Transcript Lecture 17
Evolution Macroevolution/Speciation Macroevolution • Macroevolution is the occurrence of large-scale changes in the characteristics (traits) of life -- in effect, the evolution of species and higher taxa. • It is distinct from microevolution, which is the sequence of changes that occur within a single population. • According to Charles Darwin and also the modern synthesis, this distinction is relative and purely a matter of scale. – The modern evolutionary synthesis (often referred to simply as the modern synthesis), brings together Charles Darwin's theory of the evolution of species by natural selection with the theory of genetics as the basis for biological inheritance. Proposed Mechanisms for Macroevolution • Extrapolation of microevolutionary processes......Gradualism. • The second way in which macroevolution is believed to occur is through sudden and rapid changes......Punctuated Equilibrium. Tiny microevolutions, over sufficient time, add up and accumulate in isolated populations and eventually result in new species. Based on the fact that there are critical genes in all living organisms, and a small change in them could cause drastic changes in the organism, resulting in a new species quite rapidly. Too long of a time scale! • Whilst speciation events have been demonstrated in the laboratory and observed in the field, really dramatic differences between species do not usually occur in directly observable timescales (it occurs too quickly for the process to be shown in the fossil record.) • It is argued that, since macroevolution can not be confirmed by a controlled experiment, it cannot be considered to be part of a scientific theory. • However, evolutionists counter that astronomy, geology, archaeology and the other historical sciences, like macroevolution, have to check hypotheses through natural experiments. • They confirm hypotheses by finding out if they conform or fit with the physical or observational evidence and can make valid predictions. In this way, macroevolution is testable and falsifiable. How is it studied then? • Classical evidence of macroevolutionary processes – fossils (extinction and radiation) – biogeography (global taxa distribution) – comparative biology • Phenetics and cladistics: tree-building • Molecular evolution and molecular phylogenetics The tree of life: stems and leaves extant spp biogeographic/ geological events extinct spp • We need to infer the shape of the tree (evolutionary relationships) from leaves (extant taxa) and branches (extinct or fossil taxa), and from knowledge of geological events ... 1. What was present, where? (Fossils) 2. What happened, when? (Geology, palaeoclimatology) 3. What is present, where? (Biogeography) Determining evolutionary relationships • Understanding structure similarities and differences – Homologous structures from a common ancestor based on descent – Analagous structures with similar functionality but not dervied from common ancestor – Convergent and divergent evolution • Derived characters vs. Ancestral How is it studied then? • Classical evidence of macroevolutionary processes – fossils (extinction and radiation) – biogeography (global taxa distribution) – comparative biology • Phenetics and cladistics: tree-building • Molecular evolution and molecular phylogenetics Comparative Biochemistry • Many genes are found in all living things, because we all use similar metabolism. • Genes can also be described as homologous and analogous. • Considering homologous genes, the genes of closely related species are or similar than genes from more distantly related species. Increasing time since the divergence of two species gives increasing numbers of random mutations. • Cytochrome C is part of the electron transport system in the mitochondria. It is found in all eukaryotes, and some aerobic prokaryotes as well. The number of amino acid differences between the cytochrome c found in different species is proportional to the time since they diverged. Speciation • Speciation refers to the appearance of a new species of life on earth. • Species occur when a parent species (also referred to as a common ancestor) splits into two (or more) reproductively-isolated populations – each of which then accumulates changes from sexual reproduction and/or random mutation (in addition to any other various contributors to genetic change).......... • until the populations are no longer capable of interbreeding. Macroevolution: Speciation • Process whereby new species arise • Generally one of two conditions required: – Geographic barriers – Reproductive isolation • Cumulative process of drift and selection acting over many generations Reproductive Isolation Mechanisms (Pre-zygotic (before mating) vs. post-zygotic (after mating) • Pre-zygotic – different breeding seasons (either yearly or time of day) – different signals: sight, sound, smell, behavior – mechanical problems—genitalia don’t fit together – gametic isolation: sperm don’t detect the egg or can’t fertilize it • Post-zygotic – hybrid inviability: hybrids don’t survive to birth or adulthood – hybrid sterility: as with the mule, a sterile hybrid. • Even having hybrids that are significantly less fit (able to survive and reproduce) than purebreds is an isolating mechanism, a way to prevent the two populations from fusing into one population. horse (fert.) donke y (fert.) mule (sterile) Allopatric Speciation: The Great Divide • The simplest and most common form of speciation – 2 groups of one species are isolated geographically (mountains, oceans, rivers)..... – and diverge into separate species. • Or, the barrier develops slowly as conditions change: the gradual formation of the Grand Canyon split a population into 2 isolated groups, that have diverged into separate species, the Kaibab and Albert squirrels. Sympatric Speciation • Sympatric speciation means speciation that occurs within the same geographical location. • An example: cichlid fish in Lake Barmobi Mbo in Cameroon, Africa—an isolated volcanic lake. – Nine species, all more closely related to each other (by DNA evidence) than to similar fish in other lakes. – Lake has no distinct geographical zones, and the fish can easily swim anywhere in it. – They feed in different locations, but all breed in the same location, close to the bottom. – The mechanism is not clear. Rates and results of speciation • Isolation, low dispersal, strong sexual selection and reductions in population size all favor speciation • Speciation gives rise to adaptive diversity • According to the theory of punctuated equilibria speciation has been the major source of adaptive change as opposed to phyletic gradualism