall to a v low level resulting in a loss of a large no. Of alleles + a reduction in the gene pool of the species.

Bottleneck effect explanation

Decrease in genetic diversity in the pop as alleles r lost.

Surviving pop end up breeding w close relatives- decrease in gene pool.

Hardy-Weinberg Principle- def

States that: allele frequencies remain constant from gen to gen based on these assumptions:

Pop is large. No selective pressures against on of the genotypes, no migration, mating is random, sexual reproduction, no mutations.

Hardy-Weinberg Principle- explain

Allows calc of allele frequencies.

Used to see if directional selection is acting on a pop.

Use it to determine the ratios of diff genotypes in a pop + using chi-squared test to compare them against those expected.

Frequencies r represented as proportions of the pop

Calc of H-W Principle.

p= frequency of dominant allele

q= frequency of recessive allele

p² + q² + 2pq= 1

p²= homozygous dominant= p*p

q²= homozygous recessive= q*q

2pq= chance of individual being heterozygous. Inherited dominant allele from father and recessive from mother= p*q= pq or vice versa= q*p= q-