Darwin’s Theory of Natural Selection

POINT > Review the ideas that shaped Darwin’s thinking POINT > Describe postulates that underlie natural selection POINT > Define “Descent with Modification” POINT > Describe artificial selection

Hutton and Lyell: Geologic processes occur very slowly, but the same processes occurring now have been occurring for millions of years Chilean Earthquake was Lyell’s theory in action If Earth changes slowly but significantly over time…could living things also?

WB CHECK: What ideas did Hutton and Lyell propose that helped Darwin’s thinking? a) survival of the fittest b) animals change slowly over time c) geology of earth changes slowly over time d) similar species live in similar habitats

Lamarck: Organisms change & acquire features to live more successfully in their environment (Acquired Characteristics) Acquired traits get passed to offspring (Inheritance of Acquired Characteristics)

Problems with Lamarck’s ideas: 1. Organisms don’t “strive to be more perfect” 2. Species don’t become “better” 3. Acquired traits cannot be passed to offspring

Strengths of Lamarck’s ideas: 1. Suggested species were not “fixed” and did change over time 2. Tried to explain evolution scientifically with natural processes 3. Made a connection between the environment & body structures

Malthus: Human populations are constrained by competition for space and food, and by disease and war This idea is even more applicable to other organisms that produce large numbers of offspring! Since most organisms don’t survive to reproduce, which ones do?

WB CHECK: What was a weakness of Lamarck’s ideas? Malthus would say that populations are limited by a) the desire to have enough space for everyone b) birth control c) competition for resources d) short life spans of individuals

POINT > Describe postulates that underlie natural selection Darwin’s proposal of “natural selection” is based on these three ideas: 1. Overproduction: Species produce more offspring than the environment can support 2. Variation: There is natural variation between individuals in a population 3. Competition: Some individuals have adaptations that give them a competitive advantage. “Survival of the fittest”

Struggle for Existence (from Malthus) Members of population must compete for limited resources (food, water, mating partners, etc) Most organisms produce far more offspring than can survive (overproduction)

Variation & Adaptation Individuals have natural variation in traits. Why? Some variants are better suited to their environment than others

Examples of variation & adaptation: Body parts/structures Coloration Physiological functions Behaviors

Competition: Differences in adaptations affect an individual’s “fitness” Fitness: How well an organism is adapted to survive and reproduce in its environment

Survival of the Fittest (Similar to Lamarck) Differences in adaptations affect an individual’s fitness

Survival of the Fittest Important Point: Survival does not mean just staying alive Must also reproduce, passing adaptations on to the next generation

Process by which organisms with variations most suited to their environment survive and leave more offspring

Natural selection occurs any time there are: More individuals born than can survive Heritable variations present Variable fitness among individuals

As local environments change, some adaptations may no longer be useful …but other traits may become beneficial If environments change faster then a species can adapt, it may go extinct

Over many generations, adaptations can cause successful species to evolve into new species (shown in fossil record) Living species are descended, with modifications, from common ancestors The process requires vast amounts of time

Principle of Common Descent: All species, living and extinct, are descended from common ancestors

Homework: Read 16.3 pages Assess #1-3 page 464 WB Pages 320, 324,