Evolution: drift and selection CfE Advanced Higher Biology Unit 2: Organisms and Evolution

SQA mandatory key information Processes of evolution, natural selection, sexual selection and genetic drift. Mutations can be harmful, neutral or beneficial and give rise to variation. Absolute fitness is the ratio of frequencies of a particular genotype from one generation to the next. Relative fitness is the ratio of surviving offspring of one genotype compared with other genotypes.

Key concepts Evolution is the change over time in the proportion of individuals in a population differing in one or more inherited traits. Evolution can occur through the random processes of genetic drift or the non-random processes of natural selection and sexual selection. Genetic drift is more important in small populations, as alleles are more likely to be lost from the gene pool. Variation in traits arises as a result of mutation. Mutation is the original source of new sequences of DNA. These new sequences can be novel alleles. Most mutations are harmful or neutral but in rare cases they may be beneficial to the fitness of an individual. As organisms produce more offspring than the environment can support, those individuals with variations that best fit their environment are the ones most likely to survive and breed. Through inheritance, these favoured traits are therefore likely to become more frequent in subsequent generations.

What is evolution? It is commonly defined as the change over time in the proportion of individuals in a population differing in one or more inherited traits. Other eminent scientists have also tried to define it....

Evolution John Maynard Smith ( ) – (British theoretical evolutionary biologist and geneticist) wrote in his book ‘The Theory of Evolution’ “any population of entities which has the properties of multiplication, heredity and variation” It is impossible to try to define Evolution without reference to Charles Darwin ( ). Darwin was a British naturalist whose ground breaking book ‘Of the Origin of Species’ published in 1859 changed our thinking about evolution. In the final chapter of the book, Darwin wrote.... “There is grandeur in this view of life from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved”.

Evolution Darwin rocked the scientific world with his publication in Today we continue to see evolution in action and it is important to see what is happening now with reference to those ground breaking findings in the past. John Maynard Smith summed this up in his book ‘Did Darwin get it Right’ "Of course we need to see further than Darwin, but we shall do so by standing on his shoulders, not by turning our backs on him."

Further reading Various up to date articles on evolution as well as some video clips looking at the evidence for evolution

Evolution Let’s look at evolution in more detail. It can occur through: The random process of genetic drift. The non-random processes of natural selection and sexual selection.

Genetic drift What is it? It is a random process. In small populations, unpredictable events (disease, predation, abiotic factors etc) can result in the frequency of alleles varying from one generation to the next. This tends to be masked in larger populations due to the increased number of individuals). The diagram opposite shows the effect of population size on genetic drift: Ten simulations each of random change in the frequency distribution of a single hypothetical allele over 50 generations for different sized populations.

Genetic drift – an example In the original population (Generation 1) due to unpredictable events, only some individuals of the small population are able to go on and produce fertile offspring (Generation 2). If this continues then after 5 generations, the frequencies of the red, purple and green alleles will vary considerably from the original population. In this example, by the fifth generation, the green allele is no longer present.

Genetic drift continued... This ongoing variation in allele frequency due to genetic drift is random. This contrasts with natural selection where the frequency of an allele in a population is related to the fact they allow an organism to be more adapted to its environment. Due to the random nature of allele fluctuation over time, genetic drift can result in a reduction or loss of genetic variation as some alleles are removed from the gene pool over generations (this has a negative effect on the organisms ability to evolve). Although some alleles may be lost, some are retained due to genetic drift. However, as these alleles are not selected to be beneficial to the organism, they may be harmful but still increase in frequency in the population.

Natural selection Natural selection is a non-random process. It was proposed by Charles Darwin in his book ‘On the Origin of Species’ in He wrote... ‘I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.’

What was proposed by natural selection? Darwin observed that individuals in a population show variation. Genetic characteristics are passed on from one generation to the next from parents to their offspring. All species are capable of producing more offspring than their environment can support. The majority of offspring will not survive due to lack of food or other resources. Individuals who possess variations that make them more suited to their environment are more likely to survive and pass on these successful traits to their offspring than less well suited individuals. These traits will become more frequent in subsequent generations.

What is sexual selection? There are two mechanisms, as mentioned by Charles Darwin in his book, ‘The Descent of Man and Selection in Relation to Sex’ in 1871: “The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners. ”

Sexual selection (1) Intrasexual selection refers to selection within the same sex (usually males). Individuals compete with each other with ritualised displays of strength and stamina to warn off competitors or defend his mate(s) e.g. Red deer. It is an example of dominance hierarchy. However, it has also been seen in females e.g. Ring Tailed Lemurs. Attribution: RadioFan at en.wikipedia

Sexual selection (2) Intersexual selection refers to individuals (usually female) being very selective about their choice of mate. It is sometimes called ‘mate choice’. Often bright plumage and showy courtship displays influence a female’s choice. e.g. Peacocks. The extreme difference in degree of plumage shown by the male contrasts with the smaller, much less showy, Peahen. This difference between the sexes, also shown in other forms of adornment in males such as antlers in deer is called sexual dimorphism.

Variation and mutation The cornerstone of the theory of natural selection is the fact that individuals of a population show variation. Variation in inherited traits arises as a result of mutation. Mutation is the original source of new sequences of DNA. The new DNA sequences can be new alleles. Mutations to an organisms DNA are usually harmful or they may have no effect at all and be neutral. On rare occasions, a mutation in an individual’s DNA can make it better suited to its environment and increase the fitness of an individual, increasing its chances of reproductive success.

What is fitness in evolution? Fitness is an indication of an individuals ability to be successful both at surviving and reproducing. It refers to the contribution that is made to the gene pool of the next generation by individual genotypes. As a result of natural selection we know that there is variation of alleles of genes. We know that frequencies of alleles changes through many generations and that there would be an expectation that alleles with the highest fitness would become more common in a population.

Fitness Can be defined in absolute or relative terms: Absolute fitness - the ratio of frequencies of a particular genotype from one generation to the next. Relative fitness - the ratio of surviving offspring of one genotype compared with other genotypes. It is important to remember that the overall fitness of an individual is affected by its environment. The fitness of a phenotype and genotype will differ in different environments. For example, if a moth is more successful at producing offspring due to the fact it is more camouflaged from predators, this will increase the relative fitness of the moth population. However, if the moth moves to a new environment, this phenotypic adaptive advantage will no longer apply.

Summary (1) Evolution is the change over time in the proportion of individuals in a population differing in one or more inherited traits. It can occur through the random processes of genetic drift or non random processes of natural selection and sexual selection. Variation in genetic makeup can arise as a result of mutation. Mutation is the original source of new sequences of DNA. Most mutations are harmful/neutral but occasionally can be beneficial to the fitness of an individual. Fitness can be defined in absolute or relative terms.

Summary (2) Absolute fitness is the ratio of frequencies of a particular genotype from one generation to the next. Relative fitness is the ratio of surviving offspring of one genotype compared with other genotypes. Organisms produce more offspring that the environment can support. Those individuals with variations that best fit their environment are the ones most likely to survive and breed. Through inheritance, these favoured traits are therefore likely to become more frequent in subsequent generations.