Adaptation and Evolution

Every organism has a particular role in its environment - for example, it may be a producer, a predator or a decomposer - this is referred to as its ‘niche’. Organisms are adapted for their particular niche and they become more and more adapted over time through the process of evolution.

Niche

Within a habitat, different organisms each occupy a particular niche. A niche is the role that an organism plays in the ecosystem and includes its biotic (living) interactions, such as the food it eats and the predators which it needs to hide from, as well as its abiotic (non-living) interactions, such as the gases which it breathes or the sunlight it absorbs. Within the same habitat, different organisms will have different niches. If two different species try to occupy the same niche, one will be out-competed by the other until only one species survives. To increase their chances of survival, organisms will be adapted to the niche that they occupy.

For example, the niche that gorillas occupy includes the fruits and bamboo shoots which they eat, the oxygen they inhale, the carbon dioxide they exhale and the trees that they use for shelter.

Fish swim in schools to avoid being eaten by predators. This is an example of a behavioural adaptation.

Types of adaptation

Living organisms display features (adaptations) which make them suited to their niche and more likely to survive. These adaptations can be behavioural, physiological or anatomical.

Behavioural adaptations are the way that an organism acts which increase its chances of survival. For example, fish often swim in groups called schools for protection and birds migrate south during the winter to find food.
Physiological adaptations are processes which occur within the body of an organism which increase its chances of survival. For example, sloths have a low metabolism which means they can survive on food which contains a low number of calories. Plants such as the deadly nightshade produce a poison which is a defence against being eaten by animals.
Anatomical adaptations are structural features of an organism which increase its chances of survival. For example, polar bears are camouflaged against the snow and cacti have spines to prevent being eaten by animals.

Natural Selection

Natural selection is the process by which species evolve. Evolution is the change in allele frequency within a population over time. It can be broken down in the following steps:

There is variation within the population (because different individuals have different alleles). Gene mutations cause new alleles to appear in the population - some of these may be harmful but some may be beneficial.
Organisms with alleles which give them characteristics most suited to their environment are more likely to survive to reproductive age and pass on their genes to their offspring, compared to individuals which do not have the beneficial allele.
A greater proportion of individuals in the next generation have inherited the advantageous allele. These organisms are also more likely to survive and reproduce. They will also pass on the advantageous allele to their offspring.
Over time, the frequency of the advantageous allele increases in the population - this is evolution.

Reproductive Isolation and Speciation

Speciation refers to the development of a new species. It occurs when two groups of a population become reproductively isolated from each other. Reproductive isolation prevents the transfer of genes (reduced gene flow) between the two groups so natural selection acts separately on the two sub-populations. The frequency of alleles within their gene pools will change differently. Eventually the two groups will become so different that they will no longer be able to breed with each other - they are now classed as separate species.

Organisms may become reproductively isolated from each other due to the following changes:

Behavioural changes - e.g. some organisms may develop new courtship behaviours which are unattractive to the main population
Mechanical changes - e.g. changes to the structure of the genitalia may prevent some organisms from reproducing successfully
Temporal changes - e.g. some plants may begin to produce pollen at different times of the year

Allopatric and Sympatric Speciation

Speciation can be classed as either allopatric or sympatric - allopatric speciation involves organisms becoming geographically separated from one another whereas sympatric speciation occurs in the absence of a geographical barrier.

Allopatric speciation occurs when two groups of the same species become separated by a geographical barrier, such as a mountain range or a stream. The two groups become reproductively separated and cannot exchange alleles with each other. Since the two populations are living in different areas, they will experience different selection pressures so different changes to allele frequencies will occur. The two groups will evolve differently and will form two distinct species.

Sympatric Speciation

Sympatric speciation is when speciation occurs in the absence of a geographical barrier. Random mutations may occur within the population which results in behavioural, mechanical or temporal changes (see above) which cause the organisms to become reproductively isolated.

An example of sympatric speciation is polyploidy. Polyploidy occurs when a mutation causes an organism to have multiple sets of chromosomes instead of the usual diploid number - it is much more common in plants than animals. Organisms with different numbers of chromosomes cannot reproduce to form fertile offspring which means that polyploid organisms are reproductively isolated from diploid organisms. If a polyploid organism reproduces asexually then these organisms may eventually develop into a new species.

Evidence for Evolution

The theory of evolution suggests that species which are closely related have a more recent common ancestor. Their DNA sequences should share more similarities than two species which are more distantly related. This is because less time has passed for their DNA to accumulate changes. Scientists have collected evidence for this in genomic and proteomic studies:

Genomics - scientists compare the DNA or mRNA base sequences of two organisms.
Proteomics - scientists compare the amino acid sequence for a particular protein

Once evidence for evolutionary relationships has been collected, it will be evaluated by the scientific community before it is accepted. This is done in three ways:

Scientific journals - scientists will write up their results in the form of a scientific paper, which is published in a scientific journal and can be read by the scientific community. The methodology of the experiment will be written in detail, so that other scientists can replicate the experiment to see if they get the same results.
Peer review - before the paper is published in a scientific journal, the journal will send the paper to other scientists (peers) that work in the field. These scientists will check that the experiment has been carried out to a high standard and the conclusions that have been drawn from the data are valid.
Scientific conferences - these are large gatherings where scientists can present and discuss their data. Their results are presented in two forms: a lecture or a poster presentation.

Hardy-Weinberg Principle

Evolution can be described as a change in allele frequency over time. As organisms become more and more adapted to their environment, the alleles which give rise to favourable characteristics become increasingly common in the population. The prevalence of an allele in the population is referred to as allele frequency which can be given as a decimal (e.g. 0.6) or a percentage (60%).

Allele frequency can be calculated using the Hardy-Weinberg equations. The Hardy-Weinberg principle predicts that the allele frequency will not change between generations. However, it makes the following assumptions:

The population is large
No immigration or emigration is occurring
Mating is random
No new mutations have arisen
No natural selection has occurred

If the allele frequencies change from one generation to the next within a large population, then we know that mutation, migration, non-random mating or natural selection has taken place.

The Hardy-Weinberg principle uses two equations which you’ll need to learn for the exam:

Hardy-Weinberg Principle: Worked Example

Sickle cell anaemia is a recessive disorder which causes red blood cells to misfold into long, thin sickle-shaped cells which are unable to transport oxygen efficiently. Sickle cell anaemia is more prevalent in Africa compared to other parts of the world because people who are heterozygous for the sickle cell gene are less likely to develop malaria, since the malarial parasite is unable to infect the abnormally-shaped red blood cells. In a population of sub-Saharan Africa, 3 in 100 children are born with sickle cell anaemia. Calculate the frequency of the heterozygous phenotype.

Firstly, we need to work out the frequency of the recessive allele (q). We know that the frequency of the homozygous recessive phenotype is 0.03 (3 in 100), which is represented by q² in the HW equation. We can work out q by square-rooting q².
Square root of 0.03 = 0.17.
Now that we know the frequency of the recessive allele (q), we can work out the frequency of the dominant allele (p) since p + q = 1. To work out p we just need to subtract q from 1.
1 - 0.17 = 0.83.
Finally, we can calculate the frequency of the heterozygous phenotype (2pq) by doing 2 x 0.83 x 0.17 = 0.28. This tells us that 28% of the population will have the heterozygous phenotype.

Did you know…

Scientists studying lizards on the Caribbean island of Dominica found that their grip strength was ten times stronger after Hurricane Maria in September 2017 compared to before the hurricane. The scientists have speculated that the storm acted as a selection pressure, driving a very rapid form of evolution by killing off all lizards which were not strong enough to cling to branches during the high winds.

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