Lesson Notes By Weeks and Term v5 - Grade 12
Evolution by natural selection – Week 5 focus
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Evolution by natural selection – Week 5 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Life Sciences
Class: Grade 12
Term: 2nd Term
Week: 5
Theme: General lesson support
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Evolution by natural selection is the cornerstone of modern biology. It explains the diversity of life we see around us and how organisms adapt to their environments over time. Understanding this process is crucial, especially in a country like South Africa, with its incredible biodiversity facing numerous challenges such as climate change, habitat loss, and invasive species. These challenges directly impact our agricultural practices, conservation efforts, and overall well-being. By grasping the principles of natural selection, we can better understand and address these critical issues.
2.1 The Basic Principles of Natural Selection: Natural selection, proposed by Charles Darwin, is a mechanism of evolution where organisms with traits that are better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. This leads to a gradual change in the genetic makeup of a population over time. There are four main components to natural selection: Variation: Individuals within a population exhibit variation in their traits. This variation can arise from mutations (random changes in DNA), sexual reproduction (which shuffles genes), and gene flow (migration of genes between populations). Without variation, there's nothing for natural selection to act upon. Think of the different colours of beetles in a population.
Inheritance: Traits are heritable, meaning they can be passed on from parents to offspring. This is made possible by genes. Only heritable traits can be selected for. If a beetle is a certain colour due to diet rather than genetics, that colour won't be passed on.
Competition: Resources in the environment are limited, leading to competition among individuals for survival and reproduction. This competition can be for food, water, shelter, mates, or any other resource that is in short supply. The beetles that are better at finding food, or avoiding predators, are more likely to survive.
Differential Survival and Reproduction: Individuals with traits that give them an advantage in the competition are more likely to survive and reproduce. These individuals will then pass on their advantageous traits to their offspring, increasing the frequency of those traits in the population over time. If the green beetles are better camouflaged and therefore survive at a higher rate, they will reproduce more, and the green beetle trait will become more common. 2.2 Types of Natural Selection: Natural selection can act in different ways, leading to different patterns of evolution: Directional Selection: Favours individuals at one extreme of the phenotypic range. For example, in a population of birds, if larger beaks are favoured due to a change in food availability (e.g., harder seeds), the average beak size will increase over time. A real-world example in South Africa is the increasing resistance of certain mosquito populations to insecticides used in malaria control. The mosquitoes with genes allowing them to survive exposure to these chemicals are selected for, leading to insecticide resistance.
Stabilizing Selection: Favours individuals with intermediate phenotypes. This reduces variation in the population. For example, human birth weight is often under stabilizing selection. Babies that are too small or too large have a higher risk of complications, so babies with an average birth weight are more likely to survive. Think about a plant species in a stable climate. Those with traits best suited to the average conditions will thrive, while those with extreme traits will be less successful.
Disruptive Selection: Favours individuals at both extremes of the phenotypic range. This can lead to the formation of two or more distinct groups within a population. For example, imagine a bird population where small beaks are good for eating soft seeds, and large beaks are good for eating hard seeds, but medium-sized beaks are not efficient at eating either. Disruptive selection can lead to the population splitting into two groups, one with small beaks and one with large beaks. Consider a scenario where two different food sources are abundant in different areas of a South African grassland, favouring two distinct beak sizes in a bird population. 2.3 Evidence for Natural Selection: There is a wealth of evidence that supports the theory of natural selection: Fossil Record: Shows a progression of forms over time, with simpler organisms appearing earlier and more complex organisms appearing later. The fossil record can demonstrate how specific traits have changed over long periods, reflecting adaptation to changing environments.
Comparative Anatomy: Similarities in the anatomy of different organisms suggest a common ancestry. For example, the bones in the forelimbs of mammals (humans, bats, whales) have a similar structure, even though they are used for different purposes.
Comparative Embryology: Similarities in the embryonic development of different organisms also suggest a common ancestry. For example, vertebrate embryos all have gill slits and a tail at some point in their development.
Biogeography: The geographic distribution of organisms provides evidence for evolution. Organisms that are found in close proximity to each other are more likely to be closely related. For example, the unique flora and fauna of islands like Madagascar provide evidence of isolated evolutionary pathways.
Molecular Biology: Similarities in the DNA and protein sequences of different organisms provide strong evidence for common ancestry.
Example 1: Beak size in finches on an island.
A population of finches lives on an island with a varied food supply. A drought occurs, and only large, hard seeds are available.
Question: How will this environmental change affect the beak size of the finch population over time due to natural selection? Explain your reasoning.
Solution: The drought acts as a selective pressure. Finches with larger, stronger beaks will be better able to crack the hard seeds and obtain food. These finches will be more likely to survive and reproduce, passing on the genes for larger beaks to their offspring. Over time, the average beak size in the finch population will increase. This is an example of directional selection.
Commentary: This illustrates how environmental changes can drive evolutionary change. The key is to identify the selective pressure and how it affects survival and reproduction.
Example 2: Evolution of pesticide resistance in maize stalk borers.
Farmers in South Africa use a specific pesticide to control maize stalk borers. Initially, the pesticide is very effective.
However, after several years, the pesticide becomes less effective, and the stalk borers are causing significant damage to the crops.
Question: Explain how the maize stalk borers evolved resistance to the pesticide.
Solution: Initially, the stalk borer population contains some individuals with natural resistance to the pesticide (due to random mutations). When the pesticide is applied, most of the susceptible stalk borers are killed. The resistant individuals survive and reproduce, passing on their resistance genes to their offspring. Over time, the frequency of resistant individuals increases in the population, leading to a population that is largely resistant to the pesticide.
Commentary: This demonstrates the practical implications of natural selection. It highlights the importance of managing pesticide use to prevent the evolution of resistance.
Guided Practice (With Solutions)