2. Variation And Selection Flashcards Preview

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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





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


Non random