Evolution by natural selection (2) Flashcards
When does evolution occur?
Evolution occurs through different mechanisms. Darwin’s idea of evolution by natural selection is one of the basic mechanisms of evolution.
Darwin made four ______ that led him to formulate his theory about a mechanism of ____. The ideas are simple but they explain that overtime change and adaptation within species can occur.
observations
evolution
What are the four observations Darwin made? (4)
- More offspring are produced than are required
- Natural variation
- A change in the environment leads to differential reproduction
- Characteristics (traits) were heritable
Darwin observed that More offspring are produced than are required, what does this mean?
Darwin and fellow scientists knew that populations usually produced more offspring than the environment could support, yet populations generally remained relatively stable in size over the long term.
Darwin observed that natural variation occurred, what does this mean?
Natural variation, e.g. colour, size of beaks, muscular strength, etc., exists among individuals of a population, i.e. no two individuals are exactly alike.While in Darwin’s era, he and others could see variation they did not know the reason for it. The reason, we now know, is that every individual possesses a different combination of genes.
Darwin observed that a change in the environment leads to differential reproduction, what does this mean?
Darwin reasoned that the better adapted individuals would be likely to survive to reproduce while the less adapted would most likely die or fail to reproduce. This became known as differential reproduction, or ‘survival of the fittest’. The less-adapted individuals were eliminated and could become extinct. In other words of the large number of off-spring, only a few survive.
Darwin observed that Characteristics (traits) were heritable, what does this mean?
Darwin observed and noted that characteristics were heritable, ie they passed from parents to offspring.
How did Darwin explain natural selection?
Natural selection is a process by which nature (the environment) selects for survival those individuals that are best adapted to environmental conditions and, as a result, will produce the most offspring. Natural selection thus provides a mechanism for evolution, which can adapt species to the environment and ultimately may lead to the origin of new species.
Basic to the understanding of natural selection is the fact that natural selection only operates on _____ in inherited characteristics. If all individuals of a population were ____ identical, there would be no natural selection.
variation
genetically
How does natural selection occur? (8)
- Organisms produce a large number of offspring
- There is a great deal of variation among the offspring. Natural selection always acts on variation.
- When there is a change in the environmental conditions (new climatic conditions, disease, new predator) or there is competition there is a struggle for survival.
- Some off-spring will have a favourable trait that allows them to adapt to the changed environment. They are therefore more likely to survive.
- Whilst organisms without the favourable trait are less able to adapt and so will die.
- The organisms that survive will reproduce and thus pass on the favourable trait to their off-spring. The next generation will therefore have a higher proportion of individuals with the favourable trait. Natural selection can act only on heritable traits, traits that are passed from organisms to their offspring during reproduction.
- If this process continues, eventually all the individuals in the population will have the desirable trait and in this case will be hairy, i.e. evolution by natural selection will have taken place.
- Eventually, by accumulating this and other such small changes in a population, a new species might arise.
Life forms have evolved from previous life forms by natural selection. Most species are unable to survive in a new environment and become ____, but a few species may successfully adapt to a new environment and ultimately may lead to the origin of new species. Natural selection thus provides a ____ for evolution.
extinct
mechanism
Is natural selection a random process?
Natural selection is definitely not random as the organisms that are selected for survival are those that are better adapted to the environment. However, the ways by which variations arise, e.g. by mutations and recombination of genes during sexual reproduction, are definitely random.
Does natural selection result in perfection?
Natural selection is not all-powerful; it does not produce perfection; no population or organism is perfectly adapted. Think of us humans: are we perfectly adapted to our environment? It is more accurate to think of natural selection as a process rather than as a guiding hand. It is mechanical, has no goals, and is not trying to cause development or a balanced ecosystem.
What would happen of all individuals of a population were genetically identical?
There would be no natural selection. There must be some genetic variation within a population, which can influence the nature of the offspring, so that environmental conditions can select the best adapted individuals.
What was Darwin’s greatest difficulty?
Darwin’s greatest difficulty was a lack of understanding inheritance. Gregor Mendel did publish his ideas in Darwin’s lifetime, but Darwin was unaware of them.
What causes genetic variation? Variation of the genetic characteristics of eggs and sperm can be caused by: (4)
- Germ-line point mutations, which are the main source of genetic variation and have a strong influence on evolution.
- Duplication of genes or swapping their positions within chromosomes.
- Whole chromosomes may be deleted or duplicated, and even the entire chromosomal compliment can multiply in a process called polyploidy. Polyploidy is very important in the evolution of new species of plants.
- Sexual reproduction results in the formation of a new combination of alleles by the processes of meiosis, chance fertilisation and random mating.
What come as a result of genetic variation?
As a result of these processes a variety of genotypes (genotypic combinations) are formed in the offspring. It is important to realise that it is only ‘new’ genetic characteristics in the eggs or sperm that will be inherited by the offspring. Any genetic changes in the somatic (body) cells cannot be inherited by the offspring.
Why do offspring differ from their parents? Phenotypic changes in the offspring are due to both: (2)
- genetic variation (new combinations of alleles), and
- effect ofenvironmental factors such as food, temperature, pH, sunlight, etc., on the expression of the genotype. For example, even if the genotype carries genes for the production of large muscles, the animal will not develop big muscles if there is too little protein food available for the muscles to develop.
Selective forces (environmental pressures) such as: \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ will favour some phenotypes more than others, which results in differential reproduction.
competition, predation, climatic factors, disease and parasitism
Why are only some offspring selected for survival?
• Individuals with favourable phenotypic characteristics are selected. Being better suited to the environment they will be more likely to survive and reproduce successfully, so the proportion of their genes in the next generation will increase. • The individuals with unfavourable phenotypic variations will be less likely to reproduce successfully, leaving few offspring with the unfavourable characteristics.
What is the result of natural selection?
A new favourable genotypethus becomes more frequent in the population. If the genetic variation spreads throughout the whole population it is possible that a new species may be formed.
Explanation of an example of natural selection: Peppered moth (3)
- Moth – pre-industrial revolution in England:
The common colour of peppered moths was originally light, and this colour type represented the predominant form in England prior to the beginning of the industrial revolution. The moth’s light grey colour closely matched the lichen-covered trees in their environment.
- Moth – after the start of industry
Early coal-based industry was extremely dirty. Around large cities, everything was essentially covered in soot. This high level of pollution killed the light grey lichens on trees and the bark became much darker in appearance.When the light-coloured peppered moths landed on the same trees they had always landed on, they were extremely visible against the dark bark, and easy targets for predators. In this environment, the dark-coloured moth variant more closely matched to colour of the trees and was now harder for predators to spot.Over generations, the polluted environment continued to favour darker moths, and they progressively became more common. By the late 19th century, 98% of the moths near cities were black.
- Dark moth – in era of modern, cleaner air
Modern air pollution controls have cleaned up the environment compared to the early days of the industrial revolution. A cleaner environment has allowed the lichens to grown back, and the trees have returned to being lighter in colour.
