Biological Diversity A Framework for understanding Earth’s diversity of organisms Charles Darwin
Prior to Darwin, two ideas about life on Earth prevailed:
The species are fixed. They do not change.
The Earth is less than 10,000 years old and is also relatively unchanging.
Darwin’s early journal writings suggested he felt that the concept of fixed or unchanging species best described nature.
The Voyage of the Beagle
In December of 1831, the HMS Beagle set sail on a voyage around the world.
Darwin saw A LOT of variety . Forms of life were much different than in England.
Plants/Animals were well suited for their environment.
Different parts of the world contained different kinds of animals. (For example, no kangaroos in England, no rabbits in Australia Why?
Darwin found fossils . Some resembled currently living organisms while other were completely different.
Darwin visited the Galapagos Islands. The giant tortoises had different (similar) shells. He could identify which tortoise lived on which island based on its shell shape…
Darwin also saw birds…
They all had differently shaped beaks. He thought they were various sorts of birds (warblers, blackbirds, wrens).
Different finches often came from different islands…
Darwin noticed animals and plants varied from island to island… (birds and tortoises)
While on his journey on the BEAGLE, Darwin read and had time to reflect. He read articles written by James Hutton , who proposed the idea that the earth was many millions of years old. (as opposed to several thousands of years old…)
Ideas from Geology
He also read the writings of geologist Charles Lyell . Lyell proposed that gradual and observable geologic processes such as erosion could explain the physical features of today’s earth.
The processes that changed the earth in the past are the same processes that operate in the present.
Darwin began to change his mind…
1. The slow processes of mountain building and erosion suggested an earth that must be very old.
( old earth – yes; young earth - no )
2. These slow and gradual processes occurring over vast spans of time could cause enormous change on Earth.
( everything about the earth is changing constantly – it is NOT fixed )
In 1838, Darwin read an essay on human populations written by Thomas Malthus .
The essay noted that populations can grow much faster than the rate at which supplies of food and other resources can be produced. (If populations continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.)
Malthus believed that much of human suffering (disease, famine, homelessness) was due to the population’s potential to grow.
Darwin applied this idea to all living things – the production of more individuals than the environment can support leads to a struggle for existence.
The Origin of Species
In 1859, Darwin published his book entitled The Origin of Species . He made two main arguments:
1. Organisms living on earth today descended from ancestral species – life has a history of change – descent with modification
Darwin’s Tree of Life
Natural selection is the mechanism of evolution . What does that mean?
Organisms struggle to survive. Those best suited for their environment have adaptations that increase their chance for survival.
Fitness the ability of an individual to survive and reproduce in its environment.
Fit organisms pass their genes on to their offspring. “Survival of the Fittest”
The idea of natural selection was Darwin’s way of comparing nature to what he saw done with breeders…
Darwin found convincing evidence for his ideas in the results of artificial selection.
Breeders could produce a great deal of change in a species in a short time. He reasoned that over thousands of generations, natural selection could also cause major change.
‘ Evidences’ of Evolution
According to many scientists, evolution leaves observable signs.
Evidence 1 – The fossil record
The fossil record is a chronological collection of life’s remains in the rock layers, recorded during the passage of time. Fossils show organisms lived that no longer live…
Evidence 2 – Geographic Distribution
Darwin noted that different animals on different continents had similar anatomies and behaviors.
So, he reasoned that while the animals evolved separately, it made sense that they looked and behaved similarly – they lived in similar ecological conditions and experienced similar natural selection pressures…
Different animals (geographically distributed in different parts of the world) end up evolving certain features in common.
Certain similarities in structure among species provide clues to evolutionary history.
Similar structures in species sharing a common ancestor are called homologous structures .
Evidence 4 – Similarities in Development
Embryos of closely related organisms often have similar stages in development.
Evidence 5 – Molecular Biology
The sequences of bases in DNA molecules are passed from parents to offspring.
Evolutionary thinking suggests these information-rich molecules are the records of an organism’s ancestry.
If two species have DNA with sequences that match closely, biologists conclude that the sequences must have been inherited from a relatively recent common ancestor. In contrast, the greater the number of differences in DNA between species, the less likely they share as close a common ancestry.