Evolution - Chapter 8 Flashcards
Microevolution
microevolution = changes in allele frequencies and phenotypic traits within a population and species
not all evolutionary changes are the result of natural selection
evolution can occur due to catastrophic event
Natural selection
organisms that are homozygous for harmful alleles are less likely to survive and produce offspring than those that do not carry such genes
desirability of alleles depends on the local environment
harmful alleles do not disappear over time due to recurring mutations and the presence of heterozygous individuals
Stabilizing selection
intermediate phenotypes are favoured, and the traits that lie outside the norm are selected against
improves adaptation of the population to aspects of the environment that stay constant
example = human birth weight
Disruptive selection
both extreme phenotypes are favoured
example = british land snails can have dark or light shells, both forms can camouflage in their habitat, but the intermediate forms are most likely to be eaten by birds
Directional selection
one extreme phenotype is favoured
shift away from current average condition due to environment change, or new to environment
example = peppered moth
Sexual selection
males of some species often acquire frivolous characteristics to impress females
leads to sexual dimorphism (physical differences in appearance of males and females)
Genetic drift
change in the frequency of alleles due to random events
in small populations, the likelihood of losing certain alleles due to random/chance events is much higher than in large populations
Bottleneck effect
occurs when a population is subjected to near extinction
chance alone may determine which individuals survive the unfavourable times
many alleles are lost from the population, so the genetic makeup of future generations will be limited to the alleles carried by the survivors
Founder effect
when a few individuals from a large population establish a new population
alleles are found at a different frequency in an isolated population
the smaller the founding group, the less likely it will represent the genetic makeup of the original population
Gene flow
can increase variation in a population by introducing novel alleles from some other populations
unlike genetic drift, gene flow tends to reduce differences within populations
Mutations
may be small changes to a gene, or larger changes to a chromosome
larger duplications can lead to sections of the chromosome that are free to mutate and gain new function, as their original function is already taken care of
Speciation
the formation of a new species
isolation of a group of individuals from the whole population and a long period of time are key
most common forms of isolation are reproductive or geographic
Reproductive isolation
biological factors prevent two populations from interbreeding, despite sharing the same region or habitat
prezygotic isolation prevents gametes from ever meeting
postzygotic isolation prevents maturation and/or reproduction in offspring
Prezygotic isolation
ecological or habitat isolation seasonal isolation behavioural isolation mechanical isolation physiological isolation gametic mortality isolation
Postzygotic isolation
zygotic mortality
hybrid inviability
hybrid sterility
hybrid breakdown
Geographical isolation
when a single population of a species is separated into two or more isolated populations
2 modes of speciations based on geographical relationship of the new species to its ancestral species
Allopatric speciaion
speciation as a result of geographical isolation
gene flow is blocked by a geographical barrier
Sympatric speciation
speciation while still sharing the same geographical area
gene flow is blocked by genetic change
Divergent
species that were once similar diverge or become increasingly distinct
different selective pressures or random genetic flow
when divergent evolution occurs rapidly, it is called adaptive radiation
Convergent
similar traits arise because different species have independently adapted to similar environmental conditions
examples = bats and birds, ostriches and emus
Co-evolution
the evolution of one species influences another
can lead to dependence of one species on the other
Macroevolution
large scale evolutionary change significant enough to warrant the classification of groups into genera or even higher taxa level
example = Cambrian explosion
2 major theories exist for macroevolution
Theory of gradualism
the accumulation of many small and ongoing changes and processes
when new species first evolve, they appear very similar to the original species and only become more distinctive as natural selection and genetic drift act on both species
one would expect then to find many transitional species in the fossil record
lack of transitional species is explained by an incomplete fossil record, and the possibility that intermediate forms were not preserved
Theory of punctuated equilibrium
relatively rapid spurts of change followed by long periods of little or no change
species evolve very rapidly in evolutionary time
speciation usually occurs in small isolated populations and thus intermediate fossils are very rare
after the initial burst of evolution, species do not change significantly over long periods of time
Theory that is used today
both gradual and rapid evolutionary processes are at work, as both are needed to understand the fossil record
before a major extinction event, an environment may be host to many well adapted species, so evolutionary change would be slow
environmental crisis results in extinction of many species, leaving many niches empty
surviving species experience disruptive selection
once they become well adapted, stabilizing selection restarts
Traits that have contributed to human success
complex reasoning and communication ability to learn and use tools large brain bipedalism hands that are capable of coordination
Clues on the evolution of humans
skulls = demonstrates brain capacity (intelligence), the memory functions and communication abilities
hip bones = demonstrates the shift to bipedalism, a more energy efficient way of walking for long distances
stone tools = demonstrates the beginning of use of technology, improved eating/hunting abilities
footsteps = demonstrates that human ancestors walked upright long before they had large brains
Darwinian medicine
use of evolutionary theory to understand medicine
a fever is an evolved response to help fight off infection = taking a drug to lower body temperature might be unwise
coughing is an evolved mechanism to expel disease-causing organisms from lungs = taking medication that suppresses coughs might not be helpful
Hardy-Weinberg problems
allele frequencies will remain constant as long as =
no mutations occur in the alleles
alleles do not migrate into or out of the population
the population is large
mating is random
all possible offspring are equally like to survive
