Lec 11 slides and reading Flashcards
how can complex adaptations evolve?
In the evolution of organisms and their complex machinery, many aspects are modified (adapted) versions of pre-existing structures
Just as in biological evolution, many early designs were developed that seem poor by today’s standards, yet each one was an improvement on the ones before
In natural evolution, it results from what has been called ‘tinkering’ with the organism, with minor changes that make their possessors survive or reproduce better than others
In the evolution of a complex structure, several traits must evolve simultaneously, so that the different parts of the structure are well adapted to function as a whole
After enough time, the structure will differ from its ancestral state in many different ways, so that individuals in the descendant population have combinations of characteristics never seen in the ancestral population
example of adaptation evolving
An organism that could change a similar chemical into the useful one would benefit, and so an enzyme could evolve to catalyze that change
The useful chemical would now be synthesized from the related one
Thus, a short biosynthetic pathway, with a precursor and a product, would be favoured
2 sources of evidence of complex adaptations evolving in steps
-existence of intermediates in the fossil record
-present day species that show intermediate stages b/w simple and more advanced stages
In many cases, there is a complete absence of intermediates. E.g. the major divisions of multi-cellular animals(molluscs, arthropods, and vertebrates) nearly all appeared suddenly in the Cambrian with no fossil evidence concerning their ancestors
Recent DNA sequence studies of the relationships between them suggest strongly that these groups were already separate lineages long before the Cambrian era (we don’t know what they look like since they’re soft-bodied→ unlikely to fossilize)
evidence on features that don’t fossilize
Example provided by flight
No fossils connecting bats with other mammals, the first bat fossils, found in deposits over 60myo, have the same highly modified limbs as modern bats
Species that can glide evolve to have similar body structure to bats with wings
Similar to evolution of eyes
All vertebrate animals have the same design of eye, but with many variations of detail adapted to different modes of life
Many types of invertebrates have simple eyes, with no lens and don’t need to see clearly
why do we age?
To measure aging, we can study many individuals kept in a protected environment, where ‘external’ causes of mortality such as predation have been removed, so that individuals live for much longer than in nature
This increase in mortality reflects the deterioration of multiple biological functions with advancing age: almost everything seems to get worse, from muscular strength to mental ability
Aging is an unavoidable consequence of cumulative damage to the systems necessary for continued survival, and selection probably can’t prevent this
Single-celled organisms reproduce by dividing into daughter cells but don’t senesce(grow old), they continually break down damaged components and replace them with new ones
They continue to propagate(reproduce) indefinitely, provided that harmful mutations are removed by selection
evolutionary explanation of aging
These species differences indicate that aging evolves and therefore demands an evolutionary explanation
Selection favours survival and reproduction early rather than late in life since more individuals will be alive to experience the good effects
Aging evolves because of the greater selective value of variants with favorable effects on survival or fertility early in life, compared with variants that act late
2 main ways in which natural selection might work to cause aging:
Mutations with harmful effects will be most strongly selected against if they express their effects early in life
1st way: selection keeps early-acting mutations rare in populations, while allowing ones with effects late in life to become common
-many common human genetic diseases are due to mutations whose harmful effects appear late in life (Alzheimer’s disease)
2nd way: variants that have beneficial effects early in life will be more likely to spread through the population than those whose good effects come only in old age
-improvements to the early stages of life can evolve, even if these benefits come at the expense of harmful side effects later on
-e.g. higher levels of some reproductive hormones may enhance women’s fertility early in life, but at the risk of later breast and ovarian cancer
what does ev. theory predict about species with low death rates
Evolutionary theory of aging predicts that species with low externally caused death rates should evolve low rates of aging and longer life-spans, compared with species with higher external death rates
Strong relation between body size and the rate of aging, smaller species of animals tending to age faster than larger ones, and to reproduce earlier
This reflects the greater vulnerability of many small animals to accidents and predation
how are we an example of evolution of a slower rate of aging
Our closest relatives, chimpanzees, rarely live past 50 years in captivity and start reproducing earlier in life than humans
Humans have therefore substantially reduced their rate of aging since diverging from our common ancestor with apes, and postponed reproductive maturity
These changes are probably due to increased intelligence and ability to cooperate, which reduced vulnerability to external causes of death and reduced the advantage of reproducing early
how has industrialization led to a decline in mortality rates
Industrialization has led to a dramatic decline in mortality rates among adults, which is evident in census data
This changes the natural selection affecting the aging process in human populations
huntington’s disease and frequency of mutant genes
In industrialized societies, with low mortality, people quite often have children at ages when the disease could appear, and in consequence, affected people have on average 15% fewer children than unaffected individuals
If current conditions continue, selection will gradually reduce the frequencies of mutant genes with effects expressed late in reproductive life, and the survival rates of older individuals will increase
evolution of sterile social castes
Another problem for evolutionary theory is posed by the existence of sterile individuals in a number of types of social animals
In social wasps, bees and ants, some of the females in a nest are workers, who do not reproduce
Reproductive females are a small minority within the colony (often just a single queen); the worker females look after the queens’ offspring and maintain and provide for the nest
In the other main group of social insects, the termites, both males and female can behave as workers
In the advanced social insects, there are often several different ‘castes’, which perform very different roles and are distinguished by differences in behavior, size and body structure
kin selection
The members of a social group (e.g. naked mole rat) are generally close relatives, often sharing the same mother and father (concept of kin selection)
A genetic variant that causes its carriers to forgo their own reproductive success to help raise its relatives may help the relatives’ genes pass to the next gen and the relatives’ genes are often the same (bc of relatedness) as the helper individual’s own genes (in the case of a brother/sister, if one individual has a genetic variant inherited from one parent, the chance is one-half that the variant will be present in the other)
If the sacrifice by sterile individuals results in a sufficient increase in number of surviving and reproductively successful relatives, the increase in number of copies of the ‘worker gene’ can outweigh the decrease due to their own lost reproductive success
The increase needed to outweigh the loss is smaller the closer the degree of relationship
what does kin selection provide a framework for
This principle of kin selection provides a framework for understanding the origins of sterility in social animals
Accounts for many details of animal societies, including those with less extreme features than sterile castes
E.g. in some bird species, juvenile males don’t attempt to mate, but remain as ‘helpers’ at their parental nest while younger siblings are being cared for
If a colony with soldiers is better defended against enemies, and if colonies with the variant can produce more reproductives on average, the variant will _______ the success of its colony
increase
If the reproductively active members of the colony are close relatives of the workers, the genetic variant that induces some workers to become soldiers will be transmited by the colony via queens and males founding new colonies
Selection can thus act to ________ the representation of this variant among colonies in the species
increase