Darwin’s Theory of Evolution by Natural Selection Section 15-3 Darwin Presents His Case

Artificial Selection –The selection, by humans, of organisms with useful traits. –People decide which plants or animals they will let reproduce. –This leads to gradual improvements each generation.

Artificial Selection

Kuri

The obvious success of animal and plant breeding led Darwin to conclude that: Species are not uniform - there is variation Species are not fixed, they can change over time

Darwin realized that if artificial selection could make such dramatic changes in organisms there is no reason the same process shouldn’t be occurring in nature.

Variation in nature

Reading Malthus led Darwin to conclude that: Differential Reproductive Success… In nature, there must be “winners” and “losers” Those with favorable traits vs those with unfavorable traits The “winners” survive and leave more offspring relative to the “losers”. Survival of the Fittest

What are some traits that could make a plant or an animal better able to survive? More offspring Faster Larger Smaller Brighter color Longer teeth Camouflage Survive drought Spines to protect from predators Shell Able to dig Deep roots High photosynthetic rate Chemical defenses against herbivores

Evolutionary Fitness The reproductive success of an individual relative to the rest of the population.

Fitness

example A mutation occurs in an animal –causes it to be 25% more likely to be killed by a predator than similar individuals without the mutation –able to produce 50% more offspring than similar individuals without the mutation Should the frequency of this mutation increase or decrease in the population? Does the mutation increase or decrease the fitness of the animal?

Darwin proposes Natural Selection as the engine that drives evolution Sometimes referred to as “survival of the fittest”. –Note: never by Darwin

Main principles of Darwin’s theory of evolution by natural selection 1.Variation – variation exists among individuals of any species. 2.Heredity – Some of the variation is heritable; that is, it is genetic in nature and will be passed on to offspring. 3.Differential Reproduction (Fitness) – Some of the heritable traits will confer a reproductive advantage to individuals with the trait.  Individuals with the trait will have more surviving offspring (which will also have the advantageous trait)

Main principles of Darwin’s theory of evolution by natural selection If all three necessary conditions are met (variation, heritability, differential reproduction), then evolution occurs.

Implications of natural selection Darwin recognized an implication of his theory of evolution through the process of natural selection was that…  Each living species has descended, with changes, from other species over time.  Darwin referred to this as descent with modification.

Descent with Modification

Implications of natural selection Common Descent –All species, living or extinct, were derived from common ancestors. –A single “tree of life” links all living things.

Implications of natural selection

Common Descent

Checkpoint What is artificial selection? The selection, by humans, of organisms with useful traits. What are two observations/conclusions Darwin drew from artificial selection? Species are not uniform (there is variation) Species are not fixed, they can change over time

Checkpoint The driving force of evolution is… Natural selection What is fitness in an evolutionary sense? The reproductive success of an individual relative to the rest of the population.

Checkpoint What are the three necessary conditions for evolution by natural selection? Variation Heritability Differential reproduction

Checkpoint Descent with modification means… Each living species has descended, with changes, from other species over time.

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved…Charles Darwin

Chuck says… That’s enough notes for today. Nothing in biology makes sense except in the light of evolution... Theodosius Dobzhansky Theodosius Dobzhansky

Common Descent or Descent with Modification PTEROSAUR CHICKEN PENGUIN PORPOISE BAT HUMAN ANCESTOR REPTILE