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What is Evolution??? Learning Target: I can explain Natural Selection and the 4 conditions that are required for Natural Selection to take place. What is Evolution??? Change in the heritable (genetic) traits of a population over time Note: when we discuss evolution, we are talking about populations changing, not individuals Natural selection: The main mechanism by which evolution occurs 4 Conditions for Natural Selection: 1. Variation: Individuals in a population are not identical to each other. 2. Inheritance: Traits are passed to offspring; traits have a genetic basis 3. Environmental population limits: Environmental limiting factors prevent all individuals from surviving to reproduce; some die young. 4. Environmental selection: • Individuals in the population with more favorable (advantageous) traits are the ones that survive to reproduce. • Individuals without advantageous traits die before reproducing. These factors result in a change in the average trait of the population… Biologists call this EVOLUTION! 1. VARIATION Members of a population have traits similar to the average trait of the entire population, but they are not identical. YOUR TURN: Using height as an example, sketch a graph to represent the statement above. Mean (average) height Frequency • Height (cm) 2. INHERITANCE • DNA determines the traits of individuals DNA trait • Individuals inherit DNA from their parents • This causes the traits of the offspring to resemble the traits of the parents 3. ENVIRONMENTAL POPULATION LIMITS Population A. For all species, if every individual born into a population were to reproduce, the population would grow exponentially Time 3. ENVIRONMENTAL POPULATION LIMITS Population B. Environmental factors (limiting factors) prevent the majority of individuals from surviving to reproduce Time What are some examples of limiting factors? 4. SELECTION • Individuals without advantageous traits die before reproducing. Individuals with advantageous traits survive to reproduce. Frequency • These individuals survive to reproduce Characteristic Populations change, not individuals Frequency • The “average” characteristic or other measure of the population changes over generations These individuals survive to reproduce Characteristic The environment is the selective force behind evolution • The environment determines what characteristics are “favorable” • Characteristics are said to be “selected for” • Because the environment changes over time, the characteristic that is more favorable for a population changes • Therefore, characteristics of the population change, or evolution occurs Artificial Selection • Humans select for the desired trait • Example: plant & animal breeding • Not an example of evolution Why is genetic variation in a population important? • A population without much variation limits a species’ ability to further evolve. (Variation is one of the 4 factors required for natural selection to cause evolution) • If populations do not adapt to their environment, they may become extinct. Sources of genetic variation 1) SEXUAL REPRODUCTION A. Meiosis – one copy of a gene is passed on from each parent, combination may be different than someone else B. Random fertilization – only one of the millions of sperm involved in mating will fertilize the egg. The randomness of sexual reproduction explains why siblings can look so different. Sources of genetic variation 2) MUTATION Mutation means a change in DNA sequence. - Can happen when DNA is copied during cellular replication A new DNA sequence can result in new variation (not always). – – – Many mutations produce traits that are harmful (e.g. Huntington’s disease) Some mutations produce traits that are neutral (neither helpful nor harmful) Very, very few mutations produce traits that are advantageous Natural Selection • Individuals with advantageous genes survive to reproduce and pass on these genes to their offspring. • Individuals without advantages genes do not survive to reproduce, and these genes do not get passed on in the population. • “Survival of the Fittest” – best fit (able to survive and reproduce) in the environment • Natural Selection leads to adaptation!!! Conclusion • Mutation does NOT cause evolution – it is only a source of variation (just one of the factors required for natural selection) • Natural selection determines if the frequency of traits will change within a population. • Change in traits in a population = EVOLUTION