Topic 7—B: Populations and evolution- 2. Variation and selection Flashcards
What is variation?
- the differences that exist between individuals
What is variation within a species also called?
Intraspecifc variation
What does intraspecific variation mean?
That individuals in a population can show a wide range of different phenotypes
What can variation be caused by?
- genetic factors
- environmental factors
What do individuals of the same species have the same?
Genes
What do individuals of the same species have different?
Alleles
(Versions of genes)
This causes genetic variation within a species
What is the main source of this genetic variation?
- mutation e.g. when changes in the DNA base sequence lead to the production of new alleles
Where is genetic variation also introduced?
- during meiosis
- through the crossing over of chromatids and the independent segregation of chromosomes
- and because of random fertilisation of games during sexual reproduction
What can variation within a species also be caused by?
Differences in the environment e.g.
-food
- climate
- lifestyle
Most variation within a species is caused by genetic and environmental factors
What variation only results in evolution?
Genetic variation
Definition of evolution
The frequency of an allele in a population changes over time
What can evolution occur by?
- genetic drift
- natural selection
Selection pressures
Organisms face many pressures that affect their chances of surviving such as predation, disease and competition
What does selection pressure create a struggle for?
Survival
Natural selection
- because members of the same species have different alleles, there is variation between individuals meaning that some are better adapted to the selection pressures than others
- this means there are different levels of survival and reproductive success in a population
- individuals with a phenotype that increases their concentration of survival are more likely to survive, reproduce and pass on their genes (including the beneficial alleles that determine their phenotype) than individuals with a different phenotype
- this means that a greater proportion of the next generation inherit the beneficial alleles
- they in turn are more likely to survive, reproduce and pass on their genes
- so the frequency of the beneficial alleles in the gene pool increases from generation to generation
What are the 3 types of natural selection?
- Stabilising selection
- Directional selection
- Disruptive selection
Stabilising selection
- this is where individuals with alleles for characteristics towards the middle Of the range are more likely to survive and reproduce e
- it occurs when the environment isn’t changing, and it reduces the range of possible genotypes
Example (stabilising selection)
- in any mammal population there’s a range of fur length
- In a stable climate, having fur at he extremes of this range reduces the chances of serving as its harder to maintain the right body temperature so mammals with very short or very long fur have a selective disadvantage
- mammals with average fur length are most likely to survive, reproduce and pass on their alleles
- these mammals have a selective advantage so these alleles for average fur length increase in frequency
- overtime the population with average fur length increases and the range of fur decreases
- In the offspring graph, the range of fur lengths has deceased which results in a narrower graph
- the proportion with average fur length has increased resulting in a taller graph in the average fur length region
Directional selection
- this is where individuals with alleles for a single extreme phenotype are more likely to survive and resproduce
- This could be in response to an environmental change
Example (directional selection)
- cheetahs are the fastest animals on land
- its likely that this characteristic was developed through directional selection, as individuals that have alleles for increased speed are more likely to catch prey than slower individuals
- this means they are more likely to survive, reproduce and pass on their alleles
- over time, the frequency of alleles for high speed increases and the population becomes faster
- in the offspring, the average speed (dotted line) has moved towards the extreme faster end.
Disruptive selection
This is where individuals with alleles for extreme phenotypes are more likely to survive and reproduce
- its the opposite of stabilising selection because characteristics towards the middle of the range are lost
- it occurs when the environment favours more than 1 phenotype
Disruptive selection (example)
- in bird populations, there’s a range of break sizes
- birds with large beaks are specialised to eat large seeds and birds with small beaks are specialised to eat smalls seeds
- in an environment, where the majority of seeds are large or small and very few (if any) are medium sized, birds with medium sized beaks may have a reduced chance of survival
- this is because they are unable to eat large or small seeds effectively
- birds with large or small beaks are more likely than birds with medium sized beaks to survive, reproduce and pass on their alleles
- over time the alleles for a large beaak and small beak increase in frequency
- alleles for a medium sized beak decrease in frequency
-the proportion of the population that have either small or large beaks increases
