natural selection

what are variations

they’re differences in a species

some are favourable

• when subjected to different selection pressures, organisms that have these variations would hv a greater advantage, which could help them survive & reproduce

  • these variations (if heritable) are then passed on to offspring

these differences form the basis of natural selection

over a period of time, what would happen to the favourable variations (that are heritable)?

since they’re passed on to offspring, their frequency would increase. the frequency of unfavourable variations would decrease

what did Charles Darwin’s theory of natural selection say

The key components of Darwin’s theory of natural selection are as follows.

• Variations are seen among organisms in a population. Heritable or 

  genetic variations

   (see section D4.1.4–6) are passed on to the offspring.

• Due to overproduction (see later in this section), there is competition for resources leading to a struggle for existence.

• In the struggle for existence, organisms with traits that are better suited to the environment survive and reproduce. This is often referred to as ‘survival of the fittest’ (see section D4.1.4–6).

• The organisms that survive pass on these variations to their offspring.

• Over a period of time, the frequency of occurrence of favourable variations in the population increases.

• Natural selection eventually causes the population to become better adapted to its environment.

just a note - natural selection occurs continuously & over billions of years, resulting in the biodiversity of life on Earth

whats biodiversity

the diversity of life on Earth

• includes genetic diversity, species diversity & ecosystem diversity

whats genetic variation

the differences in the genomes in individuals of same species

• is an important prerequisite for natural selection

arises due to mutation & sexual reproduction

what are mutations?

already know this - they’re errors in copying genetic info during DNA replication

only mutations in the cells that produce gametes, can be inherited 

what do mutations result in

they result in alleles

how do mutations result in/cause variation

cuz mutations create alleles, and alleles introduce variation in a species

what does sexual reproduction introduce in a population

a way to introduce genetic variation in a population

in a population that has organisms that reproduce, the alleles are mixed again & again each time reproduction occurs, meaning there’s endless variation

can do this in 2 ways

what are the 2 ways that sexual reproduction can introduce genetic variation in a population?

1. formation of gametes crossing over & the independent assortment of homologous chromosomes - both of which result in unique allele combos

2. random fertilisation results/leads to unique & new combos of the alleles from both parents

so, what do mutations & sexual reproduction create? (like the relationship between them)

mutations create alleles. sexual reproduction creates new combos of these alleles, increasing genetic diversity?

what does overproduction mean?

Darwin stated that in nature, there’s a tendency towards overproduction

basically plants and animals produce more offspring than the environment can handle 

• often only a small portion of the offspring will survive, which is why there’s lots of  offspring produced in the first place

what are limiting factors?

a resource in environment that can significantly impact a population’s size, if it’s limited

• ex: food, mates, shelter

• this also determines the carrying capacity of an environment

whats carrying capacity

the maximum population size of a species that can be sustained by an environment 

what does overproduction result in

competition for resources

the better adapted organisms (organisms that hv favourable variations) will survive & reproduce, and less well-adapted organisms will likely die

this stabilizes the population’s size

how does overproduction & competition for resources, lead to natural selection

overproduction of offspring → competition for resources → individuals with favourable variations will survive & reproduce, & individuals with less favourable variations will die → natural selection will favour the beneficial variations of the survivors, & increase the frequency of these heritable variations  in the offspring → as the survivors reproduce, their offspring would make up a large portion of population→ these individuals will likely look & behave differently from ancestral population → evolution of new species 

what’re selection pressures

environmental conditions that are placed on a population, and influence the reproductive success of individuals

more ‘termy’ - factors that lead to differential survival & reproduction, which consequently results in a change in the genetic composition of population

may lead to evolutionary changes over time

includes both density-dependent & density-independent factors

what’re density-dependent factors

factors that affect the size of the population & vary in impact depending on the population density

ex: competition for resources, predation & disease

do density-dependent factors affect a denser population more than a less-dense population?

yes, this factor tends to have a stronger effect as population density increases

what’re density -independent factors?

physical factors that affect size of population regardless of population density

(are often abiotic)

ex: high or low temps affect any population, regardless of its density

• ex: polar bear

  • high temperatures mean that ice floes are melting, which affect their hunting, breeding & their dens (if pregnant)

  • global warming, and the high temperatures, are affecting the bears regardless of their population density