Meiosis and variation Lesson 11 The roles of genes and the environment in evolution

Learning objectives You should be able to: Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection Explain how genetic drift can cause large changes in small populations Explain how genetic drift can cause large changes in small populations Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms

Genotype and evolution We have been looking at population genetics and how allele frequency can be calculated assuming no advantage is conferred by a particular genotype. Very often though, genotype will affect the fitness of an organism and selection will occur

Carrying capacity and environmental resistance If all the offspring of organisms in a population survived and bred then the population would soon exceed its carrying capacity. The environmental factors that prevent this from occurring and therefore limit population size are called environmental resistance. Factors that limit a population to its carrying capacity can be divided into biotic and abiotic.

Abiotic = caused by non-living components of the environment e.g. space, light, minerals, water Biotic = caused by other living organisms e.g. predation, disease, food supply Biotic and abiotic

Intraspecific competition In practice, a population usually fluctuates around a mean level If it exceeds the carrying capacity then intraspecific competition becomes more intense, mortality increases and it drops back below its carrying capacity. If the population is below its carrying capacity then intraspecific competition is reduced, mortality decreases and the abundance of the organism increases.

So who lives and who dies? The organism that is fittest, in other words, is best adapted to its environment, is the one that is most likely to survive. This means that it is more likely to breed and therefore pass on the allele that conferred a selective advantage

Selection pressures Selection pressures are those environmental factors that give some individuals in a population a greater chance of surviving than others. For example, predation of zebra by lions might mean that slower individuals were more likely to be eaten We will be considering two kinds of selection pressure Stabilising selection Directional selection

Stabilising selection This is natural selection where allele and genotype frequency within population stay the same because the organisms are already well adapted for the environment. Birth weight in humans is a good example. Babies that are particularly large or small are less likely to survive birth.

A good example of stabilising selection. Crocodiles have changed little in 65 million years

Directional selection This is where a particular characteristic confers a clear advantage. For example, gazelles that run faster are less likely to be eaten by cheetahs. Similarly, hares with a white coat are less likely to be seen and therefore predated in snowy conditions. This leads to the frequency of alleles for the gene changing within the population Directional selection is a form of natural selection that leads to evolutionary change. In other words, it is an evolutionary force.

Isolating mechanisms Isolating mechanisms are factors that prevent individuals within a population from breeding with each other. They may be Ecological, for example different subpopulations of plant preferring different light levels and therefore growing in different areas Geographical, for example a population of tortoises might be split by a large river that none of them can cross Seasonal (temporal), such as climate change throughout a year Reproductive, for example, courtship behaviours or breeding seasons may not be compatible Isolation results in sub populations. Eventually these sub populations will become so genetically distinct that they will be different species

Red and white campion are a good example of isolation. Red campion prefers shade and usually flowers in spring. White campion prefers light and flowers in summer. They are therefore seasonally and ecologically isolated Red campion White campion

There are occasions though when red and white campion flower at the same time and in the same place. When this happens then they can interbreed to produce an intermediate pink form.

Genetic drift Also called allelic drift, this is the change in allele frequency in a population from one generation to the next, caused by random events in meiosis and fertilisation. It is one factor that results in isolated sub populations becoming genetically distinct Not all the alleles that an individual has will necessarily be passed on to its offspring. For example, two organisms with genotype Aa might have two offspring, each with genotype AA. The a allele would therefore not be passed on. The smaller a population, the greater the changes in allele frequency that will be caused by genetic drift. Alleles may be lost from a population altogether which reduces genetic diversity and reduces the potential for the population to adapt to a new environment. In extreme cases, genetic drift may lead to extinction or the production of a new species.

An example genetic drift in a small and isolated population A bottleneck is when a population suddenly becomes very small, for example because a natural disaster such as a volcanic eruption. This makes the population very susceptible to genetic drift. In 1775 a storm and famine reduced the size of the population of the Pingalep atoll in the Pacific to 30. Of their 2000 descendants, 5% have the eye defect achromatopsia, caused by two recessive alleles and very rare in other populations. One of the original survivors was heterozygous for this condition. The small size of the population allowed this allele to change rapidly in frequency over generations, despite conferring no advantage, an example of genetic drift.

Pingalep atoll

A modern inhabitant of the Pingalep atoll with achromatopsia