Evolution Chapter 6

Pre-Darwinian Theories

Darwin’s Voyage  Where did he go and why?  5 year trip around the world on the HMS Beagle in 1831  To learn about living things as the ship’s naturalist  Saw many living things different from life in England

H.M.S. Beagle

A Brief Biography of Charles Darwin

Darwin's Observations  Amazed by the diversity of life  Large sloth bones in Argentina  Many diverse species in the Galapagos islands  Species: a group of similar organisms that can mate with each other and produce fertile offspring

A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions. For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator. Variations in the Species!

Similarities and Differences  How did Darwin think plants and animals had originally come to the Galapagos Islands? He inferred:  Blown out to sea during a storm  Adrift on a fallen log  Reached the islands, reproduced  Offspring eventually became different from mainland species  Species differed island to island

The Galapagos Islands

Adaptations  Adaptation: a trait that helps an organism survive and reproduce  Tortoises’ shells  Finch beaks  Traits are controlled by genes!

Adaptations

Darwin’s "Dangerous" Idea  Perhaps the species became better adapted to the new conditions  He did not know how it happened  Thought it might be similar to selective breeding  evolution: gradual change in a species over time

Natural Selection  Explanation proposed by Darwin and Alfred Russel Wallace about how evolution occurs  Process by which individuals better adapted to environment are more likely to survive & reproduce  Factors: overproduction – competition – variations - selection

Overproduction  Most species produce more offspring that can survive  Not enough resources available for all: food; water; space  Ex: salmon lay hundreds of eggs but few survive to adulthood  Darwin wondered why?????

Variations  Difference between individuals of same species  Genetic!  EX: some salmon have a "notch" in their tails  Could help them survive

Adaptation/Competition  Indirectly for available resources  Not usually physical  Ex: fish with notch swim faster & escape predators  Some don’t escape predators

Selection  Some variations = better adapted  More likely to reproduce  Offspring may inherit helpful alleles  More individuals have trait after many generations  ENVIRONMENT does the “selecting”  Natural selection can lead to evolution over a long period of time  Ex: more salmon with notched tails

Natural Selection

The Role of Genes in Evolution  Darwin knew nothing of genes or mutations  He could not explain how traits were passed on  Now: Only inherited traits are acted upon by natural selection

Evolution in Action  1977 study on Daphne Major  Little rain led to fewer plants  Fewer plants = fewer seeds  Finches ate large, tough seed pods  Many small-beaked finches did not survive drought  Next year = more finches with larger, stronger beaks

Daphne Major & Minor

Evolution in Action  Peppered moths of England

How Do New Species Form?  Geographic isolation  A group of individuals remains isolated long enough to evolve different traits  Ex: Kaibab & Abert squirrels

Kaibab Squirrel Abert Squirrel The Kaibab squirrel is isolated in the Kaibab Plateau. South of the canyon and elsewhere, the similar Albert squirrels may be found.

How Do New Species Form? isolation = when some members of a species are cut off from the others a new species can form when a group of individuals remains separated from the rest of his species long enough to evolve different traits

Continental Drift  As continents separated species became isolated  Began to evolve independently  Australia and the marsupials and monotremes

Pangea Pangaea & Continental Drift