Evolution: A history and a process
Charles Darwin=
Father of the Theory of Evolution
Evolution= all of the changes that have transformed life over an immense time
Two ideas persisted before Darwin
Species are fixed
Earth was less than 10,000 years old and also unchanging
Adaptation= an inherited characteristic that improves an organism’s ability to survive and reproduce
in a particular environment
Ex: the muscular hind legs of a kangaroo
Before Darwin…
1800s Jean Baptiste Lamarck
Proposed that life evolves and that species are not permanent
Process of adaptation
Other scientists
Charles Lyell- geologist
Proposed the gradual and observable geologic processes
Erosion, formation of mountains
Darwin confirmed this when he witnessed an earthquake in Chile that moved a portion of land above
sea level
Thomas Malthus
Proposed that a population’s growth is influenced by resources
Darwin left England as a young graduate
He returned as a famous naturalist
1844 Darwin wrote a 200 paper essay that described his idea…but was not published
1858 Alfred Wallace came to the same conclusions as Darwin
Within a month, some of Wallace’s, as well as Darwin’s ideas, were presented to the public jointly
One year later, Darwin published his book The Origin of Species
Darwin made two points in his book:
1. Species on Earth today descended from ancestral species
These descendants spread into different habitats around the world and acquired adaptations for a
diversity of life
“Descent with modification”
Ex: jackrabbit and showshoe hare
1. Natural selection is the mechanism for evolution
Natural selection is the process by which individuals with inherited characteristics that are ideal to the
environment leave more offspring on average than do other individuals
Evolution leaves signs
Evolution leaves evidence in
The fossil record
The diverse assortment of modern species
Fossil record
Fossils= preserved remains or markings left by organisms that lived in the past
Found mostly in sedimentary rocks
Sedimentation causes rock formation as particles accumulate in layers; any given stratum (layer) is
older than the one above it, and younger than those below
Paleontologist= scientist who studies fossils
Oldest fossil evidence of life consists of chemical traces in rocks that are 3.8 billion years old
Found in Greenland
Prokaryote fossils have been found and dated as 3.5 billion years old
Fossil example:
Basilosaurus= an early whale found to have remnants of hind leg bones
Geographic Distribution
Darwin observed the similarities and differences of organisms from different parts of the world
Darwin proposed that organisms present today evolved from ancestral forms
Geographic distribution can be used as a clue for the evolution of species
Biogeography is the study of the distribution of plants and animals throughout the world
The world’s six biogeographical regions have their own distinct mix of living things
Continental drift refers to the changing positions of the continents over time
Two hundred twenty-five million years ago, all the present land masses belonged to one continent
(Pangaea)
The distribution of plants and animals is consistent with continental drift
Organisms, such as certain seed plant groups or reptiles, are widely distributed throughout the world
Other groups, such as mammals that arose after the continents broke up, have great differences in
species on different continents
Clues to evolutionary history
Similarities in Structure
Example: mammal forelimb
Homologous structures= similar structures in species haring a common ancestor
“Descent with modification”
Proposed by Darwin
Modification of structures to take on new functions
Vestigial structures= remnants of structures that may have served an important function in an
ancestral species, but have no clear function in some of the modern descendents
Often smaller in size
Ballene whale
Developmental similarities
Embryos of closely related species have similar stages of development
Bones of the skeleton form in a common pattern
The process of comparing how certain structures develop in different organisms is comparative
embryology.
Molecular biology
Comparison of DNA sequences between species
If the two species’ sequences match closely then it is thought that the two species are related to a
common ancestor
If the two species’ sequences have many differences they probably do not share common ancestry
Comparison of Hemoglobin
Artificial selection= selective breeding of domesticated plants and animals to produce offspring with
genetic traits that humans value
Munchkin cats
Artificial Selection
In contrast, natural selection favors traits that are beneficial to the organisms in their environment
The environment does the “selective breeding”
Resulting in evolutionary adaptation
Natural Selection of Flies
Darwin could not explain how variations passed from one individual to the next
Gregor Mendel to the rescue!!
Microevolution…a change in a population’s gene pool
Gene pool= consists of all the alleles in all the individuals that make up a population
Where genetic variation is stored
Example: Wild mustangs
What leads to genetic variation?
Mutations
Sexual recombination
Natural selection is not random
Why?
The environment favors combinations of genes that contribute to survival and reproductive success
Some alleles may become more common in a gene pool than others
Frequency of alleles= how often certain alleles occur in the gene pool
Usually expressed as a percentage
Microevolution
Blending of Mendel’s and Darwin’s theories to look at evolution based on genetics
Generation to generation changes (smallest scale)
Microevolution
What causes gene pools to change (besides natural selection)?
1. Genetic drift= a change in the gene pool of a population due to chance
All populations are subject to genetic drift
Bottleneck effect…a drastic reduction in the size of a population
Usually by natural disaster…and only a few remaining individuals are left to start a new population
•
Ex: cheetah population in Africa
Founder effect…genetic drift in a new colony
When a few individuals leave the original population and start a new population
Genetic Drift
2. Gene flow= movement of alleles between populations
Occurs when fertile individuals mate with individuals from other populations
Ex: a wind storm might blow pollen from a population of only red flowers to a population consisting
of only white flowers
Reduces the genetic differences between populations
Can eventually mix the two populations
So how did humans evolve?
A very lengthy process in which people originated from apelike ancestors.
The evolution of humans has occurred over the last 6 million years.
Bipedalism – walking on 2 legs – 4 million years ago
Most advanced traits have occurred over the last 100 million years.
•
Apes have a very large, very strong lower jaw to support their carnivorous eating.
•
As the shape of the human skull evolved to have a smaller lower jaw, this allowed for a larger
brain to develop.
•
With a larger brain, the capacity for language and the manufacture of tools were able to be
developed.