B2. Variation and Selection Flashcards
Variation
Variation is the ____________that exist between individuals. Variation within a species (also called ‘____________variation’) means that individuals in a population can show a wide range of different phenotypes. Variation can be caused by _________and/or _____________factors.
Although individuals of the same species have the same ____, they have different _________(versions of genes)—this causes genetic variation within a species. The main source of this genetic variation is __________, e.g. when changes in the DNA base sequence lead to the production of new alleles. But genetic variation is also introduced during ________(through the __________ _____ of _____________and the ____________ ____________ of _____________) and because of the __________ _____________ of __________during sexual reproduction.
Variation within a species can also be caused by differences in the environment, like food, climate, or lifestyle. Most variation within a species is caused by a combination of genetic and environmental factors, but only genetic variation results in evolution.
Variation is the differences that exist between individuals. Variation within a species (also called ‘intraspecific variation’) means that individuals in a population can show a wide range of different phenotypes. Variation can be caused by genetic and/or environmental factors.
Although individuals of the same species have the same genes, they have different alleles (versions of genes)—this causes genetic variation within a species. The main source of this genetic variation is mutation, e.g. when changes in the DNA base sequence lead to the production of new alleles see page 443. But genetic variation is also introduced during meiosis (through the crossing over of chromatids and the independent segregation of chromosomes) and because of the random fertilisation of gametes during sexual reproduction.
Variation within a species can also be caused by differences in the environment, like food, climate, or lifestyle. Most variation within a species is caused by a combination of genetic and environmental factors, but only genetic variation results in evolution.
Evolution
The ___________of an _________in a population changes over time-this is evolution. Evolution can occur by genetic drift or by _________selection.
Natural selection
Organisms face many pressures that affect their chances of surviving, such as ____________, __________and ___________. These are called ___________ __________. _____________pressures create a struggle for _________. Because members of the same species have different ________, there is variation between individuals, meaning that some are better adapted to the selection pressures than others. This means there are differential levels of survival and reproductive success in a population.
Individuals with a ______________that increases their chance of survival are more likely to _________, _________and pass on their _______(including the __________ _______that determine their ____________), than individuals with a different phenotype. This means that a ___________ ______________ of the next generation inherit the _____________ _________. They, in turn, are more likely to _________, _____________and ______ __ their ______. So the frequency of the beneficial alleles in the _____ _____ __________from ____________to _____________.
The frequency of an allele in a population changes over time-this is evolution. Evolution can occur by genetic drift (see page 410) or by natural selection.
Natural selection
Organisms face many pressures that affect their chances of surviving, such as predation, disease and competition. These are called selection pressures. Selection pressures create a struggle for survival. 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 differential levels of survival and reproductive success in a population.
Individuals with a phenotype that increases their chance 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.
Types of natural selection
The effect of natural selection on allele frequencies depends on the selection pressures acting on the population. There are three types of natural selection:
stabilising selection, directional selection and disruptive selection.
Types of natural selection - Stabilising selection
This is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. It occurs when the environment isn’t changing, and it reduces the range of possible phenotypes.
Types of natural selection - Stabilising selection - Example
In any mammal population there’s a range of fur length. In a stable climate, having fur at the extremes of this range reduces the chances of surviving as it’s harder to maintain the right body temperature, so mammals with very short or very long fur have a selective disadvantage. Mammals with alleles for average fur length are the most likely to survive, reproduce and pass on their alleles. These mammals have a __________advantage, so these alleles for average fur length increase in _____________.
Over time, the proportion of the population with average fur length increases and the range of fur lengths decreases - as shown in Figure 1. In the offspring graph the range of fur lengths has decreased, which results in a ___________graph. The proportion with average length fur has ___________, resulting in a _________graph in the average fur length region.
In any mammal population there’s a range of fur length. In a stable climate, having fur at the extremes of this range reduces the chances of surviving as it’s harder to maintain the right body temperature, so mammals with very short or very long fur have a selective disadvantage. Mammals with alleles for average fur length are the 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.
Over time, the proportion of the population with average fur length increases and the range of fur lengths decreases - as shown in Figure 1. In the offspring graph the range of fur lengths has decreased, which results in a narrower graph. The proportion with average length fur has increased, resulting in a taller graph in the average fur length region.
Types of natural selection - Directional selection
This is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce. This could be in response to an environmental change.
Types of natural selection - Directional selection - Example
Cheetahs are the fastest animals on land. It’s 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, meaning they’re more likely to ________, ____________and ______ __ their ________.
Over time, the ___________of _______for high speed increases and the population becomes faster as shown in Figure 2. In the offspring graph, the average speed (dotted line) has moved towards the ___________, faster end.
Cheetahs are the fastest animals on land. It’s 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, meaning they’re 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 as shown in Figure 2. In the offspring graph, the average speed (dotted line) has moved towards the extreme, faster end.
Tip: The ___________ _____________ is just the animals that are surviving, reproducing and passing on their alleles.
Tip: The breeding population is just the animals that are surviving, reproducing
and passing on their alleles.
Type of natural selection - Disruptive selection
This is where individuals with alleles for extreme phenotypes are more likely to survive and reproduce. It’s the opposite of stabilising selection because characteristics towards the middle of the range are lost. It occurs when the environment favours more than one phenotype.
Type of natural selection - Disruptive selection - Example
In bird populations there’s a range of beak sizes. Birds with large beaks are specialised to eat large seeds and 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 (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 either 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 beak and a small beak ___________in ______________, but the alleles for a medium-sized beak _________in _____________. The proportion of the population that have either small or large beaks increases - as shown in Figure 3.
In bird populations there’s a range of beak sizes. Birds with large beaks are specialised to eat large seeds and 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 (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 either 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 beak and a small beak increase in frequency, but the alleles for a medium-sized beak decrease in frequency. The proportion of the population that have either small or large beaks increases - as shown in Figure 3.
Exam Tip
If you’re asked to explain any of the three types of selection, you need to make sure you get in the phrase ‘____________________________________________________________________
Exam Tip
If you’re asked to explain any of the three types of selection, you need to make sure you get in the phrase ‘more likely to survive, reproduce and pass on their alleles’.
