2. Variation And Selection Flashcards
When does the HW principal only hold true ?
If no external factors are affecting allele frequency
Define variation
Differences that exist between individual.
Intraspecific - within species means that individuals in population can show wide range of different phenotypes
Variation caused by genetic / environmental factors
What caused intraspecific variation
Different alleles
Can be caused by mutations eg dna base sequence lead to production of new alleles
But also introduced during meiosis through crossing over of chromatids and independent segregation
Also from random fertilisation of games during sexual reproduction
And due to environment - food / climate
How do allele frequencies change over time
By evolution - which can be due to genetic drift or natural selection
What is selection presssure
Organisms face pressure that affect their chances of survival like predation / diseases / competition
It creates struggle for survival because members of the same species have different alleles which could be better adapted to the selection pressure than others
Differential levels of reproduction and Survival
Natural selection and the passing on of beneficial alleles
Individuals that have phenotype which increases chance of survival and reproduction will pass on their beneficial allele to offsping
Happens again ^
Frequency of beneficial allele in the gene pool increases from generation to generation
Name 3 types of natural selection
Stabilising
Directional
Disruptive
Fur length
In stable climate having fur of the extremes of this range reduces chances of survival as hard to maintain right body temp - selective disadvantage
Average fur length - selective advantage as can survive / reproduce and pass on alleles to offspring
Over time, average fur length will increase and range of fur lengths decreased - marrow graph and taller as proportion for average fur increases
What is the breeding population on the graph ?
Animals that are surviving / reproducing and passing on their alleles
Directional selective define
Individuals with alleles for a single extreme phentotyoe are more likely to survive and reproduce - response to environmental change
Cheetahs fastest animals
Developed through directional selection as individuals that have alleles for increased speed are more likely to catch prey then slower ones
Survive / reproduce and pass on their alleles
Over time frequency for alleles for high speed increases and population becomes faster
What happens to the mean in stabilising selection
Stays in middle
What happens to mean in directional selection
Moves left or right
What is disruptive selection
Individuals with alleles for extreme phenotype are more likely to survive and reproduce
Opposite of stabilising as characteristic towards middle of range are lost
Occur when environment favours more than one phenotype
Bird beak sizes
Large beaks eat large seeds
Small beaks eat small seeds
Medium sized beaks reduced chance is survival because they are unable to eat either large or small seeds effectively
Others more likely to survive / reproduce and pass on their alleles
Over time large / small beaks increase in population but alleles for medium sized beaks decrease in frequency
What type of mating happens in disruptive selection
Assortive
Non random
