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Meiosis and variation
Lesson 11
The roles of genes and the environment
in evolution
Learning objectives
You should be able to:
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Explain, with examples, how environmental factors
can act as stabilising or evolutionary forces of
natural selection
Explain how genetic drift can cause large changes
in small populations
Explain the role of isolating mechanisms in the
evolution of new species, with reference to
ecological (geographic), seasonal (temporal) and
reproductive mechanisms
Genotype and evolution
We have been looking at population
genetics and how allele frequency can
be calculated assuming no advantage
is conferred by a particular genotype.
Very often though, genotype will affect
the fitness of an organism and
selection will occur
Carrying capacity and
environmental resistance
If all the offspring of organisms in a population
survived and bred then the population would soon
exceed its carrying capacity.
The environmental factors that prevent this from
occurring and therefore limit population size are
called environmental resistance.
Factors that limit a population to its carrying capacity
can be divided into biotic and abiotic.
Biotic and abiotic
Abiotic = caused by non-living components
of the environment e.g. space, light,
minerals, water
Biotic = caused by other living organisms
e.g. predation, disease, food supply
Intraspecific competition
In practice, a population usually fluctuates around a
mean level
If it exceeds the carrying capacity then intraspecific
competition becomes more intense, mortality
increases and it drops back below its carrying
capacity.
If the population is below its carrying capacity then
intraspecific competition is reduced, mortality
decreases and the abundance of the organism
increases.
So who lives and who
dies?
The organism that is fittest, in other
words, is best adapted to its
environment, is the one that is most
likely to survive.
This means that it is more likely to breed
and therefore pass on the allele that
conferred a selective advantage
Selection pressures
Selection pressures are those environmental factors
that give some individuals in a population a greater
chance of surviving than others.
For example, predation of zebra by lions might mean
that slower individuals were more likely to be eaten
We will be considering two kinds of selection pressure
Stabilising selection
Directional selection
Stabilising selection
This is natural selection where allele and
genotype frequency within population stay
the same because the organisms are
already well adapted for the environment.
Birth weight in humans is a good example.
Babies that are particularly large or small
are less likely to survive birth.
A good example of stabilising selection.
Crocodiles have changed little in 65
million years
Directional selection
This is where a particular characteristic confers a clear
advantage.
For example, gazelles that run faster are less likely to be eaten
by cheetahs. Similarly, hares with a white coat are less likely
to be seen and therefore predated in snowy conditions.
This leads to the frequency of alleles for the gene changing
within the population
Directional selection is a form of natural selection that leads to
evolutionary change. In other words, it is an evolutionary
force.
Isolating mechanisms
Isolating mechanisms are factors that prevent individuals within a
population from breeding with each other.
They may be
Ecological, for example different subpopulations of plant
preferring different light levels and therefore growing in different
areas
Geographical, for example a population of tortoises might be split
by a large river that none of them can cross
Seasonal (temporal), such as climate change throughout a year
Reproductive, for example, courtship behaviours or breeding
seasons may not be compatible
Isolation results in sub populations. Eventually these sub populations
will become so genetically distinct that they will be different
species
Red and white campion are a good example of
isolation. Red campion prefers shade and usually
flowers in spring. White campion prefers light and
flowers in summer. They are therefore seasonally
and ecologically isolated
Red campion
White campion
There are occasions though when red
and white campion flower at the same
time and in the same place. When
this happens then they can interbreed
to produce an intermediate pink form.
Genetic drift
Also called allelic drift, this is the change in allele frequency in a
population from one generation to the next, caused by random
events in meiosis and fertilisation.
It is one factor that results in isolated sub populations becoming
genetically distinct
Not all the alleles that an individual has will necessarily be passed
on to its offspring. For example, two organisms with genotype
Aa might have two offspring, each with genotype AA. The a
allele would therefore not be passed on.
The smaller a population, the greater the changes in allele
frequency that will be caused by genetic drift.
Alleles may be lost from a population altogether which reduces
genetic diversity and reduces the potential for the population to
adapt to a new environment.
In extreme cases, genetic drift may lead to extinction or the
production of a new species.
An example genetic drift in a small and
isolated population
A bottleneck is when a population suddenly becomes very
small, for example because a natural disaster such as a
volcanic eruption.
This makes the population very susceptible to genetic
drift.
In 1775 a storm and famine reduced the size of the
population of the Pingalep atoll in the Pacific to 30.
Of their 2000 descendants, 5% have the eye defect
achromatopsia, caused by two recessive alleles and
very rare in other populations.
One of the original survivors was heterozygous for this
condition. The small size of the population allowed
this allele to change rapidly in frequency over
generations, despite conferring no advantage, an
example of genetic drift.
Pingalep atoll
A modern
inhabitant of
the Pingalep
atoll with
achromatopsia