MECHANISMS AND PATTERNS OF EVOLUTION
Evolution What is it again? Evolution is the change in allele frequencies of a population over generations Mechanisms of Evolution what can make evolution happen? 1. Mutation 2. Recombination (gene shuffling) 3. Natural Selection 4. Genetic Drift 5. Gene flow (migration)
Evolution of Populations Genes and Variation Gene Pool- combined genetic information of all members of a particular population Relative Frequency the number of times the allele occurs in the gene pool.
Sources of Genetic Variation- biologist have discovered two main sources 1. Mutations- change in a sequence of DNA. May affect an organisms fitness (it s ability to survive and reproduce in its environment) 2. Gene shuffling (crossing over) - most caused during production of gametes. (sexual reproduction major source of variation within many populations)
3. Natural selection Individuals with traits best suited to their environment survive and reproduce more successfully. Natural Selection does not act directly on genes. It acts on phenotypes. (it can change the relative frequencies of alleles in the population over time)
A. Polygenic Traits traits controlled by two or more genes i. Height, skin color, eye color are all examples in humans ii. A bell shaped curve is typical of polygenic traits
B. The FITNESS of individuals near each other will not be very different, but fitness may vary from one end of the curve to the other Where fitness varies, NATURAL SELECTION can act!
C. Natural selection can affect the distribution of phenotypes in 3 ways i. DIRECTIONAL Selection ii. STABILIZING Selection iii. DISRUPTIVE Selection
i. Directional Selection Low mortality, high fitness High mortality, low fitness Individuals at one end of the curve have higher fitness than individuals in middle or at other end Graph shifts as some individuals fail to survive at one end and succeed and reproduce at other
EXAMPLE OF DIRECTIONAL SELECTION Beak size varies in a population Birds with bigger beaks can feed more easily on harder, thicker shelled seeds. Suppose a food shortage causes small and medium size seeds to run low. Birds with bigger beaks would be selected for and increase in numbers in population.
ii. Stabilizing Selection Individuals in the center of the curve have higher fitness than individuals at either end Graph stays in same place but narrows as more organisms in middle are produced.
Examples of Stabilizing Selection Male birds use their plumage to attract mates. Male birds in the population with less brilliant and showy plumage are less likely to attract a mate, while male birds with showy plumage are more likely to attract a mate. Key Brightness of Feather Color Stabilizing Selection Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Male birds with showier, brightlycolored plumage also attract predators, and are less likely to live long enough to find a mate. The most fit, then, is the male bird in the middle-- showy, but not too showy.
Examples of Stabilizing Selection (2) Human babies born with low birth weight are less likely to survive. Babies born too large have difficulty being born. Average size babies are selected for.
iii. Disruptive Selection Individuals at extremes of the curve have higher fitness than individuals in middle. Can cause graph to split into two. Selection creates two distinct PHENOTYPES
Example of Disruptive Selection Suppose bird population lives in area where climate change causes medium size seeds become scarce while large and small seeds are still plentiful. Birds with bigger or smaller beaks would have greater fitness and the population may split into TWO GROUPS. One that eats small seeds and one that eats large seeds.
4. Genetic Drift- Random change in allele frequencies in a population, due to chance This may occur when a small group of individuals colonizes a new habitat. Individuals may carry alleles in different relative frequencies than the original larger population.
Genetic drift has been observed in some small human populations that have become isolated due to reasons such as religious practices and belief systems. For example, in Lancaster County, Pennsylvania, there is an Amish population of about 12,000 people who have a unique lifestyle and marry other members of their community. By chance, at least one of the original 30 Amish settlers in this community carried a recessive allele that results in short arms and legs and extra fingers and toes in offspring. Because of small gene pool, many individuals inherited the recessive allele over time. Today, the frequency of this allele among the Amish is high (1 in 14 rather than 1 in 1000 in the larger population of the U.S.)
5. Gene Flow- Movement of alleles in or out of a population (migration of individuals from group to group) Will change the allele frequencies of a population
B. Evolution Versus Genetic Equilibrium 1. Hardy-Weinberg Principle- states that allele frequencies in population will remain constant unless one or more factors cause those frequencies to change. There are 5 conditions to maintain genetic equilibrium, otherwise populations will evolve
Conditions that maintain Genetic Equilibrium (no evolution). ALL MUST BE MET! a. Random mating- populations breed randomly (mates not chosen as with humans) b. Large populations- genetic drift has less affect c. No migrations (in or out) gene pool kept together d. No mutations- if genes mutate from one form into another, new alleles may be introduced- changing frequencies of alleles e. No natural selection- no phenotype can have selective advantage over another.
Gene Frequency & the Hardy-Weinberg Equation The Hardy-Weinberg Equation p 2 + 2pq + q 2 = 1 p 2 = the frequency of homozygous dominant genotype 2pq = the frequency of heterozygous genotype q 2 = the frequency of homozygous recessive genotype
HARDY-WEINBERG PRINCIPLE = allele frequency in a population will remain constant unless one or more factors cause the frequency to change. In a situation in which allele frequencies remain constant ( = genetic equilibrium ) populations will NOT EVOLVE! It is difficult to meet all conditions of Hardy Weinberg so in most populations, EVOLUTION HAPPENS!
C. The Process of Speciation (formation of a new species) 1. Isolating mechanisms a. Reproductive isolation- Prevents two populations from interbreeding with each other and exchanging genes. This can develop in variety of waysbehavioral isolation, geographic isolation, temporal isolation The Eastern and Western Meadowlark have overlapping ranges but do not interbreed, because they have different mating songs
b. Behavioral isolation- don t interbreed because of differences in courtship rituals or other types of behavior c. Geographic isolation- two populations separated by geographic barriers (rivers, mountains, bodies of water) d. Temporal isolation- when two species reproduce at different times
Large scale evolutionary patterns and processes that occur over long periods of time = Macroevolution 1. Mass extinction 2. Adaptive radiation (Divergent evolution) 3. Convergent evolution 4. Coevolution 5. Punctuated Evolution
Mass Extinctions At several times in Earth s history large numbers of species became extinct at the same time Caused by several factors: erupting volcanoes Plate tectonics (continents were moving) Changing sea levels Asteroids hitting the Earth Global climate change EX/At the end of the Mesozoic Era more than HALF of all plants and animals were wiped out including the dinosaurs
Mass Extinctions Effects of mass extinctions: Opens habitats and provides opportunities for remaining species After mass extinctions there is often a burst of evolution that produces many new species EX: Cenozoic era that followed = Age of Mammals Mammals species increased dramatically
When a single species or small group of species has evolved through natural selection into diverse forms that live in different ways = adaptive radiation OR divergent evolution Ex: Galápagos finches More than a dozen species evolved from one species
Sometimes different organisms evolution in different places or at different times but in ecologically similar environments and end up looking very similar. Process by which unrelated organisms come to resemble each other = convergent evolution
The process by which two species evolve in response to changes in each other over time = coevolution
How fast does evolution operate? Darwin believed evolution happened slowly over a long time If biological change is at a slow pace, it is called gradualism.
Fossil record shows evolution happens more in bursts. Pattern of a long stable period interrupted by a brief period of more rapid change = Punctuated Equilibrium