EVOLUTION Flashcards
Gene pool
Range of alleles present in a population
Allele frequency
How often an allele occurs in a population
Natural selection
Individuals within a population vary as they have different alleles
Predation, competition and disease create a struggle for survival (selection pressures)
Individuals with advantageous characteristics are more likely to survive, reproduce and pass the allele on to their offspring
Greater proportion of the next generation inherit this allele
Process continues over many generations
Without a selection pressure natural selection wouldn’t take place
Stabilising selection
Environment isn’t changing much and individuals with alleles for characteristic towards the middle of the range are more likely to survive and reproduce
Reduces range of possible phenotypes
Directional selection
Change in the environment
Individuals with alleles for the extreme type are more likely to survive and reproduce
Genetic drift
Chance dictates which genes are passed on
Individuals within a population show variation within their phenotypes
By chance, the allele for one genotype is passed on to the offspring more often than others
Number of individuals with the allele increases
If by chance the same allele is passed on more often again, it can lead to evolution
Evolution by genetic drift usually has a larger effect on smaller populations where chance has more influence
Blood group evolution occurred by genetic drift in Native America
Genetic bottleneck
An event that causes a big reduction in a populations size leads to a reduction in the gene pool
Founder effect
Describes what happens when just a few organisms from a population start a new population and there are only a small number of different alleles in the initial gene pool
Can occur as a result of migration leading to geographical separation
Hardy-Weinberg principle
Predicts the frequencies of alleles in a population won’t change much from one generation to the next
Has to be a large population with no immigration, emigration, mutations or natural selection
Has to be random mating
P= frequency of dominant allele
Q= frequency of recessive allele
Artificial selection
When humans select individuals in a population to breed together to get desirable traits
Examples of artificial selection
Modern dairy cattle- farmers select a female with a high milk yield and a male whose mother has a high milk yield and breeds them. They select the offspring with the highest milk yield and breed them, over several generations this is continued until a very high milk yielding cow is produced
Bread wheat- wheat plants with high wheat yield are bred together, the offspring with the highest wheat yields are bred together and this is continued over many generations
Issues with artificial selection
Ethical issues
Reduces gene pool because organisms with similar traits and alleles are bred together
If a disease arose all would be affected
Potentially useful alleles lost
Pedigree dogs- pugs have squished noses and breathing problems
High incidence of hereditary deafness in certain dog breeds e.g dalmatians
Speciation
Development of new species
Occurs when populations of the same species become reproductively isolated and the changes in allele frequencies cause changes in phenotype that mean they can no longer breed to give fertile offspring
Types of allopatric speciation
Geographical isolation
Reproductive isolation
Geographical isolation
A physical barrier divides a population of a species (flood, volcanoes and earthquakes)
Conditions on either side of the barrier will be slightly different
Different characteristics will become more common due to natural selection
Individuals will have changed so much that they will not be able to breed together to produce fertile offspring
They have become reproductively isolated
2 groups have become separate species
