Variation and selection.

variation

differences between individuals of the same species

continuous variation

results in a range of phenotypes between two extremes

eg: body mass and body length

discontinuous variation

results in a limited number of phenotypes with no intermediate

eg: ABO blood groups, seed shape in peas and seed colour in peas

y might something be an eg of discontinuous variation

Distinct phenotypes.

Controlled by one or two genes only.

Not affected by environment.

DV vs CV

discontinuous variation – influenced by genes alone
discontinuous variation – not affected by the environment

discontinuous - results in limited number of phenotypes

discontinuous - has no intermediates

gene mutation

random change in the base sequence of DNA

ionising radiation and some chemicals increase the rate of mutation

mutation is the way in which new alleles are formed

sources of genetic variation in populations

mutation, meiosis, random mating and random fertilisation

adaptive feature

an inherited feature that helps an organism to survive and reproduce in its environment

adaptations of hydrophytes

• large and flat leaves to increase surface area to float near water surface and efficiently absorb more sunlight for photosynthesis

• small roots - roots are only for anchorage, there is no need to absorb water and mineral ions.

• stomata is found on the upper side of the leaf - to allow diffusion of gases for absorption of carbon dioxide. no stomata on lower epidermis, for diffusion happens faster in air than water.

adaptations of xerophytes

• thick waxy cuticle - to prevent water loss. longer distance for diffusion. not easy for water to pass through.

• sunken stomata - to reduce water loss. stomata in pits/grooves. stomata protected from wind, so reduces transpiration.

• small leaves - to reduce surface area for evaporation or transpiration, so less water is lost.

• thick stems or leaves - to store water for long periods of time

• extensive roots - to maximise water uptake. gain water over a large area of soil.

• root cells have low water potential; to absorb water by osmosis; from (very) salty soils/AW; roots branch many times; have many roots hairs; to give a large surface area (for absorption of water);

natural selection

* here is genetic variation within the population.
* Many offspring are produced.
* As these offspring grow, they face a struggle for survival, including competition for resources.
* There is a greater chance of reproduction by individuals that are better adapted to the environment than others.
* These individuals pass on their alleles to the next generation, leading to gradual change in the population over time.

adaptation

the process, resulting from natural selection, by which populations become more suited to their environment over many generations

antibiotic resistance

* Mutation cause change in DNA base sequence
* Some bacteria develop resistance
* Bacteria with resistance survive and bacteria without resistance die as a result of natural selection
* Surviving bacteria multiply/reproduce
* And pass on their alleles for resistance to their offspring

why is selective breeding on plants done?

Improve crop yield.

Produce crops with disease resistance.

Improve the taste and flavour of fruits.

Cultivate plants with large or unique flowers.

outline selective breedingws

selection by humans of individuals with desirable features

crossing these individuals to produce the next generation

selection of offspring showing the desirable features

keep crossing and selecting

natural vs artificial selection

natural is caused by the environment

AS occurs due to human choice, rather than environmental pressures

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random mating in NS. mating is not random in AS.

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NS takes a long time. AS selection takes less time, and only individuals with desired features are allowed to reproduce