- Organisms generally produce far more offspring than the environment can support; most will die before adulthood due to lack of resources such as food and space.
- The offspring will show variation in their characteristics. These variations will be better adapted to the environment than others. The limited resources would mean competition between the individuals, and so the individuals that have the variations which are better adapted will be more likely to survive and others will die.
- This is called ‘survival of the fittest’ or natural selection. These individuals who survive will breed and pass on their genes that helped them survive to their offspring.
- Evolution means a slow and continuous change of organisms from one generation to the next. In the early 1800s some scientists realised that species can change over time, but these theories didn’t explain all the facts. Charles Darwin however drew together several ideas to produce a new theory that was better at explaining the facts.
- Darwin knew there was competition between individuals. He also knew that pigeon breeders selected birds with certain characteristics to breed which meant that those characteristics were inherited by the offspring.
- Darwin realised that if the environment changes then different variations may be better suited to the new conditions. Individuals with those variations will be more likely to survive and pass their characteristics on to offspring.
- Therefore, the range of variation in characteristics of the population will gradually change over generations, which is evolution. If the environment changes too rapidly, however, and no individuals have adaptions that help them survive, they may die out and the species become extinct.
New evidence for Darwin’s theory
- When Darwin published his theory, one of the problems was that there was little evidence for it. Evolution takes a long time to observe.
- Warfarin is a chemical that was used to poison rats in the 1940s and 1950s. When it was first used most rats died in the first few days of eating it, but a few survived. These rats bred to reproduce more rats resistant to the chemical and within 10 years, most rats were resistant to warfarin.
- DNA research also suggests that all organisms have evolved from shared common ancestors. Closely related species diverged more recently.
- They found evolution is caused by small changes in DNA. Consequently, organisms that diverged away from each other more recently should have more similar DNA. This is exactly what scientists found e.g. humans and chimps have similar DNA.
The scientific community
- The scientific community is all the scientists around the world e.g. researchers, technicians and professors. Scientists within the scientific community accept the theory of evolution because they have shared and discussed the evidence to ensure it is valid and reliable. There are three main ways they do this:
- Scientific journals: Scientists publish their work into journals. If other scientists can repeat their experiments and get the same results the scientific community can be sure the evidence is reliable.
- Peer review: This is when scientists read and review the work of other scientists. This is to check that it is valid and that experiments are carried out to the highest possible standards.
- Scientific conferences: These are meetings that scientists attend to present and discuss their work. They are an easy way for the latest hypotheses and evidence to be shared and discussed.
- Darwin began thinking about evolution after noticing differences between mockingbirds from different Galapagos Islands. They were all closely related, but each island had its own species. He guessed that originally, individuals from one species of mockingbird had reached the islands from South America.
- The environmental conditions varied between islands, so on each island different adaptations would have been more successful. As a result, each island population evolved in a different way. Over time, the individuals on each island became so different that they couldn’t interbreed with birds from another island. They had become a new species, a process called speciation.
Genes and Alleles
- Gene: A section of DNA that codes for a specific protein.
- Allele: Each gene comes in different types called alleles. For example, a gene for eye colour may have a gene for blue type allele and a brown type allele.
- Dominant allele: Version of a gene that will always have an effect if it is present.
- Recessive allele: Version of a gene that will only have an effect if the dominant version of the gene is not present (if both are present)
- Homozygous: both alleles for a characteristic are the same the organism is homozygous for that characteristic.
- Heterozygous: if both alleles for a characteristic are different then the organism is heterozygous for that characteristic.
- Phenotype: The physical characteristics that a certain set of alleles cause.
- Genotype: The alleles for a certain characteristic that are found in an organism.
Components of DNA
- DNA is found in the nucleus of a cell and is in the formation of long strands. Each strand forms a structure called a chromosome.
- A nucleus contains different chromosomes; there are usually two copies of each chromosome.
- Chromosomes are divided up into genes and each chromosome contains a large amount of them. Each gene has a job.
- For example, many genes control variations in our characteristics. Variation caused by genes is called inherited variation because we inherit genes from our parents.
- Different forms of the same gene are called alleles. Each allele may contain slightly different instructions to create variations.
- As there are two copies of every chromosome in a somatic cell nucleus there are two copies of every gene. Each copy of a gene could be a different allele.
- Different organisms contain different numbers of chromosomes. Humans have 23 pairs of such, which contain about 25000 different genes in total. There are many alleles for each gene so it is easy to understand variation between humans.