Summary
Summary Evolution by natural selection
- Course
- Life Sciences
- Institution
- 12th Grade
Summary and notes needed for grade 12, Relative to both IEB and NSC
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HYPOTHESIS & THEORY: EVOLUTION:> Knowledge is obtained through discovery inquiry or explanatory inquiry - Theory of evolution → all things we see today arose from things that
> Discovery inquiry → observations are made, facts are collected & existed in the past but look different because things change over long
recorded, then analysed. Based on this, scientists develop generalisations. periods of time (applies to all life forms in the universe)
> Eg. Scientists observed specimens under microscopes & came up with the - Biological evolution → refers to changes that living things have undergone
generalisation that “all living organisms are made up of cells” over periods of time.
> This process of arriving at a generalisation = inductive reasoning - This means:
> Explanatory inquiry → scientists look for explanations for the > All life forms have descended from & are related to those that lived
generalisations & facts on which these generalisations were based. in the past
> Eg. Scientists are looking for an explanation for why roots always grow > Present day life forms may look different from those that they
downwards ∴ put forward ‘possible explanations’ / hypotheses & carry out descended from due to modification from generation to generation
investigations to test these hypotheses → hypothesis testing o ∴ theory of evolution = theory of “descent with modification”
> During hypothesis testing, deductive reasoning is used → generalisation is
used to predict a specific result. &
> Eg. Look at the 2 generalisations below:
• Biodiversity → range of species & no. of organisms making up each species
> All living organisms are made up of cells
• 5 Kingdom Classification (Monera, Protista, Fungi, Plantae, Animalia)
> Humans are living organisms
• In classifying living organisms, scientists have attempted to show
> ∴ we can predict that humans are made up of cells (‘If all living organisms are
made up of cells & humans are living organisms, humans are made up of cells’) evolutionary relationships.
> Scientists use models to explain generalisations / hypotheses • Evolution took extremely long ∴ Geological Time Scale was established.
> eg. Fluid-mosaic model of cell membrane (Singer & Nicolson) double-helix • According to scientists’ interpretation of fossils, cells existed from 3.4
model of DNA (Watson & Crick) billion years ago ∴ evolution of cells must have started long before then…
> larger generalisations = laws • Simplest life forms appeared during pre-Cambrian times & major groups of
> theory → explanation for something observed in nature which can be animals appeared in Cambrian period (543 - 505 mya)
supported by facts, generalisations, tested hypotheses, models & laws. • Extinctions have been occurring constantly at a low rate, usually matching
> Eg. Heliocentric theory (Earth revolves around the sun) and atomic theory speciation rate.
(regarding the stricture of matter) • 5 mass extinctions → more than 50% of earth’s species vanish in a few
million years.
, o Appearance of characteristics is affected by environmental
VARIATION WITHIN A SPECIES: conditions. (eg. Milk yield of a cow)
o Bar graph drawn = bell-shaped normal distribution curve
o Species → group of similar organisms able to interbreed to produce o Mean, median & mode are the same
fertile offspring.
o Population → organisms of the same species occupying the same habitat
at the same time & having the ability to interbreed.
↠ MUTATIONS
-Change to structure of a gene leads to altered genotype &
thus altered phenotype.
↠ CROSSING OVER
- During prophase 1 of meiosis, an exchange of chromatid
segments between homologous chromosomes = new combos
of genetic material = gametes are different from each other! DISCONTINUOUS VARIATION
↠ RANDOM ARRANGEMENT OF CHROMOSOMES o No range of different phenotypes for a particular characteristic (either
- Chromosomes arrange randomly at equator during metaphase present or not)
1 & 2 ∴ in anaphase 1 & 2, they move to poles in different o Eg. Blood groups (A,B,AB or O); flower colour (purple or white)
combinations = gametes are different from each other! o Environment has little effect
o Histogram drawn = no bell-shaped normal distribution curve
↠ RANDOM FERTILISATION OF GAMETES
- Egg & sperm cells produced are different from each other
- Random fertilisation means there will be random combos of
genetic material in offspring.
- Having a particular phenotype may allow that species to adapt better to
environmental conditions than those without this phenotype.
& - Eg in light of discontinuous variation: In polluted environment, tree bark =
dark (soot) ∴ dark-coloured moths can camouflage → prevents them from
CONTINUOUS VARIATION
being eaten by birds, but light-coloured moths cannot.
o range of different phenotypes for a particular characteristic. (eg.
- In light of continuous variation: range of phenotypes means there will be
Height, shoe size, milk yield)
different levels of adaptation to environment & thus different chances of
o Brought about by many genes working together (example of
survival.
polygenic inheritance)
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