- The Hardy-Weingberg equation is used to show that population changes over time. (they evolve)
Environmental resistance: combined action of biotic and abiotic factors that limits the growth of a popuation.
Selection pressure:
- environmental actor that confers greater chances of surviving and reproducing on some members of the population than others
e.g. Rabbits that are well camouflaged are more likely to escape predation, survive, reproduce and pass on favourable (camouflaged) genes to offspring. - Selection pressure in this case is predation
- it reduces the chances of white or black coat being passed on
- this keeps the population stable = stabilising selection
- If the environment changes - becomes colder, ore snow - rabbits with white fur would have selective advantage. White rabbits are more likely to survive, reproduce and pass white alleles to the next generation.
- frequency of white alleles would increase = directional selection
- leads to evolutionary change.
Types of selection
- Driving force behind evolution is natural selection and this occurs in three main ways
- stabilising selection
- most common
- a response to a stable environment
- e.g. birth weight
- under weight babies are less likely to survive
- overweight babies are likely to get stuck during birth killing not only themselves but also their mothers.
- The result is that the population graph gets narrower and taller as selection agaisnt mutations takes place
- bottle neck event - occurs when a population is reduced to just a few breeding individuals (e.g. cheetahs)
- though the total population may later recover, they will all be descendants of the few originals and so will have a much smaller gene pool than the original population.
- directional selection
- results ina population of new trait
- takes place whenever a change occurs in the environment
- an evolutionary force of natual selection
- e.g. resistance. Wararin resistance in rats, DDT resistance in mosquitoes or antiiotic rsistance in bacteria. Resistant individuals soon become the dominant type within the population
- disruptive selection
- less common
- results in two distinct populations
- eventually these two forms may become so distinct they become new species
Isolating mechanism
- A large population of organisms may be split into sub groups that are prevented from interbreeding by
- geographical barriers e.g. rivers, mountain range
- seasonal/temporal barriers e.g. climate
- reproductive mechanism
Genetic drift
- change in allele frequency in a population, as some alleles pass to the next generation and some disappear
- large changes in small populations
- as the size of the population decreases the degree of fluctuation increases
SPECIES
Basic classifcation system
- Domain Eukaryotes
- Kingdom Animalia
- Phylum Chordates
- Class Vertebrata
- Order Mammalia
- Family Primates
- Genus Great Apes
- Species Home sapiens
Biological species concept
- a group of similar organisms that can interbreed and produce fertile offspring
- problems with this definition
- some species do not reproduce sexually
- some members of the same species look very different (e.g. gender)
Phylogenetic species concept
- a group of organisms that have similar
- morphology
- physiology
- embryology
- behaviour
- and occupy the same ecological niche
- Closely related organisms will have similar DNA
- DNA analysis is carried out to compare particular base sequences (haplotype)
- Any differences in bases is expressed a % divergence
- A clade is a group of all the organisms that share a single common ancestor and therefore have the similar DNa and features.
- aka a monophyletic group e.g. A single ancestor and all its descendants
Cladistics
- based on evolutionary ancestry (phylogentic relationships)
- it does not use a fixed number of level such as kingdom, phylum and class
- whereas taxonomic classification focuses on similarities between organisms
- it include both monophyletic and paraphyletic groups
- Use molecule analysis e.g. DNA/RNA sequencing
- computer programs to represent evolutionary tree of life
- makes no distinction between extinct and existing species
- the cladisitic approcach has often confirmed the Linneasen classfication of orgnanisms but has sometime sled to organims being reclassified
Paraphyletic groups
- includes most recent ancestors but not all of its descendants e.g. reptiles
- Class: Reptilia. Reptiles are cold blooded vertebrates which unlike amphibians possess thick, impermeable skin covered y scaled. They have lungs, not gills and usually a three chambered heat. Their eggs are watertight shells. - Paraphyletic as it excludes birds which are descendants of reptiles
- The cl
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