Download Genes, environment and evolution

Genes, environment and
evolution
Learning objectives
You should be able to:
• 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
What is happening to this population?
deer population dynamics
20
number of deer
18
16
14
12
10
8
6
4
2
0
0
2
4
6
8
10
12
14
Year
Why doesn’t the population ever go above 18?
Carrying Capacity
The number of organisms of one
species that an environment can support
indefinitely.
18 is the Carrying Capacity for our population.
Population growth
K = carrying capacity
• Not all young survive to adulthood and
reproduce
• If they did, the graph would be J-shaped, not
S-shaped
600
500
400
300
Series1
200
100
0
0
2
4
6
8
10
12
What limits population growth?
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.
List at least 3 for each of the following types of
factors that cause environmental resistance
• Abiotic
• Biotic
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.
Logistic Growth of a Sheep Population
on the island of Tasmania, 1800–1925
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.
• Increases the chance of certain alleles (NOT genes) to
be passed on to the next generation
• 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.
• Favours intermediate phenotypes
• 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.
• Favours extreme phenotypes and cccurs when
selection pressure changes due to
environmental change
• 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.
Isolating mechanisms
• Isolating mechanisms are factors that prevent individuals
within a population from breeding with each other.
• They may be:
– Ecological e.g. different subpopulations of plant preferring
different light levels and therefore growing in different areas
– Geographical e.g.a population of tortoises might be split by a large
river that none of them can cross
– Seasonal (temporal) e.g.climate change throughout a year
– Reproductive e.g. 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.
Consequences of genetic drift
• Loss of genetic variation
– Loss of an allele
– The other allele becomes fixed
• Less potential for natural selection
– Alleles that lead to low fitness reduces ability for
adaptation & survival
– Could lead to extinction
• Different populations become genetically
different
– Different alleles fixed
– Could lead to speciation
Naturally small
populations:
Desert pup fish
• Some populations are small and remain small over long periods of time
simply because they naturally occur in small areas of good habitat.
• Desert pupfish occur in small ponds in the desert. The ponds are too small
to hold many fish, so populations are small.
• Each pond has a separate population from each other pond since, clearly,
fish are not able to get out of the pond and walk through the desert to
another pond.
• These pupfish populations are subject to large amounts of genetic drift
and have been for a very long time.
Historically large
populations recently
become small:
Bornean Orangutan Born
• In areas where there were once large areas of habitat, but where
much of that habitat has now been destroyed.
• Either through natural events, such as storms, fires, floods, disease
epidemics, etc. or more commonly, destruction due to human
activity
• We have created situations in which a lot of genetic drift occurs.
• This decreases the potential for adaptation which may make these
small populations more likely to go extinct.
Population bottleneck
• populations that may have been historically
large, go through a period of time when they
are small, and then become large again for
example because of a natural disaster such as
a volcanic eruption.
• Bottle neck effect – decreased variation even
when population has become large again.
This makes the population very susceptible to
genetic drift.
• Mutation will introduce variation again very
slowly
An example genetic drift in a small and isolated
population
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.