Fossils 

In the century prior to Darwin, fossils suggested that species had indeed changed over time

organisms evolve by the use and disuse of body parts and 

these acquired characteristics are passed on to offspring.

So for instance he suggested giraffes got their long necks because the ancestors of giraffes had lengthened their necks by stretching higher and higher into the trees to reach the leaves

suggesting that natural forces

gradually changed Earth and 

are still operating today

noting that as organisms spread into various habitats over millions of years, they accumulated diverse ? that fit them to specific ways of life in these new environments.

organisms with traits that increase their chance of surviving and reproducing in their environment tend to leave more offspring than others and 

this unequal reproduction will lead to the accumulation of favorable traits in a population over generations.

Individuals do not evolve: populations evolve.

Natural selection can amplify or diminish only heritable traits. Acquired characteristics cannot be passed on to offspring.  

Evolution is not goal directed and does not lead to perfection. Favorable traits vary as environments change.

Natural selection is more of an editing process than a creative mechanism.

A pesticide does not create an alleles that allows insects to resist it.  The presence of the pesticide leads to natural selection for insects already in the population that already have the alleles.

Natural selection is contingent on time and place, favoring those characteristics in a population that fit the current, local environment

Comparative anatomy

is the comparison of body structures in different species,

was extensively cited by Darwin, and

illustrates that evolution is a remodeling process

Comparative embryology

is the comparison of early stages of development among different organisms and

reveals homologies not visible in adult organisms.

For example, all vertebrate embryos have, at some point in their development,

a tail posterior to the anus and

pharyngeal throat pouches.

all life-forms are related,

all life shares a common DNA code for the proteins found in living cells, and 

humans and bacteria share homologous genes that have been inherited from a very distant common ancestor.

Advances in molecular biology reveal evolutionary relationships by comparing DNA and amino acid sequences between different organisms. These studies indicate that

changes in the nucleotide sequence of DNA and

the ultimate source of new alleles

Homologous chromosomes sort independently as they separate during anaphase I of meiosis.

During prophase I of meiosis, pairs of homologous chromosomes cross over and exchange genes.

Further variation arises when sperm randomly unite with eggs in fertilization

Sexual reproduction shuffles alleles to produce new combinations in three ways:

natural selection,

genetic drift, and

gene flow.

The three main causes of evolutionary change are

Natural selection

If individuals differ in their survival and reproductive success, natural selection will favor those individuals best suited for their particular environment.

Occurs when chance events cause allele frequencies to fluctuate unpredictably from one generation to the next.

The smaller the population, the more impact genetic drift will have.  An allele can be lost from a small population by such chance fluctuations.

is a drastic reduction in population size that leaves a small surviving population that is unlikely to have the same genetic makeup as the original population.

It may be caused by natural disasters such as fire, earthquakes, floods, or for many species today by human actions.

ultimately doom a population that is later faced with widespread disease.

The loss of genetic diversity in a population due to bottlenecks is a significant problem in conservation.

 When a species is reduced to relatively few individuals, and then is brought back to abundance by extraordinary efforts, the species is not fully recovered. 

The lost genetic diversity may be a prerequisite for the long-term survival of the species.

when a few individuals colonize an island or new habitat.

A small group cannot adequately represent the genetic diversity in the ancestral population.

The frequency of alleles will therefore be different between the old and new populations.

Imagine if all the students present in today’s class are the only survivors of some global catastrophe. Would this class adequately represent the biological diversity of the current human population?

explains the relatively high frequency of certain inherited disorders among some human populations established by small numbers of colonists.

An example is retinitis pigmentosa, a progressive form of blindness and an autosomal recessive disorder, in the descendants on Tristan da Cunha (group of small islands in the middle of the Atlantic Ocean.)

The incidence of the eye disease allele on these islands is 10X greater than in the British population from which the founders came.

Gene flow 

Gene flow tends to reduce differences between populations.

produce the largest number of viable, fertile offspring and

pass on the most genes to the next generation

The fittest individuals are those that

Evolution does not scrap ancestral anatomy and build each new complex structure from scratch; it co-opts existing structures and adapts them to new situations.

Birds and bats evolved from four-legged ancestors – their forelimbs became wings, leaving two limbs for walking.

The same structure often performs many functions.

Example: a blue-footed booby uses its webbed feet to swim after prey in the ocean, but these same feet are clumsy to walk on land.

Most species that have ever lived are now extinct. 

Species do not have a mechanism to obtain what they need to survive. Instead, preexisting variation from preceding generations and new variations that emerge from mutation and genetic recombination provide the raw materials for survival. 

No amount of need can influence the variety upon which selection must act. It is therefore possible that the variety in any generation may be insufficient for survival, particularly during periods of relatively fast environmental change.