SOC111 Introduction to Anthropology

EVOLUTION AND GENETICS

Evolution and Genetics

• What is evolution, and how does it occur?

• How does heredity work, and how is it studied?

• What forces contribute to genetic evolution?

Evolution

• Humans have uniquely varied ways—cultural and biological—of adapting to environmental stresses

• Many scholars became interested in biological diversity and our position within the classification of plants and animals during the 18th century

Evolution• Creationism: biological similarities and

differences originated at the Creation• Linnaeus (1707–1778) developed

the first comprehensive and still influential classification, or taxonomy, of plants and animals

• He grouped life forms on the basis of differences and similarities in physical characteristics.– Fossil discoveries during the 18th and 19th centuries

raised doubts about creationism

• If all life originated at the same time, why aren’t ancient species still around?

• Why weren’t contemporary plants and animals found in fossil records?

• Catastrophism: a modified version of creationism that accounts for the fossil record by positing divinely authored worldwide disasters that wiped out creatures represented in the fossil record.

Theory and Fact

• Evolution (descent with modification): an alternative to creationism and catastrophism was transformism

• Species arise from others through a long and gradual process of transformation.

• Charles Darwin is the

best known of evolutionists

• Darwin influenced by 2 people:– Grandfather, Erasmus Darwin, who

proclaimed a common ancestry of all animal species

– Lyell’s principle of Uniformitarianism: The present is the key to the past; explanations for past events should be sought in the long-term action of ordinary forces that still operate today.

• Natural forces (rain, earthquakes, volcanic eruptions) have gradually built and modified geological features (such as mountain ranges).

• Cappadocia

–Uniformitarianism cast doubt on the belief that the world was only 6000 years old. It would take much longer for ordinary forces such as rain and wind to produce major geological changes.

– Theory of evolution• Theory: a set of ideas formulated to explain

something

Darwin applied uniformitarianism and long-term transformation to living things and argued that all life forms are related and the number of species have increased over time.

• Natural selection: the process by which the forms most suited to survive and reproduce in a given environment do so in greater numbers.

• More than survival of the fittest, natural selection is differential reproductive succes.

• When there’s a food shortage, those with longer necks have an advantage

• If this feeding advantagepermits longer-necked giraffes to survive and reproduce more effectively, then they will transmit more of their genetic material to future generations than will those with shorter necks.

Evolution

• Physical development of the individual, not species. (babies of weight lifters)

• Evolution works as the process of natural selectiontakes advantage of the variety that is already present in a population.

Genetics

• Genetic science helps us understand the causes of biological variation– Mendelian genetics: studies the

ways in which chromosomes transmit genes across generations

– Biochemical genetics: examines structure, function, and changes in DNA

– Population genetics: investigates natural selection and the causes of genetic variation, stability, and change

Mendel’s experiments

• Austrian monk Gregor Mendel began a series of experiments that revealed the basic principle of genetics in 1856–Studied inheritance of seven

contrasting traits in pea plants–Discovered that heredity is determined

by discrete particles or units

Mendel’s experiments

• Concluded that a dominant form could mask another form in hybrid individuals, without destroying the recessive trait

• Basic genetic units Mendel described were factors (now called genes or alleles) that are located on chromosomes

Mendel’s experiments

• Chromosome: a paired length of DNA, composed of multiple genes

• Gene: a place (locus) on a chromosome that determines a particular trait

• Allele: a variant to a particular gene

Mendel’s experiments

• Independent Assortment and Recombination– Independent Assortment: chromosomes

are inherited independently of one another– Recombination: the combination of genetic

traits in an offspring; this process creates new types on which natural selection can operate

Biochemical, or Molecular, Genetics

• Mutation: changes in the DNA molecules of which genes and

chromosomes are built• Gametes: sex cells that

make new generations

Biochemical, or Molecular, Genetics

• DNA: initiates and guides the construction of hundreds of proteins necessary for bodily growth, maintenance, and repair.

Cell Division

• Mitosis: ordinary cell division, wherein one cell splits to form two identical cells

• Meiosis: the special process by which sex cells are produced

– Four cells produced from one

– Each cell carries half the genetic material of the original cell

– Fertilization allows the products of meiosis from one parent to recombine with those from the other parent

– Chromosomes sort independently

Crossing Over

• Crossing over: the process wherein homologous chromosomes exchange segments by breakage and recombination– Can occur with any chromosome pair– An important source of variety

Mutation

• Base substitution mutation: substitution of one base in a triplet by another– If mutation occurs in a sex cell, the new

organism will carry mutation in every cell

• Chromosomal rearrangement: pieces of a chromosome break off and reattach someplace else on that chromosome

Mutation

• Approximately three mutations will occur in every sex cell

– Most mutations are neutral– Evolution depends on mutations– Variants produced through mutation

can be especially significant if there is a change in environment

Population Genetics and Mechanisms of Genetic Evolution

• Population genetics studies stable and changing populations

– Gene pool: alleles and genotypes within breeding population

– Genetic evolution: the change in allele frequency in a breeding population

Natural Selection

• Genotype: the genetic makeup of an organism

• Phenotype: an organism’s evident biological traits– Natural selection acts only on phenotypes– Human biology has considerable plasticity – The environment works on

a genotype to build a phenotype

3 Different Ways of Natural Selection

1.Directional Selection2.Sexual Selection

3.Stabilizing Selection

Directional Selection

• After several generations of selection, gene frequencies change– Adaptive: favored by natural selection – Directional selection continues as long

as environmental sources stay the same– Humans do not have to delay adaptation until

a favorable mutation occurs

Sexual Selection

• Selection also operates through competition for mates

• Sexual selection: based on differential success in mating; a selection of traits that enhances mating success

Stabilizing Selection

• Balanced polymorphism: the frequencies of two or more alleles of a gene remain constant from generation to generation

Random Genetic Drift

• Random genetic drift: loss of alleles from a population’s gene pool through chance

– Lost alleles can reappear in the gene pool only through mutation

– Fixation, the replacement, for example, of blue eyes by brown eyes, is more rapid in small populations

Gene Flow

• Gene flow: exchange of genetic material between populations of the same species– Alleles spread through gene flow even when

selection not operating on the allele– Species: group of related organisms whose

members can interbreed to produce offspring that live and reproduce

– Gene flow tends to prevent speciation: the formation of new species

RECAP OF LAST CLASS

• In the 18th century, Carolus Linnaeus developed biological taxonomy. He viewed similarities and differences among organisms as part of God’s orderly plan rather than as evidence for evolution.

• Charles Darwin proposed that natural selection could explain the origin of species, biological diversity and similarities among related life forms. Natural selection requires variety in the population undergoing selection.

RECAP OF LAST CLASS

• Through breeding experiments with peas in 1856, Gregor Mendel discovered that genetic traits pass on as units which are known as chromosomes. He also formulated the law of independent assortment. Each of seven traits he studied in peas was inherited independently of all others. Independent assortment of chromosomes and their recombination provide some of the variety needed for natural selection. But the major source for such variety is mutation, an alteration in the DNA molecules of which genes are made.

RECAP OF LAST CLASS

• Genetics studies structure, function and changes in genetic material-DNA.

• Genetic changes that provide variety within a population include base substitution mutations, chromosomal rearrangements and genetic recombination.

• Natural selection is the most important mechanism of evolutionary change. Others include random genetic drift and gene flow.

• Natural selection work for characteristics already present in a population. If variety is insufficient for adaptation to environmental change, extinction happens.

The Modern Synthesis

• Currently accepted view of evolution:

– Microevolution: small-scale changes in allele frequencies over just a few generations

– Macroevolution: large-scale changes in allele frequencies in a population over a longer time period that result in the formation of new species.

The Modern Synthesis

• Punctuated equilibrium: long periods of stasis may be interrupted by evolutionary leaps

– Sudden environmental change offers possibility for the pace of evolutions to speed up

– Although species can survive radical environmental shifts, extinction is more common

Extinctions of Earth

• The earth has witnessed several mass extinctions.

• The biggest one Palezoic Period to

Masezoic Period 245 million years ago5 million species on earth 4.5 m. species died

Extinctions of Earth

• The second biggest extinction– 65 million years ago– Extinction of dinasours– One explanation: long-lasting cloud of gas and

dust arose from the impact of a giant meteorite at the end of the mesozoic period. The cloud blocked solar radiation and photosynthesis, ultimately destroying more plants and the chain of animals that fed on them.