4.4 – Evolution
4.4.1 – Define evolution
Evolution is the cumulative change in the heritable characteristics of a population.
It is the development of new types of living organisms from pre-existing types by accumulation of genetic differences over long periods of time. All the forms of life on the Earth today are the result of divergence from a common ancestor over billions of years.
1. Evolution – all life is perpetually changing, in contrast with the idea that all forms of life are fixed and unchanging
2. Common Descent – all living things share a common ancestor if traced back far enough
3. Gradualism – Evolutionary change takes place slowly and gradually
4. Multiplication of Species – diversity of life is a consequence of speciation, where populations adapt to locations and become reproductively isolated from other populations
5. Natural Selection – a two-stage process involving genetic variation and selection of the most suitable to the location
4.4.2 – Outline the evidence for evolution provided by the fossil record, selective breeding of domesticated animals and homologous structures
Evolution requires evidence showing that organisms change over time, even resulting in production of new species
Fossils are ancient remains of organisms that have been preserved though a rare, chance event. Scientists can determine the age of fossils from the age of the rock (using 14C, 40K or 40Ar radiometric dating). Fossil sequences show change over time, indicating how species have evolved. However, gaps often occur in records.
Fossils can be formed through a number of processes:
- Petrification – Organic matter is replaced by mineral ions
- Mould – Organic matter decays, space left becomes a mould, filled by mineral matter
- Trace – Impression (footprint or leaf) hardens in the layers
- Preservation – The organism is preserved, such as in amber or in anaerobic, acidic peat
These show that all life is connected, having derived traits from a common ancestor. Organisms with closer connections have similar structures
Above: pentadactyl limb of the vertebrate – humerus, radius, ulna. Bones are adapted to the animal’s locomotion. It shows divergence, which is adaption and modification from a limb structure found in their common ancestor. Convergence, on the other hand, is when organisms with different ancestors have structures fulfilling the same function, but that have evolved from different origins. i.e. Insect wings and bird wings
Useful or valuable characteristics in an organism lead to selection for breeding, such as size or colour. These characteristic will be present in the next generation in higher frequency, leading to gradual improvement. Weaker or deformed organisms may be culled.
Similarly, natural populations show phenotypic variation, are subject to natural selection pressures for advantageous characteristics, such as resistance to disease.
By producing more offspring than the environment can support, the chances of survival are increased for the population as a whole. A single death would be less devastating to a population of 1000 than of 10. The increased population causes intraspecific competition for the limited resources. Those with advantageous characteristics are ‘fitter’ and are more likely to successfully reproduce, causing the advantageous characteristics then become more frequent in the next generation. These characteristics have a genetic basis: the alleles for them are more frequent in the population.
4.4.4 – Explain that the consequence of the potential overproduction of offspring is a struggle for survival
The ‘struggle for survival‘ is a result of overpopulation, leading to competition for limited resources. Individuals with beneficial traits that increase their chances of survival will be selected for, and will be able to breed. Thus, these advantageous alleles are present in a higher frequency in the next generation. The consequent change in heritable characteristics is evolution.
4.4.5 – State that the members of a species show variation
All populations of a species will show variation, or a difference in phenotypes, which occurs in two different patterns:
When there are distinct classes of individuals. This indicates the condition is controlled by one or two genes. For example, gender, eye colour and blood group.
This shows no distinct classes, but a complete range of the characteristic. This normally indicates a polygenic condition or multiple alleles. Examples include skin colour, height and mass.
4.4.6 – Explain how sexual reproduction promotes variation in a species
Sources of Genetic Variation
Some organisms may contain a mutated gene. This may be beneficial, harmful, lethal or have no effect. This can occur in both asexual and sexual reproduction. Also, migration may cause genes that were not previously present in the population to be present in later generations, as individuals from another population bring the different genes.
Sexual Reproduction and Variation
Meiosis and the independent assortment of chromosomes creates 2n new combination of chromosomes in the next generation
n = haploid number of chromosomes
Random fertilisation increases variation to 22n again. Two haploid gametes are unified during fertilisation, leading to greater variation due to the mixing of genes. The number of genetic variations is increased further by cross-over in meiosis by 23 in addition to the above
With so many possible combinations of genes, the variation amongst a population is greatly increased. Whilst variation can still occur through asexual reproduction, during sexual reproduction, the genes of two random individuals will be mixed, which gives rise to greater variation still.
4.4.7 – Explain how natural selection leads to evolution
Not every species that has ever developed is still present on the Earth today. Many have become extinct due to changes in biotic and abiotic factors that caused them to die off. Those species that remain have been able to adapt and respond to these pressures but selecting for favourable characteristics.
Individuals with favourable heritable variations tend to have better survival and reproductive rates, which influences the types of genes that will be passed on to the next generation.
Natural selection happens in the stages:
- Overproduction – the organisms have more offspring than the environment can
- Variation – Mutations, random assortment of chromosomes and random fertilisation
lead to a range of characteristics amongst the population, some more beneficial than
- Competition – Limited resources, including habitat and food, mean that not all individuals will survive to a reproductive age
- Survival of the fittest phenotype – Individuals with more beneficial characteristics will
have an advantage in obtaining food and finding a mating partner to pass their genes on to the next generation
- Increase in the frequency of favourable genes – These beneficial alleles will become more frequent as they are more likely to be passed on, whilst individuals with unfavourable characteristics will die off before they have the chance to reproduce.
Evolution happens through the cumulative change in heritable characteristics of a population. Natural selection can still occur without speciation happening. The genetic profile of the population is adapting to changes in local conditions. Every phase is affected by variation and selection.
For example, the heights of a certain population may be distributed as follows:
However, if environmental conditions only favour those individuals at the top of this range, then those at the other end will die off due to competition.
4.4.8 – Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria
This bacterium is associated with skin and lung conditions.
It is commonly found in hospitals, and has two forms:
Methicillin-resistant Staphylococcus Aureus (MRSA) – This is resistant to the antibiotic Methicillin
Methicillin-Susceptible Staphylococcus (MSSA) – This can still be controlled by the use of Methicillin
This antibiotic used to be used to control the spread of golden staph; however, from the 1980’s to the 2000’s, MRSA became more frequent as the resistance gene became dominant.
MRSA evolved because antibiotics selectively kill only the susceptible forms of bacteria, putting selective on the population. DNA mutation would have produced a resistant gene, which could be passed on via plasmids. MSSA bacteria would have the disadvantage, and be readily killed off by the antibiotic. MRSA would survive to reproduce, making the gene more frequent in the population.
This bacterium is of concern to health professionals because it cannot be controlled by methods used previously, and can lead to infection.
New Zealand Kaka
These became isolated from their parrot family ancestor by the Tasman sea. When mountains formed in New Zealand, the environment changed and became more alpine, placing selection pressure on the birds. As a result, two new species developed: the Alpine Kea and the Lowland Kaka. Later, New Zealand split into two islands, causing further divergence of the species to produce the North Island Kaka and the South Island Kaka.