Patterns of Evolution

Patterns of Evolution Measuring the amount of change… Macroevolution: large evolutionary changes; takes a long time (ex: Reptiles evolving into birds) Microevolution: small evolutionary changes (ex: the peppered moth changing color)

Macroevolution: Is it Possible?

Patterns of Evolution Measuring the amount of time it takes… Gradualism: evolution as the slow accumulation of many small changes (ex – sharks today are basically the same as they were before the dinosaurs) Punctuated Equilibrium: sudden change in a group after years of no change (ex- mammals evolved very quickly to become large after the dinosaurs went extinct)

Patterns of Natural Selection

Natural Selection on Single Gene Traits Reminder: natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution

Natural Selection on Polygenic Traits Natural selection can affect a population in any of three ways: (leave space between each to draw a small graph!!!) 1. Directional Selection – one extreme is favored (ex: smallest survive best) 2. Stabilizing Selection – the average trait is favored (ex: medium survive best) 3. Disruptive Selection – both extremes are favored (ex: smallest AND largest survive best)

Directional Selection - individuals at one end of the curve have higher fitness than those in the middle/the other end - the entire curve moves as the character trait changes - evolution causes an increase in the # of individuals with the trait at one end of the curve

Stabilizing Selection - when individuals near the center of the curve have higher fitness than individuals at the ends - the center of the curve at its current position, but it narrows the overall graph

Disruptive Selection - the individuals at the upper and lower ends of the curve have higher fitness that individuals near the middle - acts most strongly against individuals of an intermediate type

Divergent Evolution Disruptive Natural Selection often leads to… divergent evolution: when two related groups develop more and more differences examples: polar bear – white fur for camouflage grizzly bear – brown fur for camouflage If the disrupted population is completely split , it leads to speciation – the formation of new species.

Divergent Evolution Careful! Don’t confuse divergent with convergent evolution… convergent evolution: when two UNRELATED species evolve similarities ONLY because they live in the same habitat and NOT because they are related (ex: sharks and dolphins)

Speciation

Isolating Mechanisms Definition of “Species” = group of similar individuals that can successfully and naturally interbreed - Preventing individuals from interbreeding (isolating) can cause speciation; the following can cause reproductive isolation 1. Behavioral Isolation – not attracted to one another (ex: different bird songs) 2. Geographic Isolation – don’t live in the same area (ex: island species are separated from the mainland) 3. Temporal Isolation – reproductive timing is off (ex: reproducing with the full moon instead of the half moon)

Behavioral Isolation Two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior

Geographic Isolation Two populations are separated by geographic barriers (rivers, mountains, bodies of water)

Temporal Isolation Two or more species reproduce at different times

Careful! Speciation can split a population into more than just two groups Adaptive radiation: one ancestral group gives rise to many different species all at once (ex- Darwin’s finches evolved from the same ancestor, but were adapted to MANY different food sources) Temporal Isolation

Organizing Evolution

Phylogenetic Trees Timeline/family tree of evolution created using ALL the evidence of evolution/relatedness we can find; the closer together two species are on the tree, the more closely related they are

Cladogram Chart of species based on derived (shared) characteristics only; just used to show one type of evidence, like physical similarities; every species below the character lacks that trait, every species above the character has it