Variation And Selection

Front 1

Define - variation?

Back 1

The differences that exist between individuals

Front 2

Define - intraspecific variation?

Back 2

Individuals in a population show a wide range of different phenotypes

Front 3

What is variation within a species caused by?

Back 3

Genetic (e.g. crossing over)

and/or

environmental factors (Differences in the environment like food, climate, or lifestyle (e.g. some girls are bigger than others))

Front 4

Why does genetic variation exist within a species?

Back 4

• Individuals of the same species have the same genes

But

• They have different alleles

Front 5

How does genetic variation within a species happen?

Back 5

• random mutations - produces new alleles

• crossing over (in meiosis)

• independent segregation of homologous chromosomes

• random fertilisation of gametes (during sexual reproduction)

Front 6

Most variation within a species is caused by a combination of genetic and environmental factors

e.g.

Weight - some people can be genetically disposed - but if you eat loads you will be fat

Back 6

-

Front 7

Only genetic variation results in evolution.

(Not environmental variation)

Back 7

-

Front 8

Define - Evolution?

Back 8

The change in the heritable characteristics of biological populations over successive generations

Front 9

The frequency of an allele in a population changes over time

Back 9

.

Front 10

Define - selection pressures?

Back 10

Pressures that affect an organism’s chances of survival

e.g. predation, disease, competition

Front 11

How do selection pressures cause natural selection?

Back 11

• Create a struggle for survival

• There is variation between individuals, meaning some are better adapted to the selection pressure than others (have phenotypes that increase their chance of survival)

• Produces differential levels of survival and reproductive success in a population - some alleles (beneficial alleles) are more likely to be passed on

• frequency of beneficial alleles increase in the gene pool

Front 12

How do selection pressures cause natural selection?

Back 12

• Create a struggle for survival

• There is variation between individuals, meaning some are better adapted to the selection pressure than others (have phenotypes that increase their chance of survival)

• Produces differential levels of survival and reproductive success in a population - some alleles (beneficial alleles) are more likely to be passed on

• frequency of beneficial alleles increase in the gene pool

Front 13

What are the different types of natural selection?

Back 13

• Stabilising selection

• Directional selection

• Disruptive selection

Front 14

The effect of natural selection on allele frequencies depends on the selection pressures acting on the population!

Back 14

-

Front 15

Define - stabilising selection?

Back 15

Individuals with alleles for characteristics is towards the middle of the range are more likely to survive and reproduce

Front 16

When does stabilising selection occur?

Back 16

When the environment isn’t changing

Front 17

What is the effect of stabilising selection on allele frequencies in a population?

Back 17

• Reduces the range of possible phenotypes (alleles)

Front 18

Stabilising selection example

Mammal population/fur length

Back 18

• In the population there’s a range of fur length

• In a stable climate, having fair at the extremes of this range (very short/very long) reduces the chances of surviving as it’s harder to maintain the right body temperature - these mammals have a selective disadvantage

• Mammals with average fur length are most likely to survive reproduce and pass on the alleles - selective advantage

• Average fur length alleles increase in frequency

• Over time, the proportion of the population with average fur length increases, and the range of fur lengths decreases

Front 19

Stabilising selection example

Mammal population/fur length

Back 19

• In the population there’s a range of fur length

• In a stable climate, having fair at the extremes of this range (very short/very long) reduces the chances of surviving as it’s harder to maintain the right body temperature - these mammals have a selective disadvantage

• Mammals with average fur length are most likely to survive reproduce and pass on the alleles - selective advantage

• Average fur length alleles increase in frequency

• Over time, the proportion of the population with average fur length increases, and the range of fur lengths decreases

Front 20

Draw the graph for the offspring (after stabilising selection has occurred)?

Back 20

Back (Definition)

Front 21

What is meant by an organism’s ‘fitness’ ?

Back 21

How well-adapted an organism is to survive and reproduce successfully in its environment

Front 22

Define - breeding population?

Back 22

The organisms that are surviving, reproducing and passing on their alleles

Front 23

Define - directional selection?

Back 23

Individuals with alleles for a single extreme phenotype are more likely to survive and reproduce

Front 24

When can directional selection occur?

Back 24

In response to an environmental change

Front 25

What is the effect of directional selection on allele frequencies in a population?

Back 25

The mean moves towards one extreme (directional movement of the mean)

But

Range of phenotypes stays about the same

Front 26

Example of directional selection

Cheetahs/speed

Back 26

• It’s likely that Cheetahs developed the characteristic of being very fast through directional selection

• Individuals that have alleles for increased speed are more likely to catch prey - so more likely to survive, reproduce and pass on their alleles

• Over time, the frequency of alleles for high speed increases, the population becomes faster (mean speed increases), as the slower individuals are outcompeted

Front 27

Draw the graph of the offspring (after directional selection has occurred)?

Back 27

Back (Definition)

Front 28

Define - disruptive selection?

Back 28

Individuals with alleles for extreme phenotypes are more likely to survive and reproduce

(Both extremes favoured, not just one)

Front 29

When does disruptive selection occur?

Back 29

Environmental change taking place, in which the environment favours more than one phenotype

Front 30

Disruptive selection example

Birds/beak size

Back 30

• In bird populations there’s a range of beak sizes

• Birds with large beaks are specialised to eat large seeds

• Birds with small beaks are specialised to eat small seeds

• In an environment where the majority of seeds are large or small and very few are medium-sized - birds with medium-sized beaks may have a reduced chance of survival (selective disadvantage)

• But with large or small beaks are more likely to survive, reproduce and pass on their alleles

• You get 2 discrete breeding populations, one with small birds, one with large birds

• Over time, the alleles for a large beak and a small beak increase in frequency, but the alleles for a medium-sized beak decrease in frequency (Eventually phenotypes towards the middle of the range are lost - opposite of stabilising selection)

Front 31

Disruptive selection example

Birds/beak size

Back 31

• In bird populations there’s a range of beak sizes

• Birds with large beaks are specialised to eat large seeds

• Birds with small beaks are specialised to eat small seeds

• In an environment where the majority of seeds are large or small and very few are medium-sized - birds with medium-sized beaks may have a reduced chance of survival (selective disadvantage)

• But with large or small beaks are more likely to survive, reproduce and pass on their alleles

• You get 2 discrete breeding populations, one with small birds, one with large birds

• Over time, the alleles for a large beak and a small beak increase in frequency, but the alleles for a medium-sized beak decrease in frequency (Eventually phenotypes towards the middle of the range are lost - opposite of stabilising selection)

Front 32

Explain why in the bird example you end up with 2 separate breeding populations?

Back 32

• The mating is assortative (non-random)

• Birds with small beaks are more likely to mate with other birds with small beaks and they are with large-beaked birds

Front 33

Draw the graph for the offspring (after disruptive selection has occurred)?

Back 33

Back (Definition)

Front 34

What effect does disruptive selection have on allele frequencies in a population?

Back 34

Causes phenotypes at the extreme ends of a range to increase in frequency

Phenotypes in the middle of the range decrease in frequency (eventually lost)