Transcript presentation source

Introduction to
History of Life
Biological evolution consists of change in the hereditary
characteristics of groups of organisms over the course of
generations.
• Groups of organisms, termed populations and species, are
formed by the division of ancestral populations or species,
and the descendant groups then change independently.
• From a long-term perspective, evolution is the descent,
with modification, of different lineages from common
ancestors.
• The history of evolution has two major components:
1) branching of lineages and 2) changes within lineages
Evolutionary theory is a body of statements about the
processes of evolution that are believed to have caused
the history of evolutionary events.
• Biological evolution occurs as the consequence of
random and nonrandom processes.
Random Processes
Mutation
• Variation in the characteristics of organisms in a population originates through
random mutation of DNA sequences (genes) that affect the characteristics.
• Genetic variation is augmented by recombination during sexual reproduction,
which results in new combinations of genes.
• Variation is also augmented by gene flow, the input of new genes from other
populations.
Genetic Drift
Results from random variation in the survival and reproduction of different
genotypes.
The frequencies of alleles fluctuate by pure chance, eventually one allele will
replace the others
Genetic drift is most important when the alleles of a gene are neutral—that is,
when they do not substantially differ in their effects on survival or
reproduction—and it proceeds faster, the smaller the population is.
Nonrandom Processes
Natural selection
• Any consistent (nonrandom) difference among organisms bearing different
alleles or genotypes in their rate of survival or reproduction (i.e., their fitness)
due to differences in one or more characteristics.
• In most cases, environmental circumstances affect which variant has the
higher fitness.
• A common consequence of natural selection is adaptation, an improvement
in the average ability of the population's members to survive and reproduce in
their environment.
• Natural selection tends to eliminate alleles and characteristics that reduce
fitness (such as mutations that cause severe birth defects in humans and other
species), and it also acts as a "sieve" that preserves and increases the
abundance of combinations of genes and characteristics that increase fitness,
but which would occur only rarely by chance alone.
Speciation
• If gene flow among different geographically separated populations is slight,
different genetic changes can transpire in those populations.
• Because the populations experience different histories of mutation, genetic
drift, and natural selection, they follow different paths of change, diverging in
their genetic constitutions and in the individual organisms' characteristics
• The differences that accumulate eventually cause the different populations to
be reproductively isolated; the different populations are now different species.
• The significance of this process of speciation is that the new species are
likely to evolve independently from then on.
• Some may give rise to yet other species, which ultimately may become
exceedingly different from one another.
• Successive speciation events, coupled with divergence, give rise to clusters of
branches on the phylogenetic tree of living things.
What is Microevolution and Macroevolution?
Microevolution refers to any evolutionary change below the level of
species (e.g., changes in the frequency within a population)
Macroevolution is used to refer to any evolutionary change at or
above the level of species.
• It means the splitting of a species into two (cladogenesis) or the
change of a species over time into another (anagenesis)
• Any changes that occur at higher levels, such as the evolution of new
families, phyla or genera, is also therefore macroevolution, but the term
is not restricted to the origin of those higher taxa.
• Another way to state the difference is that macroevolution is betweenspecies evolution of genes and microevolution is within-species
evolution of genes.
Conclusions: Distinguishing Patterns from Processes
• The history of evolution—the proposition that all species have
descended, with modification, from common ancestors— is supported by
overwhelming evidence
• The body of principles that describe the causal processes of evolution,
such as mutation, genetic drift, and natural selection, constitutes the theory
of evolution.
• Many of the details of the history of evolution remain to be described
• Also, the theory of evolution, like all scientific theories, continues to
develop as new information and ideas deepen our understanding.
• The major causes of evolution have been identified; however, views on
the relative importance of the various processes continue to change as new
information adds detail and modifies our understanding.