1. Genes, environment and evolution

2. Learning objectives You should be able to: Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selectionExplain, 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 populationsExplain 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 mechanismsExplain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms

3. Why doesn’t the population ever go above 18? What is happening to this population?

4. The number of organisms of one species that an environment can support indefinitely. Carrying Capacity 18 is the Carrying Capacity for our population.

5. Population growth K = carrying capacity

6. Not all young survive to adulthood and reproduce If they did, the graph would be J-shaped, not S-shaped

7. What limits population growth?

8. 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.

9. List at least 3 for each of the following types of factors that cause environmental resistance Abiotic Biotic

10. 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

11. 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.

12. Logistic Growth of a Sheep Population on the island of Tasmania, 1800–1925

13. 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

14. Selection pressures Selection pressures are those environmental factors that give some individuals in a population a greater chance of surviving than others. Selection pressures are those environmental factors that give some individuals in a population a greater chance of surviving than others. Increases the chance of certain alleles (NOT genes) to be passed on to the next generation For example, predation of zebra by lions might mean that slower individuals were more likely to be eaten 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 We will be considering two kinds of selection pressure – Stabilising selection – Directional selection

15. 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. Favours intermediate phenotypes Birth weight in humans is a good example. Babies that are particularly large or small are less likely to survive birth.

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

17. Directional selection This is where a particular characteristic confers a clear advantage. This is where a particular characteristic confers a clear advantage. Favours extreme phenotypes and cccurs when selection pressure changes due to environmental change Favours extreme phenotypes and cccurs when selection pressure changes due to environmental change 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. 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 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. Directional selection is a form of natural selection that leads to evolutionary change. In other words, it is an evolutionary force.

18. Isolating mechanisms Isolating mechanisms are factors that prevent individuals within a population from breeding with each other. They may be: – Ecological e.g. different subpopulations of plant preferring different light levels and therefore growing in different areas – Geographical e.g.a population of tortoises might be split by a large river that none of them can cross – Seasonal (temporal) e.g.climate change throughout a year – Reproductive e.g. 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

19. 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

20. 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.

21. 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.

22. Consequences of genetic drift Loss of genetic variation – Loss of an allele – The other allele becomes fixed Less potential for natural selection – Alleles that lead to low fitness reduces ability for adaptation & survival – Could lead to extinction Different populations become genetically different – Different alleles fixed – Could lead to speciation

23. Naturally small populations: Desert pup fish Some populations are small and remain small over long periods of time simply because they naturally occur in small areas of good habitat. Desert pupfish occur in small ponds in the desert. The ponds are too small to hold many fish, so populations are small. Each pond has a separate population from each other pond since, clearly, fish are not able to get out of the pond and walk through the desert to another pond. These pupfish populations are subject to large amounts of genetic drift and have been for a very long time.

24. Historically large populations recently become small: Bornean Orangutan Born In areas where there were once large areas of habitat, but where much of that habitat has now been destroyed. Either through natural events, such as storms, fires, floods, disease epidemics, etc. or more commonly, destruction due to human activity We have created situations in which a lot of genetic drift occurs. This decreases the potential for adaptation which may make these small populations more likely to go extinct.

25. Population bottleneck for example because of a natural disaster such as a volcanic eruption. populations that may have been historically large, go through a period of time when they are small, and then become large again for example because of a natural disaster such as a volcanic eruption. This makes the population very susceptible to genetic drift. Bottle neck effect – decreased variation even when population has become large again. This makes the population very susceptible to genetic drift. Mutation will introduce variation again very slowly

26. An example genetic drift in a small and isolated population 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.